Only within the past couple of decades have scientists taken seriously the idea that life on Earth has been disrupted over the course of billions of years by asteroid and comet impacts. We now have a much better appreciation for Earth's presence in an occasionally hostile environment, and for Earth's fragility in the face of external cosmic dangers. Here, the 5-km-diameter nucleus of comet Wild2 was imaged in the year 2004 as part of the Stardust mission designed to fly though a comet's tail--and ultimately to return samples of the tail to Earth. (JPL)
The asteroid belt, along with the orbits of Earth, Mars, and Jupiter (not drawn to scale). The main belt, the Trojan asteroids, and some Apollo (Earth-crossing) and Amor (Mars-crossing) orbits are shown. (We will learn more about these classes of asteroids later in the chapter.)
NEAR was launched on 17 February 1996 The aim of the NEAR mission was to: 1. Determine the physical and geological properties of a near-Earth asteroid. Eros was the target asteroid. 2. To further our knowledge on the nature and origin of the many asteroids, meteorites and comets close to Earth's orbit. 3. Further our understanding of how and under what conditions the planets formed and evolved. Near Shoemaker achieved all of its science goals during the year in orbit and conducted the first long-term close-up study of an asteroid. An additional bonus was despite being designed as an orbiter, it achieved the unbelievable by landing on asteroid Eros.
The C-type asteroid Mathilde, imaged by the NEAR spacecraft en route to the near-Earth asteroid Eros. Mathilde measures some 60 x 50 km and rotates every 17.5 days. The largest craters visible in this image are about 20 km across--much larger than the craters seen on either Gaspra or Ida. The cause may be the asteroid's low density (approximately ) and rather soft composition. (NASA)
The S-type asteroid Gaspra, as seen from a distance of 1600 km by the space probe Galileo on its way to Jupiter. (b) The S-type asteroid Ida, photographed by Galileo from a distance of 3400 km. (Ida's moon, Dactyl, is visible at the right of the photo.) The resolution in these photographs is about 100 m. True-color images showed the surfaces of both bodies to be a fairly uniform shade of gray. Sensors on board the spacecraft indicated that the amount of infrared radiation absorbed by these surfaces varies from place to place, probably because of variations in the thickness of the dust layer blanketing them. (NASA)
The NEAR-Shoemaker spacecraft entered orbit around asteroid Eros in February 2000, making a series of orbital corrections (engine burns) during April to bring it closer and closer to the surface before landing on the asteroid in February 2001. (b) A mosaic of detailed images showing the entire asteroid, which has a very odd shape, 34 x 11 x 11 km. Craters of all sizes, ranging from 50 m (the resolution of the image) to 5 km, pit Eros's surface. The inset shows a close-up image of a &quot;young&quot; section of the surface, where loose material from recent impacts has apparently filled in and erased all trace of older craters. (JHU/NASA)
The asteroid Icarus has an orbit that passes within 0.2 A.U. of the Sun, well within Earth's orbit. Icarus occasionally comes close to Earth, making it one of the best-studied asteroids in the solar system. Its motion relative to the stars makes it appear as a streak (marked) in this long-exposure photograph. (Palomar/Caltech)
The Tunguska event of 1908 leveled trees over a vast area. Although the impact of the blast was tremendous and its sound audible for hundreds of kilometers, the Siberian site is so remote that little was known about the event until scientific expeditions arrived to study it many years later. (Sovfoto/Eastfoto)
A bright streak called a meteor is produced when a fragment of interplanetary debris plunges into the atmosphere, heating the air to incandescence. Distant stars and the northern lights provide a stunning background for a bright meteor trail. (b) These meteors (and a red smoke trail) streak across the sky during the height of the Leonid meteor storm of November 2001. (P. Parviainen; J. Lodriguss)
A meteoroid swarm associated with a given comet intersects Earth's orbit at specific locations, giving rise to meteor showers at certain fixed times of the year. A portion of the comet breaks up near perihelion, at the point marked 1. The fragments continue along the comet's orbit, gradually spreading (points 2 and 3). The rate at which the debris disperses around the orbit is actually much slower than depicted here--it takes many orbits for the material to spread out as shown, but eventually the fragments extend all around the orbit, more or less uniformly. If the orbit happens to intersect Earth's, a meteor shower is seen each time Earth passes through the intersection (point 4).
This photograph, taken from orbit by the U.S. Skylab space station, clearly shows the ancient impact basin that forms Quebec's Manicouagan Reservoir. A large meteorite landed there about 200 million years ago. The central floor of the crater rebounded after the impact, forming an elevated central peak. The lake, 70 km in diameter, now fills the resulting ring-shaped depression. (NASA)
The world's second largest meteorite, the Ahnighito, on display at the American Museum of Natural History in New York, serves as a jungle gym for curious children. This 34-ton rock is so heavy that the Museum floor had to be specially reinforced to support its weight. The Willamette Meteorite on display at the American Museum of Natural History in New York City .
