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Aten asteroid
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The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered. They are defined by having semi-major axes of less than one astronomical unit (the distance from the Earth to the Sun). Note that, because asteroids' orbits can be highly eccentric, an Aten orbit need not be entirely contained within Earth's orbit; in fact, nearly all known Aten asteroids have their aphelion greater than one AU. Those that have their aphelion entirely within the Earth's orbit are known as Apohele asteroids. As of March 2008 there are only five known Apoheles Atira]], , , and ) and four suspected Apoheles (, , and ).
The smallest semi-major axis is that of , at 0.642 AU (its eccentricity of 0.688 takes it from a perihelion of 0.200 AU, well within Mercury's orbit, to an aphelion of 1.084 AU), although seems to have an even smaller one (0.635 AU; eccentricity 0.532 ranging from 0.297 to 0.973 AU —enough to cross Venus' orbit but not Mercury's).
For a brief time near the end of 2004, the asteroid 99942 Apophis (then known only by its provisional designation ) appeared to pose a threat of causing an Earth impact event in 2029, but earlier observations were found that eliminated that possibility, although a very small possibility remains for 2036.
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