The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered (2062 Aten, discovered on January 7, 1976 by Eleanor F. Helin). They are defined by having a semi-major axis of less than one astronomical unit (AU), the distance from the Earth to the Sun. Atens also have an aphelion (furthest distance from the Sun) greater than 0.983 AU. Asteroids' orbits can be highly eccentric. An Aten orbit need not be entirely contained within Earth's orbit, as nearly all known Aten asteroids have their aphelion greater than one AU although their semi-major axis is less than one AU. Observation of objects inferior to the Earth's orbit is difficult and this difficulty may be the cause of some sampling bias in the apparent preponderance of eccentric Atens. Aten asteroids account for only about 6% of the known near-Earth asteroid population. Many more Apollo-class asteroids are known than Aten-class asteroids, possibly because of the sampling bias.
Asteroids that have their aphelion entirely within the Earth's orbit are known as Apohele asteroids, or alternatively as Inner Earth Objects (IEOs) or Atira asteroids. Apoheles are traditionally listed as a subclass of Atens, but are a separate class. As of January 2015, there are only 14 known Apoheles, and 898 known Aten asteroids, and has increased to 952 by August 2015.
The shortest semi-major axis for any known Aten asteroid is (325102) 2008 EY5 at 0.626 AU. The Aten asteroid with the smallest known perihelion is also the one with the highest known eccentricity: (137924) 2000 BD19 has an orbit with an eccentricity of 0.895, which takes it from a perihelion of 0.092 AU, well within Mercury's orbit, to an aphelion of 1.66 AU.
For a brief time near the end of 2004, the asteroid 99942 Apophis (then known only by its provisional designation 2004 MN4) appeared to pose a threat of causing an Earth impact event in 2029, but earlier observations were found that eliminated that possibility. A very small possibility remained for 2036, but this was also eliminated.
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