Atira asteroids/əˈtɪrə/ or Apohele asteroids, also known as interior-Earth objects (IEOs), are Near-Earth objects whose orbits are entirely confined within Earth's orbit;[1] 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.[2]
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 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.[a] 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 confirmed Atira asteroid was 163693 Atira in 2003, discovered by the Lincoln Laboratory Near Earth Asteroid Research Team.[8] As of October 2024[update], there are 32 known Atiras, two of which are named, nine of which have received a numbered designation, and seven of which are potentially hazardous objects.[2][9][10] An additional 127 objects have aphelia smaller than Earth's aphelion (Q = 1.017 AU).[11]
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.[7] A number of known Atiras could be fragments or former moons of larger Atiras as they exhibit an unusually high level of orbital correlation.[12]
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,[b] which contributes to enhanced long-term orbital stability, since there is no libration of the perihelion.[13][14]
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.[15] 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.[15]
Related asteroid groups
ꞌAylóꞌchaxnim asteroids
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.[17] Despite their orbits placing them at a significant distance from Earth, they are still classified as near-Earth objects.[18] Observations suggest that ꞌAylóꞌchaxnim asteroids frequently have their orbits altered into Atira asteroids and vice-versa.[19]
First formally theorised to exist by William F. Bottke and Gianluca Masi in 2002 and 2003,[20][21] the first and to date only such asteroid found is 594913 ꞌAylóꞌchaxnim,[22][23] which was discovered on 4 January 2020 by the Zwicky Transient Facility. As the archetype, it subsequently gave its name to the class.[17] It has an aphelion of only 0.656 AU, the smallest of any known asteroid.[9][13]
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.[16]
Members
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.
List of known and suspected Atiras as of February 2021 (Q < 0.983 AU)[9]
^Cambridge Conference Correspondence, (2): WHAT'S IN A NAME: APOHELE = APOAPSIS & HELIOS – from Dave Tholen, Cambridge Conference Network (CCNet) DIGEST, 9 July 1998 Benny, Duncan Steel has already brought up the subject of a class name for objects with orbits interior to the Earth's. To be sure, we've already given that subject some thought. I also wanted a word that begins with the letter "A", but there was some desire to work Hawaiian culture into it. I consulted with a friend of mine that has a master's degree in the Hawaiian language, and she recommended "Apohele", the Hawaiian word for "orbit". I found that an interesting suggestion, because of the similarity to fragments of "apoapsis" and "helios", and these objects would have their apoapsis closer to the Sun than the Earth's orbit. By the way, the pronunciation would be like "ah-poe-hey-lay". Rob Whiteley has suggested "Aliʻi", which refers to the Hawaiian elite,
which provides a rich bank of names for discoveries in this class, such as Kuhio, Kalakaua, Kamehameha, Liliuokalani, and so on. Unfortunately, I think the okina (the reverse apostrophe) would be badly treated by most people. I wasn't planning to bring it up at this stage, but because Duncan has
already done so, here's what we've got on the table so far. I'd appreciate some feedback on the suggestions. --Dave
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