Atira asteroids/əˈtɪrə/orApohele asteroids, also known as interior-Earth objects (IEOs), are asteroids 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
The first suspected Atira asteroid was 1998 DK36, but the first to be confirmed was 163693 Atira in 2003, which subsequently gave its name to the class. As of February 2022[update], there are 28 known Atiras,[2] of which 24 have robust orbit determinations, of which seven have been computed with sufficient precision to receive a permanent number.[7] An additional 127 objects (not listed below) have aphelia smaller than Earth's aphelion (Q = 1.017 AU).[8]
Origins
Most Atira asteroids originated in the Asteroid Belt, being driven to their current locations as a result of gravitational perturbation, as well as other causes such as the Yarkovsky effect.[9]
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. 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.[10][11]
Related asteroid groups
ꞌAylóꞌchaxnim asteroids
ꞌAylóꞌchaxnim asteroids, which had been provisionally nicknamed "Vatira" asteroids before the first was discovered,[b] are a subclass of Atiras that orbit entirely interior to the orbit of Venus, aka 0.718 AU.[12] Despite their orbits placing them at a significant distance from Earth, they are still classified as near-Earth objects.[13]
The first and to date only such asteroid discovered is 594913 ꞌAylóꞌchaxnim,[14][15] discovered on 4 January 2020 by the Zwicky Transient Facility, which as the prototype 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.[7][10]
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.[17]
Members
The following table lists the known and suspected Atiras as of February 2022[update]. The sole known Vatira asteroid 594913 ꞌAylóꞌchaxnim has been highlighted in pink. The planets Mercury and Venus have been included for comparison (grey rows).
List of known and suspected Atiras as of February 2021 (Q < 0.983 AU)[7]
^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
^The nickname "Vatira" combined "Venus" with "Atira"
^Tholen, D. J.; Whiteley, R. J. (September 1998). "Results From NEO Searches At Small Solar Elongation". American Astronomical Society. 30: 1041. Bibcode:1998DPS....30.1604T.
^Michel, Patrick; Zappalà, Vincenzo; Cellino, Alberto; Tanga, Paolo (February 2000). "NOTE: Estimated Abundance of Atens and Asteroids Evolving on Orbits between Earth and Sun". Icarus. 143 (2): 421–424. Bibcode:2000Icar..143..421M. doi:10.1006/icar.1999.6282.
