In search of the source of asteroid (101955) Bennu: Applications of the stochastic YORP model

William F. Bottke, David Vokrouhlický, Kevin J. Walsh, Marco Delbo, Patrick Michel, Dante S. Lauretta, Humberto Campins, Harold C. Connolly, Daniel J. Scheeres, Steven R. Chelsey

Research output: Contribution to journalArticlepeer-review

81 Scopus citations


Asteroid (101955) Bennu, the target of NASA's OSIRIS-REx sample return mission, is a D≈. 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°. <. i<. 6°. 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. This "stochastic YORP" mechanism slows down how often asteroids reach YORP endstates (i.e., spinning up so fast that the asteroid sheds mass, spinning down so much the asteroid enters into a tumbling rotation state). This new model allowed us to reproduce the semimajor axis distribution of observed family members from Clarissa, Erigone, Eulalia, New Polana, and Sulamitis. In the process, we derived model family formation ages of ~60. Myr old, 130. ±. 30. Myr old, 830-100+370Myr old, 1400. ±. 150. Myr old, and. 200. ±. 40. Myr, respectively.Next, using a Monte-Carlo code to track millions of test asteroids from each of the families above to main belt escape routes capable of producing Bennu-like orbits, we found the most likely parent families for Bennu are Eulalia and New Polana. On average, more than twice as many 0.5. km objects from the New Polana family reach Bennu's orbit as those from the Eulalia family. This corresponds to the New Polana and Eulalia families having a 70-4+8% and 30-8+4% probability of producing Bennu, respectively. Comparable runs to deduce the source of the Hayabusa 2 target, the low albedo 0.87. km diameter near-Earth object (162173) 1999 JU3, produced similar probabilities for both families. The former Marco-Polo-R target, the 1.9. km asteroid (175706) 1996 FG3, however, has a 85-83+4% probability of coming from the Eulalia family and a 15-4+83% probability of coming from the New Polana family. The reason for this switch is that 1996 FG3 may have been part of Yarkovsky/YORP-produced wave of like-sized bodies that is only now reaching the terrestrial planet region. We suggest that the top-like shape of Bennu is a byproduct of mass wasting and/or mass shedding events produced by YORP spin up during its long journey across the inner main belt.

Original languageEnglish (US)
Pages (from-to)191-217
Number of pages27
StatePublished - Feb 1 2015
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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