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Neil A. Armstrong was an American astronaut. He was the first person to set foot on the
Neil A. Armstrong was an American astronaut. He was the first person to set foot on the
moon. Image credit: NASA
Born in 1930, Neil A. Armstrong, a United States astronaut, was the first person to set
foot on the moon. On July 20, 1969, Armstrong and Buzz Aldrin landed the Apollo 11
lunar module Eagle on the moon. Armstrong left the module and explored the lunar
surface. Upon taking his first step onto the moon, he said: "That's one small step for a
man, one giant leap for mankind." But the word a was lost in radio transmission.
Armstrong was born on Aug. 5, 1930, on his grandparents' farm in Auglaize County,
Ohio. He moved with his family to several Ohio communities before they settled in
Wapakoneta when Neil was 13 years old. Armstrong developed an interest in flying at an
early age. His love of airplanes grew when he went for his first plane ride in a Ford Tri-
Motor, a "Tin Goose," at the age of 6. From then on, he was fascinated by aviation.
In 1947, Armstrong entered Purdue University. He began studies in aeronautical
engineering. But in 1949, the United States Navy called him to active duty. Armstrong
became a Navy pilot and was sent to Korea in 1950, near the start of the Korean War. In
Korea, he flew 78 combat missions in Navy Panther jets.
In 1952, Armstrong returned to Purdue. He earned a bachelor's degree in aeronautical
engineering there in 1955.
Armstrong was a civilian test pilot assigned to test the X-15 rocket airplane before
becoming an astronaut in 1962. He made his first space flight in 1966 on Gemini 8 with
David R. Scott. The two men performed the first successful docking of two vehicles in
space -- the Gemini 8 and an uninhabited Agena rocket.
Armstrong resigned from the United States astronaut program in 1970. Also in 1970, he
earned a master's degree in aerospace engineering at the University of Southern
California. From 1971 to 1979, Armstrong was a professor of aerospace engineering at
the University of Cincinnati. In 1986, he was named vice chairman of a presidential
commission investigating the breakup of the space shuttle Challenger. From 1982 to
1992, Armstrong served as chairman of the board of Computing Technologies for
Aviation, a company that develops software for flight scheduling.
An aurora is a natural display of light in the sky that can be seen with the unaided eye
only at night. An auroral display in the Northern Hemisphere is called the aurora borealis,
or the northern lights. A similar phenomenon in the Southern Hemisphere is called the
aurora australis. Auroras are the most visible effect of the sun's activity on the earth's
Most auroras occur in far northern and southern regions. They appear chiefly as arcs,
clouds, and streaks. Some move, brighten, or
flicker suddenly. The most common color in an
aurora is green. But displays that occur extremely
high in the sky may be red or purple. Most
auroras occur about 60 to 620 miles (97 to 1,000
kilometers) above the earth. Some extend
lengthwise across the sky for thousands of miles
A bar magnet has a magnetic field like
Auroral displays are associated with the solar that of the sun. Field lines, which
wind, a continuous flow of electrically charged represent the field, exit the north pole
particles from the sun. When these particles reach and enter the south pole. Image credit:
the earth's magnetic field, some get trapped. World Book diagram by Precision
Many of these particles travel toward the earth's Graphics
magnetic poles. When the charged particles strike
atoms and molecules in the atmosphere, energy is released. Some of this energy appears
in the form of auroras.
Auroras occur most frequently during the most intense phase of the 11-year sunspot
cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in
number. Violent eruptions on the sun's surface, known as solar flares, are associated with
sunspots. Electrons and protons released by solar flares add to the number of solar
particles that interact with the earth's atmosphere. This increased interaction produces
extremely bright auroras. It also results in sharp variations in the earth's magnetic field
called magnetic storms. During these storms, auroras may shift from the polar regions
toward the equator.
A comet (KOM iht) is an icy body that releases gas
or dust. Most of the comets that can be seen from
Earth travel around the sun in long, oval orbits. A
comet consists of a solid nucleus (core) surrounded
by a cloudy atmosphere called the coma and one or
two tails. Most comets are too small or too faint to
be seen without a telescope. Some comets,
however, become visible to the unaided eye for
several weeks as they pass close to the sun. We can
see comets because the gas and dust in their comas
and tails reflect sunlight. Also, the gases release Halley's Comet becomes visible to
energy absorbed from the sun, causing them to the unaided eye about every 76 years
glow. as it nears the sun. Image credit:
Astronomers classify comets according to how long they take to orbit the sun. Short-
period comets need less than 200 years to complete one orbit, while long-period comets
take 200 years or longer.
Astronomers believe that comets are leftover debris from a collection of gas, ice, rocks,
and dust that formed the outer planets about 4.6 billion years ago. Some scientists believe
that comets originally brought to Earth some of the water and the carbon-based molecules
that make up living things.
Parts of a comet
The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty
snowball. The ice consists mainly of frozen water but may include other frozen
substances, such as ammonia, carbon dioxide, carbon monoxide, and methane. Scientists
believe the nucleus of some comets may be fragile because several comets have split
apart for no apparent reason.
As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on
the surface of the nucleus, spewing gas and dust particles into space. This gas and dust
forms the comet's coma. Radiation from the sun pushes dust particles away from the
coma. These particles form a tail called the dust tail. At the same time, the solar wind --
that is, the flow of high-speed electrically charged particles from the sun-converts some
of the comet's gases into ions (charged particles). These ions also stream away from the
coma, forming an ion tail. Because comet
tails are pushed by solar radiation and the
solar wind, they always point away from the
Most comets are thought to have a nucleus
that measures about 10 miles (16 kilometers)
or less across. Some comas can reach
diameters of nearly 1 million miles (1.6
million kilometers). Some tails extend to
distances of 100 million miles (160 million
The life of a comet
Comets that pass near the sun come from
Scientists think that short-period comets two groups of comets near the outer edge of
come from a band of objects called the the solar system, according to astronomers.
Kuiper belt, which lies beyond the orbit of The disk-shaped Kuiper belt contributes
Pluto. The gravitational pull of the outer comets that orbit the sun in fewer than 200
planets can nudge objects out of the Kuiper years. The Kuiper belt lies beyond Pluto's
belt and into the inner solar system, where orbit, which extends to about 4.6 billion
they become active comets. Long-period miles (7.4 billion kilometers) from the sun.
comets come from the Oort cloud, a nearly The Oort cloud provides comets that take
spherical collection of icy bodies about longer to complete their orbits. The outer
1,000 times farther away from the sun than edge of the Oort cloud may be 1,000 times
Pluto's orbit. Gravitational interactions with farther than the orbit of Pluto. Image credit:
passing stars can cause icy bodies in the World Book diagram by Terry Hadler,
Oort cloud to enter the inner solar system Bernard Thornton Artists
and become active comets.
Comets lose ice and dust each time they return to the inner solar system, leaving behind
trails of dusty debris. When Earth passes through one of these trails, the debris become
meteors that burn up in the atmosphere. Eventually, some comets lose all their ices. They
break up and dissipate into clouds of dust or turn into fragile, inactive objects similar to
The long, oval-shaped orbits of comets can cross the almost circular orbits of the planets.
As a result, comets sometimes collide with planets and their satellites. Many of the
impact craters in the solar system were caused by collisions with comets.
Scientists learned much about comets by studying Halley's Comet as it passed near Earth
in 1986. Five spacecraft flew past the comet and gathered information about its
appearance and chemical composition. Several probes flew close enough to study the
nucleus, which is normally concealed by the comet's coma. The spacecraft found a
roughly potato-shaped nucleus measuring about 9
miles (15 kilometers) long. The nucleus contains
equal amounts of ice and dust. About 80 percent
of the ice is water ice, and frozen carbon
monoxide makes up another 15 percent. Much of
the remainder is frozen carbon dioxide, methane,
and ammonia. Scientists believe that other comets
are chemically similar to Halley's Comet.
Scientists unexpectedly found the nucleus of
Halley's Comet to be extremely dark black. They
The space probe Giotto passed near
now believe that the surface of the comet, and
Halley's Comet on March 14, 1986.
perhaps most other comets, is covered with a
Giotto returned dramatic close-up
black crust of dust and rock that covers most of
images of the comet, including this
the ice. These comets release gas only when holes
one. Image credit: European Space
in this crust rotate toward the sun, exposing the
interior ice to the warming sunlight.
Another comet nucleus that has been seen by spacecraft cameras is that of Comet
Borrelly. During a flyby in 2001, the Deep Space 1 spacecraft observed a nucleus about
half the size of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped
and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas
from small areas where holes in the crust exposed the ice to sunlight.
In 1994, astronomers observed a comet named Shoemaker-Levy 9, which had split into
more than two dozen pieces, crashing into the planet Jupiter. One of the most active
comets seen in more than 400 years was Comet Hale-Bopp, which came within 122
million miles (197 million kilometers) of Earth in 1997. This was not an especially close
approach for a comet. However, Hale-Bopp appeared bright to the unaided eye because
its unusually large nucleus gave off a great deal of dust and gas. The nucleus was
estimated to be about 18 to 25 miles (30 to 40 kilometers) across.
In 2004, the U.S. spacecraft Stardust passed near the nucleus of Comet Wild 2 and
gathered samples from the comet's coma. Stardust was scheduled to return the samples to
Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta
spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014.
Rosetta carried a small probe designed to land on the comet's nucleus.
Europa, (yu ROH puh), is a large moon of Jupiter. Its
surface is made of ice, which may have an ocean of water
beneath it. Such an ocean could provide a home for living
things. The surface layer of ice or ice and water is 50 to
The surface of Europa, a moon
100 miles (80 to 160 kilometers) deep. The satellite has
of Jupiter, consists mostly of
an extremely thin atmosphere. Electrically charged
huge blocks of ice that have
particles from Jupiter's radiation belts continuously
cracked and shifted about,
suggesting that there may be
an ocean of liquid water
Europa is one of the smoothest bodies in the solar system.
underneath. Image credit:
Its surface features include shallow cracks, valleys,
ridges, pits, blisters, and icy flows. None of them extend
more than a few hundred yards or meters upward or downward. In some places, huge
sections of the surface have split apart and separated. The surface of Europa has few
impact craters (pits caused by collisions with asteroids or comets). The splitting and
shifting of the surface and disruptions from below have destroyed most of the old craters.
Europa's interior is hotter than its surface. This internal heat comes from the gravitational
forces of Jupiter and Jupiter's other large satellites, which pull Europa's interior in
different directions. As a result, the interior flexes, producing heat in a process known as
tidal heating. The core of Europa may be rich in iron, but most of the satellite is made of
Europa's diameter is 1,940 miles (3,122 kilometers), slightly smaller than Earth's moon.
Europa takes 3.55 days to orbit Jupiter at a distance of 416,900 miles (670,900
kilometers). The Italian astronomer Galileo discovered Europa in 1610. Much of what is
known about it comes from data gathered by a space probe, also named Galileo, that
orbited Jupiter from 1995 to 2003.
Global warming is an increase in the average temperature of Earth's surface. Since the
late 1800's, the global average temperature has increased about 0.7 to 1.4 degrees F (0.4
to 0.8 degrees C). Many experts estimate that the average temperature will rise an
additional 2.5 to 10.4 degrees F (1.4 to 5.8 degrees C) by 2100. That rate of increase
would be much larger than most past rates of increase.
Scientists worry that human societies and natural ecosystems might not adapt to rapid
climate changes. An ecosystem consists of the living organisms and physical
environment in a particular area. Global warming could cause much harm, so countries
throughout the world drafted an agreement called the Kyoto Protocol to help limit it.
Causes of global warming
Climatologists (scientists who study climate) have analyzed the global warming that has
occurred since the late 1800's. A majority of climatologists have concluded that human
activities are responsible for most of the warming. Human activities contribute to global
warming by enhancing Earth's natural greenhouse effect. The greenhouse effect warms
Earth's surface through a complex process involving sunlight, gases, and particles in the
atmosphere. Gases that trap heat in the atmosphere are known as greenhouse gases.
The main human activities that contribute to global warming are the burning of fossil
fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in
automobiles, in factories, and in electric power plants that provide energy for houses and
office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical
formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees
and other plants remove CO2 from the air during photosynthesis, the process they use to
produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate
at which the gas is removed from the atmosphere or by the decomposition of dead
A small number of scientists argue that the increase in greenhouse gases has not made a
measurable difference in the temperature. They say that natural processes could have
caused global warming. Those processes include increases in the energy emitted (given
off) by the sun. But the vast majority of climatologists believe that increases in the sun's
energy have contributed only slightly to recent warming.
The impact of global warming
Continued global warming could have many
damaging effects. It might harm plants and
animals that live in the sea. It could also force
animals and plants on land to move to new
habitats. Weather patterns could change, causing
flooding, drought, and an increase in damaging
storms. Global warming could melt enough polar
ice to raise the sea level. In certain parts of the
world, human disease could spread, and crop Thousands of icebergs float off the
yields could decline. coast of the Antarctic Peninsula after
1,250 square miles (3,240 square
Harm to ocean life kilometers) of the Larsen B ice shelf
disintegrated in 2002. The area of the
Through global warming, the surface waters of ice was larger than the state of Rhode
the oceans could become warmer, increasing the Island or the nation of Luxembourg.
stress on ocean ecosystems, such as coral reefs. Antarctic ice shelves have been
High water temperatures can cause a damaging shrinking since the early 1970's
process called coral bleaching. When corals because of climate warming in the
bleach, they expel the algae that give them their region. Image credit: NASA/Earth
color and nourishment. The corals turn white and, Observatory
unless the water temperature cools, they die.
Added warmth also helps spread diseases that affect sea creatures.
Changes of habitat
Widespread shifts might occur in the natural habitats of animals and plants. Many species
would have difficulty surviving in the regions they now inhabit. For example, many
flowering plants will not bloom without a sufficient period of winter cold. And human
occupation has altered the landscape in ways that would make new habitats hard to reach
or unavailable altogether.
Extreme weather conditions might become more frequent and therefore more damaging.
Changes in rainfall patterns could increase both flooding and drought in some areas.
More hurricanes and other tropical storms might occur, and they could become more
Rising sea level
Continued global warming might, over centuries, melt large amounts of ice from a vast
sheet that covers most of West Antarctica. As a result, the sea level would rise throughout
the world. Many coastal areas would experience flooding, erosion, a loss of wetlands, and
an entry of seawater into freshwater areas. High sea levels would submerge some coastal
cities, small island nations, and other inhabited regions.
Threats to human health
Tropical diseases, such as malaria and dengue, might spread to larger regions. Longer-
lasting and more intense heat waves could cause more deaths and illnesses. Floods and
droughts could increase hunger and malnutrition.
Changes in crop yields
Canada and parts of Russia might benefit from an increase in crop yields. But any
increases in yields could be more than offset by decreases caused by drought and higher
temperatures -- particularly if the amount of warming were more than a few degrees
Celsius. Yields in the tropics might fall disastrously because temperatures there are
already almost as high as many crop plants can tolerate.
Limited global warming
Climatologists are studying ways to limit global warming. Two key methods would be
(1) limiting CO2 emissions and (2) carbon sequestration -- either preventing carbon
dioxide from entering the atmosphere or removing CO2 already there.
Limiting CO2 emissions
Two effective techniques for limiting CO2 emissions would be (1) to replace fossil fuels
with energy sources that do not emit CO2, and (2) to use fossil fuels more efficiently.
Alternative energy sources that do not emit CO2 include the wind, sunlight, nuclear
energy, and underground steam. Devices known as wind turbines can convert wind
energy to electric energy. Solar cells can convert sunlight to electric energy, and various
devices can convert solar energy to useful heat. Geothermal power plants convert energy
in underground steam to electric energy.
Alternative sources of energy are more expensive to use than fossil fuels. However,
increased research into their use would almost certainly reduce their cost.
Carbon sequestration could take two forms: (1) underground or underwater storage and
(2) storage in living plants.
Underground or underwater storage would involve injecting industrial emissions of CO2
into underground geologic formations or the ocean. Suitable underground formations
include natural reservoirs of oil and gas from which most of the oil or gas has been
removed. Pumping CO2 into a reservoir would have the added benefit of making it easier
to remove the remaining oil or gas. The value of that product could offset the cost of
sequestration. Deep deposits of salt or coal could also be suitable.
The oceans could store much CO2. However, scientists have not yet determined the
environmental impacts of using the ocean for carbon sequestration.
Storage in living plants
Green plants absorb CO2 from the atmosphere as they grow. They combine carbon from
CO2 with hydrogen to make simple sugars, which they store in their tissues. After plants
die, their bodies decay and release CO2. Ecosystems with abundant plant life, such as
forests and even cropland, could tie up much carbon. However, future generations of
people would have to keep the ecosystems intact. Otherwise, the sequestered carbon
would re-enter the atmosphere as CO2.
Agreement on global warming
Delegates from more than 160 countries met in Kyoto, Japan, in 1997 to draft the
agreement that became known as the Kyoto Protocol. That agreement calls for decreases
in the emissions of greenhouse gases.
Thirty-eight industrialized nations would have to restrict their emissions of CO2 and five
other greenhouse gases. The restrictions would occur from 2008 through 2012. Different
countries would have different emissions targets. As a whole, the 38 countries would
restrict their emissions to a yearly average of about 95 percent of their 1990 emissions.
The agreement does not place restrictions on developing countries. But it encourages the
industrialized nations to cooperate in helping developing countries limit emissions
Industrialized nations could also buy or sell emission reduction units. Suppose an
industrialized nation cut its emissions more than was required by the agreement. That
country could sell other industrialized nations emission reduction units allowing those
nations to emit the amount equal to the excess it had cut.
Several other programs could also help an industrialized nation earn credit toward its
target. For example, the nation might help a developing country reduce emissions by
replacing fossil fuels in some applications.
Approving the agreement
The protocol would take effect as a treaty if (1) at least 55 countries ratified (formally
approved) it, and (2) the industrialized countries ratifying the protocol had CO2
emissions in 1990 that equaled at least 55 percent of the emissions of all 38 industrialized
countries in 1990.
In 2001, the United States rejected the Kyoto Protocol. President George W. Bush said
that the agreement could harm the U.S. economy. But he declared that the United States
would work with other countries to limit global warming. Other countries, most notably
the members of the European Union, agreed to continue with the agreement without
United States participation.
By 2004, more than 100 countries, including nearly all the countries classified as
industrialized under the protocol, had ratified the agreement. However, the agreement
required ratification by Russia or the United States to go into effect. Russia ratified the
protocol in November 2004. The treaty was to come into force in February 2005.
Analyzing global warming
Scientists use information from several sources to analyze global warming that occurred
before people began to use thermometers. Those sources include tree rings, cores
(cylindrical samples) of ice drilled from Antarctica and Greenland, and cores drilled out
of sediments in oceans. Information from these sources indicates that the temperature
increase of the 1900's was probably the largest in the last 1,000 years.
Computers help climatologists analyze past climate changes and predict future changes.
First, a scientist programs a computer with a set of mathematical equations known as a
climate model. The equations describe how various factors, such as the amount of CO2 in
the atmosphere, affect the temperature of Earth's surface. Next, the scientist enters data
representing the values of those factors at a certain time. He or she then runs the program,
and the computer describes how the temperature would vary. A computer's representation
of changing climatic conditions is known as a climate simulation.
In 2001, the Intergovernmental Panel on Climate Change (IPCC), a group sponsored by
the United Nations (UN), published results of climate simulations in a report on global
warming. Climatologists used three simulations to determine whether natural variations
in climate produced the warming of the past 100 years. The first simulation took into
account both natural processes and human activities that affect the climate. The second
simulation took into account only the natural processes, and the third only the human
The climatologists then compared the temperatures predicted by the three simulations
with the actual temperatures recorded by thermometers. Only the first simulation, which
took into account both natural processes and human activities, produced results that
corresponded closely to the recorded temperatures.
The IPCC also published results of simulations that predicted temperatures until 2100.
The different simulations took into account the same natural processes but different
patterns of human activity. For example, scenarios differed in the amounts of CO2 that
would enter the atmosphere due to human activities.
The simulations showed that there can be no "quick fix" to the problem of global
warming. Even if all emissions of greenhouse gases were to cease immediately, the
temperature would continue to increase after 2100 because of the greenhouse gases
already in the atmosphere.
A hurricane is a powerful, swirling storm that
begins over a warm sea. Hurricanes form in
waters near the equator, and then they move
toward the poles.
The winds of a hurricane swirl around a calm
central zone called the eye surrounded by a band Hurricane winds swirl about the eye, a
of tall, dark clouds called the eyewall. The eye is calm area in the center of the storm.
usually 10 to 40 miles (16 to 64 kilometers) in The main mass of clouds shown in this
diameter and is free of rain and large clouds. In photograph measures almost 250 miles
the eyewall, large changes in pressure create the (400 kilometers) across. The
hurricane's strongest winds. These winds can hurricane, named Andrew, struck the
reach nearly 200 miles (320 kilometers) per hour. Bahamas, Florida, and Louisiana in
Damaging winds may extend 250 miles (400 1992, killing 65 people and causing
kilometers) from the eye. billions of dollars in damage. Image
Hurricanes are referred to by different labels, depending on where they occur. They are
called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea,
the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons
if they occur in the Northwest Pacific Ocean, west of an imaginary line called the
International Date Line. Near Australia and in the Indian Ocean, they are referred to as
Hurricanes are most common during the summer and early fall. In the Atlantic and the
Northeast Pacific, for example, August and September are the peak hurricane months.
Typhoons occur throughout the year in the Northwest Pacific but are most frequent in
summer. In the North Indian Ocean, tropical cyclones strike in May and November. In
the South Indian Ocean, the South Pacific Ocean, and off the coast of Australia, the
hurricane season runs from December to March. Approximately 85 hurricanes, typhoons,
and tropical cyclones occur in a year throughout the world. In the rest of this article, the
term hurricane refers to all such storms.
Hurricanes require a special set of conditions, including ample heat and moisture, that
exist primarily over warm tropical oceans. For a hurricane to form, there must be a warm
layer of water at the top of the sea with a surface temperature greater than 80 degrees F
(26.5 degrees C).
Warm seawater evaporates and is absorbed by the surrounding air. The warmer the
ocean, the more water evaporates. The warm, moist air rises, lowering the atmospheric
pressure of the air beneath. In any area of low atmospheric pressure, the column of air
that extends from the surface of the water -- or land -- to the top of the atmosphere is
relatively less dense and therefore weighs relatively less.
Air tends to move from areas of high pressure to areas of low pressure, creating wind. In
the Northern Hemisphere, the earth's rotation causes the wind to swirl into a low-pressure
area in a counterclockwise direction. In the Southern Hemisphere, the winds rotate
clockwise around a low. This effect of the rotating earth on wind flow is called the
Coriolis effect. The Coriolis effect increases in intensity farther from the equator. To
produce a hurricane, a low-pressure area must be more than 5 degrees of latitude north or
south of the equator. Hurricanes seldom occur closer to the equator.
For a hurricane to develop, there must be little wind shear -- that is, little difference in
speed and direction between winds at upper and lower elevations. Uniform winds enable
the warm inner core of the storm to stay intact. The storm would break up if the winds at
higher elevations increased markedly in speed, changed direction, or both. The wind
shear would disrupt the budding hurricane by tipping it over or by blowing the top of the
storm in one direction while the bottom moved in another direction.
The life of a hurricane
Meteorologists (scientists who study weather) divide the life of a hurricane into four
stages: (1) tropical disturbance, (2) tropical depression, (3) tropical storm, and (4)
Tropical disturbance is an area where rain clouds are building. The clouds form when
moist air rises and becomes cooler. Cool air cannot hold as much water vapor as warm air
can, and the excess water changes into tiny droplets of water that form clouds. The clouds
in a tropical disturbance may rise to great heights, forming the towering thunderclouds
that meteorologists call cumulonimbus clouds.
Cumulonimbus clouds usually produce heavy rains that end after an hour or two, and the
weather clears rapidly. If conditions are right for a hurricane, however, there is so much
heat energy and moisture in the atmosphere that new cumulonimbus clouds continually
form from rising moist air.
Tropical depression is a low-pressure area surrounded by winds that have begun to blow
in a circular pattern. A meteorologist considers a depression to exist when there is low
pressure over a large enough area to be plotted on a weather map. On a map of surface
pressure, such a depression appears as one or two circular isobars (lines of equal
pressure) over a tropical ocean. The low pressure near the ocean surface draws in warm,
moist air, which feeds more thunderstorms.
The winds swirl slowly around the low-pressure area at first. As the pressure becomes
even lower, more warm, moist air is drawn in, and the winds blow faster.
When the winds exceed 38 miles (61 kilometers) per hour, a tropical storm has
developed. Viewed from above, the storm clouds now have a well-defined circular shape.
The seas have become so rough that ships must steer clear of the area. The strong winds
near the surface of the ocean draw more and more heat and water vapor from the sea. The
increased warmth and moisture in the air feed the storm.
A tropical storm has a column of warm air near its center. The warmer this column
becomes, the more the pressure at the surface falls. The falling pressure, in turn, draws
more air into the storm. As more air is pulled into the storm, the winds blow harder.
Each tropical storm receives a name. The names help meteorologists and disaster
planners avoid confusion and quickly convey information about the behavior of a storm.
The World Meteorological Organization (WMO), an agency of the United Nations, issues
four alphabetical lists of names, one for the North Atlantic Ocean and the Caribbean Sea,
and one each for the Eastern, Central, and Northwestern Pacific. The lists include both
men's and women's names that are popular in countries affected by the storms.
Except in the Northwestern and Central Pacific, the first storm of the year gets a name
beginning with A -- such as Tropical Storm Alberto. If the storm intensifies into a
hurricane, it becomes Hurricane Alberto. The second storm gets a name beginning with
B, and so on through the alphabet. The lists do not use all the letters of the alphabet,
however, since there are few names beginning with such letters as Q or U. For example,
no Atlantic or Caribbean storms receive names beginning with Q, U, X, Y, or Z.
Because storms in the Northwestern Pacific occur throughout the year, the names run
through the entire alphabet instead of starting over each year. The first typhoon of the
year might be Typhoon Nona, for example. The Central Pacific usually has fewer than
five named storms each year.
The system of naming storms has changed since 1950. Before that year, there was no
formal system. Storms commonly received women's names and names of saints of both
genders. From 1950 to 1952, storms were given names from the United States military
alphabet -- Able, Baker, Charlie, and so on. The WMO began to use only the names of
women in 1953. In 1979, the WMO began to use men's names as well.
A storm achieves hurricane status when its
winds exceed 74 miles (119 kilometers) per
hour. By the time a storm reaches hurricane
intensity, it usually has a well-developed eye
at its center. Surface pressure drops to its Hurricane winds on the ocean surface swirl
lowest in the eye. counterclockwise around a calm eye in the
Northern Hemisphere. Image credit: World
In the eyewall, warm air spirals upward, Book illustrations by Bruce Kerr
creating the hurricane's strongest winds. The
speed of the winds in the eyewall is related to the diameter of the eye. Just as ice skaters
spin faster when they pull their arms in, a hurricane's winds blow faster if its eye is small.
If the eye widens, the winds decrease.
Heavy rains fall from the eyewall and bands of dense clouds that swirl around the
eyewall. These bands, called rainbands, can produce more than 2 inches (5 centimeters)
of rain per hour. The hurricane draws large amounts of heat and moisture from the sea.
The path of a hurricane
Hurricanes last an average of 3 to 14 days. A long-lived storm may wander 3,000 to
4,000 miles (4,800 to 6,400 kilometers), typically moving over the sea at speeds of 10 to
20 miles (16 to 32 kilometers) per hour.
Hurricanes in the Northern Hemisphere usually begin by traveling from east to west. As
the storms approach the coast of North America or Asia, however, they shift to a more
northerly direction. Most hurricanes turn gradually northwest, north, and finally
northeast. In the Southern Hemisphere, the storms may travel westward at first and then
turn southwest, south, and finally southeast. The path of an individual hurricane is
irregular and often difficult to predict.
All hurricanes eventually move toward higher latitudes where there is colder air, less
moisture, and greater wind shears. These conditions cause the storm to weaken and die
out. The end comes quickly if a hurricane moves over land, because it no longer receives
heat energy and moisture from warm tropical water. Heavy rains may continue, however,
even after the winds have diminished.
Hurricane damage results from wind and water. Hurricane winds can uproot trees and
tear the roofs off houses. The fierce winds also create danger from flying debris. Heavy
rains may cause flooding and mudslides.
The most dangerous effect of a hurricane, however, is a rapid rise in sea level called a
storm surge. A storm surge is produced when winds drive ocean waters ashore. Storm
surges are dangerous because many coastal areas are densely populated and lie only a few
feet or meters above sea level. A 1970 cyclone in East Pakistan (now Bangladesh)
produced a surge that killed about 266,000 people. A hurricane in Galveston, Texas, in
1900 produced a surge that killed about 6,000 people, the worst natural disaster in United
Hurricane watchers rate the intensity of storms on a scale called the Saffir-Simpson scale,
developed by American engineer Herbert S. Saffir and meteorologist Robert H. Simpson.
The scale designates five levels of hurricanes, ranging from Category 1, described as
weak, to Category 5, which can be devastating. Category 5 hurricanes have included
Hurricane Camille, which hit the United States in 1969; Hurricane Gilbert, which raked
the West Indies and Mexico in 1988; and Hurricane Andrew, which struck the Bahamas,
Florida, and Louisiana in 1992.
