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The chances of a celestial body colliding with the earth are small, but the consequences would be catastrophic Copyright 1996 Scientific American, Inc. F. GOHIER Explorer Copyright 1996 Scientific American, Inc. MARS Copyright 1996 Scientific American, Inc. he asteroid belt, home of most asteroids, lies between the orbits of Mars and Jupiter. The chart (below ), put together from Spacewatch observations, shows that smaller asteroids, produced by fragmentation of the larger ones, are more numerous.The rocks normally remain in circular, stable orbits, but collisions, along with the gravitational influence of Jupiter, can throw them into narrow, unstable orbits. Then the asteroids may enter the inner solar system, where they pose a threat to the earth. ALFRED T. KAMAJIAN (illustration ); JOHNNY JOHNSON (graph ) 10 2 ROBERT JEDICKE University of Arizona NUMBER OF ASTEROIDS 10 3 10 3.2 12.8 6.4 1.6 ASTEROID DIAMETER (KILOMETERS) Copyright 1996 Scientific American, Inc. JUPITER NEPTUNE Copyright 1996 Scientific American, Inc. UNIVERSITY OF ARIZONA ALFRED T. KAMAJIAN ( illustration ); ROYAL GREENWICH OBSERVATORY Science Photo Library/Photo Researchers, Inc. ( photograph) omets reside beyond the orbit of Neptune in the Kuiper belt and the Oort cloud and, like asteroids, come near the earth only when dislodged from their circular paths. The Kuiper belt probably merges into the Oort cloud, which extends a fifth of the distance to the nearest star, Alpha Centauri. Comet Halley (below) is a visitor from the Oort cloud that has swung into a steeply elliptical orbit around the sun, having a period of 76 years. Copyright 1996 Scientific American, Inc.