Asteroids are dwarf planets, especially those of the inner Sol System. The larger ones have also been called planetesimals or protoplanets. The term asteroid has come increasingly to refer specifically to the small bodies of the inner Sol System out to the orbit of Jupiter. They are grouped with the outer bodies, Neptune Trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomy. In this article the term "asteroid" refers to the minor planets of the inner Sol System.
Number and LocationEdit
There are millions of asteroids within the Sol System that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the Near-Earth asteroids.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.
Asteroids vary greatly in size, from almost 1,000 km for the largest down to rocks just tens of meters across. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either surviving planetesimals or fragments of larger bodies.
Most asteroids outside of Sol's big four (Ceres, Pallas, Vesta, and Hygiea) are broadly similar in appearance, if irregular in shape. 50-km 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and 300-km 511 Davida, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida that have been surveyed also reveal a deep regolith covering the surface.