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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 diameter 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. At the end of 2010 over half a million asteroids were known and the total was growing by about 10% per year.

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. Increasing numbers are being discovered in dedicated searches such as the Catalina Sky Survey, Lincoln Near-Earth Asteroid Research, Pan-STARRS, and Spacewatch. There are thought to be between 1–2 million larger than 1 km, mostly in the main belt, although only a small percentage of these are currently known.

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. A few asteroids have small moons, the first of which was photographed in orbit around Ida by the Galileo space probe.

Some main-belt asteroids form groups with similar orbital characteristics (semimajor axis, orbital eccentricity, and inclination), for example the Cybele, Hilda, Hungaria, and Phocaea groups. Where the group seems to have originated from the break-up of a single parent body, it is called a Hirayama family. A small percentage of asteroids orbit outside the main asteroid belt. Members of the Amor group cross the orbit of Mars, while Apollo and Aten group asteroids cross that of Earth; these three groups are collectively termed near-Earth asteroids. Farther out, the Trojan asteroids orbit at Jupiter's distance, while beyond Neptune is the Kuiper Belt.

Asteroids are divided into various classes according to their reflectance spectra, which reveal differences in composition. The proportion of different asteroid classes changes markedly with increasing distance from the Sun. S-class (silicaceous) asteroids predominate in the inner main belt (at less than 2.4 AU). C-class (carbonaceous) asteroids are more prevalent in the middle and outer regions of the belt, with a peak near 3 AU. The dark asteroids near the outer edge of the main belt have a reddish tinge, and may be richer in organic components; these are the P-class asteroids. Still farther out, many of the Trojan asteroids are even redder; they are termed D-class asteroids. There is an apparent concentration of M-class (metallic) asteroids in the middle of the belt, at 2.5–3.0 AU.


Subjects: Astronomy and Astrophysics.

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