An asteroid is any one of a large number of small solid objects in the SOLAR SYSTEM, sometimes called minor planets because they orbit the Sun directly. The vast majority are found in a swarm called the asteroid belt, between the orbits of Mars and Jupiter, at average distances of 2.1 to 3.3 ASTRONOMICAL UNITS (AU) from the Sun. They are given a number (and sometimes then named) when their orbits are well established. More than 5,000 such asteroids are now known, and many more have been sighted at least once. Numbered ones include about 38 to 45 Amor asteroids, whose orbits intersect the orbit of Mars; about 40 Apollo asteroids, whose orbits intersect the Earth's orbit; about 6 Aten asteroids, with orbits smaller than the Earth's orbit; and about 96 Trojan asteroids, which precede or follow Jupiter in its orbit. One Apollo asteroid, now numbered 4581 and called Asclepius, passed within 800,000 km (500,000 mi) of Earth in 1989--the closest approach of a large asteroid since that of Hermes in 1937. Asteroids range from many kilometers to a few meters or less in diameter. The three largest, CERES, Pallas, and Vesta, are about 785, 610, and 540 km (488, 379, and 336 mi) wide, respectively. Gaspra, an irregular asteroid about 20 km (12 mi) long on its largest axis, was the first to be photographed at close range--by the space probe GALILEO in 1991, at a distance of 1,600 km (1,000 mi). Probes had earlier photographed the two tiny moons of Mars, however, and it is likely these also are asteroids, captured by that planet's gravitational field. Asteroids are thought to be remnants of the early solar system that never grew to planetary size. Collisions between such objects must have been numerous in those early times. Most of the asteroids seen at present are probably fragments of once-larger ones, as indicated by their irregular shape. While some of those found in the inner solar system may be burned-out comets, most probably have compositions in the general range found in meteorites. Collisions of larger asteroids with the Earth have been implicated in mass EXTINCTIONS over geological time, as well as with the onset of an ice age about 2.3 million years ago. Some interest has been expressed in planning ways to avert future cataclysmic encounters with the Earth. Lawrence Grossman Bibliography: Binzel, R. P., et al., Asteroids II (1990); Cunningham, C., Introduction to Asteroids (1987).