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2020 BX12: The Last Binary Asteroid Discovered by Arecibo Observatory

Presentation #207.01 in the session “Radar Observations: Zapping them before they Zap Us!”.

Published onOct 03, 2021
2020 BX12: The Last Binary Asteroid Discovered by Arecibo Observatory

Near-Earth asteroid 2020 BX12 was discovered on 2020 Jan 27 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey at Mauna Loa Observatory. Due to its estimated size, within a factor of two of 200 meters based on its absolute magnitude of 20.8, and Earth minimum orbit intersection distance of 0.0021 au (less than 1 lunar distance), this Apollo-group object is classified as a potentially hazardous asteroid. Its close approach on 2020 Feb 03 (TDB) was within 11.4 lunar distances (0.0292 au). Once 2020 BX12 moved to northern declinations, it was observed with the S-band planetary radar system (2380 MHz, 12.6 cm) at Arecibo Observatory on 2020 Feb 04 and Feb 05.

Observations of continuous-wave spectra immediately revealed a secondary as a narrow echo superimposed on the broader echo of the primary component. On the first date of observations, the primary’s bandwidth was about 1.6 Hz, and the secondary’s bandwidth was about 0.08 Hz (resolved at 0.04 Hz resolution). The difference in bandwidths is due to the differences in the sizes of the components and their rotation rates. Follow up delay-Doppler radar imaging at 0.05 us (7.5 m/px, 0.075 Hz/pixel Hz resolution) resolved the two components of the binary system unambiguously.

The primary component appears spheroidal with an estimated diameter of (160 ± 30) m based on its visible extent in high-resolution radar images, while the secondary component is roughly half the size of the primary. The mutual-orbital phase changed by about 180 degrees in radar images roughly 24 hours apart, from the secondary leading the primary to the secondary trailing the primary. This could support a tidally locked secondary and a mutual-orbit period of 45-50 hours, though a shorter orbital period of 15-16 hours has not been ruled out. The maximum separation observed for the two bodies was roughly 360 m. Based on its visible extent in radar images and its observed echo bandwidth, the estimated rotation period for the primary is 2.8 hours.

The results above are based on preliminary analyses. We will present revised system parameters and details of radar-derived dynamical and physical properties.

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