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Asteroid
Any of many thousands of small bodies, made of rock and minerals, that orbit the Sun. Most lie in a region called the asteroid belt between the orbits of Mars and Jupiter, and are thought to be fragments left over from the formation of the Solar System. About 100,000 asteroids may exist, but their total mass is only a few hundredths of the mass of the Moon. These rocky fragments range in size from 1 km/0.6 mi to 900 km/560 mi in diameter.
The largest asteroids are sometimes called minor planets; these include Ceres (the largest asteroid, 940 km/584 mi in diameter) and Vesta (which has a light-coloured surface and is the brightest as seen from Earth). Some asteroids are in orbits that bring them close to Earth and some, such as the Apollo asteroids, which include Eros and Icarus, even cross Earth's orbit. They may be remnants of former comets.
© Research Machines plc 2008. All rights reserved. Helicon Publishing is a division of Research Machines plc.
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