Meteorologists use weather balloons, satellites, and radar to watch for areas of rapidly
falling pressure that may become hurricanes. Specially equipped airplanes called
hurricane hunters investigate budding storms.
If conditions are right for a hurricane, the National Weather Service issues a hurricane
watch. A hurricane watch advises an area that there is a good possibility of a hurricane
within 36 hours. If a hurricane watch is issued for your location, check the radio or
television often for official bulletins. A hurricane warning means that an area is in danger
of being struck by a hurricane in 24 hours or less. Keep your radio tuned to a news station
after a hurricane warning. If local authorities recommend evacuation, move quickly to a
safe area or a designated hurricane shelter.
Moon is Earth's only natural satellite and the only
astronomical body other than Earth ever visited by human
beings. The moon is the brightest object in the night sky The moon's surface shows
but gives off no light of its own. Instead, it reflects light striking contrasts of light and
from the sun. Like Earth and the rest of the solar system, dark. The light areas are
the moon is about 4.6 billion years old. rugged highlands. The dark
zones were partly flooded by
The moon is much smaller than Earth. The moon's lava when volcanoes erupted
average radius (distance from its center to its surface) is billions of years ago. The lava
1,079.6 miles (1,737.4 kilometers), about 27 percent of froze to form smooth rock.
the radius of Earth. Image credit: Lunar and
The moon is also much less massive than Earth. The moon has a mass (amount of matter)
of 8.10 x 1019 tons (7.35 x 1019 metric tons). Its mass in metric tons would be written
out as 735 followed by 17 zeroes. Earth is about 81 times that massive. The moon's
density (mass divided by volume) is about 3.34 grams per cubic centimeter, roughly 60
percent of Earth's density.
Because the moon has less mass than Earth, the force due to gravity at the lunar surface is
only about 1/6 of that on Earth. Thus, a person standing on the moon would feel as if his
or her weight had decreased by 5/6. And if that person dropped a rock, the rock would
fall to the surface much more slowly than the
same rock would fall to Earth.
Despite the moon's relatively weak
gravitational force, the moon is close enough
to Earth to produce tides in Earth's waters.
The average distance from the center of
Earth to the center of the moon is 238,897
miles (384,467 kilometers). That distance is
growing -- but extremely slowly. The moon
is moving away from Earth at a speed of
about 1 1/2 inches (3.8 centimeters) per year.
The distance to the moon is measured to an
The temperature at the lunar equator ranges accuracy of 5 centimeters by a laser beam
from extremely low to extremely high -- sent from Earth. The beam bounces off a
from about -280 degrees F (-173 degrees C) laser reflector placed on the moon by
at night to +260 degrees F (+127 degrees C) astronauts, and returns to Earth. Image
credit: World Book diagram by Bensen
in the daytime. In some deep craters near the moon's poles, the temperature is always
near -400 degrees F (-240 degrees C).
The moon has no life of any kind. Compared with Earth, it has changed little over billions
of years. On the moon, the sky is black -- even during the day -- and the stars are always
A person on Earth looking at the moon with the unaided eye can see light and dark areas
on the lunar surface. The light areas are rugged, cratered highlands known as terrae
(TEHR ee). The word terrae is Latin for lands. The highlands are the original crust of the
moon, shattered and fragmented by the impact of meteoroids, asteroids, and comets.
Many craters in the terrae exceed 25 miles (40 kilometers) in diameter. The largest is the
South Pole-Aitken Basin, which is 1,550 miles (2,500 kilometers) in diameter.
The dark areas on the moon are known as maria (MAHR ee uh). The word maria is Latin
for seas; its singular is mare (MAHR ee). The term comes from the smoothness of the
dark areas and their resemblance to bodies of water. The maria are cratered landscapes
that were partly flooded by lava when volcanoes erupted. The lava then froze, forming
rock. Since that time, meteoroid impacts have created craters in the maria.
The moon has no substantial atmosphere, but small amounts of certain gases are present
above the lunar surface. People sometimes refer to those gases as the lunar atmosphere.
This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density)
zone of particles surrounding an airless body. Mercury and some asteroids also have an
In 1959, scientists began to explore the
moon with robot spacecraft. In that year,
the Soviet Union sent a spacecraft called
Luna 3 around the side of the moon that
faces away from Earth. Luna 3 took the
first photographs of that side of the moon.
The word luna is Latin for moon.
On July 20, 1969, the U.S. Apollo 11 The first people on the moon were U.S.
lunar module landed on the moon in the astronauts Neil A. Armstrong, who took this
first of six Apollo landings. Astronaut picture, and Buzz Aldrin, who is pictured next
Neil A. Armstrong became the first to a seismograph. A television camera and a
human being to set foot on the moon. United States flag are in the background.
Their lunar module, Eagle, stands at the right.
In the 1990's, two U.S. robot space Image credit: NASA
probes, Clementine and Lunar Prospector, detected evidence of frozen water at both of
the moon's poles. The ice came from comets that hit the moon over the last 2 billion to 3
billion years. The ice apparently has lasted in areas that are always in the shadows of
crater rims. Because the ice is in the shade, where the temperature is about -400 degrees F
(-240 degrees C), it has not melted and evaporated.
This article discusses Moon (The movements of the moon) (Origin and evolution of the
moon) (The exosphere of the moon) (Surface features of the moon) (The interior of the
moon) (History of moon study).
The movements of the moon
The moon moves in a variety of ways. For example, it rotates on its axis, an imaginary
line that connects its poles. The moon also orbits Earth. Different amounts of the moon's
lighted side become visible in phases because of the moon's orbit around Earth. During
events called eclipses, the moon is positioned in line with Earth and the sun. A slight
motion called libration enables us to see about 59 percent of the moon's surface at
Rotation and orbit
The moon rotates on its axis once every 29 1/2 days. That is the period from one sunrise
to the next, as seen from the lunar surface, and so it is known as a lunar day. By contrast,
Earth takes only 24 hours for one rotation.
The moon's axis of rotation, like that of Earth, is tilted. Astronomers measure axial tilt
relative to a line perpendicular to the ecliptic plane, an imaginary surface through Earth's
orbit around the sun. The tilt of Earth's axis is about 23.5 degrees from the perpendicular
and accounts for the seasons on Earth. But the tilt of the moon's axis is only about 1.5
degrees, so the moon has no seasons.
Another result of the smallness of the moon's tilt is that certain large peaks near the poles
are always in sunlight. In addition, the floors of some craters -- particularly near the south
pole -- are always in shadow.
The moon completes one orbit of Earth with respect to the stars about every 27 1/3 days,
a period known as a sidereal month. But the moon revolves around Earth once with
respect to the sun in about 29 1/2 days, a period known as a synodic month. A sidereal
month is slightly shorter than a synodic month because, as the moon revolves around
Earth, Earth is revolving around the sun. The moon needs some extra time to "catch up"
with Earth. If the moon started on its orbit from a spot between Earth and the sun, it
would return to almost the same place in about 29 1/2 days.
A synodic month equals a lunar day. As a result, the moon shows the same hemisphere --
the near side -- to Earth at all times. The other hemisphere -- the far side -- is always
turned away from Earth.
People sometimes mistakenly use the term dark side to refer to the far side. The moon
does have a dark side -- it is the hemisphere that is turned away from the sun. The
location of the dark side changes constantly, moving with the terminator, the dividing
line between sunlight and dark.
The lunar orbit, like the orbit of Earth, is shaped like a slightly flattened circle. The
distance between the center of Earth and the moon's center varies throughout each orbit.
At perigee (PEHR uh jee), when the moon is closest to Earth, that distance is 225,740
miles (363,300 kilometers). At apogee (AP uh jee), the farthest position, the distance is
251,970 miles (405,500 kilometers). The moon's orbit is elliptical (oval-shaped).
As the moon orbits Earth, an observer on Earth can see the moon appear to change shape.
It seems to change from a crescent to a circle and back again. The shape looks different
from one day to the next because the observer sees different parts of the moon's sunlit
surface as the moon orbits Earth. The different appearances are known as the phases of
the moon. The moon goes through a complete cycle of phases in a synodic month.
The moon has four phases: (1) new moon, (2) first quarter, (3) full moon, and (4) last
quarter. When the moon is between the sun and Earth, its sunlit side is turned away from
Earth. Astronomers call this darkened phase a new moon.
The next night after a new moon, a thin crescent of light appears along the moon's eastern
edge. The remaining portion of the moon that faces Earth is faintly visible because of
earthshine, sunlight reflected from Earth to the moon. Each night, an observer on Earth
can see more of the sunlit side as the terminator, the line between sunlight and dark,
moves westward. After about seven days, the observer can see half a full moon,
commonly called a half moon. This phase is known as the first quarter because it occurs
one quarter of the way through the synodic month. About seven days later, the moon is
on the side of Earth opposite the sun. The entire sunlit side of the moon is now visible.
This phase is called a full moon.
About seven days after a full moon, the observer again sees a half moon. This phase is
the last quarter, or third quarter. After another seven days, the moon is between Earth and
the sun, and another new moon occurs.
As the moon changes from new moon to full moon, and more and more of it becomes
visible, it is said to be waxing. As it changes from full moon to new moon, and less and
less of it can be seen, it is waning. When the moon appears smaller than a half moon, it is
called crescent. When it looks larger than a half moon, but is not yet a full moon, it is
called gibbous (GIHB uhs).
Like the sun, the moon rises in the east and sets in the west. As the moon progresses
through its phases, it rises and sets at different times. In the new moon phase, it rises with
the sun and travels close to the sun across the sky. Each successive day, the moon rises an
average of about 50 minutes later.
Eclipses occur when Earth, the sun, and the moon are in a straight line, or nearly so. A
lunar eclipse occurs when Earth gets directly -- or almost directly -- between the sun and
the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a
full moon. A solar eclipse occurs when the moon gets directly -- or almost directly --
between the sun and Earth, and the moon's shadow falls on Earth. A solar eclipse can
occur only during a new moon.
During one part of each lunar orbit, Earth is between the sun and the moon; and, during
another part of the orbit, the moon is between the sun and Earth. But in most cases, the
astronomical bodies are not aligned directly enough to cause an eclipse. Instead, Earth
casts its shadow into space above or below the moon, or the moon casts its shadow into
space above or below Earth. The shadows extend into space in that way because the
moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun.
People on Earth can sometimes see a small part of the far side of the moon. That part is
visible because of lunar libration, a slight rotation of the moon as viewed from Earth.
There are three kinds of libration: (1) libration in longitude, (2) diurnal (daily) libration,
and (3) libration in latitude. Over time, viewers can see more than 50 percent of the
moon's surface. Because of libration, about 59 percent of the lunar surface is visible from
Libration in longitude occurs because the moon's orbit is elliptical. As the moon orbits
Earth, its speed varies according to a law
discovered in the 1600's by the German astronomer
Johannes Kepler. When the moon is relatively
close to Earth, the moon travels more rapidly than
its average speed. When the moon is relatively far
from Earth, the moon travels more slowly than
average. But the moon always rotates about its own
axis at the same rate. So when the moon is
traveling more rapidly than average, its rotation is
too slow to keep all of the near side facing Earth.
And when the moon is traveling more slowly than
average, its rotation is too rapid to keep all of the
near side facing Earth.
Diurnal libration is caused by a daily change in the
position of an observer on Earth relative to the
moon. Consider an observer who is at Earth's
equator when the moon is full. As Earth rotates Diurnal libration enables an observer
on Earth to see around one edge of
the moon, then the other, during a
single night. The libration occurs
because Earth's rotation changes the
observer's viewpoint by a distance
equal to the diameter of Earth. Image
credit: World Book illustration
from west to east, the observer first sees the moon when it rises at the eastern horizon and
last sees it when it sets at the western horizon. During this time, the observer's viewpoint
moves about 7,900 miles (12,700 kilometers) -- the diameter of Earth -- relative to the
moon. As a result, the moon appears to rotate slightly to the west.
While the moon is rising in the east and climbing to its highest point in the sky, the
observer can see around the western edge of the near side. As the moon descends to the
western horizon, the observer can see around the eastern edge of the near side.
Libration in latitude occurs because the moon's axis of rotation is tilted about 6 1/2
degrees relative to a line perpendicular to the moon's orbit around Earth. Thus, during
each lunar orbit, the moon's north pole tilts first toward Earth, then away from Earth.
When the lunar north pole is tilted toward Earth, people on Earth can see farther than
normal along the top of the moon. When that pole is tilted away from Earth, people on
Earth can see farther than normal along the bottom of the moon.
Origin and evolution of the moon
Scientists believe that the moon formed as a result of a collision known as the Giant
Impact or the "Big Whack." According to this idea, Earth collided with a planet-sized
object 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off
Earth's surface and went into orbit around Earth. The cloud cooled and condensed into a
ring of small, solid bodies, which then gathered together, forming the moon.
The rapid joining together of the small bodies released much energy as heat.
Consequently, the moon melted, creating an "ocean" of magma (melted rock).
The magma ocean slowly cooled and solidified. As it cooled, dense, iron-rich materials
sank deep into the moon. Those materials also cooled and solidified, forming the mantle,
the layer of rock beneath the crust.
As the crust formed, asteroids bombarded it heavily,
shattering and churning it. The largest impacts may have
stripped off the entire crust. Some collisions were so
powerful that they almost split the moon into pieces. One
such collision created the South Pole-Aitken Basin, one
of the largest known impact craters in the solar system.
About 4 billion to 3 billion years ago, melting occurred in A basalt rock that astronauts
the mantle, probably caused by radioactive elements deep brought to Earth from the
in the moon's interior. The resulting magma erupted as moon formed from lava that
dark, iron-rich lava, partly flooding the heavily cratered erupted from a lunar volcano.
surface. The lava cooled and solidified into rocks known Escaping gases created the
as basalts (buh SAWLTS). holes before the lava solidified
into rock. Image credit: Lunar
and Planetary Institute
Small eruptions may have continued until as recently as 1 billion years ago. Since that
time, only an occasional impact by an asteroid or comet has modified the surface.
Because the moon has no atmosphere to burn up meteoroids, the bombardment continues
to this day. However, it has become much less intense.
Impacts of large objects can create craters. Impacts of micrometeoroids (tiny meteoroids)
grind the surface rocks into a fine, dusty powder known as the regolith (REHG uh lihth).
Regolith overlies all the bedrock on the moon. Because regolith forms as a result of
exposure to space, the longer a rock is exposed, the thicker the regolith that forms on it.
The exosphere of the moon
The lunar exosphere -- that is, the materials surrounding the moon that make up the lunar
"atmosphere" -- consists mainly of gases that arrive as the solar wind. The solar wind is a
continuous flow of gases from the sun -- mostly hydrogen and helium, along with some
neon and argon.
The remainder of the gases in the exosphere form on the moon. A continual "rain" of
micrometeoroids heats lunar rocks, melting and vaporizing their surface. The most
common atoms in the vapor are atoms of sodium and potassium. Those elements are
present in tiny amounts -- only a few hundred atoms of each per cubic centimeter of
exosphere. In addition to vapors produced by impacts, the moon also releases some gases
from its interior.
Most gases of the exosphere concentrate about halfway between the equator and the
poles, and they are most plentiful just before sunrise. The solar wind continuously sweeps
vapor into space, but the vapor is continuously replaced.
During the night, the pressure of gases at the lunar surface is about 3.9 x 10-14 pound per
square inch (2.7 x 10-10 pascal). That is a stronger vacuum than laboratories on Earth can
usually achieve. The exosphere is so tenuous -- that is, so low in density -- that the rocket
exhaust released during each Apollo landing temporarily doubled the total mass of the
The surface of the moon is covered with bowl-shaped holes called craters, shallow
depressions called basins, and broad, flat plains known as maria. A powdery dust called
the regolith overlies much of the surface of the moon.
The vast majority of the moon's craters are formed by the
impact of meteoroids, asteroids, and comets. Craters on
the moon are named for famous scientists. For example, Euler Crater has central peaks
Copernicus Crater is named for Nicolaus Copernicus, a and slumped walls. The peaks
Polish astronomer who realized in the 1500's that the almost certainly formed
planets move about the sun. Archimedes Crater is named quickly after the impact that
for the Greek mathematician Archimedes, who made produced the crater
many mathematical discoveries in the 200's B.C. compressed the ground. The
ground rebounded upward,
The shape of craters varies with their size. Small craters forming the peaks. The crater
with diameters of less than 6 miles (10 kilometers) have walls are slumped because the
relatively simple bowl shapes. Slightly larger craters original walls were too steep
cannot maintain a bowl shape because the crater wall is to withstand the force of
too steep. Material falls inward from the wall to the floor. gravity. Material fell inward,
As a result, the walls become scalloped and the floor away from the walls. This
becomes flat. crater, in Mare Imbrium (Sea
of Rains), is about 17 1/2
Still larger craters have terraced walls and central peaks. miles (28 kilometers) across.
Terraces inside the rim descend like stairsteps to the Image credit: Lunar and
floor. The same process that creates wall scalloping is Planetary Institute
responsible for terraces. The central peaks almost
certainly form as did the central peaks of impact craters on Earth. Studies of the peaks on
Earth show that they result from a deformation of the ground. The impact compresses the
ground, which then rebounds, creating the peaks. Material in the central peaks of lunar
craters may come from depths as great as 12 miles (19 kilometers).
Surrounding the craters is rough, mountainous material -- crushed and broken rocks that
were ripped out of the crater cavity by shock pressure. This material, called the crater
ejecta blanket, can extend about 60 miles (100 kilometers) from the crater.
Farther out are patches of debris and, in many cases, irregular secondary craters, also
known as secondaries. Those craters come in a range of shapes and sizes, and they are
often clustered in groups or aligned in rows. Secondaries form when material thrown out
of the primary (original) crater strikes the surface. This material consists of large blocks,
clumps of loosely joined rocks, and fine sprays of ground-up rock. The material may
travel thousands of miles or kilometers.
Crater rays are light, wispy deposits of powder that can extend thousands of miles or
kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes
the powder into the upper surface layer. Thus, craters that still have visible rays must be
among the youngest craters on the moon.
Craters larger than about 120 miles (200 kilometers) across tend to have central
mountains. Some of them also have inner rings of peaks, in addition to the central peak.
The appearance of a ring signals the next major transition in crater shape -- from crater to
Basins are craters that are 190 miles (300 kilometers) or more across. The smaller basins
have only a single inner ring of peaks, but the larger ones typically have multiple rings.
The rings are concentric -- that is, they all have the same center, like the rings of a
dartboard. The spectacular, multiple-ringed basin called the Eastern Sea (Mare Orientale)
is almost 600 miles (1,000 kilometers) across. Other basins can be more than 1,200 miles
(2,000 kilometers) in diameter -- as large as the entire western United States.
Basins occur equally on the near side and far side. Most basins have little or no fill of
basalt, particularly those on the far side. The difference in filling may be related to
variations in the thickness of the crust. The far side has a thicker crust, so it is more
difficult for molten rock to reach the surface there.
In the highlands, the overlying ejecta blankets of the basins make up most of the upper
few miles or kilometers of material. Much of this material is a large, thick layer of
shattered and crushed rock known as breccia (BREHCH ee uh). Scientists can learn about
the original crust by studying tiny fragments of breccia.
Maria, the dark areas on the surface of the moon, make up about 16 percent of the surface
area. Some maria are named in Latin for weather terms -- for example, Mare Imbrium
(Sea of Rains) and Mare Nubium (Sea of Clouds). Others are named for states of mind, as
in Mare Serenitatus (Sea of Serenity) and Mare Tranquillitatis (Sea of Tranquility).
Landforms on the maria tend to be smaller than those of the highlands. The small size of
mare features relates to the scale of the processes that formed them -- volcanic eruptions
and crustal deformation, rather than large impacts.
The chief landforms on the maria include wrinkle
ridges and rilles and other volcanic features.
Wrinkle ridges are blisterlike humps that wind
across the surface of almost all maria. The ridges
are actually broad folds in the rocks, created by
compression. Many wrinkle ridges are roughly
circular, aligned with small peaks that stick up
through the maria and outlining interior rings.
Circular ridge systems also outline buried
features, such as rims of craters that existed
before the maria formed.
Rilles are snakelike depressions that wind across A lunar rover is parked near the edge
many areas of the maria. Scientists formerly of Hadley Rille, a long channel
thought the rilles might be ancient riverbeds. probably formed by lava 4 billion to 3
However, they now suspect that the rilles are billion years ago. The slopes in the
background are part of a formation
called the Swann Hills. This photo
was taken during the Apollo 15
mission in 1971. Astronaut David R.
Scott is reaching under a seat to get a
camera. Image credit: NASA
channels formed by running lava. One piece of evidence favoring this view is the dryness
of rock samples brought to Earth by Apollo astronauts; the samples have almost no water
in their molecular structure. In addition, detailed photographs show that the rilles are
shaped somewhat like channels created by flowing lava on Earth.
Scattered throughout the maria are a variety of other features formed by volcanic
eruptions. Within Mare Imbrium, scarps (lines of cliffs) wind their way across the
surface. The scarps are lava flow fronts, places where lava solidified, enabling lava that
was still molten to pile up behind them. The presence of the scarps is one piece of
evidence indicating that the maria consist of solidified basaltic lava.
Small hills and domes with pits on top are probably little volcanoes. Both dome-shaped
and cone-shaped volcanoes cluster together in many places, as on Earth. One of the
largest concentrations of cones on the moon is the Marius Hills complex in Oceanus
Procellarum (Ocean of Storms). Within this complex are numerous wrinkle ridges and
rilles, and more than 50 volcanoes.
Large areas of maria and terrae are covered by dark material known as dark mantle
deposits. Evidence collected by the Apollo missions confirmed that dark mantling is
Much smaller dark mantles are associated with small craters that lie on the fractured
floors of large craters. Those mantles may be cinder cones -- low, broad, cone-shaped
hills formed by explosive volcanic eruptions.
The interior of the moon
The moon, like Earth, has three interior zones -- crust, mantle, and core. However, the
composition, structure, and origin of the zones on the moon are much different from
those on Earth.
Most of what scientists know about the interior of Earth and the moon has been learned
by studying seismic events -- earthquakes and moonquakes, respectively. The data on
moonquakes come from scientific equipment set up by Apollo astronauts from 1969 to
The average thickness of the lunar crust is about 43 miles (70 kilometers), compared with
about 6 miles (10 kilometers) for Earth's crust. The outermost part of the moon's crust is
broken, fractured, and jumbled as a result of the large impacts it has endured. This
shattered zone gives way to intact material below a depth of about 6 miles. The bottom of
the crust is defined by an abrupt increase in rock density at a depth of about 37 miles (60
kilometers) on the near side and about 50 miles (80 kilometers) on the far side.
The mantle of the moon consists of dense rocks that are rich in iron and magnesium. The
mantle formed during the period of global melting. Low-density minerals floated to the
outer layers of the moon, while dense minerals sank deeper into it.
Later, the mantle partly melted due to a build-up of heat in the deep interior. The source
of the heat was probably the decay (breakup) of uranium and other radioactive elements.
This melting produced basaltic magmas -- bodies of molten rock. The magmas later made
their way to the surface and erupted as the mare lavas and ashes. Although mare
volcanism occurred for more than 1 billion years -- from at least 4 billion years to fewer
than 3 billion years ago -- much less than 1 percent of the volume of the mantle ever
Data gathered by Lunar Prospector confirmed that the moon has a core and enabled
scientists to estimate its size. The core has a radius of only about 250 miles (400
kilometers). By contrast, the radius of Earth's core is about 2,200 miles (3,500
The lunar core has less than 1 percent of the mass of the moon. Scientists suspect that the
core consists mostly of iron, and it may also contain large amounts of sulfur and other
Earth's core is made mostly of molten iron and nickel. This rapidly rotating molten core
is responsible for Earth's magnetic field. A magnetic field is an influence that a magnetic
object creates in the region around it. If the core of a planet or a satellite is molten,
motion within the core caused by the rotation of the planet or satellite makes the core
magnetic. But the small, partly molten core of the moon cannot generate a global
magnetic field. However, small regions on the lunar surface are magnetic. Scientists are
not sure how these regions acquired magnetism. Perhaps the moon once had a larger,
more molten core.
There is evidence that the lunar interior formerly contained gas, and that some gas may
still be there. Basalt from the moon contains holes called vesicles that are created during
a volcanic eruption. On Earth, gas that is dissolved in magma comes out of solution
during an eruption, much as carbon dioxide comes out of a carbonated beverage when
you shake the drink container. The presence of vesicles in lunar basalt indicates that the
deep interior contained gases, probably carbon monoxide or gaseous sulfur. The
existence of volcanic ash is further evidence of interior gas; on Earth, volcanic eruptions
are largely driven by gas.
History of moon study
Some ancient peoples believed that the moon was a rotating bowl of fire. Others thought
it was a mirror that reflected Earth's lands and seas. But philosophers in ancient Greece
understood that the moon is a sphere in orbit around Earth. They also knew that
moonlight is reflected sunlight.
Some Greek philosophers believed that the moon was a world much like Earth. In about
A.D. 100, Plutarch even suggested that people lived on the moon. The Greeks also
apparently believed that the dark areas of the moon were seas, while the bright regions
In about A.D. 150, Ptolemy, a Greek astronomer who lived in Alexandria, Egypt, said
that the moon was Earth's nearest neighbor in space. He thought that both the moon and
the sun orbited Earth. Ptolemy's views survived for more than 1,300 years. But by the
early 1500's, the Polish astronomer Nicolaus Copernicus had developed the correct view
-- Earth and the other planets revolve about the sun, and the moon orbits Earth.
Early observations with telescopes
The Italian astronomer and physicist Galileo wrote the first scientific description of the
moon based on observations with a telescope. In 1609, Galileo described a rough,
mountainous surface. This description was quite different from what was commonly
believed -- that the moon was smooth. Galileo noted that the light regions were rough and
hilly and the dark regions were smoother plains.
The presence of high mountains on the moon fascinated Galileo. His detailed description
of a large crater in the central highlands -- probably Albategnius -- began 350 years of
controversy and debate about the origin of the "holes" on the moon.
Other astronomers of the 1600's mapped and cataloged every surface feature they could
see. Increasingly powerful telescopes led to more detailed records. In 1645, the Dutch
engineer and astronomer Michael Florent van Langren, also known as Langrenus,
published a map that gave names to the surface features of the moon, mostly its craters. A
map drawn by the Bohemian-born Italian astronomer Anton M. S. de Rheita in 1645
correctly depicted the bright ray systems of the craters Tycho and Copernicus. Another
effort, by the Polish astronomer Johannes Hevelius in 1647, included the moon's libration
By 1651, two Jesuit scholars from Italy, the astronomer Giovanni Battista Riccioli and
the mathematician and physicist Francesco M. Grimaldi, had completed a map of the
moon. That map established the naming system for lunar features that is still in use.
Determining the origin of craters
Until the late 1800's, most astronomers thought that volcanism formed the craters of the
moon. However, in the 1870's, the English astronomer Richard A. Proctor proposed
correctly that the craters result from the collision of solid objects with the moon. But at
first, few scientists accepted Proctor's proposal. Most astronomers thought that the
moon's craters must be volcanic in origin because no one had yet described a crater on
Earth as an impact crater, but scientists had found dozens of obviously volcanic craters.
In 1892, the American geologist Grove Karl Gilbert argued that most lunar craters were
impact craters. He based his arguments on the large size of some of the craters. Those
included the basins, which he was the first to recognize as huge craters. Gilbert also noted
that lunar craters have only the most general resemblance to calderas (large volcanic
craters) on Earth. Both lunar craters and calderas are large circular pits, but their
structural details do not resemble each other in any way.
In addition, Gilbert created small craters experimentally. He studied what happened when
he dropped clay balls and shot bullets into clay and sand targets.
Gilbert was the first to recognize that the circular Mare Imbrium was the site of a gigantic
impact. By examining photographs, Gilbert also determined which nearby craters formed
before and after that event. For example, a crater that is partially covered by ejecta from
the Imbrium impact formed before the impact. A crater within the mare formed after the
Describing lunar evolution
Gilbert suggested that scientists could determine the relative age of surface features by
studying the ejecta of the Imbrium impact. That suggestion was the key to unraveling the
history of the moon. Gilbert recognized that the moon is a complex body that was built
up by innumerable impacts over a long period.
In his book The Face of the Moon (1949), the American astronomer and physicist Ralph
B. Baldwin further described lunar evolution. He noted the similarity in form between
craters on the moon and bomb craters created during World War II (1939-1945) and
concluded that lunar craters form by impact.
Baldwin did not say that every lunar feature originated with an impact. He stated
correctly that the maria are solidified flows of basalt lava, similar to flood lava plateaus
on Earth. Finally, independently of Gilbert, he concluded that all circular maria are
actually huge impact craters that later filled with lava.
In the 1950's, the American chemist Harold C. Urey offered a contrasting view of lunar
history. Urey said that, because the moon appears to be cold and rigid, it has always been
so. He then stated -- correctly -- that craters are of impact origin. However, he concluded
falsely that the maria are blankets of debris scattered by the impacts that created the
basins. And he was mistaken in concluding that the moon never melted to any significant
extent. Urey had won the 1934 Nobel Prize in chemistry and had an outstanding scientific
reputation, so many people took his views seriously. Urey strongly favored making the
moon a focus of scientific study. Although some of his ideas were mistaken, his support
of moon study was a major factor in making the moon an early goal of the U.S. space
In 1961, the U.S. geologist Eugene M. Shoemaker founded the Branch of Astrogeology
of the U.S. Geological Survey (USGS). Astrogeology is the study of celestial objects
other than Earth. Shoemaker showed that the moon's surface could be studied from a
geological perspective by recognizing a sequence of relative ages of rock units near the
crater Copernicus on the near side. Shoemaker also studied the Meteor Crater in Arizona
and documented the impact origin of this feature. In preparation for the Apollo missions
to the moon, the USGS began to map the geology of the moon using telescopes and
pictures. This work gave scientists their basic understanding of lunar evolution.
