page_title: 163693 Atira
section_title: Binary system
hierarchical_section_title: 163693 Atira / Binary system
caption_attribution_description: English: Radar images of 163693 Atira, taken at Arecibo Observatory on 23 January 2017
context_page_description: 163693 Atira, provisional designation 2003 CP₂₀, 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 4.8 kilometers is orbited by a minor-planet moon that measures about 1 km. 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.
  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.
context_section_description: Atira came within 0.207 AU (31,000,000 km) from Earth in January 2017, the closest since its discovery in 2003. This provided an opportunity to study the asteroid by radar. Images taken at Arecibo Observatory 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 4.8±0.5 km is possibly elongated and very angular in shape. The secondary is tidally locked and has a diameter of 1.0±0.3 km. 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.
caption: Arecibo follow-up observations from 23 January 2017