Atira asteroid

File:Neo orbit types.jpg

Common orbital subgroups of Near-Earth Objects (NEOs)

Atira asteroids Template:IPAc-en or Apohele asteroids, also known as interior-Earth objects (IEOs), are Near-Earth objects whose orbits are entirely confined within Earth's orbit;<ref name="NEO-groups" /> that is, their orbit has an aphelion (farthest point from the Sun) smaller than Earth's perihelion (nearest point to the Sun), which is 0.983 astronomical units (AU). Atira asteroids are by far the least numerous group of near-Earth objects, compared to the more populous Aten, Apollo and Amor asteroids.<ref name="neo-jpl-stats" />

History

Naming

There is no official name for the class commonly referred as Atira asteroids. The term "Apohele asteroids" was proposed by the discoverers of Template:Mpl,<ref name="Tholen-1998" /> after the Hawaiian word for orbit, from apo {{#invoke:IPA|main}} 'circle' and hele {{#invoke:IPA|main}} 'to go'.<ref>(Ulukau Hawaiian Electronic Library)</ref> This was suggested partly because of its similarity to the words aphelion (apoapsis) and helios.Template:Efn Other authors adopted the designation "Inner Earth Objects" (IEOs).<ref name="Michel-2000" /> Following the general practice to name a new class of asteroids for the first recognized member of that class, which in this case was 163693 Atira, the designation of "Atira asteroids" was largely adopted by the scientific community, including by NASA.<ref name="Ribeiro">Template:Cite journal</ref><ref name="NEO-groups" />

Discovery and observation

Their location inside the Earth's orbit makes Atiras very difficult to observe, as from Earth's perspective they are close to the Sun and therefore 'drowned out' by the Sun's overpowering light.<ref name=Sparkle/> This means that Atiras can usually only be seen during twilight.<ref name=Sparkle/> The first documented twilight searches for asteroids inside Earth's orbit were performed by astronomer Robert Trumpler over the early 20th century, but he failed to find any.<ref name=Sparkle/>

The first confirmed Atira asteroid was 163693 Atira in 2003, discovered by the Lincoln Laboratory Near Earth Asteroid Research Team.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Template:As of, there are 34 known Atiras, two of which are named, nine of which have received a numbered designation, and seven of which are potentially hazardous objects.<ref name="neo-jpl-stats" /><ref name="jpl-search-by-smallest-aphelion"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Origins

Most Atira asteroids originated in the asteroid belt and were driven to their current locations as a result of gravitational perturbation, as well as other causes such as the Yarkovsky effect.<ref name=Sparkle>Template:Cite journal</ref> A number of known Atiras could be fragments or former moons of larger Atiras as they exhibit an unusually high level of orbital correlation.<ref name="de la Fuente Marcos-2023">Template:Cite journal</ref>

Orbits

Atiras do not cross Earth's orbit and are not immediate impact event threats, but their orbits may be perturbed outward by a close approach to either Mercury or Venus and become Earth-crossing asteroids in the future. The dynamics of many Atira asteroids resemble the one induced by the Kozai-Lidov mechanism,Template:Efn which contributes to enhanced long-term orbital stability, since there is no libration of the perihelion.<ref name=FuenteMarcos20190611>Template:Cite journal</ref><ref name=FuenteMarcos20190801>Template:Cite journal</ref>

Exploration

A 2017 study published in the journal Advances in Space Research proposed a low-cost space probe be sent to study Atira asteroids, citing the difficulty in observing the group from Earth as a reason to undertake the mission.<ref name=Explore>Template:Cite journal</ref> The study proposed that the mission would be powered by spacecraft electric propulsion and would follow a path designed to flyby as many Atira asteroids as possible. The probe would also attempt to discover new NEO's that may pose a threat to Earth.<ref name=Explore/>