Beginning in 1959, the Soviet Union and the United States sent a series of robot
spacecraft to examine the moon in detail. Their ultimate goal was to land people safely on
the moon. The United States finally reached that goal in 1969 with the landing of the
Apollo 11 lunar module. The United States conducted six more Apollo missions,
including five landings. The last of those was Apollo 17, in December 1972.
The Apollo missions revolutionized the understanding of the moon. Much of the
knowledge gained about the moon also applies to Earth and the other inner planets --
Mercury, Venus, and Mars. Scientists learned, for
example, that impact is a fundamental geological process
operating on the planets and their satellites.
After the Apollo missions, the Soviets sent four Luna
robot craft to the moon. The last, Luna 24, returned
samples of lunar soil to Earth in August 1976.
The Clementine orbiter used
No more spacecraft went to the moon until January 1994, radar signals to find evidence
when the United States sent the orbiter Clementine. From of a large deposit of frozen
February to May of that year, Clementine's four cameras water on the moon. The orbiter
took more than 2 million pictures of the moon. A laser sent radar signals to various
device measured the height and depth of mountains, target points on the lunar
craters, and other features. Radar signals that Clementine surface. The targets reflected
some of the signals to Earth,
where they were received by
large antennas and analyzed.
Image credit: Lunar and
bounced off the moon provided evidence of a large deposit of frozen water. The ice
appeared to be inside craters at the south pole.
The U.S. probe Lunar Prospector orbited the moon from January 1998 to July 1999. The
craft mapped the concentrations of chemical elements in the moon, surveyed the moon's
magnetic fields, and found strong evidence of ice at both poles. Small particles of ice are
apparently part of the regolith at the poles.
The SMART-1 spacecraft, launched by the European Space Agency in 2003, went into
orbit around the moon in 2004. The craft's instruments were designed to investigate the
moon's origin and conduct a detailed survey of the chemical elements on the lunar
A planet is a large, round heavenly body that orbits a star
and shines with light reflected from the star. Eight planets
orbit the sun in our solar system. In order of increasing
distance from the sun, they are: (1) Mercury, (2) Venus,
(3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus,
and (8) Neptune. Many nearly planet-sized objects, called
dwarf planets, also orbit the sun. Dwarf planets include
The sun blazes with energy.
Pluto and Ceres. Since 1992, astronomers have
On its surface, magnetic forces
discovered many planets orbiting other stars.
create loops and streams of gas
Traditionally, the term planet has had no formal definition
that extend tens of thousands
in astronomy. Millions of objects orbit the sun—the most
of miles or kilometers into
basic characteristic of a planet. But scholars have
space. This image was made
struggled to devise a simple classification system that
by photographing ultraviolet
distinguishes the smallest worlds from the largest comets,
radiation given off by atoms of
asteroids, and other bodies.
iron gas that are hotter than 9
million degrees F (5 million
The International Astronomical Union (IAU), the
degrees C). Image credit:
recognized authority in naming heavenly bodies, divides
NASA/Transition Region &
objects that orbit the sun into three major classes: (1) planets, (2) dwarf planets, and (3)
small solar system bodies. A planet orbits the sun and no other body. It has so much mass
(amount of matter) that its own gravitational pull compacts it into a round shape. In
addition, a planet has a strong enough gravitational pull to sweep the region of its orbit
relatively free of other objects. A dwarf planet also orbits the sun and is large enough to
be round. However, it does not have a strong enough gravitational pull to clear the region
of its orbit. Small solar system bodies, including most asteroids and comets, have too
little mass for gravity to round their irregular shapes. Many planets, dwarf planets, and
other bodies have smaller objects orbiting them called satellites or moons.
The planets in our solar system can be divided into two groups. The innermost four
planets—Mercury, Venus, Earth, and Mars—are
small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin
word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets
have from 38 to 95 percent of Earth's diameter and from 5.5 to 82 percent of Earth's
The outer four planets—Jupiter, Saturn, Uranus, and Neptune—are called gas giants or
Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All
four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other
materials also occur, including traces of ammonia and methane in their atmospheres.
They range from 3.9 times to 11.2 times Earth's diameter and from 15 times to 318 times
Earth's mass. Jupiter, Saturn, and Neptune give off more energy than they receive from
the sun. Most of this extra energy takes the form of infrared radiation, which is felt as
heat, instead of visible light. Scientists think the source of some of the energy is probably
the slow compression of the planets by their own gravity.
From its discovery in 1930, Pluto was generally considered a planet. However, its small
size and irregular orbit led many astronomers to question whether Pluto should be
grouped with worlds such as Earth and Jupiter. Pluto more closely resembles other icy
objects found in a region of the outer solar system called the Kuiper belt. In the early
2000’s, astronomers found several such Kuiper belt objects (KBO’s) comparable in size
to Pluto. The IAU created the “dwarf planet” classification to describe Pluto and other
nearly planet-sized objects.
Observing the planets
People have known the inner six planets of our solar system for thousands of years
because they are visible from Earth without a telescope. The outermost three planets—
Uranus and Neptune—were discovered by astronomers, beginning in the 1780's. These
planets can be seen from Earth with a telescope.
To the unaided eye, the planets look much like the background stars in the night sky.
However, the planets move slightly from night to night in relation to the stars. The name
planet comes from a Greek word meaning to wander. The planets and the moon follow
the same apparent path through the sky. This path, known as the zodiac, is about 16°
wide. At its center is the ecliptic, the apparent path of the sun. If you see a bright object
near the ecliptic at night or near sunrise or sunset, it is most likely a planet. You can even
see the brightest planets in the daytime, if you know where to look.
Planets and stars also differ in the steadiness of their light when viewed from Earth's
surface. Planets shine with a steady light, but stars seem to twinkle.
The twinkling is due to the moving layers of air that surround Earth. Stars are so far away
that they are mere points of light in the sky, even when viewed through a telescope. The
atmosphere bends the starlight passing through it. As small regions of the atmosphere
move about, the points of light seem to dance and change in brightness.
Planets, which are much closer, look like tiny disks through a telescope. The atmosphere
scatters light from different points on a planet's disk. However, enough light always
arrives from a sufficient number of points to provide a steady appearance.
Viewed from Earth's surface, the planets of the solar system and the stars appear to move
around Earth. They rise in the east and set in the west each night. Most of the time, the
planets move westward across the sky slightly more slowly than the stars do. As a result,
the planets seem to drift eastward relative to the background stars. This motion is called
prograde. For a while each year, however, the planets seem to reverse their direction.
This backward motion is called retrograde.
In ancient times, most scientists thought that the moon, sun, planets, and stars actually
moved around Earth. One puzzle that ancient scientists struggled to explain was the
annual retrograde motion of the planets. In about A.D. 150, the Greek astronomer
Ptolemy developed a theory that the planets orbited in small circles, which in turn orbited
Earth in larger circles. Ptolemy thought that retrograde motion was caused by a planet
moving on its small circle in an opposite direction from the motion of the small circle
around the big circle.
In 1543, the Polish astronomer Nicolaus Copernicus showed that the sun is the center of
the orbits of the planets. Our term solar system is based on Copernicus's discovery.
Copernicus realized that retrograde motion occurs because Earth moves faster in its orbit
than the planets that are farther from the sun. The planets that are closer to the sun move
faster in their orbits than Earth travels in its orbit. Retrograde motion occurs whenever
Earth passes an outer planet traveling around the sun or an inner planet passes Earth.
In the 1600's, the German astronomer Johannes Kepler used observations of Mars by the
Danish astronomer Tycho Brahe to figure out three laws of planetary motion. Although
Kepler developed his laws for the planets of our solar system, astronomers have since
realized that Kepler's laws are valid for all heavenly bodies that orbit other bodies.
Kepler's first law says that planets move in elliptical (oval-shaped) orbits around their
parent star—in our solar system, the sun. An ellipse is a closed curve formed around two
fixed points called foci. The ellipse is formed by the path of a point moving so that the
sum of its distances from the two foci remains the same. The orbital paths of the planets
form such curves, with the parent star at one focus of the ellipse. Before Kepler, scientists
had assumed that the planets moved in circular orbits.
Kepler's second law says that an imaginary line joining the parent star to its planet
sweeps across equal areas of space in equal amounts of time. When a planet is close to its
star, it moves relatively rapidly in its orbit. The line therefore sweeps out a short, fat,
trianglelike figure. When the planet is farther from its star, it moves relatively slowly. In
this case, the line sweeps out a long, thin figure that resembles a triangle. But the two
figures have equal areas.
Kepler's third law says that a planet's period (the time it takes to complete an orbit around
its star) depends on its average distance from the star. The law says that the square of the
planet's period—that is, the period multiplied by itself—is proportional to the cube of the
planet's average distance from its star—the distance multiplied by itself twice—for all
planets in a solar system.
The English scientist, astronomer, and mathematician Isaac Newton presented his theory
of gravity and explained why Kepler's laws work in a treatise published in 1687. Newton
showed how his expanded version of Kepler's third law could be used to find the mass of
the sun or of any other object around which things orbit. Using Newton's explanation,
astronomers can determine the mass of a planet by studying the period of its moon or
moons and their distance from the planet.
Planets rotate at different rates. One day is defined as how long it takes Earth to rotate
once. Jupiter and Saturn spin much faster, in only about 10 hours. Venus rotates much
slower, in about 243 Earth days.
Most planets rotate in the same direction in which they revolve around the sun, with their
axis of rotation standing upright from their orbital path. A law of physics holds that such
rotation does not change by itself. So astronomers think that the solar system formed out
of a cloud of gas and dust that was already spinning.
Uranus is tipped on its side, however, so that its axes lies nearly level with its paths
around the sun. Venus is tipped all the way over. Its axis is almost completely upright,
but the planet rotates in the direction opposite from the direction of its revolution around
the sun. Most astronomers think that some other objects in the solar
system must have collided with Uranus, Pluto, and Venus and tipped
The planets of our solar system
Astronomers measure distances within the solar system in Mercury was first
astronomical units (AU). One astronomical unit is the average photographed in
distance between Earth and the sun, which is about 93 million miles detail on March
(150 million kilometers). The inner planets have orbits whose 29, 1974, by the
diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the U.S. probe
gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Mariner 10. Image
Because of their different distances from the sun, the temperature, credit: NASA
surface features, and other conditions on the planets vary widely.
Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close
to the sun that temperatures on its surface can climb as high as 800 degrees F (430
degrees C). But some regions near the planet's poles may be always in shadow, and
astronomers speculate that water or ice may remain there. No spacecraft has visited
Mercury since the 1970's, when Mariner 10 photographed about half the planet's surface
at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by
Mercury three times before going into orbit around the planet in 2011.
Venus is known as Earth's twin because it resembles Earth in size and
mass, though it has no moon. Venus has a dense atmosphere that
consists primarily of carbon dioxide. The pressure of the atmosphere
on Venus's surface is 90 times that of Earth's atmosphere. Venus's
thick atmosphere traps energy from the sun, raising the surface
temperature on Venus to about 870 degrees F (465 degrees C), hot
enough to melt lead. This trapping of heat is Thick clouds of
known as the greenhouse effect. Scientists have sulfuric acid cover
warned that a similar process on Earth is causing Venus. Image
permanent global warming. Several spacecraft credit: NASA
have orbited or landed on Venus. In the 1990's, the
Magellan spacecraft used radar -- radio waves
bounced off the planet -- to map Venus in detail.
Earth, our home
Earth, our home planet, has an atmosphere that is planet, has oceans
mostly nitrogen with some oxygen. Earth has of liquid water,
oceans of liquid water and continents that rise and continents
above sea level. Many measuring devices on the that rise above sea
surface and in space monitor conditions on our level. Image
planet. In 1998, the National Aeronautics and credit:
Space Administration (NASA) launched the first of NASA/Goddard
a series of satellites called the Earth Observing Space Flight
System (EOS). The EOS satellites will carry remote-sensing instruments to measure
climate changes and other conditions on Earth's surface.
Mars is known as the red planet because of its reddish-brown
appearance, caused by rusty dust on the Martian surface. Mars is a
cold, dry world with a thin atmosphere. The atmospheric pressure
(pressure exerted by the weight of the gases in the atmosphere) on the
Martian surface is less than 1 percent the atmospheric pressure on
Earth. This low surface pressure has enabled most of the water that
Mars may once have had to escape into space.
The planet Mars
has clouds in its The surface of Mars has giant volcanoes, a huge system of canyons,
atmosphere and a and stream beds that look as if water flowed through them in the past.
deposit of ice at its Mars has two tiny moons, Phobos and Deimos. Many spacecraft have
north pole. Image landed on or orbited Mars.
NASA/JPL/Malin Jupiter, the largest planet in our solar system, has more mass than the
Space Science other planets combined. Like the other Jovian planets, it has gaseous The layers of
Systems outer layers and may have a rocky core. A huge storm system called dense clouds
the Great Red Spot in Jupiter's atmosphere is larger than Earth and has raged for around Jupiter
hundreds of years. appear in a
photograph of the
Jupiter's four largest moons -- Io, Europa, Ganymede, and Callisto -- are larger than planet taken by
Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter's equator are three the Voyager 1
thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by space probe.
Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a Image credit: JPL
probe into Jupiter's atmosphere. Galileo orbited Jupiter from 1995 to 2003.
Saturn, another giant planet, has a magnificent set of gleaming
rings. Its gaseous atmosphere is not as colorful as Jupiter's,
however. One reason Saturn is relatively drab is that its hazy upper
atmosphere makes the cloud patterns below difficult to see. Another
Saturn is encircled reason is that Saturn is farther than Jupiter from the sun. Because of
by seven major the difference in distance, Saturn is colder than Jupiter. Due to the
rings. Image credit: temperature difference, the kinds of chemical reactions that color
NASA/JPL/Space Jupiter's atmosphere occur too slowly to do the same on Saturn.
Saturn's moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of
nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up
views of Saturn and its rings and moons.
The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was
designed to be dropped into Titan's atmosphere.
Uranus was the first planet discovered with a telescope. German-
born English astronomer William Herschel found it in 1781. He Uranus appears in true
at first thought he had discovered a comet. Almost 200 years colors, left, and false
later, scientists detected 10 narrow rings around Uranus when the colors, right, in images
planet moved in front of a star and the rings became visible. produced by
Voyager 2 studied Uranus and its rings and moons close-up in combining numerous
1986. pictures taken by the
Voyager 2 spacecraft.
Neptune was first observed in 1846 by German Image credit: JPL
astronomer Johann G. Galle after other astronomers predicted its
position by studying how it affected Uranus's orbit. In 1989, Voyager 2
found that Neptune had a storm system called the Great Dark Spot,
similar to Jupiter's Great Red Spot. But five years later, in 1994, the
Hubble Space Telescope found that the Great Dark Spot had vanished.
The blue clouds Neptune has four narrow rings, one of which has clumps of
of Neptune are matter. Neptune's moon Triton is one of the largest in the solar
mostly frozen system and has volcanoes that emit plumes of frozen nitrogen.
other object The Dwarf Planets
Pluto is so far from
Neptune's moon The solar system’s dwarf planets consist primarily of rock and ice
Earth that even
Triton. Image and feature little or no atmosphere. They lack the mass to sweep powerful telescopes
credit: their orbits clear, so they tend to be found among populations of reveal little detail of its
NASA/JPL similar, smaller bodies. surface. The Hubble
Ceres ranks as the largest of millions of asteroids found between the orbits of Mars gathered the light for
and Jupiter. Ceres has a rocky composition and resembles a slightly squashed the pictures of Pluto
sphere. Its longest diameter measures 596 miles (960 kilometers). The Italian shown here. Image
astronomer Giuseppe Piazzi discovered Ceres in 1801. As with Pluto, people once credit: NASA
widely considered Ceres a planet.
The outer dwarf planets generally lie beyond the orbit of Neptune. Astronomers have had
difficulty studying bodies in this region because they are extremely far from Earth.
Dozens of them probably fit the IAU’s definition of a dwarf planet. Most of these bodies
belong to the Kuiper belt.
Some astronomers have suggested calling the outer dwarf planets plutonians in honor of
Pluto, the first one discovered. A body designated 2003 UB313 ranks as the largest dwarf
planet, with a diameter of around 1,500 miles (2,400 kilometers). Quaoar «KWAH oh
wahr» , a KBO discovered in 2002, measures roughly half the size of Pluto. Sedna,
discovered in 2004, measures about three-fourths the size of Pluto and lies nearly three
times as far from the sun. Some scientists think Sedna belongs to population of cometlike
objects called the Oort cloud, which lies beyond the Kuiper belt.
2003 UB 313 has a surface that contains methane ice and remains around –406 °F (–243
°C). 2003 UB313 has a small moon about 1⁄8 its diameter. The American astronomers
Michael E. Brown, Chadwick A. Trujillo, and David L. Rabinowitz announced the
discovery of 2003 UB313 in 2005.
Pluto was long considered the ninth planet. The American astronomer Clyde W.
Tombaugh discovered Pluto in 1930. Pluto is slightly smaller than 2003 UB313. Its
surface also features methane ice. Pluto’s largest satellite, Charon, measures about half
the dwarf planet’s diameter. The New Horizons probe, launched in 2006, was designed to
observe Pluto during a flyby in 2015.
Planets in other solar systems
Even with the most advanced telescopes, astronomers cannot see planets orbiting other
stars directly. The planets shine only by reflected light and are hidden by the brilliance of
their parent stars. The planets and their stars are also much farther away than our sun. The
nearest star is 4.2 light-years away, compared to 8 light-minutes for the sun. One light-
year is the distance that light travels in one year -- about 5.88 trillion miles (9.46 trillion
kilometers). Thus, it takes light 4.2 years to reach Earth from the nearest star beyond the
sun and only 8 minutes to reach Earth from the sun.
Scientists know of more than 100 stars other than the sun that have planets. Astronomers
cannot see planets around distant stars. However, they can detect the planets from tiny
changes in the stars' movement and tiny decreases in the amount of light coming from the
stars. The changes in a star's movement are caused by the slight pull of the planet's
gravity on its parent star. To find new planets, astronomers use a technique called
spectroscopy, which breaks down the light from stars into its component rainbow of
colors. The scientists look for places in the rainbow where colors are missing. At these
places, dark lines known as spectral lines cross the rainbow. The spectral lines change
their location in the rainbow slightly as a star is pulled by the gravity of an orbiting planet
toward and away from Earth. These apparent changes in a star's light as the star moves
are due to a phenomenon known as the Doppler effect. The changes not only show that a
planet is present but also indicate how much mass it has.
The amount of light coming from the star decreases when the planet passes in front of the
star. The planet blocks some of the starlight, dimming the star.
The first discoveries
Astronomers announced the discovery of the first planets around a star other than our sun
in 1992. The star is a pulsar named PSR B1257+12 in the constellation Virgo. Pulsars are
dead stars that have collapsed until they are only about 12 miles (20 kilometers) across.
They spin rapidly on their axes, sending out radio waves that arrive on Earth as pulses of
radio energy. Some pulsars spin hundreds of times each second. If a pulsar has a planet,
the planet pulls the star to and fro slightly as it orbits. These pulls cause slight variations
in the radio pulses. From measurements of these variations, the Polish-born American
astronomer Alexander Wolszczan and American Dale A. Frail discovered three planets in
orbit around PSR B1257+12. The star emits such strong X rays, however, that no life
could survive on its planets.
Astronomers soon began to find planets around stars more like the sun. In 1995, Swiss
astronomers Michel Mayor and Didier Queloz found the first planet orbiting a sunlike
star, 51 Pegasi, in the constellation Pegasus. American astronomers Geoffrey W. Marcy
and R. Paul Butler confirmed the discovery and found planets of their own around other
stars. In 1999, astronomers announced the first discovery of a multiple-planet system
belonging to a sunlike star. They determined that three planets orbit the star Upsilon
Andromedae, which is 44 light-years from Earth in the constellation Andromeda.
Also in 1999, American astronomer Gregory W. Henry first detected a dimming of
starlight due to the presence of a planet. The star that Henry observed is known as HD
209458, and it is located in Pegasus. Henry measured the star's brightness at the request
of Marcy, Butler, and American astronomer Steven S. Vogt, who had previously used the
spectroscopic technique to identify this star as a parent of a planet.
Some stars have a planet orbiting them at a distance at which living things could exist.
Most scientists consider liquid water essential for life, so a region that is neither too hot
nor too cold for liquid water is known as a habitable zone. Although astronomers have
found stars with planets in their habitable zones, all the planets found so far are probably
gaseous with no solid surface. But they may have solid moons.
In 2001, Marcy announced the discovery of a solar system containing an extremely
unusual object. That object and an ordinary planet orbit the star HD 168443, which is 123
light-years away in the constellation Serpens. The object is so unusual because of its
mass. It is at least 17 times as massive as Jupiter.
Astronomers are not yet sure how to classify the object. They had not thought that a
planet could be as massive as the object is. Before this discovery, the only known
heavenly bodies of such mass were dim objects called brown dwarfs. But brown dwarfs
form by means of the same process that forms stars, not planets.
Astronomers also have been surprised to find that other solar systems have huge, gaseous
planets in close orbits. In our own solar system, the inner planets are rocky and small, and
only the outer planets, except for Pluto, are huge and gassy. But several newly discovered
planets have at least as much mass as Jupiter, the largest planet in our solar system.
Unlike Jupiter, however, these massive planets race around their stars in only a few
weeks. Kepler's third law says that for a planet to complete its orbit so quickly, it must be
close to its parent star. Several of these giant planets, therefore, must travel around their
stars even closer than our innermost planet, Mercury, orbits our sun. Such close orbits
would make their surfaces too hot to support life as we know it.
Some newly discovered planets follow unusual orbits. Most planets travel around their
stars on nearly circular paths, like those of the planets in our solar system. But a planet
around the star 16 Cygni B follows an extremely elliptical orbit. It travels farther from its
star than the planet Mars does from our sun, and then draws closer to the star than Venus
does to our sun. If a planet in our solar system traveled in such an extreme oval, its
gravity would disrupt the orbits of the other planets and toss them out of their paths.
hroughout the early 2000's, astronomers continued to improve techniques for detecting
planets, enabling them to discover an increasing variety of planets around other stars. In
2004, astronomers announced the first discoveries of planets much smaller than Jupiter.
The newly discovered planets were about the size of Uranus or Neptune. Despite the
planets' huge size, astronomers theorized that some of them might be rocky planets rather
than gas giants.
How the planets formed
Astronomers have developed a theory about how our solar system formed that explains
why it has small, rocky planets close to the sun and big, gaseous ones farther away.
Astronomers believe our solar system formed about 4.6 billion years ago from a giant,
rotating cloud of gas and dust called the solar nebula. Gravity pulled together a portion of
gas and dust at the center of the nebula that was denser than the rest. The material
accumulated into a dense, spinning clump that formed our sun.
The remaining gas and dust flattened into a disk called a protoplanetary disk swirling
around the sun. Protoplanetary disks around distant stars were first observed through
telescopes in 1983. Rocky particles within the disk collided and stuck together, forming
bodies called planetesimals. Planetesimals later combined to form the planets. At the
distances of the outer planets, gases froze into ice, creating huge balls of frozen gas that
formed the Jovian planets.
Hot gases and electrically charged particles flow from our sun constantly, forming a
stream called the solar wind. The solar wind was stronger at first than it is today. The
early solar wind drove the light elements -- hydrogen and helium -- away from the inner
planets like Earth. But the stronger gravity of the giant outer planets held on to more of
the planets' hydrogen and helium, and the solar wind was weaker there. So these outer
planets kept most of their light elements and wound up with much more mass than Earth.
Astronomers developed these theories when they thought that rocky planets always
orbited close to the parent star and giant planets farther out. But the "rule" was based only
on our own solar system. Now that astronomers have learned something about other solar
systems, they have devised new theories. Some scientists have suggested that the giant
planets in other solar systems may have formed far from their parent stars and later
moved in closer.
A rocket is a type of engine that pushes itself forward or upward by producing thrust.
Unlike a jet engine, which draws in outside air, a rocket engine uses only the substances
carried within it. As a result, a rocket can operate in outer space, where there is almost no
air. A rocket can produce more power for its size than any other kind of engine. For
example, the main rocket engine of the space shuttle weighs only a fraction as much as a
train engine, but it would take 39 train engines to produce the same amount of power.
The word rocket can also mean a vehicle or object driven by a rocket engine.
Rockets come in a variety of sizes. Some rockets that shoot fireworks into the sky
measure less than 2 feet (60 centimeters) long. Rockets 50 to 100 feet (15 to 30 meters)
long serve as long-range missiles that can be used to bomb distant targets during wartime.
Larger and more powerful rockets lift spacecraft, artificial satellites, and scientific probes
into space. For example, the Saturn 5 rocket that carried astronauts to the moon stood
about 363 feet (111 meters) tall.
Rocket engines generate thrust by expelling gas. Most rockets produce thrust by burning
a mixture of fuel and an oxidizer, a substance that enables the fuel to burn without
drawing in outside air. This kind of rocket is called a chemical rocket because burning
fuel is a chemical reaction. The fuel and oxidizer are called the propellants.
A chemical rocket can produce great power, but it burns propellants rapidly. As a result,
it needs a large amount of propellants to work for even a short time. The Saturn 5 rocket
burned more than 560,000 gallons (2,120,000 liters) of propellants during the first 2 3/4
minutes of flight. Chemical rocket engines become extremely hot as the propellants burn.
The temperature in some engines reaches o 6000 degrees F (3300 degrees C), much
higher than the temperature at which steel melts.
Jet engines also burn fuel to generate thrust. Unlike rocket engines, however, jet engines
work by drawing in oxygen from the surrounding air. For more information on jet
engines, see Jet propulsion.
Researchers have also developed rockets that do not burn propellants. Nuclear rockets
use heat generated by a nuclear fuel to produce thrust. In an electric rocket, electric
energy produces thrust.
Military forces have used rockets in war for hundreds of years. In the 1200's, Chinese
soldiers fired rockets against attacking armies. British troops used rockets to attack Fort
McHenry in Maryland during the War of 1812 (1812-1815). After watching the battle,
the American lawyer Francis Scott Key described "the rocket's red glare" in the song
"The Star-Spangled Banner." During World War I (1914-1918), the French used rockets
to shoot down enemy observation balloons. Germany attacked London with V-2 rockets
during World War II (1939-1945). In the Persian Gulf War of 1991 and the Iraq War,
which began in 2003, United States troops launched rocket-powered Patriot missiles to
intercept and destroy Iraqi missiles.
Rockets are the only vehicles powerful enough to carry people and equipment into space.
Since 1957, rockets have lifted hundreds of artificial satellites into orbit around Earth.
These satellites take pictures of Earth's weather, gather information for scientific study,
and transmit communications around the world. Rockets also carry scientific instruments
far into space to explore and study other planets. Since 1961, rockets have launched
spacecraft carrying astronauts and cosmonauts into orbit around Earth. In 1969, rockets
carried astronauts to the first landing on the moon. In 1981, rockets lifted the first space
shuttle into Earth orbit.
This article discusses Rocket (How rockets work) (How rockets are used) (Kinds of
rocket engines) (History).
How rockets work
Rocket engines generate thrust by putting a gas under pressure. The pressure forces the
gas out the end of the rocket. The gas escaping the rocket is called exhaust. As it escapes,
the exhaust produces thrust according to the laws of motion developed by the English
scientist Isaac Newton. Newton's third law of motion states that for every action, there is
an equal and opposite reaction. Thus, as the rocket pushes the exhaust backward, the
exhaust pushes the rocket forward.
The amount of thrust produced by a rocket depends on the momentum of the exhaust --
that is, its total amount of motion. The exhaust's momentum equals its mass (amount of
matter) multiplied by the speed at which it exits the rocket. The more momentum the
exhaust has, the more thrust the rocket produces. Engineers can therefore increase a
rocket's thrust by increasing the mass of exhaust it produces. Alternately, they can
increase the thrust by increasing the speed at which the exhaust leaves the rocket.
Parts of a rocket include the rocket engine and the equipment and cargo the rocket
carries. The four major parts of a rocket are (1) the payload, (2) propellants, (3) the
chamber, and (4) the nozzle.
The payload of a rocket includes the cargo, passengers, and equipment the rocket carries.
The payload may consist of a spacecraft, scientific instruments, or even explosives. The
space shuttle's payload, for example, is the shuttle orbiter and the mission astronauts and
any satellites, scientific experiments, or supplies the orbiter carries. The payload of a
missile may include explosives or other weapons. This kind of payload is called a
Propellants generally make up most of the weight of a rocket. For example, the fuel and
oxidizer used by the space shuttle account for nearly 90 percent of its weight at liftoff.
The shuttle needs such a large amount of propellant to overcome Earth's gravity and the
resistance of the atmosphere.
