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Asteroids (from Greek, ἀστήρ ”star” + εἶδος “like”, in form), sometimes grouped with centaurs, Neptune trojans and trans-Neptunian objects into minor planets or planetoids,[1] are a class of small Solar System bodies in orbit around the Sun. The term "asteroid" was historically applied to any astronomical object orbiting the Sun that was not observed to have the characteristics of an active comet or a planet, but it has increasingly come to particularly refer to the small rocky and metallic bodies of the inner Solar System and out to the orbit of Jupiter. As small objects in the outer Solar System have begun to be discovered their observed composition differs from the objects historically termed asteroids. Harbouring predominantly volatiles-based material similar to comets rather than the more familiar rocky or metallic asteroids, they are often distinguished from them.[2]

There are millions of asteroids, and like most other small Solar System bodies the asteroids are thought to be remnants of planetesimals, material within the young Sun’s solar nebula that have not grown large enough to form planets.[3] The large majority of known asteroids orbit in the main asteroid belt between the orbits of Mars and Jupiter, however many different orbital families exist with significant populations including Jupiter Trojans and near-Earth asteroids. Individual asteroids are categorized by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These are generally identified with carbon-rich, stony, and metallic compositions respectively.


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