- Solar system body intermediate in size between planets and meteorites. The largest are hundreds of kilometres across and the smallest only hundreds of metres. When first discovered, asteroids were called ‘minor planets’ and, because they mostly orbit just inside the orbit of Jupiter, were thought to be material which would have formed a real planet were it not for the gravitation of Jupiter. Now the asteroids are thought to be related to comets and to the ‘planetesimals’ from which the planets formed, kept in orbits inside that of Jupiter by the effects of Jupiter’s gravitation. Some comets have been observed to lose their tails in the inner solar system, at which point they start looking like asteroids. Asteroids are material from the earliest days of the solar system, so that information about them is of immense astronomical value. On the basis of occultation experiments, radar studies and spacecraft flybys, many asteroids seem to travel in pairs orbiting about each other. They are classified in groups on the basis of their albedo. The less reflective asteroids have albedos which seem to resemble those of chondritic meteorites, while the brighter S group seem to have properties like those of silicate rocks on the Earth. There is also a group M with metallic-type albedos. Now the classifications are being expanded as more data is obtained with modern instruments and spectral examination shows more links to meteorites and other objects. Infrared observations from orbit are a particularly powerful tool for this work. In the 1990s our knowledge of the asteroids was expanded by the visit by the Galileo spacecraft to asteroids 243 Ida and 951 Gaspra, en route to Jupiter. (Asteroids are given a number in order of discovery, as well as a name chosen by the discoverer. Number 5,000 was reached in 1989, 1 Ceres having been found on the first day of 1801.).