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Asteroids ASTEROIDS Asteroids are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter. A few asteroids approach the Sun more closely. None of the Asteroid 253 Mathilde, a Nearasteroids have atmospheres. Earth Asteroid photographed by Asteroids are also known as planetoids or minor NASA's NEAR (Near Earth planets. 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 BELT The asteroid belt is a doughnut-shaped concentration of asteroids orbiting the Sun between the orbits of Mars and Jupiter, closer to the orbit of Mars. Most asteroids orbit from between 186 million to 370 million miles (300 million to 600 million km or 2 to 4 AU) from the Sun. The asteroids in the asteroid belt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to six Earth years. The strong gravitational force of the planet Jupiter shepherds the asteroid belt, pulling the asteroids away from the Sun, keeping them from careening into the inner planets. THE KIRKWOOD GAPS The asteroid belt is not smooth; there are concentric gaps in it (known as Kirkwood gaps). These gaps are orbital radii where the gravitational forces from Jupiter do not let asteroids orbit (they would be pulled towards Jupiter). There is a gap at 3.28 AU (which corresponds to 1/2 of Jupiter's period), another at 2.50 AU (which corresponds to 1/3 of Jupiter's period), etc. The Kirkwood gaps are named for Daniel Kirkwood who discovered 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 been cataloged. There are many more smaller asteroids. The first one discovered (and the biggest) is named Ceres; it was discovered in 1801. 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 Asteroid 4 Vesta, the brightest diameter. Some asteroids even have asteroid and the fourth largest. Vesta orbiting moons. is the only asteroid that can be seen without a telescope (it is sixth magnitude). CERES: THE LARGEST ASTEROID Ceres is the largest of the asteroids. It was the first asteroid ever discovered (by the Italian astronomer Giuseppe Piazzi on January 1, 1801). Ceres is the size of the state of Texas! It is so huge in comparison with the other asteroids that its mass is equal to over one-third of the 2.3 x 1021 kg estimated total mass of all the 3,000 cataloged asteroids. Ceres is about 578 miles (930 kilometers) in diameter. 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 The asteroid 243 Ida and its tiny asteroid moons of Mars, Phobos and Deimos, moon, Dactyl. This is the first asteroid are captured asteroids. ever found with an orbiting moon. Ida's dimensions are about 56 x 24 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 BELT The asteroid belt may be material that never coalesced into a planet, perhaps because its mass was too small; the total mass of all the asteroids is only a small fraction of that of our Moon. The total mass of all the asteroids is about 2.3 x 1021 kg ); our moon's mass is 7.35 x 1022 kg; the asteroids' mass combined is about 1/30 of the mass of the Moon. A less satisfactory explanation of the origin of the asteroid belt is that it may have once been a planet that was fragmented by a collision with a huge comet. TROJAN ASTEROIDS Trojan asteroids are asteroids that orbit in gravitationally stable Lagrange points in a planet's orbit, either trailing it or preceding it (these places are where the gravitational attraction of the Sun and of the planet balance each other). Jupiter has the most Trojan asteroids; Mars also has some. Achilles was the first Trojan asteroid found. The asteroids preceding Jupiter in its orbit were named for Greek heroes; those following Jupiter in its orbit were named for Trojan heroes. Near-Earth Asteroids NEAR-EARTH ASTEROIDS Asteroids whose orbits bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are called Near-Earth Asteroids (NEA) or Earth-Approaching asteroids. These asteroids probably came from the main Eros, Asteroid asteroid belt, but were jolted from the belt by #433, is an collisions or by interactions with other elongated Nearobjects' gravitational fields (primarily Earth Asteroid Jupiter). which is 21 by 8 by 8 miles (33 by About 250 NEAs have been found so far, but 13 by 13 many, many more exist. The largest known NEA kilometers). The is 1036 Ganymede, with a diameter of 25.5 density of Eros is miles (41 kilometers). According to 2.4 grams per astronomers there are at least 1,000 NEA's cubic centimeter, whose diameter is greater than 0.6 miles (1 roughly the same kilometer) and which could do catastrophic as the density of damage to the Earth. Even smaller NEA's could Earth's crust. cause substantial destruction if they were to collide with the Earth. There are three types of NEA's: • • Amors (named for asteroid Amor, #1221): Asteroids which orbit between the orbits of Earth and Mars. Eros (#433) is an Amor. Apollos (named for asteroid Apollo, #1862): Asteroids which cross Earth's orbit and have a period longer than 1 year. They have semimajor axes greater than 1 astronomical • unit (AU), and perihelion distances less than 1.017 AU. Geographos is an Apollo. Atens (named for asteroid Aten, #2062): Atens