The space shuttle and many other chemical rockets use liquid hydrogen as fuel. Hydrogen
becomes a liquid only at extremely low temperatures, requiring powerful cooling
systems. Kerosene, another liquid fuel, is easier to store because it remains liquid at room
Many rockets, including the space shuttle, use liquid oxygen, or lox, as their oxidizer.
Like hydrogen, oxygen must be cooled to low temperatures to become a liquid. Other
commonly used oxidizers include nitrogen tetroxide and hydrogen peroxide. These
oxidizers remain liquid at room temperature and do not require cooling.
An electric or nuclear rocket uses a single propellant. These rockets store the propellant
as a gas or liquid.
The chamber is the area of the rocket where propellants are put under pressure.
Pressurizing the propellants enables the rocket to expel them at high speeds.
In a chemical rocket, the fuel and oxidizer combine and burn in an area called the
combustion chamber. As they burn, the propellants expand rapidly, creating intense
Burning propellants create extreme heat and pressure in the combustion chamber.
Temperatures in the chamber become hot enough to melt the steel, nickel, copper, and
other materials used in its construction. Combustion chambers need insulation or cooling
to survive the heat. The walls of the chamber must also be strong enough to withstand
intense pressure. The pressure inside a rocket engine can exceed 3,000 pounds per square
inch (200 kilograms per square centimeter), nearly 100 times the pressure in the tires of a
car or truck.
In a nuclear rocket, the chamber is the area where nuclear fuel heats the propellant,
producing pressure. In an electric rocket, the chamber contains the electric devices used
to force the propellant out of the nozzle.
The nozzle is the opening at the end of the chamber that allows the pressurized gases to
escape. It converts the high pressure of the gases into thrust by forcing the exhaust
through a narrow opening, which accelerates the exhaust to high speeds. The exhaust
from the nozzle can travel more than 1 mile (1.6 kilometers) per second. Like the
chamber, the nozzle requires cooling or insulation to withstand the heat of the exhaust.
Many chemical rockets work by burning propellants
in a single combustion chamber. Engineers refer to
these rockets as single-stage rockets. Missions that
require long-distance travel, such as reaching Earth
orbit, generally require multiple-stage or multistage
rockets. A multistage rocket uses two or more sets of
combustion chambers and propellant tanks. These
sets, called stages, may be stacked end to end or
attached side by side. When a stage runs out of
propellant, the rocket discards it. Discarding the
empty stage makes the rocket lighter, allowing the
remaining stages to accelerate it more strongly.
Engineers have designed and launched rockets with
as many as five separate stages. The space shuttle
uses two stages.
A two-stage rocket carries a
How rockets are used propellant and one or more rocket
engines in each stage. The first
People use rockets for high-speed, high-power stage launches the rocket. After
transportation both within Earth's atmosphere and in burning its supply of propellant,
space. Rockets are especially valuable for (1) military the first stage falls away from the
use, (2) atmospheric research, (3) launching probes rest of the rocket. The second stage
and satellites, and (4) space travel. then ignites and carries the payload
into earth orbit or even farther into
Military use space. A balloon and a rocket work
in much the same way. Gas
Rockets used by the military vary in size from small flowing from the nozzle creates
rockets used on the battlefield to giant guided unequal pressure that lifts the
missiles that can fly across oceans. The bazooka is a balloon or the rocket off the
small rocket launcher carried by soldiers for use ground. Image credit: World Book
against armored vehicles. A person using a bazooka diagram
has as much striking power as a small tank. Armies
use larger rockets to fire explosives far behind enemy lines and to shoot down enemy
aircraft. Fighter airplanes carry rocket-powered guided missiles to attack other planes and
ground targets. Navy ships use guided missiles to attack other ships, land targets, and
Powerful rockets propel a type of long-range guided missile called an intercontinental
ballistic missile (ICBM). Such a missile can travel 3,400 miles (5,500 kilometers) or
more to bomb an enemy target with nuclear explosives. An ICBM generally employs two
or three separate stages to propel it during the early part of its flight. The ICBM coasts
the rest of the way to its target.
Scientists use rockets to explore Earth's atmosphere. Sounding rockets, also called
meteorological rockets, carry such equipment as barometers, cameras, and thermometers
high into the atmosphere. These instruments collect information about the atmosphere
and send it by radio to receiving equipment on the ground.
Rockets also provide the power for experimental research airplanes. Engineers use these
planes in the development of spacecraft. By studying the flights of such planes as the
rocket-powered X-1 and X-15, engineers learned how to control vehicles flying many
times as fast as the speed of sound.
Launching probes and satellites
Rockets carry crewless spacecraft called space probes on long voyages to explore the
solar system. Probes have explored the sun, the moon, and all the planets in our solar
system except Pluto. They carry scientific instruments that gather information about the
planets and transmit data back to Earth. Probes have landed on the surface of the moon,
Venus, and Mars.
Rockets lift artificial satellites into orbit around Earth. Some orbiting satellites gather
information for scientific research. Others relay telephone conversations and radio and
television broadcasts across the oceans. Weather satellites track climate patterns and help
scientists predict the weather. Navigation satellites, such as those that make up the Global
Positioning System (GPS), enable receivers anywhere on Earth to determine their
locations with great accuracy. The armed forces use satellites to observe enemy facilities
and movements. They also use satellites to communicate, monitor weather, and watch for
missile attacks. Not only are satellites launched by rockets, but many satellites use small
rocket engines to maintain their proper orbits.
Rockets that launch satellites and probes are called launch vehicles. Most of these rockets
have from two to four stages. The stages lift the satellite to its proper altitude and give it
enough speed -- about 17,000 miles (27,000 kilometers) per hour -- to stay in orbit. A
space probe's speed must reach about 25,000 miles (40,000 kilometers) per hour to
escape Earth's gravity and continue on its voyage.
Engineers created the first launch vehicles by altering military rockets or sounding
rockets to carry spacecraft. For example, they added stages to some of these rockets to
increase their speed. Today, engineers sometimes attach smaller rockets to a launch
vehicle. These rockets, called boosters, provide additional thrust to launch heavier
Rockets launch spacecraft carrying astronauts that orbit Earth and travel into space.
These rockets, like the ones used to launch probes and satellites, are called launch
The Saturn 5 rocket, which carried astronauts to the moon, was the most powerful launch
vehicle ever built by the United States. Before launch, it weighed more than 6 million
pounds (2.7 million kilograms). It could send a spacecraft weighing more than 100,000
pounds (45,000 kilograms) to the moon. The Saturn 5 used 11 rocket engines to propel
Space shuttles are reusable rockets that can fly into space and return to Earth repeatedly.
Engineers have also worked to develop space tugs, smaller rocket-powered vehicles that
could tow satellites, boost space probes, and carry astronauts over short distances in orbit.
For more information on rockets used in space travel, see Space exploration.
People have fired rockets as distress signals from ships and airplanes and from the
ground. Rockets also shoot rescue lines to ships in
distress. Small rockets called JATO (jet-assisted take-
off) units help heavily loaded airplanes take off.
Rockets have long been used in fireworks displays.
Kinds of rocket engines
The vast majority of rockets are chemical rockets.
The two most common types of chemical rockets are
solid-propellant rockets and liquid-propellant rockets.
Engineers have tested a third type of chemical rocket,
called a hybrid rocket, that combines liquid and solid
propellants. Electric rockets have propelled space
probes and maneuvered orbiting satellites.
Researchers have designed experimental nuclear
Solid-propellant rockets burn a rubbery or plastic-like
material called the grain. The grain consists of a fuel A solid-propellant rocket burns a
and an oxidizer in solid form. It is shaped like a solid material called the grain.
cylinder with one or more channels or ports that run Engineers design most grains with
through it. The ports increase the surface area of the a hollow core. The propellant
grain that the rocket burns. Unlike some liquid burns from the core outward.
propellants, the fuel and oxidizer of a solid-propellant Unburned propellant shields the
rocket do not burn upon contact with each other. engine casing from the heat of
Instead, an electric charge ignites a smaller grain. Hot combustion. Image credit: World
exhaust gases from this grain ignite the main Book diagram by Precision
propellant surface. Graphics
The temperature in the combustion chamber of a solid-propellant rocket ranges from
3000 to 6000 degrees F (1600 to 3300 degrees C). In most of these rockets, engineers
build the chamber walls from high-strength steel or titanium to withstand the pressure
and heat of combustion. They also may use composite materials consisting of high-
strength fibers embedded in rubber or plastic. Composite chambers made from high-
strength graphite fibers in a strong adhesive called epoxy weigh less than steel or titanium
chambers, enabling the rocket to accelerate its payload more efficiently. Solid propellants
burn at a rate of about 0.6 inch (1.5 centimeters) per second.
Solid propellants can remain effective after long storage and present little danger of
combusting or exploding until ignited. Furthermore, they do not need the pumping and
injecting equipment required by liquid propellants. On the other hand, rocket controllers
cannot easily stop or restart the burning of solid propellant. This can make a solid-
propellant rocket difficult to control. One method used to stop the burning of solid
propellant involves blasting the entire nozzle section from the rocket. This method,
however, prevents restarting.
Rocket designers often choose solid propellants for
rockets that must be easy to store, transport, and
launch. Military planners prefer solid-propellant
rockets for many uses because they can be stored
for a long time and fired with little preparation.
Solid-propellant rockets power ICBM's, including
the American Minuteman 2 and MX and the
Russian RT-2. They also propel such smaller
missiles as the American Hellfire, Patriot,
Sparrow, and Sidewinder, and the French SSBS.
Solid-propellant rockets often serve as sounding
rockets and as boosters for launch vehicles and
cruise missiles. They are also used in fireworks.
Liquid-propellant rockets burn a mixture of fuel
and oxidizer in liquid form. These rockets carry A liquid-propellant rocket carries fuel
the fuel and the oxidizer in separate tanks. A and an oxidizer in separate tanks. The
system of pipes and valves feeds the propellants fuel circulates through the engine's
into the combustion chamber. In larger engines, cooling jacket before entering the
either the fuel or the oxidizer flows around the combustion chamber. This circulation
outside of the chamber before entering it. This preheats the fuel for combustion and
flow cools the chamber and preheats the propellant helps cool the rocket. Image credit:
for combustion. World Book diagram by Precision
A liquid-propellant rocket feeds the fuel and
oxidizer into the combustion chamber using either pumps or high-pressure gas. The most
common method uses pumps to force the fuel and oxidizer into the combustion chamber.
Burning a small portion of the propellants provides the energy to drive the pumps. In the
other method, high-pressure gas forces the fuel and oxidizer into the chamber. The gas
may be nitrogen or some other gas stored under high pressure or may come from the
burning of a small amount of propellants.
Some liquid propellants, called hypergols, ignite
when the fuel and the oxidizer mix. But most liquid
propellants require an ignition system. An electric
spark may ignite the propellant, or the burning of a
small amount of solid propellant in the combustion
chamber may do so. Liquid propellants continue to
burn as long as fuel and oxidizer flow into the
Engineers use thin, high-strength steel or aluminum
to construct most tanks that hold liquid propellants.
They may also reinforce tanks with composite
materials like those used in solid-propellant rocket
chambers. Most combustion chambers in liquid-
propellant rockets are made of steel or nickel.
Liquid propellants usually produce greater thrust
than do equal amounts of solid propellants burned in Launch vehicles used by European
the same amount of time. Controllers can easily nations include the European Space
adjust or stop burning in a liquid-propellant rocket Agency's Ariane 5 rocket and
by increasing or decreasing the flow of propellants Russia's A class and Proton rockets.
into the chamber. Liquid propellants, however, are These vehicles carry space probes
difficult to handle. If the fuel and oxidizer blend and artificial satellites into outer
without igniting, the resulting mixture often will space. The A Class rocket has also
explode easily. Liquid propellants also require carried people into space, and the
complicated pumping machinery. Proton rocket has carried
International Space Station
Scientists use liquid-propellant rockets for most modules. Image credit: World Book
space launch vehicles. Liquid-propellant rockets illustrations by Oxford Illustrators
serve as the main engines of the space shuttle as Limited
well as Europe's Ariane rocket, Russia's Soyuz rocket, and China's Long March rocket.
Hybrid rockets combine some of the advantages of both solid-propellant and liquid-
propellant rockets. A hybrid rocket uses a liquid oxidizer, such as liquid oxygen, and a
solid-fuel grain made of plastic or rubber. The solid-fuel grain lines the inside of the
combustion chamber. A pumping system sprays the oxidizer onto the surface of the grain,
which is ignited by a smaller grain or torch.
Hybrid rockets are safer than solid-propellant rockets because the propellants are not
premixed and so will not ignite accidentally. Also, unlike solid-propellant rockets, hybrid
rockets can vary thrust or even stop combustion by adjusting the flow of oxidizer. Hybrid
engines require only half the pumping gear of liquid-propellant rockets, making them
simpler to build.
A key disadvantage of hybrid rockets is that their fuel burns slowly, limiting the amount
of thrust they can produce. A hybrid rocket burns grain at a rate of about 0.04 inch (1
millimeter) per second. For a given amount of propellant, hybrid rockets typically
produce more thrust than solid rockets and less than liquid engines. To generate more
thrust, engineers must manufacture complex fuel grains with many separate ports through
which oxidizer can flow. This exposes more of the grain to the oxidizer.
Researchers have used hybrid rockets to propel targets used in missile testing and to
accelerate experimental motorcycles and cars attempting land speed records. Their safety
has led designers to attempt to develop hybrid rockets for use in human flight. One such
rocket would launch from an airplane to carry people to an altitude of about 60 miles
(100 kilometers). Researchers have not yet developed hybrid rockets powerful enough to
launch human beings into space. Hybrid rockets can produce enough thrust, however, to
boost planetary probes or maneuver satellites in orbit. Hybrid rockets could also power
escape mechanisms being developed for new launch vehicles that would carry crews.
The safety of hybrid rockets has led engineers to
develop them for use in human flight. The Scaled
Composites company of Mojave, California,
developed a hybrid rocket called SpaceShipOne
that launched from an airplane. On June 21,
2004, SpaceShipOne became the first privately
funded craft to carry a person into space. It
carried the American test pilot Michael Melvill
more than 62 miles (100 kilometers) above
Earth's surface during a brief test flight.
Researchers have also used hybrid rockets to
propel targets used in missile testing and to
accelerate experimental motorcycles and cars
attempting land speed records. In addition, they
have worked to develop hybrid rockets to boost
planetary probes, maneuver satellites in orbit,
and power crew escape mechanisms for launch An ion rocket is a kind of electric
vehicles. rocket. Heating coils in the rocket
change a fuel, such as xenon, into a
Electric rockets use electric energy to expel ions vapor. A hot platinum or tungsten
(electrically charged particles) from the nozzle. ionization grid changes the flowing
Solar panels or a nuclear reactor can provide the vapor into a stream of electrically
energy. charged particles called ions. Image
credit: World Book diagram by
In one design, xenon gas passes through an Precision Graphics
electrified metal grid. The grid strips electrons from the xenon atoms, turning them into
positively charged ions. A positively charged screen repels the ions, focusing them into a
beam. The beam then enters a negatively charged device called an accelerator. The
accelerator speeds up the ions and shoots them out through a nozzle.
The exhaust from such rockets travels extremely fast. However, the stream of xenon ions
has a relatively low mass. As a result, an electric rocket cannot produce enough thrust to
overcome Earth's gravity. Electric rockets used in space must therefore be launched by
chemical rockets. Once in space, however, the
low rate of mass flow becomes an advantage. It
enables an electric rocket to operate for a long
time without running out of propellant. The xenon
rocket that powered the U.S. space probe Deep
Space 1, launched in 1998, fired for a total of over
670 days using only 160 pounds (72 kilograms) of
propellant. In addition, small electric rockets
using xenon propellant have provided the thrust to
keep communications satellites in position above
Another type of electric rocket uses
electromagnets rather than charged screens to
accelerate xenon ions. This type of rocket powers
the SMART-1 lunar probe, launched by the
European Space Agency in 2003.
A nuclear rocket uses the heat from a
Nuclear rockets use the heat energy of a nuclear nuclear reactor to change a liquid fuel
reactor, a device that releases energy by splitting into a gas. Most of the fuel flows
atoms. Some proposed designs would use through the reactor. Some of the fuel,
hydrogen as propellant. The rocket would store heated by the nozzle of the rocket,
the hydrogen as a liquid. Heat from the reactor flows through the turbine. The turbine
would boil the liquid, creating hydrogen gas. The drives the fuel pump. Image credit:
gas would expand rapidly and push out from the World Book diagram by Precision
The exhaust speed of a nuclear rocket might reach four times that of a chemical rocket.
By expelling a large quantity of hydrogen, a nuclear rocket could therefore achieve high
thrust. However, a nuclear rocket would require heavy shielding because a nuclear
reactor uses radioactive materials. The shielding would weigh so much that the rocket
could not be practically used to boost a launch vehicle. More practical applications would
use small nuclear engines with low, continuous thrust to decrease flight times to Mars or
Nuclear rocket developers must also overcome public fears that accidents involving such
devices could release harmful radioactive materials. Before nuclear rockets can be
launched, engineers must convince the public that such devices are safe.
Historians believe the Chinese invented rockets, but they do not know exactly when.
Historical accounts describe "arrows of flying fire" -- believed to have been rockets --
used by Chinese armies in A.D. 1232. By 1300, the use of rockets had spread throughout
much of Asia and Europe. These first rockets burned a substance called black powder,
which consisted of charcoal, saltpeter, and sulfur. For several hundred years, the use of
rockets in fireworks displays outranked their military use in importance
During the early 1800's, Colonel William Congreve of the British Army developed
rockets that could carry explosives. Many of these rockets weighed about 32 pounds (15
kilograms) and could travel 1 3/4 miles (2.7 kilometers). British troops used Congreve
rockets against the United States Army during the War of 1812. Austria, Russia, and
several other countries also developed military rockets during the early 1800's.
The English inventor William Hale improved the accuracy of military rockets. He
substituted three fins for the long wooden tail that had been used to guide the rocket.
United States troops used Hale rockets in the Mexican War (1846-1848). During the
American Civil War (1861-1865), both sides used rockets.
Rockets of the early 1900's
The Russian school teacher Konstantin E. Tsiolkovsky first stated the correct theory of
rocket power. He described his theory in a scientific paper published in 1903.
Tsiolkovsky also first presented the ideas of the multistage rocket and rockets using
liquid oxygen and hydrogen propellants. In 1926, the American rocket pioneer Robert H.
Goddard conducted the first successful launch of a liquid-propellant rocket. The rocket
climbed 41 feet (13 meters) into the air at a speed of about 60 miles (97 kilometers) per
hour and landed 184 feet (56 meters) away.
During the 1930's, rocket research advanced in Germany, the Soviet Union, and the
United States. Hermann Oberth led a small group of German engineers and scientists that
experimented with rockets. Leading Soviet rocket scientists included Fridrikh A. Tsander
and Sergei P. Korolev. Goddard remained the most prominent rocket researcher in the
During World War II, German engineers under the direction of Wernher von Braun
developed the powerful V-2 guided missile.
Germany bombarded London and Antwerp,
Belgium, with hundreds of V-2's during the last
months of the war. American forces captured
many V-2 missiles and sent them to the United
States for use in research. After the war, von
Braun and about 150 other German scientists
moved to the United States to continue their work
with rockets. Some other German rocket experts
went to the Soviet Union.
For several years after World War II, U.S.
scientists benefited greatly by conducting
experiments with captured German V-2's. These
V-2's became the first rockets used for high-
The vehicles shown here helped the
The first high-altitude rockets designed and built United States and the Soviet Union
in the United States included the WAC Corporal, achieve milestones in the exploration
the Aerobee, and the Viking. The 16-foot (4.9- of space. The United States no longer
meter) WAC Corporal reached altitudes of about builds these rockets, but Russia
45 miles (72 kilometers) during test flights in continues to use the Soviet A Class
1945. Early models of the Aerobee climbed about design in the Soyuz rocket.
70 miles (110 kilometers). In 1949, the U.S. Navy • Jupiter C, U.S. Lifted Explorer I, the
launched the Viking, an improved liquid- first U.S. satellite, in 1958. 68 feet (21
propellant rocket based chiefly on the V-2. The meters)
Viking measured more than 45 feet (14 meters) • Mercury-Redstone, U.S. Launched
long, much longer than the Aerobee. But the first Alan Shepard in 1961. 83 feet (25
models of the Viking rose only about 50 miles (80 meters)
kilometers). • A Class (Sputnik), Soviet. Boosted
Sputnik 1, the first artificial satellite,
Rockets developed by the U.S. armed forces in 1957. 98 feet (29 meters) Image
during the 1950's included the Jupiter and the credit: WORLD BOOK illustrations
Pershing. The Jupiter had a range of about 1,600 by Oxford Illustrators Limited
miles (2,600 kilometers), and the Pershing could
travel about 450 miles (720 kilometers).
The U.S. Navy conducted the first successful launch of a
Polaris underwater missile in 1960. United States space
scientists later used many military rockets developed in the
1950's as the basis for launch vehicles.
On Oct. 14, 1947, Captain Charles E. Yeager of the U.S. Air
Force made the first supersonic (faster than sound) flight. He
flew a rocket-powered airplane called the X-1.
A rocket engine also powered the X-15, which set an
unofficial airplane altitude record of 354,200 feet (107,960
meters) in 1963. In one flight, the X-15 reached a peak speed
of 4,520 miles (7,274 kilometers) per hour -- more than six
times the speed of sound. A privately owned and developed
rocket-powered plane called the EZ-Rocket began piloted
test flights in 2001.
The vehicles shown here
helped the United States The space age began on Oct. 4, 1957, when the Soviet Union
and the Soviet Union launched the first artificial satellite, Sputnik 1, aboard a two-
achieve milestones in the stage rocket. On Jan. 31, 1958, the U.S. Army launched the
exploration of space. The first American satellite, Explorer 1, into orbit with a Juno I
United States no longer rocket.
builds these rockets, but
Russia continues to use the On April 12, 1961, a Soviet rocket put a cosmonaut, Major
Soviet A Class design in Yuri A. Gagarin, into orbit around Earth for the first time.
the Soyuz rocket. On May 5, 1961, a Redstone rocket launched Commander
• A Class (Vostok), Soviet. Alan B. Shepard, Jr., the first American to travel in space.
Carried Yuri Gagarin, the On April 12, 1981, the United States launched the rocket-
first person to orbit the powered Columbia, the first space shuttle to orbit Earth. For
earth, in 1961. 126 feet (38 more information on the history of rockets in space travel,
meters) see Space exploration.
• Saturn 5, U.S. Launched
Neil Armstrong, the first Rocket research
person to set foot on the
moon, in 1969. 363 feet In the early 2000's, engineers and scientists worked to
(111 meters) Image credit: develop lightweight rocket engines that used safer
WORLD BOOK propellants. They also searched for more efficient propellants
illustrations by Oxford that did not require refrigeration. Engineers began designing
Illustrators Limited and testing smaller rocket engines for use in smaller vehicles,
such as tiny satellites that may weigh only a few pounds or
kilograms when fully loaded.
A star is a huge, shining ball in space that
produces a tremendous amount of light and other
forms of energy. The sun is a star, and it supplies
Earth with light and heat energy. The stars look
like twinkling points of light -- except for the sun.
The sun looks like a ball because it is much closer
to Earth than any other star.
A globular cluster is a tightly grouped
The sun and most other stars are made of gas and swarm of stars held together by
a hot, gaslike substance known as plasma. But gravity. This globular cluster is one of
some stars, called white dwarfs and neutron stars, the densest of the 147 known clusters
consist of tightly packed atoms or subatomic in the Milky Way galaxy. Image
particles. These stars are therefore much more credit: NASA
dense than anything on Earth.
Stars come in many sizes. The sun's radius (distance from its center to its surface) is
about 432,000 miles (695,500 kilometers). But astronomers classify the sun as a dwarf
because other kinds of stars are much bigger. Some of the stars known as supergiants
have a radius about 1,000 times that of the sun. The smallest stars are the neutron stars,
some of which have a radius of only about 6 miles (10 kilometers).
About 75 percent of all stars are members of a binary system, a pair of closely spaced
stars that orbit each other. The sun is not a member of a binary system. However, its
nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that
also includes Alpha Centauri A and Alpha Centauri B.
The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion
kilometers). This distance is so great that light takes 4.2 years to travel between the two
stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun. One light-year,
the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46
Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies
throughout the universe at distances of 12 billion to 16 billion light-years. The sun is in a
galaxy called the Milky Way that contains more than 100 billion stars. There are more
than 100 billion galaxies in the universe, and the average number of stars per galaxy may
be 100 billion. Thus, more than 10 billion trillion stars may exist. But if you look at the
night sky far from city lights, you can see only about 3,000 of them without using
binoculars or a telescope.
Stars, like people, have life cycles -- they are born, pass through several phases, and
eventually die. The sun was born about 4.6 billion years ago and will remain much as it is
for another 5 billion years. Then it will grow to become a red giant. Late in the sun's
lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will
slowly fade to become a black dwarf.
Other stars will end their lives in different ways. Some will not go through a red giant
stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small
percentage of stars will die in spectacular explosions called supernovae.
This article discusses Star (The stars at night) (Names of stars) (Characteristics of stars)
(Fusion in stars) (Evolution of stars).
The stars at night
If you look at the stars on a clear night, you will notice that they seem to twinkle and that
they differ greatly in brightness. A much slower movement also takes place in the night
sky: If you map the location of several stars for a few hours, you will observe that all the
stars revolve slowly about a single point in the sky.
Twinkling of stars is caused by movements in Earth's atmosphere. Starlight enters the
atmosphere as straight rays. Twinkling occurs because air movements constantly change
the path of the light as it comes through the air. You can see a similar effect if you stand
in a swimming pool and look down. Unless the water is almost perfectly still, your feet
will appear to move and change their shape. This "twinkling" occurs because the moving
water constantly changes the path of the light rays that travel from your feet to your eyes.
Brightness of stars. How bright a star looks when viewed from Earth depends on two
factors: (1) the actual brightness of the star -- that is, the amount of light energy the star
emits (sends out) -- and (2) the distance from Earth to the star. A nearby star that is
actually dim can appear brighter than a distant star that is really extremely brilliant. For
example, Alpha Centauri A seems to be slightly brighter than a star known as Rigel. But
Alpha Centauri A emits only 1/100,000 as much light energy as Rigel. Alpha Centauri A
seems brighter because it is only 1/325 as far from Earth as Rigel is -- 4.4 light-years for
Alpha Centauri A, 1,400 light-years for Rigel.
Rising and setting of stars
When viewed from Earth's Northern Hemisphere, stars rotate counterclockwise around a
point called the celestial north pole. Viewed from the Southern Hemisphere, stars rotate
clockwise about the celestial south pole. During the day, the sun moves across the sky in
the same direction, and at the same rate, as the stars. These movements do not result from
any actual revolution of the sun and stars. Rather, they occur because of the west-to-east
rotation of Earth about its own axis. To an observer standing on the ground, Earth seems
motionless, while the sun and stars seem to move in circles. But actually, Earth moves.
Names of stars
Ancient people saw that certain stars are arranged in patterns shaped somewhat like
human beings, animals, or common objects. Some of these patterns, called constellations,
came to represent figures of mythological characters. For example, the constellation
Orion (the Hunter) is named after a hero in Greek mythology.
Today, astronomers use constellations, some of which were described by the ancients, in
the scientific names of stars. The International Astronomical Union (IAU), the world
authority for assigning names to celestial objects, officially recognizes 88 constellations.
These constellations cover the entire sky. In most cases, the brightest star in a given
constellation has alpha -- the first letter of the Greek alphabet -- as part of its scientific
name. For instance, the scientific name for Vega, the brightest star in the constellation
Lyra (the Harp), is Alpha Lyrae. Lyrae is Latin for of Lyra.
The second brightest star in a constellation is usually designated beta, the second letter of
the Greek alphabet, the third brightest is gamma, and so on. The assignment of Greek
letters to stars continues until all the Greek letters are used. Numerical designations
But the number of known stars has become so large that the IAU uses a different system
for newly discovered stars. Most new names consist of an abbreviation followed by a
group of symbols. The abbreviation stands for either the type of star or a catalog that lists
information about the star. For example, PSR J1302-6350 is a type of star known as a
pulsar -- hence the PSR in its name. The symbols indicate the star's location in the sky.
The 1302 and the 6350 are coordinates that are similar to the longitude and latitude
designations used to indicate locations on Earth's surface. The J indicates that a
coordinate system known as J2000 is being used.
Characteristics of stars
A star has five main characteristics: (1) brightness, which astronomers describe in terms
of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass
(amount of matter). These characteristics are related to one another in a complex way.
Color depends on surface temperature, and brightness depends on surface temperature
and size. Mass affects the rate at which a star of a given size produces energy and so
affects surface temperature. To make these relationships easier to understand,
astronomers developed a graph called the Hertzsprung-Russell (H-R) diagram. This
graph, a version of which appears in this article, also helps astronomers understand and
describe the life cycles of stars.
Magnitude and luminosity
Magnitude is based on a numbering system invented by the Greek astronomer
Hipparchus in about 125 B.C. Hipparchus numbered groups of stars according to their
brightness as viewed from Earth. He called the brightest stars first magnitude stars, the
next brightest second magnitude stars, and so on to sixth magnitude stars, the faintest
Modern astronomers refer to a star's brightness as viewed from Earth as its apparent
magnitude. But they have extended Hipparchus's system to describe the actual brightness
of stars, for which they use the term absolute magnitude. For technical reasons, they
define a star's absolute magnitude as what its apparent magnitude would be if it were 32.6
light-years from Earth.
