We observed the near-Earth asteroid (101955) Bennu from the ground in 1999 and 2005, and with the Hubble Space Telescope in 2012, to constrain its rotation rate. The data reveal an acceleration of 2.64±1.05×10−6degday−2, which could be due to a change in the moment of inertia of Bennu or to spin up from the YORP effect or other source of angular momentum. The best solution is within 1 sigma of the period determined by Nolan et al. (2013). The OSIRIS-REx mission will determine the rotation state independently in 2019. Those measurements should show whether the change in rotation rate is a steady increase (due, for example, to the YORP effect) or some other phenomenon. The precise shape and surface properties measured by the OSIRIS-REx science team will allow for a better understanding of variations in rotation rate of small asteroids.
@article{arxiv.1903.00096,
title = {Detection of Rotational Acceleration of Bennu using HST Lightcurve Observations},
author = {Michael C. Nolan and Ellen S. Howell and Daniel J. Scheeres and Jay W. McMahon and Oleksiy Golubov and Carl W. Hergenrother and Joshua P. Emery and Keith S. Noll and Steven R. Chesley and Dante S. Lauretta},
journal= {arXiv preprint arXiv:1903.00096},
year = {2019}
}