The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered (2062 Aten, discovered January 7 1976 by Eleanor F. Helin). They have semi-major axes of less than one astronomical unit, placing them inside the orbit of Earth.
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 May 2004 there are only two known Apoheles: 2003 CP20 and 2004 JG6.
The smallest semi-major axis is that of (66391) 1999 KW4, 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 2004 JG6 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, 2004 MN4 appeared to pose a threat of causing an Earth impact event in 2029, but earlier observations were found that eliminated that possibility.
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