Astronomers have also extended the system of magnitude numbers to include stars
brighter than first magnitude and dimmer than sixth magnitude. A star that is brighter
than first magnitude has a magnitude less than 1. For example, the apparent magnitude of
Rigel is 0.12. Extremely bright stars have magnitudes less than zero -- that is, their
designations are negative numbers. The brightest star in the night sky is Sirius, with an
apparent magnitude of -1.46. Rigel has an absolute magnitude of -8.1. According to
astronomers' present understanding of stars, no star can have an absolute magnitude much
brighter than -8. At the other end of the scale, the dimmest stars detected with telescopes
have apparent magnitudes up to 28. In theory, no star could have an absolute magnitude
much fainter than 16.
Luminosity is the rate at which a star emits energy. The scientific term for a rate of
energy emission is power, and scientists generally measure power in watts. For example,
the luminosity of the sun is 400 trillion trillion watts. But astronomers do not usually
measure a star's luminosity in watts. Instead, they express luminosities in terms of the
luminosity of the sun. They often say, for instance, that the luminosity of Alpha Centauri
A is about 1.3 times that of the sun and that Rigel is roughly 150,000 times as luminous
as the sun.
Luminosity is related to absolute magnitude in a simple way. A difference of 5 on the
absolute magnitude scale corresponds to a factor of 100 on the luminosity scale. Thus, a
star with an absolute magnitude of 2 is 100 times as luminous as a star with an absolute
magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a
star whose absolute magnitude is 2 and 10,000 times as luminous as a star that has an
absolute magnitude of 7.
Color and temperature
If you look carefully at the stars, even without binoculars or a telescope, you will see a
range of color from reddish to yellowish to bluish. For example, Betelgeuse looks
reddish, Pollux -- like the sun -- is yellowish, and Rigel looks bluish.
A star's color depends on its surface temperature. Astronomers measure star temperatures
in a metric unit known as the kelvin. One kelvin equals exactly 1 Celsius degree (1.8
Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin
scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees
C, or -459.67 degrees F. A temperature of 0
degrees C (32 degrees F) equals 273.15 K.
Dark red stars have surface temperatures of about
2500 K. The surface temperature of a bright red
star is approximately 3500 K; that of the sun and
other yellow stars, roughly 5500 K. Blue stars
range from about 10,000 to 50,000 K in surface A spectacular explosion on the star Eta
temperature. Carinae about 150 years ago produced
three huge clouds of gas and dust --
two puffy lobes and a thin disk.
Astronomers call Eta Carinae a
Although a star appears to the unaided eye to have luminous blue variable star because of
a single color, it actually emits a broad spectrum its color and because it often becomes
(band) of colors. You can see that starlight very bright -- as it did when the
consists of many colors by using a prism to explosion occurred. Image credit:
separate and spread the colors of the light of the NASA
sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These
colors range from red, produced by the photons (particles of light) with the least energy;
to violet, produced by the most energetic photons.
Visible light is one of six bands of electromagnetic radiation. Ranging from the least
energetic to the most energetic, they are: radio waves, infrared rays, visible light,
ultraviolet rays, X rays, and gamma rays. All six bands are emitted by stars, but most
individual stars do not emit all of them. The combined range of all six bands is known as
the electromagnetic spectrum.
Astronomers study a star's spectrum by separating it, spreading it out, and displaying it.
The display itself is also known as a spectrum. The scientists study thin gaps in the
spectrum. When the spectrum is spread out from left to right, the gaps appear as vertical
lines. The spectra of stars have dark absorption lines where radiation of specific energies
is weak. In a few special cases in the visible spectrum, stars have bright emission lines
where radiation of specific energies is especially strong.
An absorption line appears when a chemical element or compound absorbs radiation that
has the amount of energy corresponding to the line. For example, the spectrum of the
visible light coming from the sun has a group of absorption lines in the green part of the
spectrum. Calcium in an outer layer of the sun absorbs light rays that would have
produced the corresponding green colors.
Although all stars have absorption lines in the visible band of the electromagnetic
spectrum, emission lines are more common in other parts of the spectrum. For instance,
nitrogen in the sun's atmosphere emits powerful radiation that produces emission lines in
the ultraviolet part of the spectrum.
Astronomers measure the size of stars in terms of the sun's radius. Alpha Centauri A,
with a radius of 1.05 solar radii (the plural of radius), is almost exactly the same size as
the sun. Rigel is much larger at 78 solar radii, and Antares has a huge size of 776 solar
A star's size and surface temperature determine its luminosity. Suppose two stars had the
same temperature, but the first star had twice the radius of the second star. In this case,
the first star would be four times as bright as the second star. Scientists say that
luminosity is proportional to radius squared -- that is, multiplied by itself. Imagine that
you wanted to compare the luminosities of two stars that had the same temperature but
different radii. First, you would divide the radius of the larger star by the radius of the
smaller star. Then, you would square your answer.
Now, suppose two stars had the same radius but the first star's surface temperature --
measured in kelvins -- was twice that of the second star. In this example, the luminosity
of the first star would be 16 times that of the second star. Luminosity is proportional to
temperature to the fourth power. Imagine that you wanted to compare the luminosities of
stars that had the same radius but different temperatures. First, you would divide the
temperature of the warmer star by the temperature of the cooler star. Next, you would
square the result. Then, you would square your answer again.
Astronomers express the mass of a star in terms of the solar mass, the mass of the sun.
For example, they give the mass of Alpha Centauri A as 1.08 solar masses; that of Rigel,
as 3.50 solar masses. The mass of the sun is 2 Ž 1030 kilograms, which would be written
out as 2 followed by 30 zeros.
Stars that have similar masses may not be similar in size -- that is, they may have
different densities. Density is the amount of mass per unit of volume. For instance, the
average density of the sun is 88 pounds per cubic foot (1,400 kilograms per cubic meter),
about 140 percent that of water. Sirius B has almost exactly the same mass as the sun, but
it is 90,000 times as dense. As a result, its radius is only about 1/50 of a solar radius.
The Hertzsprung-Russell diagram displays the main characteristics of stars. The diagram
is named for astronomers Ejnar Hertzsprung of Denmark and Henry Norris Russell of the
United States. Working independently of each other, the two scientists developed the
diagram around 1910.
Points representing the brightest stars appear toward the top of the H-R diagram; points
corresponding to the dimmest stars, toward the bottom. These points appear in groups
that correspond to different kinds of stars. In the 1930's, American astronomers William
W. Morgan and Philip C. Keenan invented what came to be known as the MK luminosity
classification system for these groups. Astronomers revised and extended this system in
1978. In the MK system, the largest and brightest classes have the lowest classification
numbers. The MK classes are: Ia, bright supergiant; Ib, supergiant; II, bright giant; III,
giant; IV, subgiant; and V, main sequence or dwarf.
Because temperature also affects the luminosity of a star, stars from different luminosity
classes can overlap. For example, Spica, a class V star, has an absolute magnitude of -3.2;
but Pollux, a class III star, is dimmer, with an absolute magnitude of 0.7.
Points representing the stars with the highest surface temperatures appear toward the left
edge of the H-R diagram; points representing the coolest stars, toward the right edge. In
the MK system, there are eight spectral classes, each corresponding to a certain range of
surface temperature. From the hottest stars to the coolest, these classes are: O, B, A, F, G,
K, M, and L. Each spectral class, in turn, is made up of 10 spectral types, which are
designated by the letter for the spectral class and a numeral. The hottest stars in a spectral
class are assigned the numeral 0; the coolest stars, the numeral 9.
A complete MK designation thus includes symbols for luminosity class and spectral type.
For example, the complete designation for the sun is G2V. Alpha Centauri A is also a
G2V star, and Rigel's designation is B8Ia.
Fusion in stars
A star's tremendous energy comes from a process known as nuclear fusion. This process
begins when the temperature of the core of the developing star reaches about 1 million K.
A star develops from a giant, slowly rotating cloud that consists almost entirely of the
chemical elements hydrogen and helium. The cloud also contains atoms of other elements
as well as microscopic particles of dust.
Due to the force of its own gravity, the cloud begins to collapse inward, thereby
becoming smaller. As the cloud shrinks, it rotates more and more rapidly, just as spinning
ice skaters turn more rapidly when they pull in their arms. The outermost parts of the
cloud form a spinning disk. The inner parts become a roughly spherical clump, which
continues to collapse.
The collapsing material becomes warmer, and its pressure increases. But the pressure
tends to counteract the gravitational force that is responsible for the collapse. Eventually,
therefore, the collapse slows to a gradual contraction. The inner parts of the clump form a
protostar, a ball-shaped object that is no longer a cloud, but is not yet a star. Surrounding
the protostar is an irregular sphere of gas and dust that had been the outer parts of the
When the temperature and pressure in the protostar's core become high enough, nuclear
fusion begins. Nuclear fusion is a joining of two atomic nuclei to produce a larger
Nuclei that fuse are actually the cores of atoms. A complete atom has an outer shell of
one or more particles called electrons, which carry a negative electric charge. Deep inside
the atom is the nucleus, which contains almost all the atom's mass. The simplest nucleus,
that of the most common form of hydrogen, consists of a single particle known as a
proton. A proton carries a positive electric charge. All other nuclei have one or more
protons and one or more neutrons. A neutron carries no net charge, and so a nucleus is
electrically positive. But a complete atom has as many electrons as protons. The net
electric charge of a complete atom is therefore zero -- the atom is electrically neutral.
However, under the enormous temperatures and pressures near the core of a protostar,
atoms lose electrons. The resulting atoms are known as ions, and the mixture of the free
electrons and ions is called a plasma.
Atoms in the core of the protostar lose all their electrons, and the resulting bare nuclei
approach one another at tremendous speeds. Under ordinary circumstances, objects that
carry like charges repel each other. However, if the core temperature and pressure
become high enough, the repulsion between nuclei can be overcome and the nuclei can
fuse. Scientists commonly refer to fusion as "nuclear burning." But fusion has nothing to
do with ordinary burning or combustion.
Converting mass to energy
When two relatively light nuclei fuse, a small amount of their mass turns into energy.
Thus, the new nucleus has slightly less mass than the sum of the masses of the original
nuclei. The German-born American physicist Albert Einstein discovered the relationship
E = mc-squared (E=mc 2) that indicates how much energy is released when fusion
occurs. The symbol E represents the energy; m, the mass that is converted; and c-squared
(c2), the speed of light squared.
The speed of light is 186,282 miles (299,792 kilometers) per second. This is such a large
number that the conversion of a tiny quantity of mass produces a tremendous amount of
energy. For example, complete conversion of 1 gram of mass releases 90 trillion joules of
energy. This amount of energy is roughly equal to the quantity released in the explosion
of 22,000 tons (20,000 metric tons) of TNT. This is much more energy than was released
by the atomic bomb that the United States dropped on Hiroshima, Japan, in 1945 during
World War II. The energy of the bomb was equivalent to the explosion of 13,000 tons
(12,000 metric tons) of TNT.
Destruction of light nuclei
In the core of a protostar, fusion begins when the temperature reaches about 1 million K.
This initial fusion destroys nuclei of certain light elements. These include lithium 7
nuclei, which consist of three protons and four neutrons. In the process involving lithium
7, a hydrogen nucleus combines with a lithium 7 nucleus, which then splits into two
parts. Each part consists of a nucleus of helium 4 -- two protons and two neutrons. A
helium 4 nucleus is also known as an alpha particle.
After the light nuclei are destroyed, the protostar continues to contract. Eventually, the
core temperature reaches about 10 million K, and hydrogen fusion begins. The protostar
is now a star.
In hydrogen fusion, four hydrogen nuclei fuse to form a helium 4 nucleus. There are two
general forms of this reaction: (1) the proton-proton (p-p) reaction and (2) the carbon-
nitrogen-oxygen (CNO) cycle.
The p-p reaction can occur in several ways, including the following four-step process:
(1) Two protons fuse. In this step, two protons collide, and then one of the protons loses
its positive charge by emitting a positron. The proton also emits an electrically neutral
particle called a neutrino.
A positron is the antimatter equivalent of an electron. It has the same mass as an electron
but differs from the electron in having a positive charge. By emitting the positron, the
proton becomes a neutron. The new nucleus therefore consists of a proton and a neutron
-- a combination known as a deuteron.
(2) The positron collides with an electron that happens to be nearby. As a result, the two
particles annihilate each other, producing two gamma rays.
(3) The deuteron fuses with another proton, producing a helium 3 nucleus, which consists
of two protons and one neutron. This step also produces a gamma ray.
(4) The helium 3 nucleus fuses with another helium 3 nucleus. This step produces a
helium 4 nucleus, and two protons are released.
The CNO cycle differs from the p-p reaction mainly in that it involves carbon 12 nuclei.
These nuclei consist of six protons and six neutrons. During the cycle, they change into
nuclei of nitrogen 15 (7 protons and 8 neutrons) and oxygen 15 (8 protons and 7
neutrons). But they change back to carbon 12 nuclei by the end of the cycle.
Fusion of other elements
Helium nuclei can fuse to form carbon 12 nuclei. However, the core temperature must
rise to about 100 million K for this process to occur. This high temperature is necessary
because the helium nuclei must overcome a much higher repulsive force than the force
between two protons. Each helium nucleus has two protons, so the repulsive force is four
times as high as the force between two protons.
The fusion of helium is called the triple-alpha process because it combines three alpha
particles to create a carbon 12 nucleus. Helium fusion also produces nuclei of oxygen 16
(8 protons and 8 neutrons) and neon 20 (10 protons and 10 neutrons).
At core temperatures of about 600 million K, carbon 12 can fuse to form sodium 23 (11
protons, 12 neutrons), magnesium 24 (12 protons, 12 neutrons), and more neon 20.
However, not all stars can reach these temperatures.
As fusion processes produce heavier and heavier elements, the temperature necessary for
further processes increases. At about 1 billion K, oxygen 16 nuclei can fuse, producing
silicon 28 (14 protons, 14 neutrons), phosphorus 31 (15 protons, 16 neutrons), and sulfur
32 (16 protons, 16 neutrons).
Fusion can produce energy only as long as the new nuclei have less mass than the sum of
the masses of the original nuclei. Energy production continues until nuclei of iron 56 (26
protons, 30 neutrons) begin to combine with other nuclei. When this happens, the new
nuclei have slightly more mass than the original nuclei. This process therefore uses
energy, rather than producing it.
Evolution of stars
The life cycles of stars follow three general patterns, each associated with a range of
initial mass. There are (1) high-mass stars, which have more than 8 solar masses; (2)
intermediate-mass stars, with 0.5 to 8 solar masses -- the group that includes the sun; and
(3) low-mass stars, with 0.1 to 0.5 solar mass. Objects with less than 0.1 solar mass do
not have enough gravitational force to produce the core temperature necessary for
The life cycles of single stars are simpler than those of binary systems, so this section
discusses the evolution of single stars first. And because astronomers know much more
about the sun than any other star, the discussion begins with the development of
A cloud that eventually develops into an intermediate-mass star takes about 100,000
years to collapse into a protostar. As a protostar, it has a surface temperature of about
4000 K. It may be anywhere from a few times to a few thousand times as luminous as the
sun, depending on its mass.
When hydrogen fusion begins, the protostar is still surrounded by an irregular mass of
gas and dust. But the energy produced by hydrogen fusion pushes away this material as a
protostellar wind. In many cases, the disk that is left over from the collapse channels the
wind into two narrow cones or jets. One jet emerges from each side of the disk at a right
angle to the plane of the disk. The protostar has become a T-Tauri star, a type of object
named after the star T in the constellation Taurus (the Bull). A T-Tauri star is a variable
star, one that varies in brightness.
The T-Tauri star contracts for about 10 million years. It stops contracting when its
tendency to expand due to the energy produced by fusion in its core balances its tendency
to contract due to gravity. By this time, hydrogen fusion in the core is supplying all the
star's energy. The star has begun the longest part of its life as a producer of energy from
hydrogen fusion, the main-sequence phase. The name of this phase comes from a part of
the H-R diagram.
Any star -- whatever its mass -- that gets all its energy from hydrogen fusion in its core is
said to be "on the main sequence" or "a main-sequence star." The amount of time a star
spends there depends on its mass. The greater a star's mass, the more rapidly the
hydrogen in its core is used up, and therefore the shorter is its stay on the main sequence.
An intermediate-mass star remains on the main sequence for billions of years.
Red giant phase
When all the hydrogen in the core of an intermediate-mass star has fused into helium, the
star changes rapidly. Because the core no longer produces fusion energy, gravity
immediately crushes matter down upon it. The resulting compression quickly heats the
core and the region around it. The temperature becomes so high that hydrogen fusion
begins in a thin shell surrounding the core. This fusion produces even more energy than
had been produced by hydrogen fusion in the core. The extra energy pushes against the
star's outer layers, and so the star expands enormously.
As the star expands, its outer layers become cooler, so the star becomes redder. And
because the star's surface area expands greatly, the star also becomes brighter. The star is
now a red giant.
Horizontal branch phase
Eventually, the core temperature reaches 100 million K, high enough to support the
triple-alpha process. This process begins so rapidly that its onset is known as helium
As the triple-alpha process continues, the core expands, but its temperature drops. This
decrease in temperature causes the temperature of the hydrogen-burning shell to drop.
Consequently, the energy output of the shell decreases, and the outer layers of the star
contract. The star becomes hotter but smaller and fainter than it had been as a red giant.
This change occurs over a period of about 100 million years.
At the end of this period, the star is in its horizontal branch phase, named for the position
of the point representing the star on the H-R diagram. The star steadily burns helium and
hydrogen, and so its temperature, size, and luminosity do not change significantly. This
phase lasts for about 10 million years.
Asymptotic giant phase
When all the helium in the core has fused, the core contracts and therefore becomes
hotter. The triple-alpha process begins in a shell surrounding the core, and hydrogen
fusion continues in a shell surrounding that. Due to the increased energy produced by the
burning in the shells, the star's outer layers expand. The star becomes a giant again, but it
is bluer and brighter than it was the first time.
On the H-R diagram, the point representing the star has moved upward and to the right
along a line known as the asymptotic (as ihm TOT ihk) giant branch (AGB). The star is
therefore called an AGB star.
An AGB star's core is so hot and its gravitational
grip on its outermost layers is so weak that those
layers blow away in a stellar wind. As each layer
blows away, a hotter layer is exposed. Thus, the
stellar wind becomes even stronger. Out in space,
a succession of new, fast winds slam into old,
slow winds that are still moving away from the
star. The collisions produce dense shells of gas,
some of which cool to form dust.
White dwarf phase
In just a few thousand years, all but the hot core
of an AGB star blows away, and fusion ceases in
the core. The core illuminates the surrounding
shells. Such shells looked like planets through the
crude telescopes of astronomers who studied them A planetary nebula with an unusual
in the 1800's. As a result, the astronomers called textured appearance, the cause of
which is unknown. This photo was
taken by the Hubble Space Telescope.
Image credit: NASA
the shells planetary nebulae -- and today's astronomers still do. The word nebulae is Latin
After a planetary nebula fades from view, the remaining core is known as a white dwarf
star. This kind of star consists mostly of carbon and oxygen. Its initial temperature is
about 100,000 K.
Black dwarf phase
Because a white dwarf star has no fuel remaining for fusion, it becomes cooler and
cooler. Over billions of years, it cools more and more slowly. Eventually, it becomes a
black dwarf -- an object too faint to detect. A black dwarf represents the end of the life
cycle of an intermediate-mass star.
High-mass stars, those with more than 8 solar masses, form quickly and have short lives.
A high-mass star forms from a protostar in about 10,000 to 100,000 years.
High-mass stars on the main sequence are hot and blue. They are 1,000 to 1 million times
as luminous as the sun, and their radii are about 10 times the solar radius. High-mass stars
are much less common than intermediate- and low-mass stars. Because they are so bright,
however, high-mass stars are visible from great distances, and so many are known.
A high-mass star has a strong stellar wind. A star of 30 solar masses can lose 24 solar
masses by stellar wind before its core runs out of hydrogen and it leaves the main
As a high-mass star leaves the main sequence, hydrogen begins to fuse in a shell outside
its core. As a result, its radius increases to about 100 times that of the sun. However, its
luminosity decreases slightly. Because the star is now emitting almost the same amount
of energy from a much larger surface, the temperature of the surface decreases. The star
therefore becomes redder.
As the star evolves, its core heats up to 100 million K, enough to start the triple-alpha
process. After about 1 million years, helium fusion ends in the core but begins in a shell
outside the core. And, as in an intermediate-mass star, hydrogen fuses in a shell outside
that. The high-mass star becomes a bright red supergiant.
When the contracting core becomes sufficiently hot, carbon fuses, producing neon,
sodium, and magnesium. This phase lasts only about 10,000 years. A succession of
fusion processes then occur in the core. Each successive process involves a different
element and takes less time. Whenever a different element begins to fuse in the core, the
element that had been fusing there continues to fuse in a shell outside the core. In
addition, all the elements that had been fusing in shells continue to do so. Neon fuses to
produce oxygen and magnesium, a process that lasts about 12 years. Oxygen then fuses,
producing silicon and sulfur for about 4 years. Finally, silicon fuses to make iron, taking
about a week.
At this time, the radius of the iron core is about 1,900 miles (3,000 kilometers). Because
further fusion would consume energy, the star is now doomed. It cannot produce any
more fusion energy to balance the force of gravity.
When the mass of the iron core reaches 1.4 solar masses, violent events occur. The force
of gravity within the core causes the core to collapse. As a result, the core temperature
rises to nearly 10 billion K. At this temperature, the iron nuclei break down into lighter
nuclei and eventually into individual protons and neutrons. As the collapse continues,
protons combine with electrons, producing neutrons and neutrinos. The neutrinos carry
away about 99 percent of the energy produced by the crushing of the core.
Now, the core consists of a collapsing ball of neutrons. When the radius of the ball
shrinks to about 6 miles (10 kilometers), the ball rebounds like a solid rubber ball that has
All the events from the beginning of the collapse of the core to the rebounding of the
neutrons occur in about one second. But more violence is in store. The rebounding of the
ball of neutrons sends a spherical shock wave outward through the star. Much of the
energy of the wave causes fusion to occur in overlying layers, creating new elements. As
the wave reaches the star's surface, it boosts temperatures to 200,000 K. As a result, the
star explodes, hurling matter into space at speeds of about 9,000 to 25,000 miles (15,000
to 40,000 kilometers) per second. The brilliant explosion is known as a Type II
Supernovae enrich the clouds of gas and dust from which new stars eventually form. This
enrichment process has been going on since the first supernovae billions of years ago.
Supernovae in the first generation of stars enriched the clouds with materials that later
went into making newer stars.
Three generations of stars may exist. Astronomers have not found any of what would be
the oldest generation, Population III, stars. But they have found members of the other two
generations. Population II stars, which would be the second generation, contain relatively
small amounts of heavy elements. The more massive ones aged and died quickly, thereby
contributing more nuclei of heavy elements to the clouds. For this reason, Population I
stars, the third generation, contain the largest amounts of heavy elements. Yet these
quantities are tiny compared with the amount of hydrogen and helium in Population I
stars. For example, elements other than hydrogen and helium make up from 1 to 2
percent of the mass of the sun, a Population I star.
After a Type II supernova blast occurs, the stellar core remains behind. If the core has
less than about 3 solar masses, it becomes a neutron star. This object consists almost
entirely of neutrons. It packs at least 1.4 solar masses into a sphere with a radius of about
6 to 10 miles (10 to 15 kilometers).
Neutron stars have initial temperatures of 10 million K, but they are so small that their
visible light is difficult to detect. However, astronomers have detected pulses of radio
energy from neutron stars, sometimes at a rate of almost 1,000 pulses per second.
A neutron star actually emits two continuous beams of radio energy. The beams flow
away from the star in opposite directions. As the star rotates, the beams sweep around in
space like searchlight beams. If one of the beams periodically sweeps over Earth, a radio
telescope can detect it as a series of pulses. The telescope detects one pulse for each
revolution of the star. A star that is detected in this way is known as a pulsar.
If the stellar core remaining after the supernova explosion has about 3 or more solar
masses, no known force can support it against its own gravitation. The core collapses to
form a black hole, a region of space whose gravitational force is so strong that nothing
can escape from it. A black hole is invisible because it traps even light. All its matter is
located at a single point in its center. This point, known as a singularity, is much smaller
than an atomic nucleus.
Low-mass stars, ranging from 0.1 to 0.5 solar mass, have surface temperatures less than
about 4,000 K. Their luminosities are less than 2 percent of the solar luminosity. Low-
mass stars use hydrogen fuel so slowly that they may shine as main-sequence stars for
100 billion to 1 trillion years. This life span is longer than the present age of the universe,
believed to be 10 billion to 20 billion years. Therefore, no low-mass star has ever died.
Nevertheless, astronomers have determined that
low-mass stars will never fuse anything but
hydrogen. Thus, as these stars die, they will not
pass through a red-giant phase. Instead, they will
merely cool to become white dwarfs, then black
Binary stars develop from two protostars that
form near each other. More than 50 percent of
what seem to the unaided eye to be single stars
are actually binaries.
One star in a binary system can affect the life Transfer of mass occurs in a binary
cycle of the other if the two stars are sufficiently star system. Matter flows from a
close together. Between the stars is a location sunlike star, in the background in this
called the Lagrange point, named for the French illustration, to a disk orbiting a white
dwarf star, then to the surface of the
dwarf. Image credit: Space Telescope
mathematician Joseph Louis Lagrange, where the star's gravitational forces are exactly
equal. If one of the stars expands so much that its outer layers pass the Lagrange point,
the other star will begin to strip away those layers and accumulate them on its surface.
This process, called mass transfer, can take many forms. Mass transfer from a red giant
onto a main-sequence companion can add absorption lines of carbon or other elements to
the spectrum of the main- sequence star. But if the stars are close together, the material
will flow in the opposite direction when the giant star becomes a white dwarf. The matter
will spiral in toward the dwarf, forming a hot disk around it. The disk will flare brilliantly
in visible and ultraviolet radiation.
If the giant star leaves behind a neutron star or a black hole instead of a white dwarf, an
X-ray binary may form. In this case, the matter transferred from the main-sequence star
will become extremely hot. When this matter strikes the surface of the neutron star or is
pulled into the black hole, it will emit X rays.
In a third case, the red giant becomes a white dwarf, and the main-sequence star becomes
a red giant. When enough gas from the giant accumulates on the dwarf's surface, gas
nuclei will fuse violently in a flash called a nova. In some cases, so much gas will
accumulate that its weight will cause the dwarf to collapse. Almost instantly, the dwarf's
carbon will fuse, and the entire dwarf will explode in a Type I supernova. This kind of
explosion is so bright that it can outshine an entire galaxy for a few months.
Uranus, (YUR uh nuhs or yu RAY nuhs), is the
seventh planet from the sun. Only Neptune and
Uranus appears in true colors, left, and
Pluto are farther away. Uranus is the farthest
false colors, right in images produced
planet that can be seen without a telescope. Its
by combining numerous pictures taken
average distance from the sun is about
by the Voyager 2 spacecraft. The false
1,784,860,000 miles (2,872,460,000 kilometers),
colors emphasize bands of smog
a distance that takes light about 2 hours 40
around the planet's south pole. The
minutes to travel.
small spots are shadows of dust specks
in the camera. Image credit: JPL
Uranus is a giant ball of gas and liquid. Its
diameter at the equator is 31,763 miles (51,118 kilometers), over four times that of Earth.
The surface of Uranus consists of blue-green clouds made up of tiny crystals of methane.
The crystals have frozen out of the planet's atmosphere. Far below the visible clouds are
probably thicker cloud layers made up of liquid water and crystals of ammonia ice.
Deeper still -- about 4,700 miles (7,500 kilometers) below the visible cloud tops -- may
be an ocean of liquid water containing dissolved ammonia. At the very center of the
planet may be a rocky core about the size of Earth. Scientists doubt Uranus has any form
Uranus was the first planet discovered since ancient times. British astronomer William
Herschel discovered it in 1781. Johann E. Bode, a German astronomer, named it Uranus
after a sky god in Greek mythology. Most of our information about Uranus comes from
the flight of the United States spacecraft Voyager 2. In 1986, that craft flew within about
50,000 miles (80,000 kilometers) of the planet's cloud tops.
Orbit and rotation
Uranus travels around the sun in an elliptical (oval-shaped) orbit, which it completes in
30,685 Earth days, or just over 84 Earth years. As it orbits the sun, Uranus also rotates on
its axis, an imaginary line through its center. The planet's interior (ocean and core) takes
17 hours 14 minutes to spin around once on its axis. However, much of the atmosphere
rotates faster than that. The fastest winds on Uranus, measured about two-thirds of the
way from the equator to the south pole, blow at about 450 miles per hour (720 kilometers
per hour). Thus, this area toward the south pole makes one complete rotation every 14
Uranus is tilted so far on its side that its axis lies nearly level with its path around the sun.
Scientists measure the tilt of a planet relative to a line at a right angle to the orbital plane,
an imaginary surface touching all points of the orbit. Most planets' axes tilt less than 30¡.
For example, the tilt of Earth's axis is about 23 1/2. But Uranus's axis tilts 98 degrees, so
that the axis lies almost in the orbital plane. Many astronomers think that a collision with
an Earth-sized planet may have knocked Uranus on its side soon after it was formed.
