163693 Atira
163693 Atira 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 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 near Socorro, New Mexico, in the United States. To date, it is the only confirmed binary asteroid that orbits inside Earth's orbit.
It is the namesake and the first numbered body of the Atira asteroids, a new subclass of near-Earth asteroids, which have their orbits entirely within that of Earth and are therefore alternatively called Interior-Earth Objects. 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 smaller than that of Earth. However, and contrary to Aten asteroids, the aphelion for Atiras is always smaller than Earth's perihelion, 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. Its orbit has an eccentricity of 0.32 and an inclination of 26° with respect to the ecliptic. With a perihelion of 0.50 AU the body also classifies as a Venus-crosser 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. As no precoveries were found, Atiras observation arc begins with its discovery observation in 2003. It has a rotation period of 3.3984 hours with a brightness variation of 0.36 magnitude and a very low albedo 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.