2020 BX₁₂—The Last Binary Asteroid Discovered at Arecibo

Radar observations of 2020 BX₁₂ conducted with the S-band planetary radar system (2380 MHz, 12.6 cm) at the Arecibo Observatory on 2020 February 4 and 5 revealed that this potentially hazardous asteroid is a binary system. Spectroscopic observations with the 10.4 m Gran Telescopio Canarias on 2024 February 16 indicate that 2020 BX₁₂ is an S-complex asteroid (Bus-DeMeo taxonomy). We present the results of shape modeling and orbit fitting based on the radar observations. The system consists of a primary of a diameter ∼205 m and a ∼50 m secondary revolving around their common center of mass. This size places the system among the smallest 10% of known binary asteroid systems. The orbital period of the system is >40 hr. The semimajor axis is >375 m. This binary system, like many other binary near-Earth asteroids, features a spheroidal primary spinning near the breakup point, indicating likely formation through spin-up and fission and migration from the main belt. 2020 BX₁₂ was the last binary asteroid discovered at Arecibo.