II.ASTEROIDS COMPARED TO OTHER SOLAR SYSTEM OBJECTS


Solar System Tour

Asteroids are sometimes called planetoids or minor planets. Most rotate on their axes every 5 to 20 hours. Some smaller asteroids may orbit larger asteroids as satellites, forming binary pairs. Asteroids differ from true planets mainly by their much smaller size—only about 200 asteroids have diameters of more than 97 km (60 mi). Planets are defined as bodies that have settled into a rounded shape because of the inward pull of their own gravitation. Asteroids are not thought to have enough mass to settle into rounded shapes in the same way as planets. With their low masses, asteroids also do not have atmospheres.In 2006 the International Astronomical Union (IAU) designated a new category of solar system objects called dwarf planets. The dividing line between asteroids and dwarf planets is still being clarified. Dwarf planets have rounded shapes but are not massive enough to clear other bodies from around their orbits. Major or “classical” planets had enough mass to clear their neighborhoods of small bodies, either by pulling such objects in as the planets formed or by throwing the small bodies into distant orbits or out of the solar system. Dwarf planets orbit the Sun in regions of the solar system that contain swarms of small bodies such as the asteroid belt or the Kuiper Belt. The largest object in the asteroid belt, 1 Ceres, is now classified as a dwarf planet because of its rounded shape. Future research may show that additional large asteroids also qualify as dwarf planets because their shape was rounded by their own gravitation.Because of their small size and primitive composition, asteroids share many basic properties with comets. Comets typically have much more elliptical orbits than asteroids and actively shed gas and dust. Although asteroids are mainly rocky, some may also contain water-ice material and so are not clearly distinct from objects that can become comets if heated by enough sunlight. It is also possible that some objects that are considered asteroids are remains of dead comets that have lost their gas and dust. In 2006 astronomers announced finding a number of icy comet-like objects orbiting in the main asteroid belt, suggesting that asteroids and comets can occur together. The term asteroid is sometimes extended to the small icy bodies found in the outer system beyond Jupiter. Unlike rocky asteroids, these more distant bodies are mainly made of ice, along with dust and rock. Closely related to comets, these solar system bodies are now generally called Kuiper Belt Objects (KBOs) or Trans-Neptunian Objects (TNOs). Most of these objects orbit the Sun beyond Neptune in a disk called the Kuiper Belt. A special population of these icy objects called centaurs have left the Kuiper Belt and have unstable orbits that lie between Neptune and Jupiter. Chiron, the first centaur discovered, was originally classified as an asteroid. Like asteroids and dwarf planets, KBOs and centaurs are listed in the official catalog of minor planets.