Minor Planets

Not all asteroids reside in the main belt. The asteroid Hidalgo, discovered by American astronomer Walter Baade in 1920, for example, travels along an orbit which takes it from the inner edge of the main belt (2.0 AU from the Sun) to beyond the orbit of Saturn (9.7 AU). Likewise, the strange asteroid Chiron, discovered by American astronomer Charles Kowal (1940–) in 1977, moves along an orbit which brings it no closer than 8.5 AU to the Sun, but takes it all the way out to the orbit of Uranus (18.9 AU). Chiron is unusual because it occasionally shows cometlike behavior, emitting thin gases through evaporation from its surface. It is probably a former member of the Kuiper belt, ejected from its earlier orbit by random gravitational interactions with other asteroids and with the outer planets.

The Trojan asteroids are an interesting group since they orbit the Sun at a distance of 5.2 AU—the same distance as the planet Jupiter. These asteroids move in a special way, and keep a near constant angle of 60° between themselves, Jupiter, and the Sun (Fig. 2). This relative configuration of objects was shown to be gravitationally stable by the French mathematician Joseph Louis Lagrange (1736–1813) in 1772, and the Trojans, with one group trailing and the other group leading Jupiter, occupy the so-called fourth and fifth Lagrange points. Several hundred Trojan asteroids have now been identified, the largest of which have diameters of 93–124 mi (150–200 km). The best-studied Trojan asteroid, 624 Hektor, is unusual in that it appears to be nearly twice as long as it is wide. It has been suggested that 624 Hektor is in fact a binary asteroid, with the two components revolving around each other in near contact. (Several other asteroids have been found to be double.)