Editing Near-Earth object (section)

==== Orbital classification ====
[[File:Neo orbit types.jpg|thumb|upright=1.4|NEA orbital groups (NASA/JPL)]]
Near-Earth asteroids are divided into groups based on their [[semi-major axis]] (a), [[Apsis|perihelion]] distance (q), and [[Apsis|aphelion]] distance (Q):<ref name="CNEOS-NEO-groups"/><ref name="NEOCC-DA"/>
* The ''[[Atira asteroid|Atiras]]'' or ''Apoheles'' have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983&nbsp;AU). That is, {{nowrap|Q < 0.983 AU}}, which implies that the asteroid's semi-major axis is also less than 0.983 AU.<ref name="atiras">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |date=August 1, 2019 |title=Understanding the evolution of Atira-class asteroid 2019 AQ<sub>3</sub>, a major step towards the future discovery of the Vatira population |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume= 487 |issue= 2 |pages= 2742–2752 |arxiv=1905.08695 |bibcode=2019MNRAS.487.2742D |doi=10.1093/mnras/stz1437 |doi-access=free |s2cid=160009327}}</ref> This group includes asteroids on orbits that never get close to Earth, including the sub-group of [[Atira asteroid#ꞌAylóꞌchaxnim asteroids|ꞌAylóꞌchaxnims]], which orbit the Sun entirely within the orbit of [[Venus (planet)|Venus]]<ref>{{cite journal |last1=Bolin |first1=Bryce T. |display-authors=et al |date=November 2022 |title=The discovery and characterization of (594913) 'Ayló'chaxnim, a kilometre sized asteroid inside the orbit of Venus |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=517 |issue=1 |pages=L49–L54 |doi=10.1093/mnrasl/slac089 |doi-access=free |arxiv=2208.07253}}</ref> and which include the hypothetical sub-group of [[Vulcanoid]]s, which have orbits entirely within the orbit of [[Mercury (planet)|Mercury]].<ref>{{cite news |first=David |last=Dickinson |title=Astronomers Discover Asteroid that Flies Close to the Sun |date=August 25, 2021 |work=Sky & Telescope |url=https://skyandtelescope.org/astronomy-blogs/astronomy-space-david-dickinson/astronomers-discover-asteroid-that-flies-close-to-the-sun/ |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241130073024/https://skyandtelescope.org/astronomy-blogs/astronomy-space-david-dickinson/astronomers-discover-asteroid-that-flies-close-to-the-sun/ |archive-date=November 30, 2024}}</ref>
* The ''[[Aten asteroid|Atens]]'' have a semi-major axis of less than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a < 1.0 AU}} and {{nowrap|Q > 0.983 AU}}. (0.983 AU is Earth's perihelion distance.)
* The ''[[Apollo asteroid|Apollos]]'' have a semi-major axis of more than 1&nbsp;AU and cross Earth's orbit. Mathematically, {{nowrap|a > 1.0 AU}} and {{nowrap|q < 1.017 AU}}. (1.017&nbsp;AU is Earth's aphelion distance.)
* The ''[[Amor asteroid|Amors]]'' have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017&nbsp;AU). Amor asteroids are also near-Earth objects so {{nowrap|q < 1.3 AU}}. In summary, {{nowrap|1.017 AU < q < 1.3 AU}}. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017&nbsp;AU.) Some Amor asteroid orbits cross the orbit of Mars.

Some authors define Atens differently: they define it as being all the asteroids with a semi-major axis of less than 1&nbsp;AU.<ref>{{cite web |title=Unusual Minor Planets |publisher=IAU/MPC |url=https://minorplanetcenter.net/iau/Unusual.html |access-date=January 2, 2025 |url-status=live |archive-url=https://web.archive.org/web/20241130124156/https://minorplanetcenter.net/iau/Unusual.html |archive-date=November 30, 2024}}</ref><ref name="galache"/> That is, they consider the Atiras to be part of the Atens.<ref name="galache">{{cite web |first=J. L. |last=Galache |title=Asteroid Classification I – Dynamics |publisher=IAU/MPC |date=March 5, 2011 |url=http://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ |access-date=March 9, 2018 |url-status=dead |archive-url=https://web.archive.org/web/20160303235814/http://minorplanetcenter.net/blog/asteroid-classification-i-dynamics/ |archive-date=March 3, 2016}}</ref> Historically, until 1998, there were no known or suspected Atiras, so the distinction wasn't necessary.

Atiras and Amors do not cross the Earth's orbit and are not immediate impact threats, but their orbits may change to become Earth-crossing orbits in the future.<ref name="MorbidelliAstIII"/><ref>{{Cite journal |last1=Ribeiro |first1=A. O. |last2=Roig |first2=F.|last3=De Prá |first3=M. N. |last4=Carvano |first4=J. M. |last5=DeSouza |first5=S. R. |title=Dynamical study of the Atira group of asteroids |date=March 17, 2016 |journal=Monthly Notices of the Royal Astronomical Society |volume=458 |issue=4 |pages=4471–4476 |doi=10.1093/mnras/stw642 |issn=0035-8711 |doi-access=free}}</ref>

{{As of|2024|12|30|df=US}}, 34 Atiras, 2,952 Atens, 21,132 Apollos and 13,137 Amors have been discovered and cataloged.<ref name="neo-jpl-stats" />