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 Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

's orbit; that is, their orbit has an

aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

(farthest point from the Sun) smaller than Earth's

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

(nearest point to the Sun), which is 0.983

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...

s (AU). Atira asteroids are by far the least numerous group of

near-Earth object A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical units (AU). ...

s, compared to the more populous

Aten Aten also Aton, Atonu, or Itn ( egy, jtn, ''reconstructed'' ) was the focus of Atenism, the religious system established in ancient Egypt by the Eighteenth Dynasty pharaoh Akhenaten. The Aten was the disc of the sun and originally an aspect of ...

Apollo Apollo, grc, Ἀπόλλωνος, Apóllōnos, label=genitive , ; , grc-dor, Ἀπέλλων, Apéllōn, ; grc, Ἀπείλων, Apeílōn, label=Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, Áploun, la, Apollō, la, Apollinis, label= ...

and 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 word for

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

, 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 The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

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 The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

and as such are 'drowned out' due to the Sun's overpowering light. This means that Atiras can usually only be seen during

twilight Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this il ...

. 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 The Mauna Kea Observatories (MKO) are a group of independent astronomical research facilities and large telescope observatories that are located at the summit of Mauna Kea on the Big Island of Hawaiʻi, United States. The facilities are located ...

, 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 The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...

and were driven to their current locations as a result of

gravitational perturbation In astronomy, perturbation is the complex motion of a massive body subjected to forces other than the gravitational attraction of a single other massive body. The other forces can include a third (fourth, fifth, etc.) body, resistance, as from ...

, as well as other causes such as the

Yarkovsky effect The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km i ...

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, Kozai ...

, 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 Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

, 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, which was discovered on 4 January 2020 by the

Zwicky Transient Facility The Zwicky Transient Facility (ZTF, obs. code: I41) is a wide-field sky astronomical survey using a new camera attached to the Samuel Oschin Telescope at the Palomar Observatory in California, United States. Commissioned in 2018, it supersedes ...

. 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, 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 Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

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