Related asteroid groups

ꞌAylóꞌchaxnim asteroids

ꞌAylóꞌchaxnim asteroids, which had been provisionally nicknamed "Vatira" asteroids before the first was discovered,Template:Efn are a subclass of Atiras that orbit entirely interior to the orbit of Venus, aka 0.718 AU.<ref name=Bolin/> Despite their orbits placing them at a significant distance from Earth, they are still classified as near-Earth objects.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Observations suggest that ꞌAylóꞌchaxnim asteroids frequently have their orbits altered into Atira asteroids and vice versa.<ref>Template:Cite journal</ref>

First formally theorised to exist by William F. Bottke and Gianluca Masi in 2002 and 2003,<ref name=Bottke2002>Template:Cite journal</ref><ref name=Masi2003>Template:Cite journal</ref> the first and to date only such asteroid found is 594913 ꞌAylóꞌchaxnim,<ref name=vt20200109>Template:Cite news</ref><ref name="Popescuetal2020"/> which was discovered on 4 January 2020 by the Zwicky Transient Facility. As the archetype, it subsequently gave its name to the class.<ref name=Bolin>Template:Cite journal</ref> It has an aphelion of only 0.656 AU, the smallest of any known asteroid.<ref name="jpl-search-by-smallest-aphelion" /><ref name=FuenteMarcos20190611/>

Vulcanoids

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} No asteroids have yet been discovered to orbit entirely inside the orbit of Mercury (q = 0.307 AU). Such hypothetical asteroids would likely be termed vulcanoids, although the term often refers to asteroids which more specifically have remained in the intra-Mercurian region over the age of the Solar System.<ref name=Greenstreet2012/>

Members

The following table lists the known and suspected Atiras Template:As of. 594913 ꞌAylóꞌchaxnim, due to its unique classification, has been highlighted in pink. The interior planets Mercury and Venus have been included for comparison as grey rows.

List of known and suspected Atiras as of November 2024 (Q < 0.983 AU)<ref name="jpl-search-by-smallest-aphelion" />

