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Asteroids Asteroids are rocky and metallic objects with an irregular shape that orbit the Sun but are too small and numerous to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within the asteroid belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona. Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never formed into a planet. Asteroids are composed of several different materials and differ in composition. This is how scientists classify asteroids. Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. Meteors, Meteoroids, & Meteorites The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the solar system falls into Earth's atmosphere creating temporary light resulting from atmospheric friction. This typically occurs at heights of 80 to 110 kilometers (50 to 68 miles) above Earth's surface. A meteoroid is a chunk of metal, rock, or dust revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. In fact, meteors, meteoroids, and meteorites can be smaller than a grain of sand. Meteors are meteoroids that pass through and burn up in the Earth’s atmosphere and are often called shooting stars. A meteorite is a meteoroid that passes through Earth’s atmosphere and hits the Earth’s surface. One of the primary goals of studying meteorites is to determine the history and origin of the celestial object they broke off of. Meteorites are classified based on their composition. Several meteorites sampled from Antarctica since 1981 have been shown to have originated from the moon. Astronomers compared the composition of these meteorites to moon rocks obtained by the Apollo missions of 1969-1972 and found that they are made of the same types of materials. Sources of other specific meteorites remain unproven, although another set of eight meteorites are suspected to have come from Mars. All other groups are presumed to have formed from asteroids or comets; the majority of meteorites are believed to be fragments of asteroids. Meteoroids may become detached from passing comets or form from collisions between asteroids. Meteorites fall all over Earth. Most of them look just like stones, so nobody notices them. A few meteorites are made almost entirely of iron and nickel, and so are unusually heavy for their size. This makes them more likely to be identified as meteorites than as Earth rocks. Comets Comets are small, fragile, irregularly shaped bodies composed of chunks of ice and dust. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto. Comets can range in size from 0.7 to 20 km in diameter. As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers away from the Sun. When far from the Sun, the center is very cold and its material is frozen solid. In this state comets are sometimes referred to as a "dirty snowball," since over half of their material is ice. When a comet approaches the Sun, the surface of the center begins to warm, forming the comet's coma (outer layer) of gas and dust. When the center is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to glow. Closer to the Sun, it produces a glow that is more intense than reflected light off it’s surface. This means that comets can produce their own light when a coma develops. The sun’s corona produces a stream of particles called the solar wind. Solar wind pushes the gas from a comet away from the sun. Gas and dust form the comet’s tail. The tail looks like hair; in fact, the name comet means “long– haired star” in Greek. A comet’s tail can be hundreds of millions of kilometers long and stretch across most of the sky. The material is stretched out very thinly, however, so there isn’t much mass in a comet tail. In 1705, Edmond Halley, an English astronomer, calculated the orbits of 24 comets that people had observed over hundreds of years. Halley realized that several of the comets seemed to have the same orbit and suggested that they were actually the same comet. Halley calculated that this comet appeared about every 76 years, and predicted that it would reappear in 1758. When this prediction came true, the comet was named Halley’s Comet. In 1986, the last time Halley’s Comet appeared in our sky, scientists all over the world awaited the arrival of Halley’s comet.