Abstract
Asteroid (101955) Bennu, the target of NASA's OSIRIS-REx sample return mission, is a D approximate to 0.5 km diameter low albedo near-Earth object. It has a spectral signature consistent with primitive carbonaceous chondrites, and an orbit similar to that of the Earth.
A plausible evolution scenario for Bennu is that it migrated inward across the inner main belt from a low albedo family by Yarkovsky thermal forces over many hundreds of Myr. Eventually, it entered a resonance that took it into the terrestrial planet region, where a combination of planetary encounters and resonances took it to its current orbit over a few Myr to tens of Myr.
When it departed the main belt, Bennu probably had an eccentricity 0.1 < e < 0.2 and an inclination 1 degrees < i < 6 degrees. Several low albedo families have the appropriate dynamical, color, albedo, and broad spectral characteristics to produce Bennu: Clarissa, Erigone, Eulalia, New Polana, and Sulamitis.
Here we used a suite of numerical simulations to determine the ages of the families above, how Bennu reached its current orbit, and the most probable source family for Bennu. Specifically, we tracked test Bennu-like asteroids evolving in semimajor axis by the coupled Yarkovsky/YORP effects, incorporating a new formalism for how YORP torques modify the spin vector evolution of small asteroids.
Using results and insights provided by Statler (Statler, T.S. [2009]. Icarus 202, 502-513), we assumed that modest shape changes to asteroids, produced by a variety of processes (e.g., crater formation, changes to asteroid rotational angular momentum by YORP), caused the test asteroids' spin rates, but not their obliquities, to undergo a random walk.