Apohele asteroid

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 asteroid, Amor asteroids.

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 , after the Hawaiian language, Hawaiian word for orbit, from ''apo'' 'circle' and ''hele'' 'to go'. This was suggested partly because of its similarity to the words ''aphelion (apoapsis)'' and ''helios''. 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.

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 as such are 'drowned out' due to 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 , 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. , there are 28 known Atiras, of which 24 have robust orbit determinations; seven of these 24 have been computed with sufficient precision to receive a permanent minor planet designation. An additional 127 objects have aphelia smaller than Earth's aphelion (Q = 1.017 AU).

Origins

Most Atira asteroids originated in the asteroid belt and were driven to their current locations as a result of Perturbation (astronomy), gravitational perturbation, as well as other causes such as the Yarkovsky effect.

Orbits

Atiras do not cross Earth's orbit and are not immediate impact event threats, but their orbits may be Perturbation (astronomy), 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 mechanism, Kozai-Lidov mechanism, which contributes to enhanced long-term orbital stability, since there is no libration of the perihelion.

Related asteroid groups

ꞌAylóꞌchaxnim asteroids

ꞌAylóꞌchaxnim asteroids, which had been provisionally nicknamed "Vatira" asteroids before the first was discovered, 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.

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.

Vulcanoids

No asteroids have yet been discovered to orbit entirely inside the orbit of Mercury (planet), Mercury (q = 0.307 AU). Such hypothetical asteroids would likely be termed vulcanoid asteroid, 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.

Members

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

:^(A) All diameter estimates are based on an assumed Geometric albedo, 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

Notes

References

External links

List Of Aten Minor Planets
Minor Planet Center

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Atira asteroids, *
Lists of asteroids