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The double asteroid, 90 AntiopeCircling the Sun between the orbits of Mars and Jupiter are millions of small, planet-like objects called asteroids or minor planets (sometimes also called planetoids). The largest, Ceres, discovered in 1801—and, since 2006, upgraded to the status of "dwarf planet"—measures just over 1000 kilometres (600 miles) across, but only a few asteroids have diameters greater than 100 kilometres (60 miles). About 4000 have been discovered and named so far, but it is thought there may be up to a million objects with a diameter of 1 kilometre or more. Many more are tiny specks too small to be identified.

A close-up view of a typical asteroid, irregular in shape and with a heavily-cratered surface
Plan view of the asteroid belt

Orbital paths

Most asteroids lie in bands between Mars and Jupiter, although some have strayed far from here. The Trojan asteroids, for example, share Jupiter’s orbital path, while Apollo, Icarus and Adonis actually pass inside Earth’s orbit. In fact, more than 7000 near-Earth asteroids have been identified, of which more than 500 are estimated to measure over 1 kilometre (0.6 miles) in diameter.

The name asteroid, originally proposed by German-born British astronomer Sir William Herschel, is from the Greek asteroeides, meaning "star-shaped".


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