163693 Atira

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 (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 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 0.2059 AU (30,800,000 km) 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 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 (which orbits between 0.31 and 0.47 AU). As no precoveries were found, Atira's 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 (U=2) 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.