Ephemeris and hazard assessment for near-Earth asteroid (101955) Bennu based on OSIRIS-REx data

Small bodies such as the near-Earth asteroid Bennu drift in their orbit due to thermalradiation forces (the Yarkovsky eect). Ground-based observations have indicateda nonzero probability of Bennu impacting Earth, depending on how its orbit evolves.Thus, among the goals of the OSIRIS-REx (Origins, Spectral Interpretation, ResourceIdentication, and Security-Regolith Explorer) mission to Bennu were to precisely measurethe Yarkovsky eect and rene the impact hazard assessment for this body. Herewe address these objectives. Using OSIRIS-REx spacecraft tracking data, we derivemeter-level constraints on the distance between Earth and Bennu from January 2019to October 2020. While these data greatly improve the knowledge of the trajectoryof Bennu, they also require an unprecedented delity for the modeling of an asteroid'strajectory. In particular, special care is needed to take into account the contribution of343 small-body perturbers and the uncertainty in their masses. Radiation eects suchas the Poynting-Robertson drag, so far only considered for interplanetary dust dynamics,now become a consideration for modeling the trajectory of a 500-m asteroid such asBennu. By employing a thermophysical model based on OSIRIS-REx's characterizationof Bennu, we estimate a semimajor axis drift of 􀀀284:60:2 m/yr (signal-to-noise ratio1400) at epoch 2011 January 1 caused by the Yarkovsky eect. The largest source ofmodeling error is solar wind drag, which may lower the magnitude of the semimajor axisdrift from the Yarkovsky eect by up to 0.16 m/yr. The Yarkovsky-related semimajoraxis drift varies by roughly 1 m/yr as the orbit of Bennu evolves due to planetaryperturbations from 1900 to 2135. The Yarkovsky model proves to be extremely accurateby predicting a bulk density estimate within 0.1% of that estimated through gravity scienceanalysis. Compared to the information available before the OSIRIS-REx mission,the knowledge of the circumstances of the scattering Earth encounter that will occur in2135 improves by a factor of 20, thus allowing us to rule out many previously possibleimpact trajectories. However, there remain some impact trajectories compatible withthe data. Prior to the spacecraft encounter, the overall impact probability through 2200was 3:710􀀀4 (1 in 2700). As a result of our analysis, the cumulative impact probabilitythrough 2300 becomes 5:710􀀀4 (1 in 1750) and the most signicant individual impactsolution is for September 2182, with an impact probability of 3:7 10􀀀4 (1 in 2700).Both Bennu and (29075) 1950 DA have a Palermo scale value of 􀀀1:42 and share thedistinction as the most hazardous object in the asteroid catalog.