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 smallest group of near-Earth objects, compared to the Aten, Apollo and Amor asteroids.
The first suspected was 1998 DK36, and the first confirmed, was 163693 Atira in 2003. As of August 2021[update], there are 26 known Atiras, of which 19 have robust orbit determinations, of which six have been computed with sufficient precision to receive a permanent number. An additional 92 objects (not listed below) have aphelia smaller than Earth's aphelion (Q = 1.017 AU).
The Near Earth Object Surveillance Satellite is intended to find more.
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. Although the dynamics of many of these objects somehow resemble the one induced by the Kozai-Lidov mechanism (coupled oscillations in eccentricity and inclination), which contributes to enhanced long-term stability, there is no libration of the value of the argument of perihelion.
Vatira asteroids are a subclass of Atiras that orbit entirely interior to the orbit of Venus (whose perihelion q = 0.718 AU). The name "Vatira" is provisional, combining "Venus" with "Atira". They were theorized to exist at least since 2012, and in early 2020, the first Vatira asteroid was discovered: (594913) 2020 AV2. It was discovered on 4 January 2020 by the Zwicky Transient Facility. Its aphelion distance is only 0.656 AU. It will receive a name that will be used to refer to this population. As of January 2020[update], it is the asteroid with the smallest known aphelion, followed by 2019 AQ3 with Q = 0.774 AU and 2019 LF6 with Q = 0.794 AU.
No asteroids have yet been discovered to orbit entirely inside the orbit of Mercury (q = 0.307 AU). Such asteroids would likely be termed vulcanoids, although the term currently often refers only to asteroids that have remained in the intra-Mercurian region over the age of the solar system.
There is no standard name for the class. Following the general practice to name a new class of asteroids for the first recognized member of that class, some astronomers, and this article, use the designation Atira asteroids.
The name Apohele was proposed by the discoverers of 1998 DK36, and is the Hawaiian word for orbit, from apo [ˈɐpo] 'circle' and hele [ˈhɛlɛ] 'to go'; it was chosen partially because of its similarity to the words aphelion (apoapsis) and helios.[a]
Other authors adopted the designation Inner Earth Objects (IEOs).
The following table lists the known and suspected Atiras as of August 2021. The sole known Vatira asteroid (594913) 2020 AV2 has been highlighted in pink. The planets Mercury and Venus have been included for comparison (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||0.6352||0.973||0.5312||18.94||185||4035||18.4||740||LONEOS||List|
MPC · JPL
|(413563) 2005 TG45||0.428||0.6814||0.935||0.3722||23.34||205||4744||17.6||1,100||Catalina Sky Survey||List|
MPC · JPL
(aka 2006 KZ39)
|0.262||0.6008||0.940||0.5642||10.76||170||2893||20.1||340||Mount Lemmon Survey
|MPC · JPL|
|2006 WE4||0.641||0.7847||0.928||0.1829||24.77||254||4081||18.9||590||Mount Lemmon Survey||MPC · JPL|
|(418265) 2008 EA32||0.428||0.6159||0.804||0.3050||28.26||177||3126||16.5||1,800||Catalina Sky Survey||List|
MPC · JPL
|(481817) 2008 UL90||0.431||0.6950||0.959||0.3798||24.31||212||3441||18.7||650||Mount Lemmon Survey||List|
MPC · JPL
|2010 XB11||0.288||0.618||0.948||0.5339||29.88||177||1811||19.9||450||Mount Lemmon Survey||MPC · JPL|
|2012 VE46||0.455||0.7129||0.971||0.3615||6.67||220||1135||20.2||320||Pan-STARRS||MPC · JPL|
|2013 TQ5||0.653||0.7737||0.894||0.1556||16.40||249||805||19.8||390||Mount Lemmon Survey||MPC · JPL|
|2014 FO47||0.548||0.7521||0.956||0.2711||19.20||238||1407||20.3||310||Mount Lemmon Survey||MPC · JPL|
|2015 DR215||0.352||0.6664||0.981||0.4716||4.09||199||404||20.3||310||Pan-STARRS||MPC · JPL|
|2017 XA1||0.646||0.8096||0.973||0.2015||17.18||266||1084||21.2||200||Pan-STARRS||MPC · JPL|
(aka 2016 XJ24)
|MPC · JPL|
|2018 JB3||0.485||0.6832||0.882||0.2905||40.39||206||419||17.6||1,070||Catalina Sky Survey||MPC · JPL|
|2019 AQ3||0.404||0.5887||0.774||0.3143||47.22||165||1199||17.4||1,200||Zwicky Transient Facility||MPC · JPL|
|2019 LF6||0.317||0.5554||0.794||0.4293||29.51||151||358||17.2||1,300||Zwicky Transient Facility||MPC · JPL|
|(594913) 2020 AV2||0.457||0.5554||0.654||0.1770||15.87||151||560||16.4||2,000||Zwicky Transient Facility||MPC · JPL|
|2020 HA10||0.694||0.8204||0.947||0.1544||49.66||271||5||19.1||540||Mount Lemmon Survey||MPC · JPL|
|2020 OV1||0.475||0.6375||0.800||0.2543||32.58||186||18||18.7||650||Zwicky Transient Facility||MPC · JPL|
|2021 BS1||0.402||0.6015||0.801||0.3310||31.23||170||8||18.5||710||Zwicky Transient Facility||MPC · JPL|
|2021 LJ4||0.416||0.6748||0.933||0.4161||9.83||202||5||20.0||360||Scott S. Sheppard||MPC · JPL|
|2021 PB2||0.606||0.7160||0.826||0.1535||25.14||221||4||18.7||640||Zwicky Transient Facility||MPC · JPL|
|2021 PH27||0.134||0.4614||0.789||0.7093||31.66||114||5||17.7||1,020||Scott S. Sheppard||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.