Asteroid is any of the many small rocky or metallic objects in the Solar System, mostly lying in a zone (the asteroid belt) between the orbits of Mars and Jupiter; also known as a minor planet. They range in size from almost 1000 km for Ceres (the first asteroid discovered, in 1801) down to less than 10 m for the smallest so far detected. The total mass of all asteroids is 4 × 1021 kg, about one-twentieth the mass of the Moon (Barnes-Svarney, 1996).
When an asteroid is discovered it is given a temporary designation, consisting of the year of discovery followed by two letters; the first indicates the half-month during which the asteroid was discovered and the second the order of discovery within that half-month. Only when an accurate orbit has been determined is it assigned a permanent number, and the discoverer then has the right to name it. By the end of 1996, orbits were known for over 7200 asteroids. Increasing numbers are being discovered in dedicated searches such as the Near-Earth Asteroid Tracking (NEAT) system on Mount Haleakala, Hawaii, and the Spacewatch Program. In all, there are thought to be at least a million asteroids, of which 90-95% is in the asteroid belt.
The orbits of most asteroids have higher eccentricities and inclinations than those of the major planets. Within the main asteroid belt, orbital eccentricities average about 0.15 and inclinations about 10°; occasionally they exceed 0.5 and 30°, respectively, more typical of the orbits of short-period comets. Indeed, some objects classified as asteroids may be defunct cometary nuclei. Rotation periods of asteroids range from a few hours to several weeks, but are typically 6-24 hours. The larger asteroids are roughly spherical, but those smaller than 150 km are commonly elongated or irregular. Radar studies of a few asteroids have revealed that some may be dumbbell-shaped or possibly double; these include Castalia and Toutatis........
