<P> Asteroids are minor planets, especially those of the inner Solar System . The larger ones have also been called planetoids . These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet . As minor planets in the outer Solar System were discovered and found to have volatile - based surfaces that resemble those of comets, they were often distinguished from asteroids of the asteroid belt . In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter . </P> <P> There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula 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 objects . Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C - type, M - type, and S - type . These were named after and are generally identified with carbon - rich, metallic, and silicate (stony) compositions, respectively . The size of asteroids varies greatly, the largest is almost 1,000 km (625 mi) across . </P> <P> Asteroids are differentiated from comets and meteoroids . In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are composed of dust and ice . In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice . The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter . Finally, meteoroids can be composed of either cometary or asteroidal materials . </P>

Relatively small bodies which generally orbit the sun between mars and jupiter are called