Designation	Perihelion (AU)	Semi-major axis (AU)	Aphelion (AU)	Eccentricity	Inclination (°)	Period (days)	Observation arc (days)	(H)	Diameter(A) (m)	Discoverer	Ref
Mercury Template:Small	0.307	0.3871	0.467	0.2056	7.01	88	NA	−0.6	Template:Sort	NA
Venus Template:Small	0.718	0.7233	0.728	0.0068	3.39	225	NA	−4.5	Template:Sort	NA
Template:Mpl	0.404	0.6923	0.980	0.4160	2.02	210	1	25.0	Template:Sort	David J. Tholen	Template:M+J
163693 Atira	0.502	0.7410	0.980	0.3222	25.62	233	6601	16.3	Template:Sort	LINEAR	Template:LoMP Template:M+J
Template:Mp	0.337	0.6176	0.898	0.4546	2.95	177	3564	20.4	Template:Sort	David J. Tholen	Template:LoMP Template:M+J
Template:Mpl	0.298	0.6353	0.973	0.5311	18.94	185	6227	18.5	Template:Sort	LONEOS	Template:LoMP Template:M+J
Template:Mp	0.428	0.6814	0.935	0.3722	23.33	205	5814	17.6	Template:Sort	Catalina Sky Survey	Template:LoMP Template:M+J
Template:Mpl Template:Small	0.262	0.6008	0.940	0.5641	10.76	170	5110	20.1	Template:Sort	Mount Lemmon Survey Pan-STARRS	Template:M+J
Template:Mp	0.641	0.7848	0.928	0.1829	24.77	254	4995	18.9	Template:Sort	Mount Lemmon Survey	Template:LoMP Template:M+J
Template:Mp	0.428	0.6159	0.804	0.3050	28.26	177	4794	16.5	Template:Sort	Catalina Sky Survey	Template:LoMP Template:M+J
Template:Mp	0.431	0.6951	0.959	0.3798	24.31	212	4496	18.6	Template:Sort	Mount Lemmon Survey	Template:LoMP Template:M+J
Template:Mp	0.288	0.6180	0.948	0.5339	29.89	177	1811	19.9	Template:Sort	Mount Lemmon Survey	Template:M+J
Template:Mp	0.455	0.7131	0.971	0.3613	6.67	220	2225	20.2	Template:Sort	Pan-STARRS	Template:M+J
Template:Mp	0.653	0.7737	0.894	0.1557	16.40	249	2269	19.8	Template:Sort	Mount Lemmon Survey	Template:M+J
Template:Mp	0.548	0.7522	0.956	0.2712	19.20	238	2779	20.3	Template:Sort	Mount Lemmon Survey	Template:M+J
Template:Mpl	0.352	0.6665	0.981	0.4716	4.08	199	2156	20.4	Template:Sort	Pan-STARRS	Template:M+J
Template:Mp	0.646	0.8095	0.973	0.2017	17.18	266	1084	21.3	Template:Sort	Pan-STARRS	Template:M+J
Template:Mp Template:Small	0.328	0.6343	0.940	0.4825	19.85	185	1127	18.4	Template:Sort	Spacewatch ATLAS	Template:M+J
Template:Mp	0.485	0.6832	0.882	0.2904	40.39	206	2037	17.7	Template:Sort	Catalina Sky Survey	Template:M+J
Template:Mpl	0.404	0.5887	0.774	0.3143	47.22	165	2175	17.5	Template:Sort	Zwicky Transient Facility	Template:M+J
Template:Mpl	0.317	0.5554	0.794	0.4293	29.51	151	796	17.3	Template:Sort	Zwicky Transient Facility	Template:M+J
594913 ꞌAylóꞌchaxnim	0.457	0.5554	0.654	0.1770	15.87	151	609	16.2	Template:Sort	Zwicky Transient Facility	Template:M+J
Template:Mpl	0.692	0.8196	0.947	0.1552	49.65	271	3248	18.9	Template:Sort	Mount Lemmon Survey	Template:M+J
Template:Mpl	0.476	0.6376	0.800	0.2541	32.58	186	1169	18.9	Template:Sort	Zwicky Transient Facility	Template:M+J
Template:Mp	0.396	0.5984	0.800	0.3377	31.73	169	46	18.5	Template:Sort	Zwicky Transient Facility	Template:M+J
Template:Mp	0.416	0.6748	0.933	0.3834	9.83	202	5	20.1	Template:Sort	Scott S. Sheppard	Template:M+J
Template:Mp	0.610	0.7174	0.825	0.1501	24.83	222	3392	18.8	Template:Sort	Zwicky Transient Facility	Template:M+J
Template:Mpl	0.133	0.4617	0.790	0.7117	31.93	115	1515	17.7	Template:Sort	Scott S. Sheppard	Template:M+J
Template:Mp	0.313	0.5339	0.755	0.4138	18.06	143	1012	18.0	Template:Sort	Zwicky Transient Facility	Template:M+J
Template:Mp	0.590	0.7852	0.981	0.2487	15.83	254	102	19.6	Template:Sort	Kitt Peak-Bok	Template:M+J
2023 EL	0.579	0.7676	0.956	0.2453	13.63	246	9	18.9	Template:Sort	Scott S. Sheppard	Template:M+J
Template:Mp	0.398	0.6033	0.809	0.3978	35.55	171	6	19.9	Template:Sort	Kitt Peak-Bok	Template:M+J
Template:Mpl	0.321	0.6436	0.966	0.5010	24.63	189	3	20.5	Template:Sort	Moonbase South Observatory	Template:M+J
Template:Mp	0.803	0.8608	0.919	0.0675	21.14	291	5	20.8	Template:Sort	Mount Lemmon Survey	Template:M+J

^(A) All diameter estimates are based on an assumed albedo of 0.14 (except 163693 Atira, for which the size has been directly measured)

^(B) Binary asteroid

See also

• List of minor planet groups
• List of minor planets

External links

• List Of Aten Minor Planets, Minor Planet Center

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