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 are defined by having semi-major axes of less than one astronomical unit (the distance from the Earth to the Sun). 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 even though their semi-major axis is less than one AU. Observation of objects inferior to the Earth's orbit is difficult and may be the cause of some bias in the apparent preponderance of eccentric Atens.
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 treated as a subclass of Atens, but sometimes are separated. As of August 2012, there are only eleven known Apoheles, and 716 more known Aten asteroids. The smallest aphelion is that of 2008 EB26), at 0.804 AU. The shortest semi-major axis for any known Aten asteroid is that of another Apohele, 2007 EA32), at 0.550 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 1.661 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, although a very small possibility remains for 2036.
See also