are asteroids that are always closer to the Sun than the Earth is; they have a period shorter than 1 year (the semi-major axis is smaller than Earth's). Ra-Shalom is an Aten. Cruithne is an asteroid about 3 miles (5 kilometers) in diameter that is co-orbital with the Earth, which means that it shares roughly the same orbit as the Earth. It is a Near-Earth asteroid (NEA 3753). From the Earth, it appears to have a horseshoeshaped orbit; it gets close to the Earth, then it moves away again. Its orbit is highly inclined to Earth's orbit. At its closest approach (which happens every 100,000 years), Cruithne comes to within 10 million miles (15 million km) of Earth (40 times the distance from the Earth to the Moon). Cruithne was named for the first Celtic tribal group that settled in the British Isles. Cruithne was discovered on October 10, 1986 by D. Waldron at Siding Spring Observatory, Coonabarabran, Australia. Composition of Asteroids COMPOSITION OF ASTEROIDS Percentage Albedo Type Composition of (reflectivity) Asteroids Carbon (Ctype) Carbon over 75 percent 0.03-0.09 (Very dark) Silicate Metallic 17 percent 0.10 -0.22 The asteroids are (Siron mixed (Relatively varied in their type) with ironbright) composition. Most silicates are made of rock, and but some are magnesiumcomposed of silicates metals and other Metallic 0.10-0.18 Iron/ less than 7 materials. (M(Relatively nickel percent type) bright) Most of what we Water know about the ice/frozen 0.05 composition of Dark carbon less than 1 (Relatively asteroids is from (Dmonoxide percent dark and studying the type) mixed with reddish) asteroids that rock have fallen to Earth; thay are called meteorites at this point. Although some meteorites come from the moon and from comets, most are asteroids. 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 when they're near the sun in their highly eccentric orbits. PARTS OF A COMET Nucleus: The nucleus is the frozen center of a comet's head. It is composed of ice, gas, and dust. The nucleus contains most of the comet's 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 a million km across. The coma is comprised of water vapor, carbon dioxide gas, ammonia, dust, and neutral gases 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 (ions streaming from the sun at high velocities) pushes it away (it is also called the plasma tail). When the comet 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 million km long. Dust Tail: The dust tail is a long, wide tailcomposed of microscopic dust particles that are buffeted by photons emitted from the Sun; this tail curves slightly due to the comet's motion. The tail fades as the comet moves far from the Sun. Hydrogen Envelope: Hydrogen gas surrounds the coma of the comet and trails along for millions of miles (it is usually between the ion tail and the dust tail). The hydrogen envelope is about 10 million km across at the nucleus of the comet and about 100 million km long. It is bigger when the comet is near the Sun. A COMET'S 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; these comets are called sungrazers. COMET EXPLORATION NASA's 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 return to 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 comet's nucleus is about 3 miles (5 km) across. Wild 2 orbits the Sun every 6.39 years; its elliptical orbit ranges from about Mars' orbit to Jupiter's orbit. COSMIC SNOWBALLS There is a new and very controversial theory that comets (composed of frozen water) are constantly bombarding the Earth. These "cosmic snowballs" have (perhaps) been seen by the visible imaging system of the Polar Satellite. In theory, these frozen comets vaporize in the atmosphere, adding water vapor to the environment. WHERE COMETS ORIGINATE Long-period Comets (comets with an orbital period over 200 years and up to 30 million years): The Oort Cloud is a cloud of rocks and dust that may surround our solar system. This cloud may be where long-period comets originate. The Oort Cloud was named for Jan H. Oort, who proposed its existence in 1950. It has been hypothesized that the Oort Cloud is responsible for the periodic mass extinctions on Earth. Short-period Comets (comets with an orbital period under 200 years): The Kuiper belt is a region beyond Neptune in which at least 70,000 small objects orbit. This belt is located from 30 to 50 (?) A.U.'s and was discovered in 1992. It is a region where the planet-building process was stopped in before any large objects were formed; there are only primitive remnants from the early accretion disk of the solar system, 4.5 billion years ago. The Kuiper belt may be the source of the short-period comets (like Halley's comet). The Kuiper belt was named for the Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951. The Kuiper Belt The Kuiper belt is a region beyond the planet Neptune in which at least 70,000 small, icy, slow-moving objects orbit. These relatively dark objects, "trans-Neptunians," are perhaps from 6-30 miles (10-50 km) in diameter. This belt is located from roughly 30 to over 50 A.U.'s from the Sun. It is a region where the planet-building