CometsA comet is a celestial body that revolves around the sun. Comets have a long tail. Early comets were describedas a phonomenon. The Danish astronomer, Tycho Brahe, proved that comets were celestial bodies. A comet ismade of a sharp nucleus, or center. It is also described as a "dirty snowball" because of the ice and dust on thecomet.When a comet comes toward the sun, the solar heat evaporates so that the ice on the comet brightens. Whenthis happens, the comet has a tail that might sometimes lenghten to a point of being many millions ofkilometers in space. The tail of the comet is usually turned away from the sun. Comets also have ellipticalorbits.Sometimes comets travel in a group that has nearly the same orbit. They are said to be members of a cometgroup. The most famous group is Ikeya-Seki of 1956. Comets have been known to be important events. Thepresence of a comet has also given fear of a collision between the comet and the earth. The earth has passedthrough the tails of comets without much effect. Although a collision of a comet with a city would destroy thecity, it will probably never occur again. AsteroidsAsteroids are one of the many minor planets of the solar system. The largest asteroids are Ceres, Pallas, andVesta. The mass of an asteroid of is much less than the mass of the Moon. Few scientists now believe thatasteroids are the remains of a former planet. They think that asteroids have a place in the solar system wherea planet couldve formed, but the gravity of Jupiter prevented it from happening. Some unusual asteroids aremade of iron-nickel alloy.Asteroids, like meteorites, can be classified into specific types. Most asteroids are related to the meteoritesknown as "irons". A few asteroids, such as Vesta, are probably related to the rarest meteorite class of all. It isthe achondrites. These asteroids look like it has the surface composition of many lunar and terrestrial lavaflows.
MeteorsA meteor is a solid body that enters a planets atmosphere from space. Brilliant meteors, consists of a radianthead and then it is followed by a tail of light, like a comet. Other meteors, called bolides, have been seen toexplode. When bolides explode, they sound like thunder. Fainter meteors, called shooting stars, occuroccasionally. At some times, hundreds of meteors can occur at once. These swarms are called meteor showers.Some appear annually on the same days of the year. They are called periodic showers. Others occur atdifferent times each year. A meteor that reaches the surface of the earth or another planet is called ameteorite. A meteoroid is a solid body orbiting the sun. It will become a meteor if it enters earthsatmosphere. The majority of meteoroids are the size of grains of dust, but they can be larger without anylimit.
AsteroidsASTEROIDSAsteroids are rocky or metallic objects, most of which orbit the Sun inthe asteroid belt between Mars and Jupiter. A few asteroids approachthe Sun more closely. None of the asteroids have atmospheres.Asteroids are also known as planetoids or minor planets. Asteroid 253 Mathilde, a Near- Earth Asteroid photographed by NASAs NEAR (Near Earth Asteroid Rendezvous) mission in June 1997. Mathilde is about 60 km in diameter and orbits in the asteroid belt between Mars and Jupiter.THE ASTEROID BELTThe asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits ofMars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids in the asteroidbelt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three tosix Earth years.The strong gravitational force of the planet Jupiter shepherds the asteroid belt, pulling the asteroids awayfrom the Sun, keeping them from careening into the inner planets.THE KIRKWOOD GAPSThe asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps areorbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulledtowards Jupiter). For example, an orbit in which an asteroid orbited the Sun exactly three times for eachJovian orbit would experience great gravitational forces each orbit, and would soon be pulled out of thatorbit. There is a gap at 3.28 AU (which corresponds to 1/2 of Jupiters period), another at 2.50 AU (whichcorresponds to 1/3 of Jupiters period), etc. The Kirkwood gaps are named for Daniel Kirkwood whodiscovered them in 1866.
HOW MANY ASTEROIDS ARE THERE?There are about 40,000 known asteroids that are over 0.5 miles (1 km)in diameter in the asteroid belt. About 3,000 asteroids have beencataloged. There are many more smaller asteroids. The first onediscovered (and the biggest) is named Ceres; it was discovered in1801. Gaspra, Asteroid #951.THE SIZES OF ASTEROIDS Asteroids range in size from tiny pebbles to about 578 miles (930 kilometers) in diameter (Ceres). Sixteen of the 3,000 known asteroids are over 150 miles (240 km) in diameter. Some asteroids even have orbiting moons. Asteroid 4 Vesta, the brightest asteroid and the fourth largest. Vesta is the only asteroid that can be seen without a telescope (it is sixth magnitude).CERES: THE LARGEST ASTEROIDCeres is the largest of the asteroids. It was the first asteroid ever discovered (by theItalian astronomer Giuseppe Piazzi on January 1, 1801). Ceres is the size of the state of Texas! It is sohuge in comparison with the other asteroids that its mass is equal to over one-third of the 2.3 x 1021 kgestimated total mass of all the 3,000 cataloged asteroids. Ceres is about 578 miles (930 kilometers) indiameter. Ceres is now considered to be a dwarf planet.ASTEROIDS BECOMING MOONS Asteroids can be pulled out of their solar orbit by the gravitational pull of a planet. They would then orbit that planet instead of orbiting the Sun. Astronomers theorize that the two moons of Mars, Phobos and Deimos, are captured asteroids. The asteroid 243 Ida and its tiny asteroid moon, Dactyl. This is the first asteroid ever found withan orbiting moon. Idas dimensions are about 56 x24 x 21 kilometers (35 x 15 x 13 miles). Dactyl is only about 1.2 x 1.4 x 1.6 km (0.75 x 0.87 x 1 mile) across.
