163693 Atira , provisional designation , is a stony

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. ...

, dwelling in the interior of Earth's orbit. It is classified as a

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). ...

. ''Atira'' is a binary asteroid, a system of two asteroids orbiting their common barycenter. The primary component with a diameter of approximately is orbited by a minor-planet moon that measures about . ''Atira'' was discovered on 11 February 2003, by astronomers with the Lincoln Near-Earth Asteroid Research at

Lincoln Laboratory's Experimental Test Site The MIT Lincoln Laboratory, located in Lexington, Massachusetts, is a United States Department of Defense federally funded research and development center chartered to apply advanced technology to problems of national security. Research and dev ...

near Socorro, New Mexico, in the United States. It is the namesake and the first numbered body of the

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

, a new subclass of near-Earth asteroids, which have their orbits entirely within that of Earth and are therefore alternatively called Interior-Earth Objects (IEO). As of 2019, there are only 36 known members of the Atira group of asteroids. Atiras are similar to the larger group of Aten asteroids, as both are near-Earth objects and both have a

semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...

smaller than that of Earth (< 1.0 AU). However, and contrary to Aten asteroids, the aphelion for Atiras is always smaller than Earth's perihelion (< 0.983 AU), which means that they do not approach Earth as close as Atens do in general. ''Atira'' has an Earth minimum orbit intersection distance of or approximately 80.1 lunar distances.

Physical properties

''Atira'' is a S-type asteroid and orbits the Sun at a distance of 0.5–1.0 AU once every 8 months (233 days). Its orbit has an eccentricity of 0.32 and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

of 26 ° with respect to the ecliptic. With a perihelion of 0.50 AU the body also classifies as a

Venus-crosser A Venus-crosser is an asteroid whose orbit crosses that of Venus. There are 2,809

as Venus orbits the Sun at a distance of 0.72–0.73 AU but does not get as close to the Sun as Mercury (which orbits between 0.31 and 0.47 AU). As no precoveries were found, ''Atira''s

observation arc In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...

begins with its discovery observation in 2003. It has a

rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...

of 3.3984 hours with a brightness variation of 0.36 magnitude () and a very low

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 0.0231. With a diameter of 4.8 kilometers, ''Atira'' is one of the largest Near-Earth objects. Early estimates of its size ranged from 1 to 2 kilometers, but those were based on an assumed higher albedo of 0.20. Its larger size and low albedo were discovered when ''Atira'' was imaged by radar in early 2017. These radar images also revealed that ''Atira'' is a binary asteroid.

Binary system

Atira came within from Earth in January 2017, the closest since its discovery in 2003. This provided an opportunity to study the asteroid by

radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

. Images taken at

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 ...

on 20 January 2017 revealed that ''Atira'' is a synchronous binary asteroid with a minor-planet moon in orbit. The primary with a diameter of is possibly elongated and very angular in shape. The secondary is tidally locked and has a diameter of . Additional images taken on 23 January 2017 showed that the two components are orbiting each other at a distance of about 6 km with an orbital period of 15.5 hours.

Atira class

Knowing that traditionally the first known object in a new class of asteroids will become the name of the new class of asteroids, due consideration was given to the name for (163693). The other classes of near-Earth asteroids are Amors, Apollos, and Atens ''(as mentioned above)'', named after a Roman, Greek, and Egyptian god, so a preference was given to a god or goddess beginning with the letter "A". Given (163693) was discovered by the LINEAR program which operates out of the southwestern United States, preference was also given to a name of local origin. The minor planet was named after Atira , an epithet of the Earth goddess of the Native American

Pawnee people The Pawnee are a Central Plains Indian tribe that historically lived in Nebraska and northern Kansas but today are based in Oklahoma. Today they are the federally recognized Pawnee Nation of Oklahoma, who are headquartered in Pawnee, Oklahoma. T ...

. Atira is the wife of the creator god, Tirawa, and goddess of Earth and the evening star. The official naming citation was published by the

Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function ...

on 22 January 2008 ().

Physical properties

Binary system

Atira class

See also

References

External links