View source for Aten asteroid

{{Short description|Group of near-Earth asteroids}}
[[File:Neo orbit types.jpg|thumb|upright=2.4|Common orbital subgroups of [[Near-Earth Object]]s (NEOs)]]

The '''Aten asteroids''' are a [[List of minor-planet groups|dynamical group]] of [[asteroid]]s whose orbits bring them into [[Near-Earth object|proximity with Earth]]. By definition, Atens are [[Earth-crossing asteroid]]s {{nowrap|([[Semi-major axis|a]]&thinsp;&lt;&thinsp;1.0&nbsp;[[Astronomical unit|AU]] and [[Aphelion|Q]]&thinsp;&gt;&thinsp;0.983&nbsp;AU)}}.<ref name="NEO-Basics" /> The group is named after [[2062&nbsp;Aten]], the first of its kind, discovered on 7 January 1976 by American astronomer [[Eleanor Helin]] at [[Palomar Observatory]]. As of January 2025, 2,966 Atens have been discovered, of which 271 are numbered, 14 are named, and 197 are classified as [[potentially hazardous asteroids]].<ref>{{cite web |title=Small-Body Database Query |url=https://ssd.jpl.nasa.gov/tools/sbdb_query.html#!#results |website=Solar System Dynamics - Jet Propulsion Laboratory |publisher=NASA - California Institute of Technology |access-date=2025-01-17}}</ref><ref name="Aten list" /<ref name="MPC-Aten" /><ref name="neo-jpl-stats" />

== Description ==
{{See also|List of Aten asteroids|Category:Aten asteroids}}

Aten asteroids are defined by having a [[semi-major axis]] (a) of less than 1.0 [[astronomical unit]] (AU), the roughly average distance from the [[Earth]] to the [[Sun]]. They also have an [[aphelion]] (Q; furthest distance from the Sun) greater than 0.983&nbsp;AU.<ref name="NEO-Basics" /> This defines them as [[Earth-crossing asteroid]]s as the orbit of Earth varies between 0.983 and 1.017&nbsp;AU.

Asteroids' orbits can be highly eccentric. Nearly all known Aten asteroids have an [[aphelion]] greater than 1&nbsp;AU. Observation of objects [[inferior and superior planets|inferior]] to the Earth's orbit is difficult, and this difficulty may contribute to [[sampling bias]] in the apparent preponderance of eccentric Atens. Aten asteroids account for only about 7.4% of the known [[near-Earth Object|near-Earth asteroid population]].<ref name="neo-jpl-stats" /> Many more [[List of Apollo asteroids|Apollo-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&nbsp;AU, for object {{mp|2016 XK|24}}.<ref name="MPC-Aten" /> The Aten asteroid with the smallest known perihelion is also the one with the highest known [[eccentricity (orbit)|eccentricity]]: {{mpl|(137924) 2000 BD|19}} has an orbit with an eccentricity of 0.895, which takes it from a perihelion of 0.092&nbsp;AU, well within [[Mercury (planet)|Mercury]]'s orbit, to an aphelion of 1.66&nbsp;AU, which is greater than the semi-major axis of [[Mars]] (1.53 AU).

Aten asteroids are generally named after [[Egyptian deities]].<ref name="BAN315">{{Citation|url=https://badastronomy.beehiiv.com/p/ban-315-apophis-and-stargate|title=Apophis and Stargate|work=Bad Astronomy Newsletter|first=Phil|last=Phait|issue=315|date=19 April 2021|access-date=11 August 2025}}</ref>

== NEO types==
{| class="wikitable" style="width: 330px; text-align: center;"
|+ Definition of [[Near-Earth object|NEO]] subgroups in [[Astronomical unit|AU]]<ref name="NEO-Basics" />
|-
! Group !! [[Perihelion|q]] !! [[Semi-major axis|a]] !! [[Aphelion|Q]] !! [[Earth-crossing asteroid|ECA]]
|-
| align=left | [[Amor asteroid|Amors]] || > 1.017 || >1.0 || –  || {{n}}
|-
| align=left | [[Apollo asteroid|Apollos]] || < 1.017 || >1.0 || – || {{y}}
|-
| align=left | '''Atens''' || – || <1.0 || > 0.983 || {{y}}
|-
| align=left | [[Atira asteroid|Atiras]] || – || <1.0 || < 0.983 || {{n}}
|-
! colspan=5 style="font-size: small; font-weight: normal; text-align: left;" | For all NEOs q is < 1.3&nbsp;AU; The orbit of [[Earth]] varies between 0.983 and 1.017&nbsp;AU
|}

== See also ==
* [[Alinda asteroid]]
* [[Amor asteroid]]
* [[Apollo asteroid]]
* [[Atira asteroid]]
* [[List of minor planets]]

== References ==
{{reflist|30em|refs=

<ref name="MPC-Aten">{{cite web
  |title      = List Of Aten Minor Planets (by perihelion distance)
  |publisher  = Minor Planet Center
  |date       = 17 May 2017
  |url        = https://minorplanetcenter.net/iau/lists/t_atens.html
  |access-date = 17 May 2017}}</ref>

<ref name="NEO-Basics">{{cite web
  |title      = NEO Basics
  |publisher  = NASA/JPL CNEOS
  |url        = https://cneos.jpl.nasa.gov/about/neo_groups.html
  |access-date = 17 May 2018}}</ref>

<!-- unused <ref name="Aten list">{{cite web
  |title      = JPL Small-Body Database Search Engine
  |publisher  = NASA/JPL
  |url        = https://ssd.jpl.nasa.gov/sbdb_query.cgi#x
  |access-date = 10 October 2024}}</ref> -->

<ref name="neo-jpl-stats">{{cite web
  |title      = Discovery Statistics – Cumulative Totals
  |publisher  = NASA/JPL CNEOS
  |url        = https://cneos.jpl.nasa.gov/stats/totals.html
  |date       = 10 October 2024
  |access-date = 10 October 2024}}</ref>

}} <!-- end reflist -->

{{Use dmy dates|date=October 2019}}
{{Asteroids}}
{{Small Solar System bodies}}
{{Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System}}

[[Category:Aten asteroids|*]]