Uranus has a mass (quantity of matter) 14 1/2 times larger than that of Earth. However,
the mass of Uranus is only about 1/20 as large as that of the largest planet, Jupiter.
Uranus has an average density of 1.27 grams per cubic centimeter, or about 1 1/4 times
the density of water. Density is the amount of mass in a substance divided by the volume
of the substance. The density of Uranus is 1/4 that of Earth, and is similar to that of
The force of gravity at the surface of Uranus is about 90 percent of that at the surface of
Earth. Thus, an object that weighs 100 pounds on Earth would weigh about 90 pounds on
The atmosphere of Uranus is composed of about 83 percent hydrogen, 15 percent helium,
2 percent methane, and tiny amounts of ethane and other gases. The atmospheric pressure
beneath the methane cloud layer is about 19 pounds per square inch (130 kilopascals), or
about 1.3 times the atmospheric pressure at the surface of Earth. Atmospheric pressure is
the pressure exerted by the gases of a planet's atmosphere due to their weight.
The visible clouds of Uranus are the same pale blue-green all over the surface of the
planet. Images of Uranus taken by Voyager 2 and
processed for high contrast by computers show very faint
bands within the clouds parallel to the equator. These
bands are made up of different concentrations of smog
produced as sunlight breaks down methane gas. In
addition, there are a few small spots on the planet's
surface. These spots probably are violently swirling
masses of gas resembling a hurricane.
The temperature of the atmosphere is about -355 degrees
F (-215 degrees C). In the interior, the temperature rises
rapidly, reaching perhaps 4200 degrees F (2300 degrees
Miranda, a satellite of Uranus,
C) in the ocean and 12,600 degrees F (7000 degrees C) in
has three regions called ovoids
the rocky core. Uranus seems to radiate as much heat into
whose outer ridges resemble
space as it gets from the sun. Because Uranus is tilted 98¡
race tracks. Internal geological
on its axis, its poles receive more sunlight during a
activity created the ovoids,
Uranian year than does its equator. However, the weather
probably in the past 2 billion
system seems to distribute the extra heat fairly evenly
years. Image credit: JPL
over the planet.
Uranus has 21 known satellites. Astronomers discovered the
5 largest satellites between 1787 and 1948. Photographs by
Voyager 2 in 1985 and 1986 revealed 10 additional satellites.
Astronomers later discovered more satellites by using Earth-
Miranda, the smallest of the five large satellites, has certain
surface features that are unlike any other formation in the
solar system. These are three oddly shaped regions called
ovoids. Each ovoid is 120 to 190 miles (200 to 300
kilometers) across. The outer areas of each ovoid resemble a
race track, with parallel ridges and canyons wrapped about
the center. But in the center, ridges and canyons crisscross Uranus has a number of
one another randomly. rings around it. Ten of
them are dark and narrow,
Magnetic field ranging in width from less
than 3 miles (5 kilometers)
to 60 miles (100
kilometers). They are no
more than 33 feet (10
meters) thick. Image credit:
Uranus has a strong magnetic field. The axis of the field (an imaginary line connecting its
north and south poles) is tilted 59 degrees from the planet's axis of rotation.
The magnetic field has trapped high-energy, electrically charged particles -- mostly
electrons and protons -- in radiation belts around the planet. As these particles travel back
and forth between the magnetic poles, they send out radio waves. Voyager 2 detected the
waves, but they are so weak that they cannot be detected on Earth.
Contributors: Peter J. Gierasch, Ph.D., Professor of Astronomy, Cornell University.
Philip D. Nicholson, Ph.D., Professor of Astronomy, Cornell University.
An artificial satellite is a manufactured object that continuously orbits Earth or some
other body in space. Most artificial satellites orbit Earth. People use them to study the
universe, help forecast the weather, transfer telephone calls over the oceans, assist in the
navigation of ships and aircraft, monitor crops and other resources, and support military
Artificial satellites also have orbited the moon, the sun, asteroids, and the planets Venus,
Mars, and Jupiter. Such satellites mainly gather information about the bodies they orbit.
Piloted spacecraft in orbit, such as space capsules, space shuttle orbiters, and space
stations, are also considered artificial satellites. So, too, are orbiting pieces of "space
junk," such as burned-out rocket boosters and empty fuel tanks that have not fallen to
Earth. But this article does not deal with these kinds of artificial satellites.
Artificial satellites differ from natural satellites, natural objects that orbit a planet. Earth's
moon is a natural satellite.
The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Since then, the
United States and about 40 other countries have developed, launched, and operated
satellites. Today, about 3,000 useful satellites and 6,000 pieces of space junk are orbiting
Satellite orbits have a variety of shapes. Some are circular, while others are highly
elliptical (egg-shaped). Orbits also vary in altitude. Some circular orbits, for example, are
just above the atmosphere at an altitude of about 155 miles (250 kilometers), while others
are more than 20,000 miles (32,200 kilometers) above Earth. The greater the altitude, the
longer the orbital period -- the time it takes a satellite to complete one orbit.
A satellite remains in orbit because of a balance between the satellite's velocity (speed at
which it would travel in a straight line) and the gravitational force between the satellite
and Earth. Were it not for the pull of gravity, a satellite's velocity would send it flying
away from Earth in a straight line. But were it not for velocity, gravity would pull a
satellite back to Earth.
To help understand the balance between gravity and velocity, consider what happens
when a small weight is attached to a string and swung in a circle. If the string were to
break, the weight would fly off in a straight line. However, the string acts like gravity,
keeping the weight in its orbit. The weight and string can also show the relationship
between a satellite's altitude and its orbital period. A long string is like a high altitude.
The weight takes a relatively long time to complete one circle. A short string is like a low
altitude. The weight has a relatively short orbital period.
Many types of orbits exist, but most artificial satellites orbiting Earth travel in one of four
types: (1) high altitude, geosynchronous; (2) medium altitude, (3) sun-synchronous,
polar; and (4) low altitude. Most orbits of these four types are circular.
A high altitude, geosynchronous orbit lies above the equator at an altitude of about
22,300 miles (35,900 kilometers). A satellite in this orbit travels around Earth's axis in
exactly the same time, and in the same direction, as Earth rotates about its axis. Thus, as
seen from Earth, the satellite always appears at the same place in the sky overhead. To
boost a satellite into this orbit requires a large, powerful launch vehicle.
A medium altitude orbit has an altitude of about 12,400 miles (20,000 kilometers) and an
orbital period of 12 hours. The orbit is outside Earth's atmosphere and is thus very stable.
Radio signals sent from a satellite at medium altitude can be received over a large area of
Earth's surface. The stability and wide coverage of the orbit make it ideal for navigation
A sun-synchronous, polar orbit has a fairly low altitude and passes almost directly over
the North and South poles. A slow drift of the orbit's position is coordinated with Earth's
movement around the sun in such a way that the satellite always crosses the equator at the
same local time on Earth. Because the satellite flies over all latitudes, its instruments can
gather information on almost the entire surface of Earth. One example of this type of
orbit is that of the TERRA Earth Observing System's NOAA-H satellite. This satellite
studies how natural cycles and human activities affect Earth's climate. The altitude of its
orbit is 438 miles (705 kilometers), and the orbital period is 99 minutes. When the
satellite crosses the equator, the local time is always either 10:30 a.m. or 10:30 p.m.
A low altitude orbit is just above Earth's atmosphere, where there is almost no air to
cause drag on the spacecraft and reduce its speed. Less energy is required to launch a
satellite into this type of orbit than into any other orbit.
Satellites that point toward deep space and provide
scientific information generally operate in this type of
orbit. The Hubble Space Telescope, for example, operates
at an altitude of about 380 miles (610 kilometers), with an
orbital period of 97 minutes.
Types of artificial satellites
A weather satellite called the
Artificial satellites are classified according to their Geostationary Operational
mission. There are six main types of artificial satellites: Environmental Satellite
(1) scientific research, (2) weather, (3) communications, observes atmospheric
(4) navigation, (5) Earth observing, and (6) military. conditions over a large area to
help scientists study and
Scientific research satellites gather data for scientific forecast the weather. Image
analysis. These satellites are usually designed to perform credit: NASA
one of three kinds of missions. (1) Some gather
information about the composition and effects of the space near Earth. They may be
placed in any of various orbits, depending on the type of measurements they are to make.
(2) Other satellites record changes in Earth and its atmosphere. Many of them travel in
sun-synchronous, polar orbits. (3) Still others observe planets, stars, and other distant
objects. Most of these satellites operate in low altitude orbits. Scientific research satellites
also orbit other planets, the moon, and the sun.
Weather satellites help scientists study weather patterns
and forecast the weather. Weather satellites observe the
atmospheric conditions over large areas.
Some weather satellites travel in a sun-synchronous, polar
orbit, from which they make close, detailed observations
of weather over the entire Earth. Their instruments
measure cloud cover, temperature, air pressure,
precipitation, and the chemical composition of the A communications satellite,
atmosphere. Because these satellites always observe Earth such as the Tracking and Data
at the same local time of day, scientists can easily Relay Satellite (TDRS) shown
compare weather data collected under constant sunlight here, relays radio, television,
conditions. The network of weather satellites in these and other signals between
orbits also function as a search and rescue system. They different points in space and
are equipped to detect distress signals from all on Earth. Image credit: NASA
commercial, and many private, planes and ships.
Other weather satellites are placed in high altitude, geosynchronous orbits. From these
orbits, they can always observe weather activity over nearly half the surface of Earth at
the same time. These satellites photograph changing cloud formations. They also produce
infrared images, which show the amount of heat coming from Earth and the clouds.
Communications satellites serve as relay stations, receiving radio signals from one
location and transmitting them to another. A communications satellite can relay several
television programs or many thousands of telephone calls at once. Communications
satellites are usually put in a high altitude, geosynchronous orbit over a ground station. A
ground station has a large dish antenna for transmitting and receiving radio signals.
Sometimes, a group of low orbit communications satellites arranged in a network, called
a constellation, work together by relaying information to each other and to users on the
ground. Countries and commercial organizations, such as television broadcasters and
telephone companies, use these satellites continuously.
Navigation satellites enable operators of aircraft, ships,
and land vehicles anywhere on Earth to determine their
locations with great accuracy. Hikers and other people on
foot can also use the satellites for this purpose. The
satellites send out radio signals that are picked up by a
computerized receiver carried on a vehicle or held in the
Navigation satellites operate in networks, and signals
from a network can reach receivers anywhere on Earth.
The receiver calculates its distance from at least three
A navigation satellite, like this satellites whose signals it has received. It uses this
Global Positioning System information to determine its location.
(GPS) satellite, sends signals
that operators of aircraft, ships, Earth observing satellites are used to map and monitor
and land vehicles and people our planet's resources and ever-changing chemical life
on foot can use to determine cycles. They follow sun-synchronous, polar orbits. Under
their location. Image credit: constant, consistent
NASA illumination from the sun,
they take pictures in different colors of visible light and
non-visible radiation. Computers on Earth combine and
analyze the pictures. Scientists use Earth observing
satellites to locate mineral deposits, to determine the
location and size of freshwater supplies, to identify
sources of pollution and study its effects, and to detect the
spread of disease in crops and forests.
An Earth observing satellite
Military satellites include weather, communications, surveys our planet's resources.
navigation, and Earth observing satellites used for This satellite, Aqua, helps
military purposes. Some military satellites -- often called scientists study ocean
evaporation and other aspects
of the movement and
distribution of Earth's water.
Image credit: NASA
"spy satellites" -- can detect the launch of missiles, the course of ships at sea, and the
movement of military equipment on the ground.
The life and death of a satellite
Building a satellite
Every satellite carries special instruments that enable it to perform its mission. For
example, a satellite that studies the universe has a telescope. A satellite that helps forecast
the weather carries cameras to track the movement of clouds.
In addition to such mission-specific instruments, all satellites have basic subsystems,
groups of devices that help the instruments work together and keep the satellite operating.
For example, a power subsystem generates, stores, and distributes a satellite's electric
power. This subsystem may include panels of solar cells that gather energy from the sun.
Command and data handling subsystems consist of computers that gather and process
data from the instruments and execute commands from Earth.
A satellite's instruments and subsystems are designed, built, and tested individually.
Workers install them on the satellite one at a time until the satellite is complete. Then the
satellite is tested under conditions like those that the satellite will encounter during
launch and while in space. If the satellite passes all tests, it is ready to be launched.
Launching the satellite
Space shuttles carry some satellites into space, but most satellites are launched by rockets
that fall into the ocean after their fuel is spent. Many satellites require minor adjustments
of their orbit before they begin to perform their function. Built-in rockets called thrusters
make these adjustments. Once a satellite is placed into a stable orbit, it can remain there
for a long time without further adjustment.
Performing the mission
Most satellites operate are directed from a control center on Earth. Computers and human
operators at the control center monitor the satellite's position, send instructions to its
computers, and retrieve information that the satellite has gathered. The control center
communicates with the satellite by radio. Ground stations within the satellite's range send
and receive the radio signals.
A satellite does not usually receive constant direction from its control center. It is like an
orbiting robot. It controls its solar panels to keep them pointed toward the sun and keeps
its antennas ready to receive commands. Its instruments automatically collect
Satellites in a high altitude, geosynchronous orbit are always in contact with Earth.
Ground stations can contact satellites in low orbits as often as 12 times a day. During
each contact, the satellite transmits information and receives instructions. Each contact
must be completed during the time the satellite passes overhead -- about 10 minutes.
If some part of a satellite breaks down, but the satellite remains capable of doing useful
work, the satellite owner usually will continue to operate it. In some cases, ground
controllers can repair or reprogram the satellite. In rare instances, space shuttle crews
have retrieved and repaired satellites in space. If the satellite can no longer perform
usefully and cannot be repaired or reprogrammed, operators from the control center will
send a signal to shut it off.
Falling from orbit
A satellite remains in orbit until its velocity decreases and gravitational force pulls it
down into a relatively dense part of the atmosphere. A satellite slows down due to
occasional impact with air molecules in the upper atmosphere and the gentle pressure of
the sun's energy. When the gravitational force pulls the satellite down far enough into the
atmosphere, the satellite rapidly compresses the air in front of it. This air becomes so hot
that most or all of the satellite burns up.
In 1955, the United States and the Soviet Union announced plans to launch artificial
satellites. On Oct. 4, 1957, the Soviet Union launched Sputnik 1, the first artificial
satellite. It circled Earth once every 96 minutes and transmitted radio signals that could
be received on Earth. On Nov. 3, 1957, the Soviets launched a second satellite, Sputnik 2.
It carried a dog named Laika, the first animal to soar in space. The United States
launched its first satellite, Explorer 1, on Jan. 31, 1958, and its second, Vanguard 1, on
March 17, 1958.
In August 1960, the United States launched the first communications satellite, Echo I.
This satellite reflected radio signals back to Earth. In April 1960, the first weather
satellite, Tiros I, sent pictures of clouds to Earth. The U.S. Navy developed the first
navigation satellites. The Transit 1B navigation satellite first orbited in April 1960. By
1965, more than 100 satellites were being placed in orbit each year.
Since the 1970's, scientists have created new and more effective satellite instruments and
have made use of computers and miniature electronic technology in satellite design and
construction. In addition, more nations and some private businesses have begun to
purchase and operate satellites. By the early 2000's, more than 40 countries owned
satellites, and nearly 3,000 satellites were operating in orbit.
Aviation is a term that includes all the activities involved in building and flying aircraft,
especially airplanes. The first successful airplane flights did not take place until 1903.
Yet today, airplanes affect the lives of people almost everywhere. Giant airliners carry
passengers and cargo between the world's major cities in a matter of hours. Planes and
helicopters rush medicine and other supplies to the farthest islands and deepest jungles.
Farmers use airplanes to seed fields, count livestock, and spray crops. Aviation has also
changed the way nations make war. Modern warfare depends on the instant striking
power of jet fighters and bombers and the rapid supply capabilities of jet transports.
Helicopters and other special aircraft are also important in military aviation.
Hundreds of thousands of airplanes are used throughout the world. They range from
small planes with room for only a pilot to enormous jumbo jets, which can carry
hundreds of passengers. To produce and operate all these airplanes requires the skills of
millions of workers in many countries -- from the engineers who design the planes to the
mechanics and pilots who service and fly them. Many government agencies also work to
make flying safer and more dependable. All these activities together make up the aviation
industry. The industry's two major activities are (1) the manufacture of aircraft and
aircraft components, such as engines, and (2) the operation of airlines. The manufacture
of aircraft, together with the manufacture of spacecraft, missiles, and related electronic
equipment, is often called the aerospace industry.
The aviation industry began on Dec. 17, 1903, near Kitty Hawk, North Carolina. That
day, Orville and Wilbur Wright -- two brothers who operated a bicycle-manufacturing
shop in Dayton, Ohio -- made the world's first successful piloted airplane flights. They
had built their airplane after studying the writings of other aviation pioneers and after
experimenting with gliders, kites, and wind tunnels.
Within a few years, several small factories in Europe and the United States were
producing airplanes. Daredevil fliers bought many of these planes and used them to put
on thrilling air shows. The governments of various countries also began to buy airplanes
to build small air forces. The daring feats of the early fliers and the development of
military airplanes greatly encouraged the growth of the aviation industry.
By the late 1930's, airplanes had become an important means of transportation. Then, in
the 1950's, engineers developed jet airliners -- and air travel grew at an even faster rate.
In 1960, the world's airlines carried about 100 million passengers. By the early 2000's,
they carried about 1 1/2 billion people annually.
Almost from the beginning of the aviation industry, the governments of most nations
have been deeply involved in its activities. Airplanes have such great importance as
weapons of war that many countries have encouraged and financed improvements in
airplane design for military reasons. Most nations have also supported the development
of civil aviation (the operation of nonmilitary aircraft).
Although aviation includes all types of heavier-than-air craft, this article deals chiefly
with airplanes. To learn about the two other main types of heavier-than-air craft. The
Airplane article traces the history of human efforts to fly and the development of the
airplane. It also describes how a plane flies, how pilots navigate, and how planes are
This article discusses Aviation (The aviation industry) (Aviation agencies and
organizations) (History of the aviation industry) (Careers in aviation).
The aviation industry
The aviation industry can be divided into five branches: (1) aircraft manufacturing, (2)
general aviation activities, (3) airline operations, (4) airport operations, and (5) aviation
Aircraft companies produce chiefly airplanes, but many also manufacture gliders,
helicopters, and parts for spacecraft. Some parts factories and assembly plants are owned
by conglomerates, enormous corporations that control a number of firms in largely
unrelated fields. Most of the aircraft used around the world are manufactured in the
The Russian aerospace industry produces aircraft and equipment for use throughout the
former Eastern bloc -- that is, the former Soviet Union and its Eastern European allies.
Russia also exports military aircraft to many other countries. British Aerospace is the
United Kingdom's major manufacturer of aircraft. Europe's other leading aircraft
manufacturing countries are France, Germany, Italy, and Spain. Other countries with
important aerospace industries include Brazil, Canada, China, India, Israel, Japan, and
South Africa. Many other nations have facilities for aircraft repair and maintenance.
Manufacturers produce three main types of airplanes: (1) general aviation planes, (2)
commercial transport planes, and (3) military planes. General aviation activities range
from business and personal flying to rescue services. Most general aviation planes are
small propeller driven airplanes with one or two engines. Many businesses use jets.
Commercial transport planes are large airplanes used to carry both passengers and cargo
or cargo only. Airlines operate these planes. The smallest commercial transports carry
from 20 to 100 passengers, and the largest, called jumbo jets or airbuses, carry several
hundred. Most commercial transports are jet planes with two, three, or four engines.
Military planes include bombers, fighters, and military transports owned by the
governments of various countries and operated by their armed forces.
In some countries, the government wholly or partly owns some or all aircraft companies.
All aircraft companies in the United States and some other countries are privately owned.
But many depend heavily on government orders for military planes, engines, missiles, or
spacecraft. Many U.S. manufacturers -- such as the Boeing Company, General Electric
Company, and the Lockheed Martin, Northrop Grumman, and United Technologies
corporations -- receive large government contracts.
A modern jet airliner costs millions of dollars to build. A small company cannot afford to
build such a plane, and even large companies often have trouble acquiring the necessary
funds. Many companies have merged (combined) to cut costs. These mergers have
produced some of the world's largest aerospace companies, including Boeing, British
Aerospace, and the European Aeronautic Defence and Space Company.
A number of European nations have cooperated in special aircraft-manufacturing
projects. For example, the British and French governments formed a partnership called a
consortium to share the cost of building a supersonic transport (SST), the Concorde.
SST's were designed to carry passengers at speeds faster than that of sound.
General aviation activities include pleasure flying, land surveying, giving flying
instructions, inspecting telephone lines, scattering seed, and spraying crops. Another
important general aviation activity is using light planes to provide transportation. Most
air taxi services, also called commuter airlines, use compact, twin-engine planes to carry
passengers -- usually fewer than 20 -- on short flights. They serve small communities and
provide connecting flights to large airports. Some air taxi services have planes large
enough to carry more than 20 passengers. Some large airlines also provide air taxi
Many businesses have their own aircraft that are used to fly officials and salespeople to
out-of-town assignments. General aviation planes also carry cargo and passengers in
areas of the world that do not have highways or railroads.
In Australia, a specialized aviation service called the Royal Flying Doctor Service
supplies medical treatment to people living in remote areas. People who are ill or require
medical advice use radio to contact a doctor at the nearest base. The doctor may advise
the patient by radio or may arrange for a light plane to pick up the patient. Air
ambulances in other parts of the world provide specially equipped airplanes to fly
patients to hospitals.
Almost every country has at least one airline. In some countries, the government owns
one or more airlines. For example, Alitalia, Italy's national airline, is mostly state-owned.
During the 1990's, many governments encouraged privatization of airlines to curb costs
and increase efficiency.
There are two main types of airline service -- scheduled flights and nonscheduled flights.
Scheduled flights are made over certain routes according to a timetable. Nonscheduled
flights are mainly charter flights for customers who want to hire a plane to fly to a
particular place at a particular time.
In the United States, airlines must receive permission from the federal government to use
commercial transport planes for scheduled flights. The airlines that the government
approves for such flights are called certificated airlines. The term scheduled airlines is
often used in the United States for the certificated airlines, though these lines may also
make some nonscheduled flights. To receive government certification, an airline's planes
and pilots must meet government standards.
Most airlines carry both passengers and cargo. Airliners usually carry a certain amount of
freight on passenger flights. Many passenger airlines also operate transport planes that
carry only cargo. A few certificated airlines in the United States specialize in carrying
cargo and do not make any passenger flights.
Sometimes, airlines have financial problems due to low passenger traffic, debts from
purchasing new aircraft, and increasing costs, such as the rising cost of jet fuel. In the
1970's, many airlines cut their airfares and developed various bargain ticket plans to
attract passengers. These steps led to huge increases in passenger traffic. High operating
costs led many small airlines to merge with larger airlines. By the late 1990's, many
airlines had also formed alliances for ticketing and for scheduling certain routes.
In most European countries, the government has combined two or more airlines to form a
large national airline. Various European airlines have also formed consortiums to help cut
expenses. The members of an airline consortium cooperate in such matters as purchasing
aircraft and training pilots.
Airports provide the fuel and the runways, navigation aids, and other ground facilities
needed for air travel. Generally, only a few of a country's airports have the facilities to
handle large passenger planes. Additional small airfields serve light planes or specialized
aircraft, such as helicopters or seaplanes. Cities or public corporations own most large
airports. Most small airports are private airfields owned by organizations or individuals.
Aviation support industries provide a wide variety of supplies and services to airlines,
airports, pilots, and passengers. Some companies furnish repair services or fuel for
airplanes. Freight forwarders make arrangements for shipping air cargo. Various food
services prepare meals to be served on passenger flights. Some insurance brokers
specialize in flight insurance, and some lawyers specialize in air law. Private weather
bureaus supply pilots with specialized information not provided by government weather
Aviation agencies and organizations
Aviation agencies Most countries have government agencies that enforce air safety
regulations and handle various economic matters relating to aviation. In the United
States, the Federal Aviation Administration (FAA) establishes the rules that all planes
must follow when flying in the United States. One of the agency's most important jobs is
to operate a network of air route traffic control centers throughout the United States and
its territories. Each control center uses radar and radio communications to help planes in
its vicinity follow the airways, also called air routes, to which they are assigned. The
FAA also issues licenses to pilots. In addition, every newly manufactured airplane must
be issued an FAA certificate of airworthiness before it may be flown. This certificate
states that the airplane has been inspected and is in good flying condition.
Almost every U.S. state has an agency to regulate and improve aviation within its
borders. These agencies handle airport construction, registration of airplanes and pilots,
and similar matters. Many local governments also have aviation agencies. These agencies
deal mainly with the operation and maintenance of local airports.
In Canada, the federal government regulates civil aviation. The director of civil aviation,
under the supervision of the Department of Transport, deals mainly with such matters as
registration of aircraft, licensing of pilots, and establishment of air navigation facilities.
The Canadian Transport Commission handles the economic regulation of Canadian
Similar regulatory activities are carried out by national agencies in other countries. Such
agencies include the United Kingdom's Civil Aviation Authority (CAA). These agencies
are involved in such issues as air-traffic control and registration of airplanes and pilots.
The International Civil Aviation Organization is an agency of the United Nations (UN).
Almost every country belongs to the ICAO. The organization sets up common air safety
standards among member countries and tries to increase cooperation in other matters
concerning international aviation.
Other aviation organizations include various groups that were formed to further their own
special interests. These groups include airline operators, airplane manufacturers, and
pilots. For example, U.S. and Canadian airline operators belong to the Air Transport
Association of America. Operators of international airlines in countries throughout the
world belong to the International Air Transport Association.
History of the aviation industry
The successful piloted flights of a powered airplane by Orville and Wilbur Wright in
1903 marked the beginning of the practical aviation industry. After these flights, the
Wright brothers tried to sell the design for their plane to the U.S. and various European
governments. But they had never made an official public flight, and government leaders
were not convinced that their plane could fly.
Meanwhile, a few European inventors had also built airplanes. In the 1890's, the German
glider pioneer Otto Lilienthal had manufactured a limited production series of special
gliders for experimental use. In 1905, two French fliers, the brothers Charles and Gabriel
Voisin, started the world's first airplane-manufacturing company. They began making a
few made-to-order planes at a small factory outside Paris. Within a few years, other
European fliers also started manufacturing companies. They included Louis Bleriot and
the brothers Henri and Maurice Farman in France; and Frederick Handley Page, A. V.
Roe, and T. O. M. Sopwith in the United Kingdom.
In 1907, Glenn H. Curtiss, an American flier and airplane designer, started the first
airplane company in the United States, in Hammondsport, New York. Curtiss sold his
first plane to the newly organized Aeronautic Society of New York for $5,000. This was
the first sale of a commercial airplane in the United States.
The Wright brothers had made their first official public flight in 1908 and amazed the
world with their airplane's flying ability. That same year, the U.S. Army Signal Corps
ordered a specially built Wright plane, for which the government paid $30,000. This was
the world's first military plane. In November 1909, a group of wealthy Americans lent the
Wright brothers money to start a manufacturing firm, the Wright Company. The company
had its factory in Dayton, Ohio, and its headquarters in New York City. In the autumn of
1909, a young automobile mechanic and salesman named Glenn L. Martin began to
manufacture airplanes in an abandoned church in Santa Ana, California. Within a few
years, his company became a leading U.S. producer of military planes.
The world's first great aviation meeting was held in 1909 near Reims, France.
Manufacturers displayed 38 airplanes. Several of the planes on show were offered for
sale to the public -- a sign of growing confidence in the airplane.
The first flying regulations
In 1905, a group of French flying enthusiasts established the Federation Aeronautique
Internationale (FAI) in Paris. One of the FAI's main duties was to regulate the sport of
flying. It also ruled on world speed, altitude, and other flying records. The FAI still has
this function. The Aero Club of America was also founded in 1905. It regulated flying in
the United States, sponsored exhibitions and races, and issued licenses to U.S. pilots.
In 1908, Kissimmee, Florida, passed the world's first law regulating airplanes. The law
required the registration of local aircraft and regulated their speed and altitude when
flying over the town. In 1911, Connecticut passed the first state law regulating aviation.
The law required anyone who owned or operated an airplane within the state to register
the plane and obtain a pilot's license.
World War I (1914-1918)
When World War I began in Europe, even the largest airplane factories turned out only a
few planes a year. But the factories quickly increased their production to meet the
demands of the warring nations. Airplane builders used newly designed engines to put
fighters and bombers into the skies. Such well-known manufacturers as Farman, Handley
Page, and Voisin built many of these planes. Other European manufacturers also became
famous for their warplanes. They included Morane-Saulnier and Nieuport in France;
Fokker and Junkers in Germany; and Bristol, de Havilland, Hawker, Short, and Vickers
in the United Kingdom. By the end of the war, designers had created such aircraft as the
British Vickers Vimy bomber and the American Curtiss NC-4, both of which flew across
the Atlantic Ocean in 1919.
The United States entered the war in 1917 with about 110 military planes. The
government immediately set a production goal of 29,000 airplanes a year. But the
airplane companies had little or no experience with mass-production methods. The
nation's automobile manufacturers, on the other hand, had developed assembly lines
before the war and used them to turn out thousands of cars yearly. Various automakers
helped set up assembly lines in the airplane factories.
