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 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 \pm 70 { kg / m ^ { 3 } } , which indicates a macroporosity in the range 40 \pm 10 % for the bulk densities of likely analog meteorites , suggesting a rubble-pile internal structure .
The associated mass estimate is ( 7.8 \pm 0.9 ) \times 10 ^ { 10 } kg and GM = 5.2 \pm 0.6 { 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 .