process was stopped in before any large objects were formed; there are only primitive remnants from the early accretion disk of the solar system, 4.5 billion years ago. Pluto and Charon (Pluto's moon) may be extremely large members of the Kuiper belt. Another large KBO (Kuiper Belt Object) is Quaoar. Comets and the Kuiper Belt The Kuiper belt may be the reservoir of the short-period comets (like Halley's comet). Short-period comets orbit the sun in less than 200 years. The Kuiper belt objects may be redirected into an eccentric orbit that comes quite close to Sun. Occasionally, one of the icy objects in the Kuiper belt is disturbed by the gravitational forces of one of the gas giant planets, causing it to approach Neptune, which sometimes propels the small object into a new, very elliptical solar orbit, where it may eventually approach the Sun (and near the Sun, the characteristic comet tail is visible). The Discovery of the Kuiper Belt The Kuiper belt was discovered in 1992. The Kuiper belt was named for the Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951. CROSSING A COMET'S ORBIT: METEOR SHOWERS The Earth passes through the orbit of some comets. When this happens, the left-over comet debris (rocks, etc.) bombards the Earth and they burn up in our atmosphere. This is called a meteor shower. In it, many meteors fall through the atmosphere in a relatively short time and in approximately parallel trajectories. Each meteor shower occurs at a predictable time each year. Meteor showers are named after the constellation they seem to originate from (their radiant). The next HUGE meteor shower (a meteor storm) will be the Leonid shower of November, 1999, which is expected to produce up to thousands of meteorite each hour. 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 Earth's atmosphere, usually making a fiery trail as it falls. It is sometimes called a shooting star or a falling star. Just before a meteor enter's the Earth's atmosphere, it is moving at roughly 70 kilometers per second. The friction between the fast-moving meteor and the gas in the Earth's 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). METEOR SHOWERS A meteor shower is a phenomenon in which many meteors fall through the atmosphere in a relatively short time and in approximately parallel trajectories. A very intense meteor shower is called a meteor storm. What Causes a Meteor Shower? Meteor showers (and meteor storms) occur when the Earth passes through a comet's orbit, and left-over comet debris (rocks, etc.) bombards the Earth. Each meteor shower occurs at a predictable time each year, as the Earth intersects that particular comet's orbit. How are Meteor Showers Named? Meteor showers are named for the constellation they seem to originate from (this is called their radiant, the point in the sky from which the shower seems to be coming from). Meteor showers have nothing to do with constellations or stars except for their names. For example, the Orionids meteor shower is named for the constellation Orion because the meteor shower seems to originate in the constellation Orion (although the meteors in this meteor shower are actually debris from Halley's comet and have nothing to do with the constellation Orion). METEORITES METEORITES FROM MARS Many meteorites from Mars have landed on Earth, one A meteorite is a meteor that with a has fallen to Earth. At least suspected 100 meteorites hit the Earth microbial CHONDRITIC every year. Meteorites have fossil. survived a fiery fall through METEOR the Earth's atmosphere and Chondritic have lost a lot of mass in that meteors are stony meteors process. Most meteorites burn with chondrules, up in the Earth's atmosphere; all that is left is a bit of dust. tiny glass spheres. These Every day, about 3000 tons of meteoroid dust falls to Earth. meteors are unchanged since their formation, There are different types of meteorites: shortly after the • Stony (92.8%): formation of the o Chondrites stony meteorites - 4.55 sun. These billion years old (from the formation meteors consist of the solar system) of elements also o Carbonaceous Chondrites common in the o Achondrites (7.1%): stony Earth's core. meteorites with chunks of differentiated matter that has melted and recrystallized • Iron (5.7%) : crystalline iron-nickel alloy o Pallasites: the mineral olivine enclosed in metal o Mesosiderites • Stony-iron: (1.5%) iron-nickel alloy and non-metallic mineral matter. When meteorites hit the Earth they cause a crater to form. The bigger the meteorite, the bigger the crater. The Biggest Meteorites The biggest meteorite ever found on Earth is the Hoba meteorite. It weighed over 100 tons. This iron meteorite was found in Namibia, Barringer Crater Africa. A lot of it has rusted away since it in Arizona. This landed on Earth; it is still in its impact crater impact crater in Namibia formed roughly 49,000 years The largest meteorite ever found in the United when a 300,000 States is Willamette meteorite (named for the ton meteorite hit city in Oregon where it landed). This iron the Earth and meteorite weighed fifteen tons and was found disintegrated. in 1902.