The United States had no designs of its own for bombers or fighters. But American
engineers designed a powerful airplane engine called the Liberty. Several U.S. companies
began to mass-produce the United Kingdom's de Havilland D.H. 4 bombers and equip
them with Liberty engines. The principal producer was the Dayton Wright Aeroplane
Company, which was organized in 1917. Wilbur Wright had died of typhoid fever in
1912, and Orville sold his interest in the Wright Company to a group of investors in
1915. Although Orville had no financial interest in the Dayton Wright Company, he
allowed the firm to use the Wright name in its title. The companies founded by Curtiss
and Martin also became major producers of military planes during the war. Although
U.S. factories did not meet their production goal of 29,000 planes a year, they had built
almost 15,000 military planes by the end of the war.
In 1916, two airplane companies were established on the West Coast of the United States.
They were the Boeing Company, founded in Seattle by William E. Boeing, and the
Lockheed Corporation (now Lockheed Martin Corporation), founded in Santa Barbara,
California, by the brothers Allan and Malcolm Loughead. The Boeing and Lockheed
companies were too small to make many planes during the war. But in time, they became
two of the nation's leading aircraft manufacturers.
The first airlines
The Wright brothers and other early fliers occasionally took passengers for short plane
rides. In 1910, a Wright airplane flew 70 pounds (32 kilograms) of silk from Dayton to
Columbus, Ohio -- perhaps the first air freight shipment in history. The world's first
regular airplane passenger service began in the United States in 1914, but it lasted only a
few months. A pilot named Tony Jannus used a small seaplane to fly passengers across
Tampa Bay, between St. Petersburg and Tampa, Florida. On May 15, 1918, the U.S.
government started the world's first permanent airmail service. Army pilots flew the mail
between New York City, Philadelphia, and Washington, D.C.
After World War I, thousands of military planes became available for civilian use. In
1919, bombers were used to start nearly 20 small passenger airlines in France, Germany,
the United Kingdom, and several other European countries. One of these airlines,
founded by Henri and Maurice Farman, began the world's first regular international
airline service. The company used old Farman bombers to make weekly passenger flights
between Paris and Brussels, Belgium.
By 1924, passenger airlines were operating in 17 European countries as well as in Africa,
Australia, and South America. Several of these airlines are still active. They include
KLM Royal Dutch Airlines (now part of Air France-KLM) of the Netherlands,
Germany's Lufthansa, and Australia's Queensland and Northern Territory Aerial Services
(QANTAS). Beginning in the mid-1920's, the governments of many countries started to
combine two or more private airlines to form a large national airline. In 1924, the United
Kingdom became the first major power to form a national, government-owned airline,
Many small passenger airlines were formed during the early 1920's. But most lasted only
a few months because they could not attract enough customers. Most people considered
flying a dangerous sport rather than a safe means of transportation.
In the United States, the federal government's main interest in aviation was to improve
airmail service. In 1920, airmail routes extended from New York City to San Francisco.
Mail planes operated only during the day, however. To help the mail pilots fly their open-
cockpit planes at night, the government installed beacon lights at airports along the
transcontinental route. Each light could be seen as far as 50 miles (80 kilometers) away.
By 1924, night-flying techniques enabled planes to get mail from New York City to San
Francisco in 24 hours.
In 1925, the U.S. Congress passed the Kelly Air Mail Act, which gave private airlines the
job of flying the mail. The government then signed contracts with 11 companies formed
to carry the mail. Henry Ford, the famous automobile maker, owned one of these airlines.
In 1926, Ford's airline became the first airline to carry U.S. mail. Within a few months,
all 11 companies were flying mail between major U.S. cities. Some of the airlines also
began carrying passengers. In 1926, airlines in the United States carried only about 6,000
passengers. In 1930, they carried more than 400,000.
Several U.S. aircraft companies were also started during the 1920's. In 1920, an engineer
named Donald Douglas helped organize an aircraft company in Santa Monica, California.
It became the Douglas Company the following year, later part of McDonnell Douglas
Corporation, and later still part of the Boeing Company. In 1923, the Consolidated
Aircraft Corporation was founded in East Greenwich, Rhode Island. It took over the
airplane designs of the Dayton Wright Company. The Pratt and Whitney Company began
making aircraft engines in Hartford, Connecticut, in 1925. In 1929, the Curtiss and
Wright companies merged to form the Curtiss-Wright Corporation. Grumman Aircraft
(now part of Northrop Grumman Corporation) also started business in 1929 on Long
Island, New York.
The rapid increase in aviation activity led Congress to pass the Air Commerce Act in
1926. This act was the first federal law to regulate aviation in the United States. It
provided for a system of airways and navigation aids across the country. The act also
called for rules governing the manufacture of airplanes and the licensing of airplanes and
pilots. A Bureau of Air Commerce was set up to carry out these measures.
The industry comes of age
Air transport continued to grow during the early 1930's. By 1935, the United States had
four major domestic airlines -- American, Eastern, Transcontinental and Western Air
(later called Trans World Airlines), and United. Smaller regional airlines included
Braniff, Delta, and Northwest. The country also had a major international airline -- Pan
American World Airways (Pan Am) -- which flew to Latin America. Many European
governments continued to form large national airlines, such as Air France (now part of
Air France-KLM) and Italy's Ala Littoria (now Alitalia).
To meet the growing demand for faster, larger airliners, manufacturers began to produce
twin-engine planes, such as the Boeing 247 and the Douglas DC-3. The DC-3 appeared in
1935 and soon became the world's most popular transport plane. A number of companies,
including Martin (now Lockheed Martin Corporation) in the United States and Short in
the United Kingdom, started to make large, four-engine seaplanes called flying boats. In
the 1930's, flying boats made the first passenger flights across oceans. New firms were
also started in the 1930's, such as North American Aviation and United Aircraft (now
United Technologies), which took over production of Pratt and Whitney engines.
By the late 1930's, flying was an important means of travel in most of the world. In 1938,
the world's airlines carried nearly 3 1/2 million passengers.
The rapid growth of civil aviation created a need for more effective government
regulation. In 1938, the U.S. Congress established the Civil Aeronautics Authority to deal
with every aspect of civil aviation. The authority included a five-member board, which,
in 1940, became the Civil Aeronautics Board. It also included an administrative office,
which became the Civil Aeronautics Administration (CAA) in 1940.
World War II (1939-1945)
The peace treaty that ended World War I prohibited the manufacture of military aircraft
in Germany. Nevertheless, several German aircraft firms were founded during the 1920's.
They included the famous Heinkel and Messerschmitt companies. In the mid-1930's,
Heinkel, Messerschmitt, and other German firms, such as Dornier and Junkers, secretly
made hundreds of bombers and fighters for the German air force. On Sept. 1, 1939,
German dive bombers attacked Poland, and World War II began. One European country
after another fell to the Germans. Finally, the United Kingdom was left nearly alone to
fight off the German air force. British aircraft companies, such as Avro, de Havilland,
Handley Page, Hawker, and Supermarine, quickly increased their production of
The United States produced about 2,100 military planes in 1939. Both Germany and
Japan had larger air forces. The huge Mitsubishi corporation produced many of Japan's
warplanes, including the famous Zero fighter. After the United States entered the war in
December 1941, U.S. airplane production increased greatly. More than 40 companies
took part in a gigantic effort to supply the United States and its allies with military
planes. Many companies enlarged their factories and hired additional workers. Assembly
lines began working round the clock. By 1944, production had reached nearly 100,000
transport planes, bombers, and fighters a year.
By the end of the war, U.S. factories had built more than 300,000 aircraft. Germany,
Japan, the Soviet Union, and the United Kingdom had also produced many thousands of
planes. During the war, aircraft production had become the world's leading
A new age of flight
In 1937, British inventor Frank Whittle built the first successful jet engine. The first jet
airplanes were developed for military use. Germany flew the first jet aircraft in 1939. By
1942, both the United Kingdom and the United States had developed experimental jet
planes for military use.
After World War II, aircraft manufacturers began the development of jet airliners. In
1952, British Overseas Airways Corporation (BOAC), now British Airways, started jet
passenger flights with de Havilland Comets. But the flights were stopped after several
Comets exploded in the air. Investigators discovered serious flaws in the plane's
structure. De Havilland engineers then designed an improved Comet. In 1958, BOAC
used the new Comets to begin jet passenger service across the Atlantic Ocean. American
companies also built successful jet transports in the late 1950's, and these aircraft quickly
dominated international air transportation. In 1959, American Airlines used the first of
these -- the Boeing 707 -- to start transcontinental jet service from New York City to Los
The beginning of jet airline service created new challenges. Large jetliners carried nearly
200 passengers, and the crash of one of these planes could cause heavy loss of life. In
addition, new hazards were created along the world's air routes as airplanes flew faster
and in greater numbers than ever before. In 1958, the U.S. government combined the
CAA and several other agencies to form the Federal Aviation Agency. The agency was
given the job of establishing and enforcing air safety regulations and air traffic
procedures in the United States. It was renamed the Federal Aviation Administration in
By 1970, jet transports had replaced propeller-driven planes on most major airlines. In
1970, Pan Am became the first airline to offer jumbo jet service, using Boeing 747's.
France and the United Kingdom began passenger service with their SST, the Concorde,
Beginning in the 1950's, several large aerospace companies were formed by mergers. In
1954, the General Dynamics Corporation took control of Consolidated Vultee (Convair).
In 1967, McDonnell Aircraft merged with Douglas Aircraft to form the McDonnell
Douglas Corporation, and North American Aviation and Rockwell-Standard merged,
forming the North American Rockwell Corporation. In 1973, this firm merged with
Rockwell Manufacturing Company to become Rockwell International Corporation.
Internationalization became an important trend in the aviation industry beginning in the
1960's. The term refers to cooperative manufacturing programs in which firms from
different nations share research, development, and production costs. The consortium
formed by the British and French to build the Concorde SST was an early program of this
type. Another successful program has been Airbus. This consortium, which manufactures
commercial transport aircraft, has involved most countries in Western Europe.
United States aviation firms moved slowly into internationalization in the 1970's.
Manufacturers in Canada, Italy, Japan, and the United Kingdom produced major parts of
the McDonnell Douglas DC-10 transport, which began commercial service in 1971.
Some U.S. firms have formed partnerships with foreign companies to manufacture
European-designed aircraft in the United States. For example, during the 1980's,
McDonnell Douglas produced the British-designed Harrier -- a V/STOL (Vertical/Short
Take-Off and Landing plane) -- in partnership with British Aerospace.
Airline safety concerns
Beginning in the 1960's, airliner hijacking, also called air piracy, became a serious
problem. In 1970, hijackers throughout the world seized 49 airliners and forced the pilots
to fly to destinations off their routes, often to other countries. In the 1980's, terrorist
sabotage became a serious risk as several airliners were blown up in flight.
In response to the hijackings, aviation authorities tightened airport security regulations.
These regulations include the inspection of aircraft, passengers, and baggage for hidden
guns, bombs, or other weapons. Most countries have laws against hijacking and
terrorism. But laws differ from country to country. The ICAO develops procedures to
help member countries establish consistent methods to prevent and investigate hijackings.
Deregulation of the U.S. airlines
In the late 1970's, the Civil Aeronautics Board began to ease its controls over airline fares
and routes in the United States to encourage greater competition and better service. In
1978, Congress passed the Airline Deregulation Act. This law provided for the gradual
removal of economic controls of the airline industry. The Civil Aeronautics Board was
dissolved in 1984. New airlines soon began to form, and existing ones rapidly expanded
Deregulation in the United States allowed domestic airlines to compete in many
international markets. Many U.S. airlines formed alliances with overseas carriers to
simplify ticketing. Many U. S. airlines also developed hub and spoke systems. In such a
system, many flights connect at a central airport. In manufacturing, several mergers in the
1990's led to the disappearance of historic U.S. airplane builders, such as McDonnell
Douglas, which merged into Boeing. International partnerships became increasingly
significant, with Airbus capturing one-third of the world market in jet airliner sales in the
On Sept. 11, 2001, terrorists hijacked four commercial airplanes, deliberately crashing
two into the towers of the World Trade Center in New York City and one into the
Pentagon Building outside Washington, D.C. The fourth hijacked plane crashed in
Somerset County, Pennsylvania. After the hijackings, U.S. airports and airlines sought
new ways to protect against terrorist attacks. Congress passed legislation requiring
federal employees to handle all passenger and baggage inspection in U.S. airports by the
end of 2002. A newly created agency, the Transportation Security Administration, took
over air safety functions from the FAA.
Fears of terrorism and a sluggish world economy contributed to a decline in air travel in
the early 2000's. In 2003, British Airways and Air France discontinued all Concorde
flights because the flights were no longer profitable.
Careers in aviation
The aviation industry employs many kinds of skilled workers. They include aeronautical
engineers, computer specialists, electricians, flight attendants, flight engineers, flying
instructors, mechanics, pilots, radar specialists, and radio operators. In the United States,
many jobs in the aviation industry require certification from the FAA. For example, air
traffic controllers, aviation mechanics, flight engineers, and pilots must have FAA
Some schools offer courses in preparation for such careers as aviation mechanic,
computer specialist, and radio operator. Aeronautical engineering and some other highly
skilled professions require a college education. Most pilots obtain their training at flying
schools or in military service. Some high schools and colleges also offer courses in
Jobs in general aviation
Many pilots work for air taxi services, business firms, and other organizations that use
light planes. In many countries, flying light planes for commercial purposes requires a
commercial pilot license. In the United States, the FAA issues these licenses to pilots 18
years old or over who have at least 200 hours of flying experience and who pass the
physical, written, and flight examinations.
Jobs with airlines and airports
In most countries, airline pilots and copilots must obtain a special license. They must pass
a thorough physical examination, as well as written and flight examinations. In the
United States, airline pilots and copilots must have an FAA airline transport pilot license.
To obtain this license, they must be at least 23 years old, and have a commercial pilot
license and 1,500 hours of flight time.
Some airlines use flight engineers. On long flights, the engineers watch the many
instruments in the cockpit that tell how the engines are operating. Most airlines require
their flight engineers to have a commercial pilot license. Airlines prefer to hire flight
attendants who have some college, business, or nursing training. Skilled mechanics are
needed for airliner maintenance.
Jobs in the aircraft industry
Aircraft manufacturers hire electricians, machine tool operators, mechanics, and other
skilled workers to make and assemble the many parts of airplanes. The industry also
employs various types of engineers to design aircraft and experienced pilots to test-fly
Jobs with government agencies. Government agencies in many countries hire radar and
radio operators to work at air route traffic control centers and airport control towers. They
also hire mechanics and pilots to serve as safety agents. Many local aviation agencies
also require engineers, mechanics, pilots, and other skilled people. Some large cities hire
pilots to serve as flying police officers or to perform rescue services.
A constellation (KON stuh LAY shuhn) is a group of stars visible within a particular
region of the night sky. The word constellation also refers to the region in which a
specific group of stars appears. Astronomers have divided the sky into 88 areas, or
The ancient Greeks, Romans, and people of various other early civilizations observed
groups of stars in the northern two-thirds of the sky. They named these groups of stars
after animals and mythological characters. For example, the constellation Leo was named
for a lion, Pisces for two fish, and Taurus for a bull. The constellations Andromeda,
Cassiopeia, Orion, and Perseus are named for heroines and heroes in Greek mythology.
Between the early 1400's and the mid-1700's, European navigators explored the Southern
Hemisphere and observed many constellations in the southernmost third of the sky.
Mapmakers and explorers named these star groups for scientific instruments and other
things as well as for animals. For example, the constellation Telescopium was named for
the telescope. Musca was named for the fly, and Tucana for the toucan, a large-billed bird
of Central and South America.
Some well-known groups of stars form only part of a constellation. Such smaller groups
are called asterisms. For example, the Big Dipper is an asterism that lies in the
constellation Ursa Major (Great Bear).
Some constellations can be seen only during certain seasons due to the earth's annual
revolution around the sun. The part of the sky visible at night at a particular place
gradually changes as the earth moves around the sun. Also, observers at different
latitudes see different parts of the sky. An observer at the equator can view all the
constellations during the course of a year, but an observer at the North or the South Pole
can see only a single hemisphere of constellations.
Extraterrestrial (EHKS truh tuh REHS tree uhl) intelligence is intelligent life that
developed somewhere other than the Earth. No life has been discovered on any planet
other than the Earth. However, many scientists have concluded that intelligent life may
exist on planets orbiting some of the hundreds of billions of stars in our galaxy, the Milky
Way. These scientists base their conclusion on research in such fields as astronomy,
biology, planetary science, and paleontology (the study of prehistoric life through
fossils). The effort to find evidence that there is extraterrestrial intelligence is often called
SETI, which stands for Search for Extraterrestrial Intelligence.
SETI researchers believe that the best way to discover other intelligent life in the galaxy
is to look for evidence of technology developed by that life. In the belief that intelligent
beings on other worlds would eventually develop radio technology, researchers have used
large radio telescopes to search the sky. In 1960, the first SETI experiment
unsuccessfully examined two stars at a single radio frequency. After several dozen
additional searches, the National Aeronautics and Space Administration (NASA) in 1992
began a two-part project known as the High Resolution Microwave Survey. Researchers
searched for weak microwave (short radio wave) signals originating near specific stars
that are similar to the sun. They also started to scan the entire sky for strong microwave
signals. In 1993, the United States Congress, in a budget-cutting measure, instructed
NASA to end the project. SETI research continues in the United States under private
In 1998, astronomers began to search for pulses of laser light. The astronomers reasoned
that intelligent beings on a planet orbiting a distant star might have developed powerful
lasers. The beings might have transmitted brief pulses of laser light into space as a signal
to observers on other planets. They would have used pulses so that the observers could
distinguish the laser light from the bright, steady light coming from their star.
Astronomers on the earth would be able to distinguish powerful pulses that were a few
billionths of a second in duration.
Gravitation is the force of attraction that acts between all objects because of their mass.
An object's mass is its amount of matter. Because of gravitation, an object that is near
Earth falls toward the surface of the planet. An object that is already on the surface
experiences a downward force due to gravitation. We experience this force on our bodies
as our weight. Gravitation holds together the hot gases that make up the sun, and it keeps
the planets in their orbits around the sun. Another term for gravitation is the force of
People misunderstood gravitation for centuries. In the 300's B.C., the Greek philosopher
and scientist Aristotle taught the incorrect idea that heavy objects fall faster than light
objects. People accepted that idea until the early 1600's, when the Italian scientist Galileo
corrected it. Galileo said that all objects fall with the same acceleration unless air
resistance or some other force acts on them. An object's acceleration is the rate of change
of its velocity (speed in a particular direction). Thus, a heavy object and a light object that
are dropped from the same height will reach the ground at the same time.
Newton's law of gravitation
Ancient astronomers measured the movements of the moon and planets across the sky.
However, no one correctly explained those motions until the late 1600's. At that time, the
English scientist Isaac Newton described a connection between the movements of the
celestial bodies and the gravitation that attracts objects to Earth.
In 1665, when Newton was 23 years old, a falling apple caused him to question how far
the force of gravity reaches. Newton explained his discovery in 1687 in a work called
Philosophiae naturalis principia mathematica (Mathematical Principles of Natural
Philosophy). Using laws of planetary motion discovered by the German astronomer
Johannes Kepler, Newton showed how the sun's force of gravity must decrease with the
distance from the sun. He then assumed that Earth's gravitation decreases in the same
way with the distance from Earth. Newton knew that Earth's gravitation holds the moon
in its orbit around Earth, and he calculated the strength of Earth's gravitation at the
distance of the moon. Using his assumption, he calculated what the strength of that
gravitation would be at Earth's surface. The calculated result was the same as the strength
of the gravitation that would accelerate an apple.
Newton's law of gravitation says that the gravitational force between two objects is
directly proportional to their masses. That is, the larger either mass is, the larger is the
force between the two objects. The law also says that the gravitational force between two
objects is inversely (oppositely) proportional to the distance between the two objects
squared (multiplied by itself). For example, if the distance between the two objects
doubles, the force between them becomes one-fourth of its original strength. Newton's
law is given by the equation F = m1m2 / d 2, where F is the gravitational force between
two objects, m1 and m2 are the masses of the objects, and d2 is the distance between
Until the early 1900's, scientists had observed only one movement that could not be
described mathematically using Newton's law -- a tiny variation in the orbit of the planet
Mercury around the sun. Mercury's orbit -- like the orbits of the other planets -- is an
ellipse, a geometric figure with the shape of a flattened hoop. The sun is not at the exact
center of the ellipse. So one point in each orbit is closer to the sun than all other points in
that orbit. But the location of the closest point changes slightly each time Mercury
revolves around the sun. Astronomers refer to that variation as a precession.
Scientists used Newton's law to calculate the precession. The calculated amount differed
slightly from the observed amount.
Einstein's theory of gravitation
In 1915, the German-born physicist Albert Einstein announced his theory of space, time,
and gravitation, the general theory of relativity. Einstein's theory completely changed
scientists' way of thinking about gravitation. However, it expanded upon Newton's law,
rather than contradicting it.
In many cases, Einstein's theory produced results that differed only slightly from results
based on Newton's law. For example, when Einstein used his theory to calculate the
precession of Mercury's orbit, the result agreed exactly with the observed motion. That
agreement was the first confirmation of Einstein's theory.
Einstein based his theory on two assumptions. The first is related to an entity known as
space-time, and the second is a rule known as the principle of equivalence.
In the complex mathematics of relativity, time and space are not absolutely separate.
Instead, physicists refer to space-time, a combination of time and the three dimensions of
space -- length, width, and height. Einstein assumed that matter and energy can distort
(change the shape of) space-time, curving it; and that gravitation is an effect of the
The principle of equivalence states that the effects of gravity are equivalent to the effects
of acceleration. To understand this principle, suppose you were in a rocket ship so far
from any planet, star, or other celestial object that the ship experienced virtually no
gravitation. Imagine that the ship was moving forward, but not accelerating -- in other
words, that the ship was traveling at a constant speed and in a constant direction. If you
held out a ball and released it, the ball would not fall. Instead, it would hover beside you.
But suppose the rocket accelerated by increasing its speed. The ball would appear to fall
toward the rear of the ship exactly as if gravity had acted upon it.
Predictions of general relativity
In the years since the calculation of Mercury's precession confirmed Einstein's theory,
several observations have verified predictions made with the theory. Some examples
include predictions of the bending of light rays and radio waves, the existence of gravity
waves and black holes, and the expansion of the universe.
Bending of light rays
Einstein's theory predicts that gravity will bend the path of a light ray as the ray passes
near a massive body. The bending will occur because the body will curve space-time. The
sun is massive enough to bend rays by an observable amount, and scientists first
confirmed this prediction during a total eclipse of the sun in 1919.
Bending and slowing of radio waves
The theory also predicts that the sun will bend radio waves and slow them down.
Scientists have measured the sun's bending of radio waves emitted (sent out) by quasars,
extremely powerful objects at the centers of some galaxies. The measurements agree well
with the prediction.
Researchers measured a delay of radio waves that pass near the sun by sending signals
between Earth and the Viking space probes that reached Mars in 1976. Those
measurements still represent one of the most precise confirmations of general relativity.
General relativity also indicates that massive bodies in orbit around each other will emit
waves of energy known as gravitational waves. Since 1974, scientists have confirmed the
existence of gravitational waves indirectly by observing an object known as a binary
pulsar. A binary pulsar is a rapidly rotating neutron star that orbits a similar, but
unobserved, companion star. A neutron star consists mostly of tightly packed neutrons,
particles that ordinarily occur only in the nuclei of atoms.
A pulsar emits two steady beams of radio waves that flow away in opposite directions. As
the star rotates on its own axis, the beams sweep around in space like searchlight beams.
If one of the radio beams periodically sweeps over Earth, a radio telescope can detect the
beam as a series of pulses. By closely observing changes in the pulse rate of a binary
pulsar, scientists can determine the pulsar's orbital period -- the time it takes the two stars
to completely orbit each other.
Observations of the binary pulsar called PSR 1913 + 16 indicate that its orbital period is
decreasing, and astronomers have measured the amount of the decrease. Scientists have
also used equations of general relativity to calculate the amount by which the orbital
period would decrease if the binary pulsar was radiating away energy as gravitational
waves. The calculated amount agrees with the measured amount.
In addition, the pulsar's orbit precesses as the pulsar revolves around the companion star.
General relativity predicts the precession rate, and measurements match the prediction
with great precision.
Einstein's theory predicts the existence of objects called black holes. A black hole is a
region of space whose gravitational force is so strong that not even light can escape from
it. Researchers have found strong evidence that most very massive stars eventually
evolve into black holes, and that most large galaxies have a gigantic black hole at their
Expansion of the universe
In a paper published in 1917, Einstein applied general relativity to cosmology, the study
of the universe as a whole. The theory showed that the universe must either expand or
contract. In 1917, however, scientists had not yet found any evidence of expansion or
contraction. To prevent his theory from disagreeing with the available evidence, Einstein
added a term, the cosmological constant, to the theory. That term represented a repulsion
(pushing away) of every point in space by the surrounding points, preventing contraction.
But in 1929, the American astronomer Edwin Hubble discovered that distant galaxies are
moving away from Earth and that, the more distant a galaxy, the more rapidly it is
moving away. Hubble's discovery indicated that the universe is expanding. In response to
Hubble's discovery and confirming observations by other astronomers, Einstein
abandoned the cosmological constant, calling it his greatest blunder.
The discovery of the expansion of the universe, together with other observations, led to
the development of the big bang theory of the origin of the universe. According to that
theory, the universe began with a hot, explosive event -- a "big bang." At the beginning
of the event, all the matter in the part of the universe we can see was smaller than a
marble. Matter then expanded rapidly, and it is still expanding.
Although Einstein called the cosmological constant his greatest blunder, it may turn out
to be one of his greatest achievements. Measurements reported in 1998 suggest that the
universe is expanding more and more rapidly. Furthermore, the rate of expansion has
been increasing as predicted by general relativity with a cosmological constant.
Until the measurements were reported, astronomers generally thought that the rate of
expansion was decreasing due to the gravitational attraction of galaxies for one another.
The measurements showed that exploding stars known as supernovae in distant galaxies
were dimmer than expected and that the galaxies therefore were farther away then
expected. But the galaxies could be so far away only if the rate of expansion had begun to
increase in the past.
Astronomers have concluded that the increase in the expansion rate is due to an entity
that presently opposes gravitation. That entity could be a cosmological constant or
something much like it called dark energy. Scientists have not yet developed theories to
account for the existence of dark energy, but they know how much of it probably exists.
The amount of dark energy in the universe is about twice as much as the amount of
The matter in the universe includes both visible matter and a mysterious substance known
as dark matter. Scientists do not know the composition of dark matter. But measurements
of the motion of stars and gas clouds in galaxies have led scientists to believe that it
exists. Those measurements show that the masses of galaxies are many times larger than
the masses of the visible objects in them. These and other observations suggest that the
universe has at least 30 times as much dark matter as visible matter.
Gravitation and the age of the universe
Other observations have helped show that the theory of general relativity applies to the
whole universe. Cosmologists have calculated the age of the universe using equations of
general relativity, the measured rate of expansion of the universe, and estimates of the
amounts of dark energy and dark matter. The calculated age, about 14 billion years,
agrees well with results determined by two methods that do not involve general relativity:
(1) calculations based on the evolution of stars and (2) the radioactive dating of old stars.
As a star evolves, its surface temperature and its brightness change in a well-understood
way. Astronomers can determine the ages of certain stars by measuring their temperature
and brightness, then performing calculations based on their knowledge of stellar
evolution. By means of such techniques, astronomers have found stars that may be about
13 billion years old -- but no stars that are clearly older than that.
Radioactive dating of stars is based on the fact that certain chemical elements undergo
radioactive decay. In radioactive decay, an isotope (form) of an element turns into an
isotope of another element. Radioactive isotopes decay at known rates.
In 2001, scientists working with the European Southern Observatory's Very Large
Telescope in Chile applied the radioactive dating technique to an old star in our galaxy,
the Milky Way. The researchers studied the isotope uranium 238, whose nucleus contains
92 protons and 146 neutrons. The scientists knew how much uranium the star must have
had when it formed, and they measured how much it has now. They then applied their
knowledge of decay rates to calculate the age of the star. The most likely age of the star is
12.5 billion years, so the universe is probably older than that. Measurements of the ages
of many old stars using another element, thorium, gave similar results.
Christiaan Huygens, (HY guhnz), (1629-1695), was a Dutch physicist, astronomer, and
mathematician. In 1678, Huygens proposed that light consists of series of waves. He used
this theory in investigating the refraction (bending) of light.
Huygens's wave theory competed for many years with the corpuscular theory of the
English scientist Isaac Newton. Newton maintained that light is made up of particles.
Today, scientists believe that light behaves as both a particle and a wave.
Huygens was born on April 14, 1629, in The Hague, the Netherlands. He studied
mathematics and law at the University of Leiden and the College of Orange at Breda.
Huygens worked with his brother Constantijn to develop skill in grinding and polishing
spherical lenses. With these lenses, they built the most powerful telescopes of their time.
Huygens also discovered Saturn's moon Titan and asserted that what astronomers called
"Saturn's arms" was a ring. In mathematics, he refined the value of pi . In the 1650's,
Huygens invented a clock with a freely suspended pendulum. He died on July 8, 1695.
