Asteroids are small planets, especially of the inside Solar System. Big asteroids are also called planetoids. These expressions have historically been applied to any astronomical body orbiting the Sun that did not look like a planet-like disc and was not seen to have characteristics of a lively comet such as a tail. As small planets in the outer Solar System were discovered they were naturally found to have volatile - rich tops like comets. As a result, they were frequently famed from objects found in the main asteroid belt., the word 'asteroid' refers to the small planets of the inner Solar System.There are zillions of asteroids, many assumed to be the crushed leftovers of planetesimals, bodies within the young Sun's solar nebula that never grew big enough to become planets. The massive majority of known asteroids orbit within the key asteroid belt located between the orbits of Mars and Jupiter or is co-orbital with Jupiter (the Jupiter Trojans). However, several other orbital families exist with significant populations, including the near-Earth objects. Single asteroids are categorized by their typical spectra, with the majority falling into three key groups: C-type, M-type, and S-type. These were termed after and are usually identified with carbon-rich, metallic, and silicate (stony) configurations, respectively. The sizes of asteroids vary greatly; the largest, Ceres, is almost 1,000 km (625 mi) across.Asteroids are separated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly made of mineral and rock, comets are mainly composed of dust and ice. Furthermore, asteroids are formed closer to the sun, preventing the progress of cometary ice. The difference between asteroids and meteoroids is mainly of size: meteoroids have a radius of one meter or less, whereas asteroids have a radius greater than one meter. Finally, meteoroids can be made of either cometary or asteroidal materials.