More than 7000 asteroids have been discovered. Several hundred more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from the Earth. There are 26 known asteroids larger than 200 km in diameter. Our census of the largest ones is now fairly complete: we probably know 99% of the asteroids larger than 100 km in diameter. Of those in the 10 to 100 km range we have cataloged about half. But we know very few of the smaller ones; perhaps as many as a million 1 km sized asteroids may exist.
The total mass of all the asteroids is less than that of the Moon.
243 Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR mission flew by 253 Mathilde (left) on 1997 June 27 returning many images. They are the only asteroids which have been studied closely so far. NEAR will enter orbit around 433 Eros in January 1999.
The largest asteroid by far is 1 Ceres. It is 933 km in diameter and contains about 25% of the mass of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter. All other known asteroids are less than 340 km across.
There is some debate as to the classification of asteroids, comets and moons. There are many planetary satellites that are probably better thought of as captured asteroids. Mars's tiny moons Deimos and Phobos, Jupiter's outer eight moons, Saturn's outermost moon, Phoebe, and perhaps some of the newly discovered moons of Uranus and Neptune are all more similar to asteroids than to the larger moons.
Asteroids are classified into a number of types according to their spectra and albedo:
Asteroids are also categorized by their position in the solar system:
There also a few "asteroids" (designated as "Centaurs") in the outer
solar system: 2060 Chiron orbits between Saturn and Uranus; the orbit of
5335 Damocles ranges from near Mars to beyond Uranus; 5145 Pholus orbits
from Saturn to past Neptune. There are probably many more, but such planet-crossing
orbits are unstable and they are likely to be perturbed in the future.
The composition of these objects is probably more like that of comets or
the Kuiper Belt objects than that of ordinary asteroids.