Bayesian Approach to Light Curve Inversion of 2020 SO

Near Earth Object (NEO) 2020 SO is believed to be a Centaur rocket booster from the mid 1960’s that was temporarily recaptured by the Earth. 2020 SO entered Earth’s Hill sphere in November 2020, with close approaches in December 2020 and February 2021, where it became bright enough (approximately 14 V magnitude) to be observed by Raven-class (< 1 m) telescopes. In this paper, 2020 SO’s spin state and reflective properties are estimated using data collected from multiple telescope sites around the world during both close approaches. The 95% Highest Posterior Density (HPD) region and Maximum A Posteriori (MAP) spin state and reflective properties of 2020 SO are estimated using Bayes’ theorem via Markov Chain Monte Carlo (MCMC) sampling of a predictive light curve simulation that is based on an anisotropic Phong reflection model. We estimate ten parameters at the start of an observation epoch: attitude quaternion (4), angular velocity vector (3), and diffusive/specular reflectivity parameters (3). Using a Fourier fitting and least squares minimization technique we find a joint-estimated period of 9.328 ± 0.275 s at a 2σ confidence level in the light curves of 2020 SO that further provides support for it being an artificial object as the current most rapidly rotating known asteroid is 2017 QG18 with a period over 1.3 times slower. The method of light curve inversion employed in this paper can be applied directly to other NEOs given photometric observations with a high enough temporal density and knowledge of some approximate physical properties of the object.