TheAten asteroids are adynamical group ofasteroids whose orbits bring them intoproximity with Earth. By definition, Atens areEarth-crossing asteroids(a < 1.0 AU andQ > 0.983 AU).[1] The group is named after2062 Aten, the first of its kind, discovered on 7 January 1976 by American astronomerEleanor Helin atPalomar Observatory. As of January 2025, 2,966 Atens have been discovered, of which 271 are numbered, 14 are named, and 197 are classified aspotentially hazardous asteroids.[2][3][4]
Aten asteroids are defined by having asemi-major axis (a) of less than 1.0astronomical unit (AU), the roughly average distance from theEarth to theSun. They also have anaphelion (Q; furthest distance from the Sun) greater than 0.983 AU.[1] This defines them asEarth-crossing asteroids as the orbit of Earth varies between 0.983 and 1.017 AU.
Asteroids' orbits can be highly eccentric. Nearly all known Aten asteroids have anaphelion greater than 1 AU. Observation of objectsinferior to the Earth's orbit is difficult, and this difficulty may contribute tosampling bias in the apparent preponderance of eccentric Atens. Aten asteroids account for only about 7.4% of the knownnear-Earth asteroid population.[4] Many moreApollo-class asteroids are known than Aten-class asteroids, possibly because of the sampling bias.
The shortest semi-major axis for any known Aten asteroid is 0.580 AU, for object2016 XK24.[3] The Aten asteroid with the smallest known perihelion is also the one with the highest knowneccentricity:(137924) 2000 BD19 has an orbit with an eccentricity of 0.895, which takes it from a perihelion of 0.092 AU, well withinMercury's orbit, to an aphelion of 1.66 AU, which is greater than the semi-major axis ofMars (1.53 AU).
| Group | q | a | Q | ECA |
|---|---|---|---|---|
| Amors | > 1.017 | >1.0 | – |  N |
| Apollos | < 1.017 | >1.0 | – |  Y |
| Atens | – | <1.0 | > 0.983 | Y |
| Atiras | – | <1.0 | < 0.983 | N |
| For all NEOs q is < 1.3 AU; The orbit ofEarth varies between 0.983 and 1.017 AU | ||||
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