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 Earthto 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 apheliongreater than one AU. Those that have their aphelion entirely within the Earth's orbit are known as Apohele asteroids. As of March 2008there are only five known Apoheles ( (163693) Atira, mpl|(164294) 2004 XZ|130, mpl|2004 JG|6, mpl|2005 TG|45 and mpl|2006 WE|4) and four suspected Apoheles (mpl|1998 DK|36, mpl|2006 KZ|39, mpl|2007 EB|26 and mpl|2008 EA|32).
The smallest semi-major axis is that of mpl|(66391) 1999 KW|4, 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 mp|2004 JG|6 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 designationmp|2004 MN|4) appeared to pose a threat of causing an Earth impact eventin 2029, but earlier observations were found that eliminated that possibility, although a very small possibility remains for 2036.
List of numbered Aten asteroids
Aten asteroid records
* [http://cfa-www.harvard.edu/iau/lists/Atens.html List of Aten Minor Planets]
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