Recent work provides some evidence of YORP-driven evolution of Bennu. Rick Binzel and Francesca DeMeo detected variations in the spectral slope as a function of latitude on the asteroid surface. These data are consistent with a gradient from pole to equator in composition or grain size. From multiple observations at different aspect angles, they detect a slight but reliable spectral-slope difference between the polar and equatorial regions of Bennu. While observational effects are not yet ruled out as the cause for some differences in slope, if real they could be interpreted as compositional or grain-size effects. Such latitudinal spectral variations are consistent with regolith migration driven by YORP-induced spin up.

OSIRIS-REx will provide the first ground-truth assessment of the Yarkovsky and YORP effects as they relate to the chemical nature and dynamical state of an individual asteroid. During encounter, precision tracking of the OSIRIS-REx spacecraft, in combination with modeling of the spacecraft motion relative to Bennu, will provide the most accurate determination of the Yarkovsky effect ever accomplished. Detailed measurement of the rotation state during the encounter, compared to the period determined from ground-based lightcurve measurements, will allow us to detect the YORP effect to within 1/1000th of a degree per day per year.

OSIRIS-REx will also develop a comprehensive thermophysical model of the asteroid using data obtained during the asteroid encounter. Comparison of the Yarkovsky and YORP effects predicted from this first-principles approach to the direct measurement of the resulting asteroid acceleration and change in rotation state will test our understanding of these phenomena and lead to a substantial improvement in our knowledge of the fundamental parameters that give rise to these effects. Finally, the thermal conductivity and heat capacity of the returned samples will be directly measured in the laboratory. These fundamental physical parameters, combined with the state of the regolith on the asteroid surface, drive the thermal inertia and the resulting strength of the YORP and Yarkovsky effects. Thus, OSIRIS-REx will benefit future studies of near-Earth objects as well as main-belt asteroids in many different ways.