ORIGIN OF THE ASTEROID BELTThe asteroid belt may be material that never coalesced into a planet, perhaps because its mass was toosmall; the total mass of all the asteroids is only a small fraction of that of our Moon. The total mass of allthe asteroids is about 2.3 x 1021 kg ); our moons mass is 7.35 x 1022 kg; the asteroids mass combined isabout 1/30 of the mass of the Moon. A less satisfactory explanation of the origin of the asteroid belt isthat it may have once been a planet that was fragmented by a collision with a huge comet.TROJAN ASTEROIDSTrojan asteroids are asteroids that orbit in gravitationally stable Lagrange points in a planets orbit, eithertrailing it or preceding it (these places are where the gravitational attraction of the Sun and of the planetbalance each other). Jupiter has the most Trojan asteroids; Mars also has some. Achilles was the firstTrojan asteroid found. The asteroids preceding Jupiter in its orbit were named for Greek heroes; thosefollowing Jupiter in its orbit were named for Trojan heroes. Introduction to Comets A comet is a small, icy celestial body that orbits around the sun. It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor, CO2, and other gases) and a long tail (made of dust and ionized gases). The tail develops when the comet is near the Sun. Its long ion tail of always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million km long, and is most of what we see. Comets are only visible whentheyre near the sun in their highly eccentric orbits.PARTS OF A COMETNucleus: The nucleus is the frozen center of a comets head. It is composed of ice, gas, and dust. Thenucleus contains most of the comets mass but is very small (about 1 to 10 km across - or more).Coma: The coma is the roughly spherical blob of gas that surrounds the nucleus of a comet; it is about amillion km across. The coma is comprised of water vapor, carbon dioxide gas, ammonia, dust, and neutralgases that have sublimed from the solid nucleus. The coma and the nucleus form the head of a comet.Ion Tail: A tail of charged gases (ions) always faces away from the sun because the solar wind (ionsstreaming from the sun at high velocities) pushes it away (it is also called the plasma tail). When thecomet is approaching the Sun, the ion tail trails the comet: when the comet is leaving of the Sun, the ion
tail leads. The tail fades as the comet moves far from the Sun. The ion tail can be well over 100 millionkm long.Dust Tail: The dust tail is a long, wide tailcomposed of microscopic dust particles that are buffeted byphotons emitted from the Sun; this tail curves slightly due to the comets motion. The tail fades as thecomet moves far from the Sun. Hydrogen Envelope: Hydrogen gas surrounds the coma of the comet andtrails along for millions of miles (it is usually between the ion tail and the dust tail). The hydrogenenvelope is about 10 million km across at the nucleus of the comet and about 100 million km long. It isbigger when the comet is near the Sun.A COMETS ORBIT Comets orbit the Sun in highly elliptical orbits. Their velocity increases greatly when they are near the Sun and slows down at the far reaches of the orbit. Since the comet is light only when it is near the Sun (and is it vaporizing), comets are dark (virtually invisible) throughout most of their orbit. The solar wind pushes the tail away from the Sun.Some comets crash into the Sun or get so close that they burn up; thesecomets are called sungrazers.COMET EXPLORATION NASAs Stardust Mission will visit the Comet Wild 2 in 2004. It will take a sample of comet particles and return them to Earth. The small spacecraft (about 770 pounds = 350 kg) was launched February 7, 1999 and rendezvoused with comet Wild 2 in January, 2004. It will returnto Earth on January 15, 2006, and land in western Utah, USA. Comet Wild 2 (aka Comet 81P) is a short-period comet that was discovered by the Swiss astronomer Paul Wild on January 6, 1978. The cometsnucleus is about 3 miles (5 km) across. Wild 2 orbits the Sun every 6.39 years; its elliptical orbit rangesfrom about Mars orbit to Jupiters orbit.COSMIC SNOWBALLSThere is a new and very controversial theory that comets (composed of frozen water) are constantlybombarding the Earth. These "cosmic snowballs" have (perhaps) been seen by the visible imaging systemof the Polar Satellite. In theory, these frozen comets vaporize in the atmosphere, adding water vapor to theenvironment.
METEOROIDS Meteoroids are small bodies that travel through space. Meteoroids are smaller than asteroids; most are smaller than the size of a pebble. Meteoroids have many sources. Most meteoroids come from asteroids that are broken apart by impacts with other asteroids. Other meteoroids come from the Moon, from comets, and from the planet Mars. METEORS A meteor is a meteoroid that has entered the Earths atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star or a falling star. Just before a meteor enters the Earths atmosphere, it is moving at roughly 70 kilometers per second. The friction between the fast-moving meteor and the gas in the Earths atmosphere causes intense heat; the meteor glows with heat and then burns. Most meteors burn up before hitting the Earth. Only large meteors can survive the trip through our atmosphere.A fireball is any meteor that is brighter than Venus (magnitude -4).A meteor shower is a phenomenon in which many meteors fall through theatmosphere in a relatively short time and in approximately parallel trajectories.A very intense meteor shower is called a meteor storm.METEORITESA meteorite is a meteor that has fallen to Earth. These rare objects have survived a fiery fall through theEarths atmosphere and have lost a lot of mass during that process. Meteorites are made up of rock and/ormetals Sources: http://www.enchantedlearning.com/subjects/astronomy/asteroids/index.shtml http://library.thinkquest.org/J001665/cam.html#