A stony (silicate) meteorite often has a dark fusion crust, created when the surface of the incoming meteoroid is melted by the tremendous heat generated during its passage through the atmosphere. (b) Iron meteorites, much rarer than stony ones, usually contain some nickel as well. Most show characteristic crystalline patterns when their surfaces are cut, polished, and etched with weak acid. (Science Graphics)
Diagram of a typical comet, showing the nucleus, coma, hydrogen envelope, and tail. The tail is not a sudden short-lived streak across the sky, as in the case of meteors or fireworks. Instead, it travels through space along with the rest of the comet (as long as the comet is sufficiently close to the Sun for the tail to exist). (b) Halley's comet in 1986, about one month before it rounded the Sun at perihelion. (NOAO)
A comet with a primarily ion tail. Called comet Giacobini-Zinner and seen here in 1959, its coma measured 70,000 km across; its tail was well over 500,000 km long. (b) Photograph of a comet having both an ion tail (dark blue) and a dust tail (white blue), showing the gentle curvature and inherent fuzziness of the dust. (See also Discovery 14-3.) This is comet Hale-Bopp in 1997. At the comet's closest approach to the Sun, its tail stretched nearly 40° across the sky. (US Naval Observatory; Aaron Horowitz/Corbis)
As it approaches the Sun, a comet develops an ion tail, which is always directed away from the Sun. Closer in, a curved dust tail, also directed generally away from the Sun, may appear. Notice that although the ion tail always points directly away from the Sun on both the inbound and the outgoing portions of the orbit, the dust tail has a marked curvature, always tending to &quot;lag behind&quot; the ion tail. (Cf. with photo of a real comet, Figure 4.10.)
Halley's comet has a smaller orbital path and a shorter period than most comets, but its orbital orientation is not typical of a short-period comet. Sometime in the past, the comet must have encountered a jovian planet (probably Jupiter itself), which threw it into a tighter orbit that extends not to the Oort cloud, but merely a little beyond Neptune. Edmund Halley applied Newton's law of gravity to predict this comet's return.
The Giotto spacecraft resolved the nucleus of Halley's comet, showing it to be very dark, although heavy dust in the area obscured any surface features. The resolution here is about 50 m--half the length of a football field. At the time this image was made, in March 1986, the comet was within days of perihelion, and the Sun was toward the right. The brightest areas are jets of evaporated gas and dust spewing from the comet's nucleus. (b) A diagram of Halley's nucleus, showing its size, shape, jets, and other physical and chemical properties. (ESA/Max Planck Institute)
Diagram of the Oort cloud, showing a few cometary orbits. Most Oort-cloud comets never come close to the Sun. Of the orbits shown, only the most elongated ellipse represents a comet that will actually enter the solar system (which, on the scale of this drawing, is much smaller than the dot at the center of the figure) and possibly become visible from Earth. (See also Figure 14.8.) (b) The Kuiper belt, the source of short-period comets, whose orbits tend to hug the plane of the ecliptic.
NASA's Stardust spacecraft captured this image (a) of comet Wild-2 in January 2004, just before the craft passed through the comet's coma. Onboard is a detector made of a foamlike gel that is 99.8% air, yet is strong enough to stop and store cometary dust particles for study upon return of the craft to Earth in 2006. (NASA)
C-type asteroid Mathilde The NEAR spacecraft visited the C-type asteroid Mathilde , on its way to its main target, Eros. Mathilde, like many other asteroids, has a very low density , and is probably not solid.
S-type Asteroids Gaspra and Ida Two small S-type asteroids, Gaspra and Ida , were visited by the Galileo probe. Gaspra (left) is in false color; it is really gray. Note that Ida (right) has a small moon, Dactyl :
Apollo Asteroids Some asteroids have orbits so eccentric that they cross Earth’s orbit. They are called Apollo asteroids, and raise the concern of a possible collision. 2600 such asteroids have been discovered so far, of which about 600 have been designated as potentially hazardous, due to their size.
Trojan Asteroids Some asteroids, called Trojan asteroids , orbit at the L 4 and L 5 Lagrangian points of Jupiter’s orbit: Lagrangian points – places where the gravity of the Sun and a planet balance
associated with a given comet intersects Earth's orbit at specific locations,
giving rise to meteor showers at certain fixed times of the year
Meteor Showers 2011 Just past full night of December 13 Geminids Rises around midnight night of November 17 Leonids Rises after midnight night of October 21 Orionids Sets around midnight night of October 8 Draconids Full night of August 13 Perseids Sets in early evening night of May 5 Eta Aquarids Rises after midnight night of April 21 Lyrids New night of January 3 Quadrantids Moon Date of Peak Name
Impact Craters on Earth bosumtwi crater, ghana gosses bluff, australia vredefort crater, johannesburg, south africa
Impact Craters on Earth Nordlingen, Germany Odessa, Texas Middlesboro, KY
Comets Comets have very eccentric orbits. Long-period comets have periods of hundreds of thousands, or even millions, of years. Short-period comets are less common, and have periods of less than 200 years.
Comets Halley’s comet is one of the most famous; it has a period of 76 years and has been observed since antiquity. Its most recent visit, in 1986, was not spectacular. Left: The comet in 1910, as seen with the naked eye. Right: the comet in 1986, seen through a telescope.
Comets Halley’s comet has a shorter period than most comets, but its orbit is not in the plane of the solar system, probably due to an encounter with a larger object.
Comets Typical cometary mass: 10 12 to 10 16 kg Each trip close to the Sun removes some material; Halley’s comet, for example, is expected to last about another 40,000 years. Sometimes a comet’s nucleus can disintegrate violently, as comet LINEAR did: