Atira asteroids or Apohele asteroids, also known as interior-Earth objects (IEOs), are

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

s 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 Amor ("love" in Latin, Spanish and Portuguese) may refer to: Music Albums * ''Amor'' (Julio Iglesias album), 1982 * ''Amor'' (Andrea Bocelli album), 2006 Songs * "Amor" (Los Auténticos Decadentes song), 2000 * "Amor" (Cristian Castro song), 199 ...

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 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 163693 Atira , provisional designation , is a stony asteroid, dwelling in the interior of Earth's orbit. It is classified as a near-Earth object. ''Atira'' is a binary asteroid, a system of two asteroids orbiting their common barycenter. The prim ...

, 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 Robert Julius Trumpler (until 1915 Robert Trümpler, born October 2, 1886 in Zürich, Switzerland; died September 10, 1956 in Berkeley, California, Berkeley, United States) was a Switzerland, Swiss-United States, American astronomer. Career After ...

over the early 20th century, but he failed to find any. The first suspected Atira asteroid was , which was discovered by

David J. Tholen David James Tholen (born 1955) is an American astronomer at the Institute for Astronomy of the University of Hawaii. He holds a 1984 PhD from the University of Arizona, and specializes in planetary and Solar System astronomy. He is a discovere ...

of the Mauna Kea Observatory, but the first to be confirmed as such was 163693 Atira in 2003, discovered by the

Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...

. , 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 A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur, trans-Neptunian object and dwarf planet but not comet). Such designation always features a leading number (catalog or ...

. 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 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 An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...

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 In celestial mechanics, the Kozai mechanism is a dynamical phenomenon affecting the orbit of a binary system perturbed by a distant third body under certain conditions. It is also known as the von Zeipel-Kozai-Lidov, Lidov–Kozai mechanism, Koza ...

, which contributes to enhanced long-term orbital stability, since there is no

libration In lunar astronomy, libration is the wagging or wavering of the Moon perceived by Earth-bound observers and caused by changes in their perspective. It permits an observer to see slightly different hemispheres of the surface at different tim ...

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 Brett James Gladman (born April 19, 1966) is a Canadian astronomer and a full professor at the University of British Columbia's Department of Physics and Astronomy in Vancouver, British Columbia. He holds the Canada Research Chair in planetary a ...

in 2012, the first and to date only such asteroid found is

594913 ꞌAylóꞌchaxnim 594913 ꞌAylóꞌchaxnim (provisional designation ) is a large near-Earth asteroid discovered by the Zwicky Transient Facility on 4 January 2020. It is the first asteroid discovered to have an orbit completely within Venus's orbit, and is thus ...

, 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 Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

(q = 0.307 AU). Such hypothetical asteroids would likely be termed

vulcanoids The vulcanoids are a hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, which was proposed on the basis of irregulariti ...

, 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 interior planets Mercury and Venus have been included for comparison as grey rows. :^(A) All diameter estimates are based on an assumed albedo of 0.14 (except 163693 Atira, for which the size has been directly measured) :^(B) Binary asteroid

Notes

References

External links

List Of Aten Minor Planets
Minor Planet Center {{DEFAULTSORT:Apohele Asteroid * Lists of asteroids