Asteroids are small rocky objects ranging in size from the largest, about one quarter the diameter of the Moon, to the smallest, the size of dust particles.
Many thousands of asteroids are known. It is believed that there could be half a million which are the size of a mountain (diameter larger than 1.6 kilometers). But the total mass of all asteroids is less than one-thousandth the mass of the Earth.
Most asteroid orbits lie in the asteroid belt between Mars and Jupiter. There are some groups of asteroids nearer the Sun, and other groups more distant from the Sun.
Many asteroids and meteorites probably have similar compositions, so asteroids may be the parent bodies of meteorites.
In general, asteroids are irregularly shaped and pitted with craters.
Asteroids may have been formed from a planet originally lying between Mars and Jupiter which was destroyed by the gravity of its giant neighbor, Jupiter.
Image copyright Joe Tucciarone
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