Naming

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,^[3] after the Hawaiian word for orbit, from apo [ˈɐpo] 'circle' and hele [ˈhɛlɛ] 'to go'.^[4] This was suggested partly because of its similarity to the words aphelion (apoapsis) and helios.^{[lower-alpha 1]} Other authors adopted the designation "Inner Earth Objects" (IEOs).^[5] 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.^[6][1]

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.^[7] This means that Atiras can usually only be seen during twilight.^[7] 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.^[7]

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.^[2][8][9] An additional 127 objects have aphelia smaller than Earth's aphelion (Q = 1.017 AU).^[10]

ꞌAylóꞌchaxnim asteroids

ꞌAylóꞌchaxnim asteroids, which had been provisionally nicknamed "Vatira" asteroids before the first was discovered,^{[lower-alpha 3]} are a subclass of Atiras that orbit entirely interior to the orbit of Venus, aka 0.718 AU.^[16] Despite their orbits placing them at a significant distance from Earth, they are still classified as near-Earth objects.^[17] Observations suggest that ꞌAylóꞌchaxnim asteroids frequently have their orbits altered into Atira asteroids and vice-versa.^[18]

First formally theorised to exist by William F. Bottke and Gianluca Masi in 2002 and 2003,^[19][20] the first and to date only such asteroid found is 594913 ꞌAylóꞌchaxnim,^[21][22] which was discovered on 4 January 2020 by the Zwicky Transient Facility. As the archetype, it subsequently gave its name to the class.^[16] It has an aphelion of only 0.656 AU, making it the asteroid with the smallest known aphelion.^[8][12]

Vulcanoids

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.^[15]