A small rocky object in the solar system. The largest, Ceres, is nearly 1,000 kilometres (600 miles) across and they range in size down to dust particles. Many thousands have been individually identified and it is believed that there could be half a million with diameters larger than 1.6 kilometres (1 mile). However, the total mass of all asteroids is less than one-thousandth the mass of the Earth. Most asteroid orbits are concentrated in theasteroid belt between Mars and Jupiter at distances ranging from 2.0 to 3.3 AU from the Sun.
Asteroids can be classified according to their spectra of reflected sunlight: 75 per cent are very dark, carbonaceous C-type asteroid, 15 per cent are greyish, silicaceous (stony) S-types asteroid and the remaining 10 per cent consist of the M-types asteroid (metallic) and a number of rare varieties. The classes are linked to the known types of meteorite. The evidence suggests that many asteroids and meteorites have similar compositions, so asteroids may be the parent bodies of meteorites.
The darkest asteroids reflect only 3 or 4 per cent of the sunlight falling on them, while the brightest reflect up to 40 per cent. Many vary regularly in brightness as they rotate. In general, they are irregularly shaped; the smallest asteroids rotate the most rapidly and are the most irregular in shape.
Asteroids are believed to be the remnants of the material from which the solar system formed. This view is supported by the way the predominating asteroid type changes with increasing distance from the Sun. High-speed collisions between asteroids are now gradually resulting in their break-up. They are believed to be dusty, cratered objects, similar to the moons of Mars, which are probably captured asteroids.
