Discovery Announcement of Binary System (163693) Atira

E. G. Rivera-Valentin, P. A. Taylor, A. Virkki, and B. Aponte-Hernandez report that Arecibo (2380 MHz, 12.6 cm) delay-Doppler radar images obtained on 2017 January 20 and 23 reveal near-Earth asteroid (163693) Atira is a binary system. Visible range extents of the components in images with 150 m/pixel resolution suggest a primary up to 4.8 ± 0.5 km in diameter and a secondary 1.0 ± 0.3 km in diameter. The primary is possibly elongated and very angular in shape. On 2017 January 20, the system was observed near maximum separation in delay (range) suggesting a mutual-orbit scale of at least 5.7 km. On 2017 January 23, the system was observed near maximum separation in Doppler frequency suggesting an orbital period of roughly 16 hrs. A preliminary circular orbit fit finds a (projected) semimajor axis of 6 km and a period of 15.5 hrs.