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

The rotation rate of (101955) Bennu has been observed to increase, providing evidence of the YORP effect in action. Bennu is a rubble pile with little strength. At the current spin-up rate, the rotation would result in large-scale disruption in <1 My. Such an extreme scenario is predicated on the YORP torque continuing to increase the rotation. However, YORP is sensitive to the shape and can change on a short timescale as small episodes of failure can increase oblateness, reduce spin rate, and redistribute rubble on the surface. A more comprehensive model of the shape and spin evolution of Bennu is required to understand its past and future. Here, we calculate the YORP torque on a shape model of Bennu. For a random distribution of rubble, the torques on individual blocks should cancel, and the large-scale structure should control the YORP response. However, we find the calculated torque is strongly dependent on the resolution of the shape model used, suggesting that the smaller material has an influence. As the surface roughness of the model increases, the magnitude of the torque and even its sign may change. Spin rate increases that more closely match measurements are obtained with increasing small-scale roughness. Simulated models that are coarser in resolution, but possess greater roughness than the equivalent lower-resolution shape model from observations, likewise are more consistent with the observed spin-up rate. We find that surface roughness with a non-random orientation controlled by large-scale structure determines the YORP torque. Following [1], we model the evolution of a rubble pile with Bennu’s shape subject to YORP using the granular modeling tool pkdgrav and explore how the torques change as the object is deformed. The YORP torques are calculated on the present shape and applied until particles begin to move. The torques are then recomputed on the new shape, and the iteration continues. We find negligible change in the torque until the rotation period decreases to 3.6 hr from its current 4.3-hr period. At 3.53 hr, the asteroid starts to lose mass from the equator. Our results suggest that the deformation of the asteroid’s shape due to YORP does not strongly alter rotation, and that if the initial shape is known to sufficient accuracy, the future shape and spin can be predicted. [1] Cotto-Figueroa D. et al. (2015) ApJ 803, 25.

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.