The European Space Agency honored Huygens's discovery of Titan by naming a space
probe after him. The Huygens probe, designed to drop through Titan's atmosphere, was
launched aboard the Cassini spacecraft in 1997.
Mars is the fourth planet from the sun. The planet
is one of Earth's "next-door neighbors" in space.
Earth is the third planet from the sun, and Jupiter is
the fifth. Like Earth, Jupiter, the sun, and the The planet Mars, like Earth, has
remainder of the solar system, Mars is about 4.6 clouds in its atmosphere and a
billion years old. deposit of ice at its north pole. But
unlike Earth, Mars has no liquid
Mars is named for the ancient Roman god of war. water on its surface. The rustlike
The Romans copied the Greeks in naming the color of Mars comes from the large
planet for a war god; the Greeks called the planet amount of iron in the planet's soil.
Ares (AIR eez). The Romans and Greeks Image credit: NASA/JPL/Malin
associated the planet with war because its color Space Science Systems
resembles the color of blood. Viewed from Earth,
Mars is a bright reddish-orange. It owes its color to iron-rich minerals in its soil. This
color is also similar to the color of rust, which is composed of iron and oxygen.
Scientists have observed Mars through telescopes based on Earth and in space. Space
probes have carried telescopes and other instruments to Mars. Early probes were
designed to observe the planet as they flew past it. Later, spacecraft orbited Mars and
even landed there. But no human being has ever set foot on Mars.
Scientists have found strong evidence that water once flowed on the surface of Mars. The
evidence includes channels, valleys, and gullies on the planet's surface. If this
interpretation of the evidence is correct, water may still lie in cracks and pores in
subsurface rock. A space probe has also discovered vast amounts of ice beneath the
surface, most of it near the south pole.
In addition, a group of researchers has claimed to have found evidence that living things
once dwelled on Mars. That evidence consists of certain materials in meteorites found on
Earth. But the group's interpretation of the evidence has not convinced most scientists.
The Martian surface has many spectacular features, including a canyon system that is
much deeper and much longer than the Grand Canyon in the United States. Mars also has
mountains that are much higher than Mount Everest, Earth's highest peak.
Above the surface of Mars lies an atmosphere that is about 100 times less dense than the
atmosphere of Earth. But the Martian atmosphere is dense enough to support a weather
system that includes clouds and winds. Tremendous dust storms sometimes rage over the
Mars is much colder than Earth. Temperatures at the Martian surface vary from as low as
about -195 degrees F (-125 degrees C)
near the poles during the winter to as
much as 70 degrees F (20 degrees C) at
midday near the equator. The average
temperature on Mars is about -80 degrees
F (-60 degrees C).
A sunset on Mars creates a glow due to the
Mars is so different from Earth mostly
presence of tiny dust particles in the
because Mars is much farther from the
atmosphere. This photo is a combination of
sun and much smaller than Earth. The
four images taken by Mars Pathfinder, which
average distance from Mars to the sun is
landed on Mars in 1997. Image credit: NASA/
about 141,620,000 miles (227,920,000
kilometers). This distance is roughly 1 1/2
times the distance from Earth to the sun. The average radius (distance from its center to
its surface) of Mars is 2,107 miles (3,390 kilometers), about half the radius of Earth.
Characteristics of Mars
Orbit and rotation
Like the other planets in the solar system, Mars travels around the sun in an elliptical
(oval) orbit. But the orbit of Mars is slightly more "stretched out" than the orbits of Earth
and most of the other planets. The distance from Mars to the sun can be as little as about
128,390,000 miles (206,620,000 kilometers) or as much as about 154,860,000 miles
(249,230,000 kilometers). Mars travels around the sun once every 687 Earth days; this is
the length of the Martian year.
The distance between Earth and Mars depends on the positions of the two planets in their
orbits. It can be as small as about 33,900,000 miles (54,500,000 kilometers) or as large as
about 249,000,000 miles (401,300,000 kilometers).
Like Earth, Mars rotates on its axis from west to east. The Martian solar day is 24 hours
39 minutes 35 seconds long. This is the length of time that Mars takes to turn around
once with respect to the sun. The Earth day of 24 hours is also a solar day.
The axis of Mars is not perpendicular to the planet's orbital plane, an imaginary plane that
includes all points in the orbit. Rather, the axis is tilted from the perpendicular position.
The angle of the tilt, called the planet's obliquity, is 25.19¡ for Mars, compared with
23.45¡ for Earth. The obliquity of Mars, like that of Earth, causes the amount of sunlight
falling on certain parts of the planet to vary widely during the year. As a result, Mars, like
Earth, has seasons.
Mass and density
Mars has a mass (amount of matter) of 7.08 X 1020 tons (6.42 X 1020 metric tons). The
latter number would be written out as 642 followed by 18 zeroes. Earth is about 10 times
as massive as Mars. Mars's density (mass divided by volume) is about 3.933 grams per
cubic centimeter. This is roughly 70 percent of the density of Earth.
Because Mars is so much smaller and less dense than Earth, the force due to gravity at the
Martian surface is only about 38 percent of that on Earth. Thus, a person standing on
Mars would feel as if his or her weight had decreased by 62 percent. And if that person
dropped a rock, the rock would fall to the surface more slowly than the same rock would
fall to Earth.
Physical features of Mars
Scientists do not yet know much about the interior of Mars. A good method of study
would be to place a network of motion sensors called seismometers on the surface. Those
instruments would measure tiny movements of the surface, and scientists would use the
measurements to learn what lies beneath. Researchers commonly use this technique to
study Earth's interior.
Scientists have four main sources of information on the interior of Mars: (1) calculations
involving the planet's mass, density, gravity, and rotational properties; (2) knowledge of
other planets; (3) analysis of Martian meteorites that fall to Earth; and (4) data gathered
by orbiting space probes. They think that Mars probably has three main layers, as Earth
has: (1) a crust of rock, (2) a mantle of denser rock beneath the crust, and (3) a core made
mostly of iron.
Scientists suspect that the average thickness of the Martian crust is about 30 miles (50
kilometers). Most of the northern hemisphere lies at a lower elevation than the southern
hemisphere. Thus, the crust may be thinner in the north
than in the south.
Much of the crust is probably composed of a volcanic
rock called basalt (buh SAWLT). Basalt is also common
in the crusts of Earth and the moon. Some Martian crustal The surface of Mars was
rocks, particularly in the northern hemisphere, may be a sampled for signs of life by the
form of andesite. Andesite is also a volcanic rock found Viking 2 lander in 1976. A
on Earth, but it contains more silica than basalt does. mechanical sampling arm dug
Silica is a compound of silicon and oxygen. the grooves near the round
rock at the lower left. The
Mantle cylinder at the right covered
the sampling device and was
The mantle of Mars is probably similar in composition to ejected after landing. The
Earth's mantle. Most of Earth's mantle rock is peridotite cylinder is about 12 inches (30
(PEHR uh DOH tyt), which is made up chiefly of silicon, centimeters) long. Image
oxygen, iron, and magnesium. The most abundant credit: NASA/National Space
mineral in peridotite is olivine (OL uh veen). Science Data Center
The main source of heat inside Mars must be the same as that inside Earth: radioactive
decay, the breakup of the nuclei of atoms of elements such as uranium, potassium, and
thorium. Due to radioactive heating, the average temperature of the Martian mantle may
be roughly 2700 degrees F (1500 degrees C).
Mars probably has a core composed of iron, nickel, and sulfur. The density of Mars gives
some indication of the size of the core. Mars is much less dense than Earth. Therefore,
the radius of Mars's core relative to the overall radius of Mars must be smaller than the
radius of Earth's core relative to the overall radius of Earth. The radius of the Martian
core is probably between 900 and 1,200 miles (1,500 and 2,000 kilometers).
Unlike Earth's core, which is partially molten (melted), the core of Mars probably is
solid. Scientists suspect that the core is solid because Mars does not have a significant
magnetic field. A magnetic field is an influence that a magnetic object creates in the
region around it. Motion within a planet's molten core makes the core a magnetic object.
The motion occurs due to the rotation of the planet.
Data from Mars Global Surveyor show that some of the planet's oldest rocks formed in
the presence of a strong magnetic field. Thus, in the distant past, Mars may have had a
hotter interior and a molten core.
Mars has many of the kinds of surface features that are common on Earth. These include
plains, canyons, volcanoes, valleys, gullies, and polar ice. But craters occur throughout
the surface of Mars, while they are rare on Earth. In addition, fine-grained reddish dust
covers almost all the Martian surface.
Many regions of Mars consist of flat, low-lying plains. Most of these areas are in the
northern hemisphere. The lowest of the northern regions are among the flattest, smoothest
places in the solar system. They may be so smooth because they were built up from
deposits of sediment (tiny particles that settle to the
bottom of a liquid). There is ample evidence that water
once flowed across the Martian surface. The water would
have tended to collect in the lowest spots on the planet
and thus would have deposited sediments there.
Along the equator lies one of the most striking features on
the planet, a system of canyons known as the Valles
Marineris. The name is Latin for Valleys of Mariner; a
space probe called Mariner 9 discovered the canyons in The Valles Marineris system
1971. The canyons run roughly east-west for about 2,500 of valleys is about 2,500 miles
miles (4,000 kilometers), which is close to the width of (4,000 kilometers) long --
Australia or the distance from Philadelphia to San Diego. roughly one-fifth the distance
Scientists believe that the Valles Marineris formed mostly around the planet Mars. Parts
by rifting, a splitting of the crust due to being stretched. of the system are 6 miles (10
kilometers) deep. Image credit:
Individual canyons of the Valles Marineris are as much as NASA/National Space Science
60 miles (100 kilometers) wide. The canyons merge in Data Center
the central part of the system, in a region that is as much as 370 miles (600 kilometers)
wide. The depth of the canyons is enormous, reaching 5 to 6 miles (8 to 10 kilometers) in
Large channels emerge from the eastern end of the canyons, and some parts of the
canyons have layered sediments. The channels and sediments indicate that the canyons
may once have been partly filled with water.
Mars has the largest volcanoes in the solar system. The tallest one, Olympus Mons (Latin
for Mount Olympus), rises 17 miles (27 kilometers) above the surrounding plains. It is
about 370 miles (600 kilometers) in diameter. Three other large volcanoes, called Arsia
Mons, Ascraeus Mons, and Pavonis Mons, sit atop a broad uplifted region called Tharsis.
All these volcanoes have slopes that rise gradually, much like the slopes of Hawaiian
volcanoes. Both the Martian and Hawaiian volcanoes are shield volcanoes. They formed
from eruptions of lavas that can flow for long distances before solidifying.
Mars also has many other types of volcanic landforms. These range from small, steep-
sided cones to enormous plains covered in solidified lava. Scientists do not know how
recently the last volcano erupted on Mars -- some minor eruptions may still occur.
Craters and impact basins
Many meteoroids have struck Mars over its history, producing impact craters. Impact
craters are rare on Earth for two reasons: (1) Those that formed early in the planet's
history have eroded away, and (2) Earth developed a dense atmosphere, preventing
meteorites that could have formed craters from reaching the planet's surface.
Martian craters are similar to craters on Earth's moon, the planet Mercury, and other
objects in the solar systems. The craters have deep, bowl-shaped floors and raised rims.
Large craters can also have central peaks that form when the crater floor rebounds
upward after an impact.
On Mars, the number of craters varies dramatically from place to place. Much of the
surface of the southern hemisphere is extremely old, and so has many craters. Other parts
of the surface, especially in the northern hemisphere, are younger and thus have fewer
Some volcanoes have few craters, indicating that they erupted recently. The lava from the
volcanoes would have covered any craters that existed at the time of the eruptions. And
not enough time has passed since the eruptions for many new craters to form.
Some of the impact craters have unusual-looking deposits of ejecta, material thrown out
of the craters at impact. These deposits resemble mudflows that have solidified. This
appearance suggests that the impacting bodies may have encountered water or ice
beneath the ground.
Mars has a few large impact craters. The largest is
Hellas Planitia in the southern hemisphere.
Planitia is a Latin word that can mean low plain
or basin; Hellas Planitia is also known as the
Hellas impact basin. The crater has a diameter of
about 1,400 miles (2,300 kilometers). The crater
floor is about 5.5 miles (9 kilometers) lower than
the surrounding plain.
Channels, valleys, and gullies occur in many
regions of Mars, apparently as a result of water Channels in a Martian crater, in an
image taken in 2000 by the Mars
Global Surveyor, suggest to scientists
that liquid water may have flowed
across the surface of Mars in recent
times. Image credit: NASA
erosion. The most striking of these features are known as outflow channels. These
channels can be as wide as 60 miles (100 kilometers) and as long as 1,200 miles (2,000
kilometers). They appear to have been carved by enormous floods that rushed across the
surface. In many cases, the water seems to have escaped suddenly from underground.
Many of the channels do not look like river systems on Earth, with the main river formed
from smaller rivers and streams. Rather, those Martian channels arise fully formed from
Other regions of Mars have much smaller features called valley networks. These
networks look more like river systems on Earth. Martian valley networks are up to a few
miles or kilometers wide and up to a few hundred miles or kilometers long. The networks
are mostly ancient features. They suggest that the Martian climate may once have been
warm enough to enable water to exist as a liquid.
The gullies are smaller still. Most of them lie at high latitudes. They may be a result of a
leakage of a small amount of ground water to the surface within the past few million
The most interesting features in the polar regions of Mars are thick stacks of finely
layered deposits of material. Scientists believe that the layers consist of mixtures of water
ice and dust. The deposits extend from the poles to latitudes of about 80 degrees in both
The atmosphere probably deposited the layers over long periods. The layers may provide
evidence of seasonal weather activity and long-term changes in the Martian climate. One
possible cause of climate changes is variation in the planet's obliquity. This variation
alters the amount of sunlight falling on different parts of Mars. The variation in sunlight,
in turn, may change the climate. Past climate changes could have affected the rate at
which the atmosphere deposited dust and ice into layers.
Lying atop much of the layered deposits in both hemispheres are caps of water ice that
remain frozen all year. The layers and overlying caps are several miles or kilometers
In the wintertime, additional seasonal caps form from layers of frost. The seasonal caps
are clearly visible through Earth-based telescopes. The frost consists of solid carbon
dioxide (CO2) -- also known as "dry ice" -- that has condensed from CO2 gas in the
atmosphere. In the deepest part of the winter, the frost extends from the poles to latitudes
as low as 45 degrees -- halfway to the equator.
The atmosphere of Mars contains much less oxygen (O2) than that of Earth. The O2
content of the Martian atmosphere is only 0.13 percent, compared with 21 percent in
Earth's atmosphere. Carbon dioxide makes up 95.3 percent of the gas in the atmosphere
of Mars. Other gases include nitrogen (N2), 2.7 percent; argon (Ar), 1.6 percent; carbon
monoxide (CO), 0.07 percent; and water vapor (H2O), 0.03 percent.
At the surface of Mars, the atmospheric pressure is typically only about 0.10 pound per
square inch (0.7 kilopascal). This is roughly 0.7 percent of the atmospheric pressure at
Earth's surface. When the seasons change on Mars, the atmospheric pressure at the
surface there varies by 20 to 30 percent.
Each winter, the condensation of CO2 at the poles removes much gas from the
atmosphere. When this happens, the atmospheric pressure due to CO2 gas decreases
sharply. The opposite process occurs each summer. In addition, the atmospheric pressure
varies as the weather changes during the day, much as on Earth.
The atmosphere of Mars is coldest at high altitudes, from about 40 to 78 miles (65 to 125
kilometers) above the surface. At those altitudes, typical temperatures are below -200
degrees F (-130 degrees C). The temperature increases toward the surface, where daytime
temperatures of -20 to -40 degrees F (-30 to -40 degrees C) are typical. In the lowest few
miles or kilometers of the atmosphere, the temperature varies widely during the day. It
can reach -150 degrees F (-100 degrees C) late at night, even near the equator.
Atmospheric temperatures can be warmer than normal when the atmosphere contains
much dust. The dust absorbs sunlight and then transfers much of the resulting heat to the
In the Martian atmosphere, thin clouds made up of particles of frozen CO2 can form at
high altitudes. In addition, clouds, haze, and fog composed of particles of water ice are
common. Haze and fog are especially frequent in the early morning. At that time,
temperatures are the lowest, and water vapor is therefore most likely to condense.
The Martian atmosphere, like that of Earth, has a general circulation, a wind pattern that
occurs over the entire planet. Scientists have studied the global wind patterns of Mars by
observing the motions of clouds and changes in the appearance of wind-blown dust and
sand on the surface.
Global-scale winds occur on Mars as a result of the same process that produces such
winds on Earth. The sun heats the atmosphere more at low latitudes than at high latitudes.
At low latitudes, the warm air rises, and cooler air flows in along the surface to take its
place. The warm air then travels toward the cooler regions at higher latitudes. At the
higher latitudes, the cooler air sinks, then travels toward the equator.
On Mars, the condensation and evaporation of CO2 at the poles influence the general
circulation. When winter begins, atmospheric CO2 condenses at the poles, and more CO2
flows toward the poles to take its place. When spring arrives, CO2 frost evaporates, and
the resulting gas flows away from the poles.
Surface winds on Mars are mostly gentle, with typical speeds of about 6 miles (10
kilometers) per hour. Scientists have observed wind gusts as high as 55 miles (90
kilometers) per hour. However, the gusts exert much less force than do equally fast winds
on Earth. The winds of Mars have less force because of the lower density of the Martian
Some of the most spectacular weather occurs on Mars when dust blows in the wind.
Small, swirling winds can lift dust off the surface for brief intervals. These winds create
dust devils, tiny storms that look like tornadoes.
Large dust storms begin when wind lifts dust into the atmosphere. The dust then absorbs
sunlight, warming the air around it. As the warmed air rises, more winds occur, lifting
still more dust. As a result, the storm becomes stronger.
At larger scales, dust storms can blanket areas from more than 200 miles (320 kilometers)
to a few thousand miles or kilometers across. The largest storms can cover the entire
surface of Mars. Storms of that size are unusual, but they can last for months. The
strongest storms can block almost the entire surface from view. Such storms occurred in
1971 and 2001.
Dust storms are most common when Mars is closest to the Sun. More storms occur then
because that is when the sun heats the atmosphere the most.
Mars has two tiny moons, Phobos and Deimos. The American astronomer Asaph Hall
discovered them in 1877 and named them for the sons of Ares. Both satellites are
irregularly shaped. The largest diameter of Phobos is about 17 miles (27 kilometers); that
of Deimos, about 9 miles (15 kilometers).
The two satellites have many craters that formed when meteoroids struck them. The
surface of Phobos also has a complicated pattern of grooves. These may be cracks that
developed when an impact created the satellite's largest crater.
Scientists do not know where Phobos and Deimos formed. They may have come into
existence in orbit around Mars at the same time the planet formed. Another possibility is
that the satellites formed as asteroids near Mars. The gravitational force of Mars then
pulled them into orbit around the planet. The color of both satellites is a dark gray that is
similar to the color of some kinds of asteroids.
Evolution of Mars
Scientists know generally how Mars evolved after it formed about 4.6 billion years ago.
Their knowledge comes from studies of craters and other surface features. Features that
formed at various stages of the planet's evolution still exist on different parts of the
surface. Researchers have developed an evolutionary scenario that accounts for the sizes,
shapes, and locations of those features.
Researchers have ranked the relative ages of surface regions according to the number of
impact craters observed. The greater the number of craters in a region, the older the
However, scientists have not yet determined exactly when the various evolutionary stages
occurred. To do that, they would need to know the ages of rocks of surface features
representing those stages. They could determine how old such rocks are if they could
analyze samples of them in a laboratory. But no space probe has ever brought Martian
rocks to Earth.
Scientists have divided the "lifetime" of Mars into three periods. From the earliest to the
most recent, the periods are: (1) The Noachian (noh AY kee uhn), (2) the Hesperian, and
(3) the Amazonian. Each period is named for a surface region that was created during that
The Noachian Period is named for Noachis Terra, a vast highland in the southern
hemisphere. During the Noachian Period, a tremendous number of rocky objects of all
sizes, ranging from small meteoroids to large asteroids, struck Mars. The impact of those
objects created craters of all sizes. The Noachian was also a time of great volcanic
In addition, water erosion probably carved the many small valley networks that mark
Mars's surface during the Noachian Period. The presence of those valleys suggests that
the climate may have been warmer during the Noachian Period than it is today.
The Hesperian Period
The intense meteoroid and asteroid bombardment of the Noachian Period gradually
tapered off, marking the beginning of the Hesperian Period. This period is named for
Hesperia Planum, a high plain in the lower latitudes of the southern hemisphere.
During the Hesperian Period, volcanic activity continued. Volcanic eruptions covered
over Noachian craters in many parts of Mars. Most of the largest outflow channels on the
planet are of Hesperian age.
The Amazonian Period, which is characterized by a low rate of cratering, continues to
this day. The period is named for Amazonis Planitia, a low plain that is in the lower
latitudes of the northern hemisphere.
Volcanic activity has occurred throughout the Amazonian Period, and some of the largest
volcanoes on Mars are of Amazonian age. The youngest geologic materials on Mars,
including the ice deposits at the poles, are also Amazonian.
Possibility of life
Mars might once have harbored life, and living things might exist there even today. Mars
almost certainly has three ingredients that scientists believe are necessary for life: (1)
chemical elements such as carbon, hydrogen, oxygen, and nitrogen that form the building
blocks of living things, (2) a source of energy that living organisms can use, and (3)
The essential chemical elements likely were present throughout the planet's history.
Sunlight could be the energy source, but a second source of energy could be the heat
inside Mars. On Earth, internal heat supports life in the deep ocean and in cracks in the
Liquid water apparently carved Mars's large channels, its
smaller valleys, and its young gullies. In addition, there
are vast quantities of ice within about 3 feet (1 meter) of
the surface near the south pole and perhaps near the north
pole. Thus, water apparently has existed near the surface
over much of the planet's history. And water is probably
present beneath the surface today, kept liquid by Mars's
A curved, rodlike structure
In 1996, scientists led by David S. McKay, a geologist at shown in the center of this
the National Aeronautics and Space Administration's photo has been referred to as a
Johnson Space Center in Houston, reported that scientists fossilized Martian creature by
there had found evidence of microscopic Martian life. some scientists. The structure
They discovered this evidence inside a meteorite that had is about 200 billionths of a
made its way to Earth. The meteorite had been blasted meter long and is part of a
from the surface of Mars, almost certainly by the impact Martian rock that was found
on Earth. Image credit: NASA/
Johnson Space Center
of a much larger meteorite. The small meteorite had then journeyed to Earth, attracted by
Earth's gravity. The trip may have taken millions of years.
The evidence included complex organic molecules, grains of a mineral called magnetite
that can form within some kinds of bacteria, and tiny structures that resemble fossilized
microbes. The scientists' conclusions are controversial, however. There is no general
scientific agreement that Mars has ever harbored life.
History of Mars study
Observation from Earth
Observing Mars through Earth-based telescopes, early astronomers discovered polar caps
that grow and shrink with the seasons. They also found light and dark markings that
change their shape and location.
In the late 1800's, the Italian astronomer Giovanni V. Schiaparelli reported that he saw a
network of fine dark lines. He called these lines canali, which is Italian for channels. But
canali was generally mistranslated as canals. Many other astronomers also reported
seeing such features. Among those observers was the American astronomer Percival
Lowell, who referred to the features as canals. Lowell speculated that the canals had been
built by a Martian civilization.
The canals turned out not to exist. In some cases, the observers had misinterpreted dark,
blurry regions that they had actually seen. In other cases, there was no relationship
between "canals" and real features.
However, the changing dark and light markings were real. Some scientists thought that
the changing patterns might result from the growth and death of vegetation. Much later,
other scientists suspected correctly that the cause was the Martian winds. Light and dark
materials blow to and fro across the surface.
Observation by spacecraft
Robotic spacecraft began detailed observation of Mars in the 1960's. The United States
launched Mariner 4 to Mars in 1964 and Mariners 6 and 7 in 1969. Each flew by Mars
about half a year after its launch. The craft took pictures showing that Mars is a barren
world, with craters like those on the moon. There was no sign of liquid water or life. The
spacecraft observed few of the planet's most interesting features because they happened
to fly by only heavily cratered regions.
In 1971, Mariner 9 went into orbit around Mars. This craft mapped about 80 percent of
Mars. It made the first discoveries of the planet's canyons and volcanoes. It also found
what appear to be dry riverbeds.
The next major mission to Mars was Viking, launched by
the United States in 1975. Viking consisted of two
orbiters and two landers. Its main goal was to search for The Sojourner Rover examines
life. The orbiters scouted out landing sites for the landers, a rock on Mars. The rover
which touched down in July and September 1976. The traveled from Earth aboard the
landers took the first close-up pictures of the Martian Mars Pathfinder space probe,
surface, and they sampled the soil. They found no strong then rolled down a ramp to the
evidence for life. surface. Sojourner is only 24
3/4 inches (63 centimeters)
The next two successful probes were Mars Pathfinder, long. Image credit: NASA
which was a lander, and Mars Global Surveyor, an
orbiter. The United States
launched both craft in 1996. The
main objective of Pathfinder was
to demonstrate a new landing
system. Inflated air bags
cushioned the probe's landing in
July 1997. Pathfinder also carried
a small roving vehicle called
Sojourner. The rover rolled down
a ramp to the surface, and then
moved from rock to rock. Mars Global Surveyor
Pathfinder sent spectacular photos studied the
back to Earth, and Sojourner composition of the
analyzed rocks and soil. People Martian surface,
throughout the world watched photographed the
television pictures of Sojourner surface in detail, and
doing its work. measured its elevation.
The space probe went
Mars Global Surveyor carried a into orbit around Mars
group of sophisticated scientific in 1997. Image credit:
instruments. A laser altimeter NASA/JPL
used laser beams to determine the elevation of the Martian surface. This instrument
produced maps of the entire surface that are accurate to within 1 yard or meter of
elevation. An infrared spectrometer determined the composition of some of the minerals
on the surface. A high-resolution camera revealed a host of new geologic features. These
include layered sediments that may have been deposited in liquid water, and small gullies
that appear to have been carved by water.
In April 2001, the United States launched the Mars Odyssey probe. The probe carried
instruments to analyze the chemical composition of the Martian surface and the rocks just
below the surface, to determine whether there is water ice on or beneath the surface, and
to study the radiation near Mars. Mars Odyssey went into orbit around the planet in
October 2001. In 2002, the probe discovered vast amounts of water ice beneath the
surface. Most of the ice found is in the far southern part of the planet, south of 60 degrees
south latitude. Scientists also suspect that there are large amounts of water ice north of 60
degrees north latitude. However, when the discovery was made,
CO2 frost covered most of that area, preventing the probe from
detecting underlying ice.
The water ice found in the south is in the upper 3 feet (1 meter) of
soil. That soil is more than 50 percent water ice by volume. The
total volume of the water ice discovered is roughly 2,500 cubic
miles (10,400 cubic kilometers), more than enough to fill Lake
photographed by the
The probe cannot detect evidence of water at depths greater than
3 feet. Thus, scientists cannot yet determine the total depth or the
Telescope in August
total volume of all the water ice on Mars.
2003 as the planet
passed closer to Earth
Mars passed closer to Earth in August 2003 than it had in nearly
than it had in nearly
60,000 years. In that year, scientists launched three new probes.
60,000 years. The
The European Space Agency's Mars Express mission included an
orbiter that carried scientific instruments and a lander designed to
many features of the
analyze the planet's soil for evidence of life. The United States
launched two rovers, nicknamed Spirit and Opportunity, to
including dark, circular
explore different regions of the planet's surface.
impact craters and the
bright ice of the
In December 2003, Mars Express went into orbit around the
southern polar cap.
planet and released its lander, Beagle 2. Mars Express
Image credit: NASA,
immediately began transmitting pictures and other information
J. Bell (Cornell U.)
about the planet, but mission managers could not contact Beagle
and M. Wolff (SSI)
2 and feared it was lost. In early January 2004, the U.S. rover
Spirit landed safely in an area called Gusev Crater. The rover Opportunity landed later
that month in an area called Meridiani Planum. The rovers transmitted detailed
photographs of Martian ground features and began analyzing rocks and soil for evidence
that large amounts of liquid water once existed on the planet's surface.
In March 2004, U.S. scientists announced that they had concluded that Meridiani Planum
once held large amounts of liquid water. Their evidence came from an outcropping of
Martian bedrock found in the small crater in which Opportunity landed. The rover's
analysis showed that the rock contained large amounts of sulfate salts, which contain
sulfur and oxygen. On Earth, such high concentrations of sulfate salts occur only in rocks
that formed in water or were exposed to water for long periods. The outcropping's surface
also bore tiny pits similar to those found on Earth where salt crystals formed in wet rock
and later dissolved or eroded away.
The rover mission was scheduled to last only 90 days, but it was
extended because Spirit and Opportunity continued to function
well. In June 2004, Opportunity descended into a large crater that
mission managers called Endurance and analyzed the layers of The rover Spirit rests
on Mars in a
bedrock there. Also in June, Spirit arrived at a group of hills,
called Columbia Hills, after a drive of over 2 miles (3 composite image made
kilometers). The rovers continued to explore these sites for up of photographs
several months. taken by a camera
mounted above the
Contributor: Steven W. Squyres, Ph.D., Professor of Astronomy, rover's body. Spirit
Cornell University. landed on Mars in
early January 2004.
The pole at the lower
left is one of the
antennas Spirit uses to
Image credit: NASA