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

Near-Earth objects A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit aro ...

whose orbits are entirely confined within

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

'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. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

(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. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

(nearest point to the Sun), which is 0.983

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

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 orbiting the Sun whose closest approach to the Sun ( perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit a ...

s, compared to the more populous

Aten Aten, also Aton, Atonu, or Itn (, reconstructed ) was the focus of Atenism, the religious system formally established in ancient Egypt by the late Eighteenth Dynasty pharaoh Akhenaten. Exact dating for the Eighteenth Dynasty is contested, thou ...

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

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 (also known as orbital revolution) 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 ...

, 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 (; Minor planet provisional designation, 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 orbiti ...

, 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 agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

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 centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

and therefore 'drowned out' by the Sun's overpowering light. This means that Atiras can usually only be seen during

twilight Twilight is daylight illumination produced by diffuse sky radiation when the Sun is below the horizon as sunlight from the upper atmosphere is scattered in a way that illuminates both the Earth's lower atmosphere and also the Earth's surf ...

. 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. The first confirmed Atira asteroid was 163693 Atira in 2003, discovered by the Lincoln Laboratory Near Earth Asteroid Research Team. , there are 34 known Atiras, two of which are named, nine of which have received a numbered designation, and seven of which are

potentially hazardous object A potentially hazardous object (PHO) is a near-Earth object – either an asteroid or a comet – with an orbit that can make close approaches to the Earth and which is large enough to cause significant regional damage in the event of impact. Th ...

Origins

Most Atira asteroids originated in the

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

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 fro ...

, 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 ...

. A number of known Atiras could be fragments or former moons of larger Atiras as they exhibit an unusually high level of orbital correlation.

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 been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effe ...

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

libration In lunar astronomy, libration is the cyclic variation in the apparent position of the Moon that is perceived by observers on the Earth and caused by changes between the orbital and rotational planes of the moon. It causes an observer to see ...

of the perihelion.

Exploration

A 2017 study published in the journal '' Advances in Space Research'' proposed a low-cost

space probe Uncrewed spacecraft or robotic spacecraft are spacecraft without people on board. Uncrewed spacecraft may have varying levels of autonomy from human input, such as remote control, or remote guidance. They may also be autonomous, in which th ...

be sent to study Atira asteroids, citing the difficulty in observing the group from Earth as a reason to undertake the mission. The study proposed that the mission would be powered by

spacecraft electric propulsion Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generating thrust to modify the velocity of a ...

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.

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 often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

, aka 0.718 AU. Despite their orbits placing them at a significant distance from Earth, they are still classified as near-Earth objects. Observations suggest that ꞌAylóꞌchaxnim asteroids frequently have their orbits altered into Atira asteroids and vice versa. First formally theorised to exist by William F. Bottke and Gianluca Masi in 2002 and 2003, 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 thu ...

, which was discovered on 4 January 2020 by the

Zwicky Transient Facility The Zwicky Transient Facility (ZTF, List of observatory codes, obs. code: IAU code#I41, I41) is a wide-field sky astronomical survey using a new camera attached to the Samuel Oschin Telescope at Palomar Observatory in San Diego County, Californi ...

. As the archetype, it subsequently gave its name to the class. It has an aphelion of only 0.656 AU, the smallest of any known asteroid.

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 The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

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 fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

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