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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 2019, 1452 Atens have been discovered, of which 12 are named.<ref name="Aten list" /> Many Atens are classified as [[potentially hazardous asteroids]].<ref name="MPC-Aten" /><ref name="neo-jpl-stats" />
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 2019, 1490 Atens have been discovered, of which 12 are named.<ref name="Aten list" /> Many Atens are classified as [[potentially hazardous asteroids]].<ref name="MPC-Aten" /><ref name="neo-jpl-stats" />


== Description ==
== Description ==
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{{See also|Category:Aten asteroids}}
{{See also|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 makes them [[Earth-crossing asteroid|Earth-crossing asteroids]] as the orbit of Earth varies between 0.983 and 1.017&nbsp;AU.
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|Earth-crossing asteroids]] as the orbit of Earth varies between 0.983 and 1.017&nbsp;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 1&nbsp;AU although their semi-major axis is less than 1 AU. Observation of objects [[inferior and superior planets|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 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.
Asteroids' orbits can be highly eccentric. An Aten orbit need not be entirely contained within Earth's orbit, and nearly all known Aten asteroids have an [[aphelion]] greater than 1&nbsp;AU although their semi-major axis is less than 1 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 {{mp|2016 XK|24}} at 0.580&nbsp;AU.<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. For a brief time near the end of 2004, the asteroid [[99942&nbsp;Apophis]] (then known only by its [[provisional designation]] {{mp|2004 MN|4}}) apparently posed a threat of [[impact event|impacting]] Earth in 2029 or 2036, but earlier observations were found that eliminated those possibilities.<ref name="Apophis-Risk" />
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 perihelion of [[Mars]] (1.38 AU). For a brief time near the end of 2004, the asteroid [[99942&nbsp;Apophis]] (then known only by its [[provisional designation]] {{mp|2004 MN|4}}) apparently posed a threat of [[impact event|impacting]] Earth in 2029 or 2036, but earlier observations were found that eliminated those possibilities.<ref name="Apophis-Risk" />


== Atiras ==
== Atiras ==


There are also 18 known [[Atira asteroid]]s (also known as Apoheles asteroids),<ref name="neo-jpl-stats" /><ref name="JPL-Atiras-query" /> traditionally listed as a subgroup of Atens, such as by the [[Minor Planet Center]],<ref name="MPC-Aten" /> but often regarded a separate class of their own.<ref name="NEO-Basics" /> Unlike Atens ([[Aphelion|Q]]&thinsp;&gt;&thinsp;0.983&nbsp;AU), Atiras permanently stay within Earth's orbit and do not cross it ([[Aphelion|Q]]&thinsp;&lt;&thinsp;0.983&nbsp;AU).<ref name="NEO-Basics" />
As of 2019 there are 18 known [[Atira asteroid]]s (also known as Apoheles asteroids),<ref name="neo-jpl-stats" /><ref name="JPL-Atiras-query" /> traditionally listed as a subgroup of Atens,<ref name="MPC-Aten" /> but often regarded as a separate asteroid class.<ref name="NEO-Basics" /> Unlike Atens ([[Aphelion|Q]]&thinsp;&gt;&thinsp;0.983&nbsp;AU), Atiras permanently reside within, and do not cross, Earth's orbit.([[Aphelion|Q]]&thinsp;&lt;&thinsp;0.983&nbsp;AU).<ref name="NEO-Basics" />


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Revision as of 20:29, 3 April 2019

The Aten group compared to the orbits of the terrestrial planets of the Solar System.
  Mars (M)
  Venus (V)
  Mercury (H) 		  Sun
  Aten asteroids
  Earth (E)

The Aten asteroids are a dynamical group of asteroids whose orbits bring them into proximity with Earth. By definition, Atens are Earth-crossing asteroids (a < 1.0 AU and Q > 0.983 AU).[1] The group is named after 2062 Aten, the first of its kind, discovered on 7 January 1976 by American astronomer Eleanor Helin at Palomar Observatory. As of 2019, 1490 Atens have been discovered, of which 12 are named.[2] Many Atens are classified as potentially hazardous asteroids.[3][4]

Description

See also: List of Aten asteroids
See also: 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 AU.[1] This defines them as Earth-crossing asteroids as the orbit of Earth varies between 0.983 and 1.017 AU.

Asteroids' orbits can be highly eccentric. An Aten orbit need not be entirely contained within Earth's orbit, and nearly all known Aten asteroids have an aphelion greater than 1 AU although their semi-major axis is less than 1 AU. Observation of objects 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 asteroid population.[4] Many more 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 AU, for object 2016 XK24.[3] 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, which is greater than the perihelion of Mars (1.38 AU). For a brief time near the end of 2004, the asteroid 99942 Apophis (then known only by its provisional designation 2004 MN4) apparently posed a threat of impacting Earth in 2029 or 2036, but earlier observations were found that eliminated those possibilities.[5]

Atiras

As of 2019 there are 18 known Atira asteroids (also known as Apoheles asteroids),[4][6] traditionally listed as a subgroup of Atens,[3] but often regarded as a separate asteroid class.[1] Unlike Atens (Q > 0.983 AU), Atiras permanently reside within, and do not cross, Earth's orbit.(Q < 0.983 AU).[1]

Definition of NEO subgroups in AU [1]
Group q a = 1 Q ECA
Amors > 1.017 larger – Red XN
Apollos < 1.017 larger – Green tickY
Atens – smaller > 0.983 Green tickY
Atiras – smaller < 0.983 Red XN
For all NEOs q is < 1.3 AU; The orbit of Earth varies between 0.983 and 1.017 AU

See also

References

  1. ^ a b c d e "NEO Basics". NASA/JPL CNEOS. Retrieved 17 May 2018.
  2. ^ "JPL Small-Body Database Search Engine". NASA/JPL. Retrieved 19 January 2019.
  3. ^ a b c "List Of Aten Minor Planets (by perihelion distance)". Minor Planet Center. 17 May 2017. Retrieved 17 May 2017.
  4. ^ a b c "Discovery Statistics – Cumulative Totals". NASA/JPL CNEOS. 16 May 2018. Retrieved 17 May 2018.
  5. ^ "99942 Apophis (2004 MN4) Earth Impact Risk Summary". NASA. Archived from the original on 11 May 2013. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  6. ^ "JPL Small-Body Database Search Engine: Q < 0.983 (AU)". JPL Solar System Dynamics. Retrieved 17 May 2018.

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