| Mars (M) |
| Sun |
Atira asteroids // or Apohele asteroids, also known as interior-Earth objects (IEOs), are asteroids whose orbits are entirely confined within Earth's orbit; 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.
There is no official name for the class commonly referred as Atira asteroids. The term "Apohele asteroids" was proposed by the discoverers of 1998 DK36, after the Hawaiian word for orbit, from apo [ˈɐpo] 'circle' and hele [ˈhɛlɛ] 'to go'. This was suggested partly because of its similarity to the words aphelion (apoapsis) and helios.[a] Other authors adopted the designation "Inner Earth Objects" (IEOs). 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.
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. This means that Atiras can usually only be seen during twilight. 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.
The first suspected Atira asteroid was 1998 DK36, which was discovered by David J. Tholen of the Mauna Kea Observatory, but the first to be confirmed as such was 163693 Atira in 2003, discovered by the Arecibo Observatory. As of February 2023[update], there are 28 known Atiras, two of which are named, eight of which have received a numbered designation, and six of which are potentially hazardous objects. An additional 127 objects have aphelia smaller than Earth's aphelion (Q = 1.017 AU).
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.
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,[b] which contributes to enhanced long-term orbital stability, since there is no libration of the perihelion.
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. 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.
ꞌAylóꞌchaxnim asteroids, which had been provisionally nicknamed "Vatira" asteroids before the first was discovered,[c] are a subclass of Atiras that orbit entirely interior to the orbit of Venus, aka 0.718 AU. Despite their orbits placing them at a significant distance from Earth, they are still classified as near-Earth objects. Observations suggest that ꞌAylóꞌchaxnim asteroids frequently have their orbits altered into Atira asteroids and vice-versa.
First formally theorised to exist by Sarah Greenstreet, Henry Ngo, and Brett Gladman in 2012, the first and to date only such asteroid found is 594913 ꞌAylóꞌchaxnim, which was discovered on 4 January 2020 by the Zwicky Transient Facility. As the archetype, it subsequently gave its name to the class. It has an aphelion of only 0.656 AU, making it the asteroid with the smallest known aphelion.
Main article: Vulcanoid
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.
The following table lists the known and suspected Atiras as of November 2023[update]. 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.
|1998 DK36||0.404||0.6923||0.980||0.4160||2.02||210||1||25.0||35||David J. Tholen||MPC · JPL|
MPC · JPL
|(164294) 2004 XZ130||0.337||0.6176||0.898||0.4546||2.95||177||3564||20.4||300||David J. Tholen||List|
MPC · JPL
|(434326) 2004 JG6||0.298||0.6353||0.973||0.5311||18.94||185||6227||18.5||710||LONEOS||List|
MPC · JPL
|(413563) 2005 TG45||0.428||0.6814||0.935||0.3722||23.33||205||5814||17.6||1,100||Catalina Sky Survey||List|
MPC · JPL
(aka 2006 KZ39)
|0.262||0.6008||0.940||0.5641||10.76||170||5110||20.1||340||Mount Lemmon Survey
|MPC · JPL|
|(613676) 2006 WE4||0.641||0.7848||0.928||0.1829||24.77||254||4995||18.9||590||Mount Lemmon Survey||List|
MPC · JPL
|(418265) 2008 EA32||0.428||0.6159||0.804||0.3050||28.26||177||4794||16.5||1,800||Catalina Sky Survey||List|
MPC · JPL
|(481817) 2008 UL90||0.431||0.6951||0.959||0.3798||24.31||212||4496||18.6||680||Mount Lemmon Survey||List|
MPC · JPL
|2010 XB11||0.288||0.6180||0.948||0.5339||29.89||177||1811||19.9||370||Mount Lemmon Survey||MPC · JPL|
|2012 VE46||0.455||0.7131||0.971||0.3613||6.67||220||2225||20.2||320||Pan-STARRS||MPC · JPL|
|2013 TQ5||0.653||0.7737||0.894||0.1557||16.40||249||2269||19.8||390||Mount Lemmon Survey||MPC · JPL|
|2014 FO47||0.548||0.7522||0.956||0.2712||19.20||238||2779||20.3||310||Mount Lemmon Survey||MPC · JPL|
|2015 DR215||0.352||0.6665||0.981||0.4716||4.08||199||2156||20.4||300||Pan-STARRS||MPC · JPL|
|2017 XA1||0.646||0.8095||0.973||0.2017||17.18||266||1084||21.3||200||Pan-STARRS||MPC · JPL|
(aka 2016 XJ24)
|MPC · JPL|
|2018 JB3||0.485||0.6832||0.882||0.2904||40.39||206||2037||17.7||1,020||Catalina Sky Survey||MPC · JPL|
|2019 AQ3||0.404||0.5887||0.774||0.3143||47.22||165||2175||17.5||1,120||Zwicky Transient Facility||MPC · JPL|
|2019 LF6||0.317||0.5554||0.794||0.4293||29.51||151||796||17.3||1,230||Zwicky Transient Facility||MPC · JPL|
|Zwicky Transient Facility||MPC · JPL|
|2020 HA10||0.692||0.8196||0.947||0.1552||49.65||271||3248||18.9||590||Mount Lemmon Survey||MPC · JPL|
|2020 OV1||0.476||0.6376||0.800||0.2541||32.58||186||1169||18.9||590||Zwicky Transient Facility||MPC · JPL|
|2021 BS1||0.396||0.5984||0.800||0.3377||31.73||169||46||18.5||710||Zwicky Transient Facility||MPC · JPL|
|2021 LJ4||0.416||0.6748||0.933||0.3834||9.83||202||5||20.1||340||Scott S. Sheppard||MPC · JPL|
|2021 PB2||0.610||0.7174||0.825||0.1501||24.83||222||3392||18.8||620||Zwicky Transient Facility||MPC · JPL|
|2021 PH27||0.133||0.4617||0.790||0.7117||31.93||115||1515||17.7||1,020||Scott S. Sheppard||MPC · JPL|
|2021 VR3||0.313||0.5339||0.755||0.4138||18.06||143||1012||18.0||890||Zwicky Transient Facility||MPC · JPL|
|2022 BJ8||0.590||0.7852||0.981||0.2487||15.83||254||102||19.6||430||Kitt Peak-Bok||MPC · JPL|
|2023 EL||0.579||0.7676||0.956||0.2453||13.63||246||9||18.9||580||Scott S. Sheppard||MPC · JPL|
|2023 EY2||0.398||0.6033||0.809||0.3978||35.55||171||6||19.9||370||Kitt Peak-Bok||MPC · JPL|
|2023 WK3||0.321||0.6436||0.966||0.5010||24.63||189||3||20.5||280||Moonbase South Observatory||MPC · JPL|
We have provisionally named objects with 0.307 < Q < 0.718 AU Vatiras, because they are Atiras which are decoupled from Venus. Provisional because it will be abandoned once the first discovered member of this class will be named.