  The target asteroid of the OSIRIS-REx asteroid sample return mission,
(101955) Bennu (formerly 1999 RQ$_{36}$), is a half-kilometer near-Earth
asteroid with an extraordinarily well constrained orbit. An extensive data set
of optical astrometry from 1999--2013 and high-quality radar delay measurements
to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect,
with a mean semimajor axis drift rate $da/dt = (-19.0 \pm 0.1)\times 10^{-4}$
au/Myr or $284\pm 1.5\;\rm{m/yr}$. The accuracy of this result depends
critically on the fidelity of the observational and dynamical model. As an
example, neglecting the relativistic perturbations of the Earth during close
approaches affects the orbit with $3\sigma$ significance in $da/dt$.
  The orbital deviations from purely gravitational dynamics allow us to deduce
the acceleration of the Yarkovsky effect, while the known physical
characterization of Bennu allows us to independently model the force due to
thermal emissions. The combination of these two analyses yields a bulk density
of $\rho = 1260\pm70\,\rm{kg/m^3}$, which indicates a macroporosity in the
range $40\pm10$% for the bulk densities of likely analog meteorites, suggesting
a rubble-pile internal structure. The associated mass estimate is
$(7.8\pm0.9)\times 10^{10}\, \rm{kg}$ and $GM = 5.2\pm0.6\,\rm{m^3/s^2}$.
  Bennu's Earth close approaches are deterministic over the interval
1654--2135, beyond which the predictions are statistical in nature. In
particular, the 2135 close approach is likely within the lunar distance and
leads to strong scattering and therefore numerous potential impacts in
subsequent years, from 2175--2196. The highest individual impact probability is
$9.5\times 10^{-5}$ in 2196, and the cumulative impact probability is
$3.7\times 10^{-4}$, leading to a cumulative Palermo Scale of -1.70.
