Many boulders on (101955) Bennu, a near-Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu’s surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue-driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures, and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from ~1 mm to 10 cm and disaggregated particles have ejection speeds up to ~2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.

NASA’s OSIRIS-REx mission observed millimeter- to centimeter-scale pebbles being ejected from the surface of asteroid (101955) Bennu, indicating that Bennu is an active asteroid. About 30% of these particles escape from Bennu, and the minimum orbital intersection distance (MOID) between Bennu and Earth suggest the possibility of a ‘Bennuid’ particle flux at Earth. We characterize the evolution of Bennu’s particle stream and potential for meteor flux by simulating weekly particle ejections between the years 1780 - 2135 continuing their dynamical evolution until 2200. Ejections are modelled as a discrete release of 95 particles every week. The meteoroid stream is found to circularize in 80 +/- 40 years. Individual particles and streams remain associable to Bennu for the entire 420 years simulated. Particle flux at Earth is predicted to begin in 2101, as the Bennu-Earth MOID reaches minimum values. The year of highest particle flux, 2182, experiences 161 Earth intersections and accounts for ~1/4 of our predicted meteors. Our methods can be expanded to study the history and structure of the general meteoroid population and to estimate flux from specific near-Earth asteroids.

On January 6, 2019, OSIRIS-REx first observed particles ejecting from the surface of near-Earth asteroid (101955) Bennu. This ejection event was unexpected and was only captured by chance in a pair of optical navigation images taken by the OSIRIS-REx NavCam 1 imager. With this limited dataset of only two observations per ejected particle, traditional orbit determination to reconstruct the particles’ trajectories was not possible. Therefore, a new technique was developed for reconstruction of the ejection event based on some simplifying assumptions that the particles all ejected from the same location at the same time and that their velocities remained constant after ejection (a reasonable approximation for fast-moving particles given Bennu’s weak gravity). This technique was then applied to reconstruct those particle events observed by the OSIRIS-REx spacecraft at Bennu from January 2019 through June 2019 by Pelgrift et al. (2020). We present a follow-on to that work that applies the same technique to reconstruct the particle ejection events observed in the latter half of OSIRIS-REx proximity operations at Bennu, covering the time span from July 2019 through July 2020. We reconstructed 8 additional events, bringing the total number of Bennu particle ejection events reconstructed using this technique to 19. The new dataset includes the largest event observed to-date, with over 350 individual tracked particles. The new events have particle ejection velocities similar to the previous events, ranging from 5 cm/s to 1.8 m/s. In the full dataset of 19 events, we observed the same trend noted in the original work where the majority of events were estimated to have occurred at mid-latitudes and afternoon local solar times (LST). Reference: Pelgrift, J. Y., Lessac-Chenen, E. J., Adam, C. D., Leonard, J. M., Nelson, D. S., McCarthy, L., et al. (2020). Reconstruction of Bennu particle events from sparse data. Earth and Space Science. 7, e2019EA000938. https://doi.org/10.1029/2019EA000938

On October 20, 2020, NASA’s OSIRIS-REx spacecraft performed its Touch-and-Go (TAG) activity, in which it briefly contacted the surface of the asteroid Bennu and successfully collected a sample of regolith. Subsequent images of the sampling mechanism showed that thousands of small regolith particles were escaping from it, apparently in conjunction with movements of the mechanical arm and wrist joint by which the sampling mechanism is attached to the spacecraft. The escaping particles could be tracked from one image to another, and across multiple images, which allowed the OSIRIS-REx optical navigation team to detect, associate, and track particles using a combination of manual and automated techniques. The associated tracks each represent a unique particle that was further analyzed to estimate its ejection time, 3D trajectory, and velocity, as well as its photometric properties, which were used to compute its brightness, size, and mass. Compiling the aggregate photometric and physical data for all of the particles leads to an estimate of total sample mass lost during the post-TAG imaging sequences. These results further inform an understanding of the sample escape mechanisms and sample loss that occurred before the sample head was stowed in the return capsule on October 28, 2020. The material presented here is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program.