  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 $2.64 \pm 1.05 \times 10^{-6}
\mathrm{deg\ day}^{-2}$, 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.
