Episodes of particle ejection from the surface of the active asteroid (101955) Bennu

Bennu ejects material from its surface Most asteroids appear inert, but remote observations show that a small number experience mass loss from their surfaces. Lauretta and Hergenrother et al. describe close-range observations of mass loss on the near-Earth asteroid Bennu (see the Perspective by Agarwal). Shortly after arriving at Bennu, navigation cameras on the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security—Regolith Explorer) spacecraft detected objects 1 to 10 centimeters in diameter moving above the surface. Analysis of the objects' trajectories showed that they originated in discrete ejection events from otherwise unremarkable locations on Bennu. Some objects remained in orbit for several days, whereas others escaped into interplanetary space. The authors suggest multiple plausible mechanisms that could underlie this activity. Science, this issue p. eaay3544; see also p. 1192 The near-Earth asteroid Bennu ejects groups of solid objects from its surface in discrete events. INTRODUCTION Active asteroids are small bodies in the Solar System that show ongoing mass loss, such as the ejection of dust, which may be caused by large impacts, volatile release, or rotational acceleration. Studying them informs our understanding of the evolution and destruction of asteroids and the origin of volatile materials such as water on Earth. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft has rendezvoused with the near-Earth asteroid (101955) Bennu. The selection of Bennu as the OSIRIS-REx target was partially based on its spectral similarity to some active asteroids. Observations designed to detect mass loss at Bennu were conducted from Earth and during the spacecraft’s approach, but no signs of asteroid activity were found. However, when the spacecraft entered orbit in January 2019, we serendipitously observed particles in the vicinity of Bennu that had apparently been ejected from its surface. RATIONALE We analyzed the properties and behavior of particles ejected from Bennu to determine the possible mechanisms of ejection and provide understanding of the broader population of active asteroids. Images obtained by the spacecraft indicate multiple discrete ejection events with a range of energies and resultant particle trajectories. We characterized three large ejection events that respectively occurred on 6 January, 19 January, and 11 February 2019. Tracking of individual particles across multiple images by means of optical navigation techniques provided the initial conditions for orbit determination modeling. By combining these approaches, we estimated the locations and times of ejection events and determined initial velocity vectors of particles. We estimated the particle sizes and the minimum energies of the ejection events using a particle albedo and density consistent with observations of Bennu. RESULTS Particles with diameters from <1 to ~10 cm were ejected from Bennu at speeds ranging from ~0.05 to >3 m s–1. Estimated energies ranged from 270 mJ for the 6 January event to 8 mJ for the 11 February event. The three events arose from widely separated sites, which do not show any obvious geological distinction from the rest of Bennu’s surface. However, these events all occurred in the late afternoon, between about 15:00 and 18:00 local solar time. In addition to discrete ejection events, we detected a persistent background of particles in the Bennu environment. Some of these background particles have been observed to persist on temporary orbits that last several days—in one case, with a semimajor axis >1 km. The orbital characteristics of these gravitationally bound objects make it possible to determine the ratio of their cross-sectional area to their mass. Combined with their photometric phase functions, this information constrains the parameter space of the particles’ diameters, densities, and albedos. CONCLUSION Plausible mechanisms for the large ejection events include thermal fracturing, volatile release through dehydration of phyllosilicates, and meteoroid impacts. The late-afternoon timing of the events is consistent with any of these mechanisms. Bennu’s boulder geology indicates that thermal fracturing, perhaps enhanced by volatile release, could occur on the asteroid surface. Smaller events, especially those that occur on the night side of Bennu, could be attributable to reimpacting particles. Our observations classify Bennu as an active asteroid. Active asteroids are commonly identified by major mass loss events observable with telescopes, on scales much greater than we observed at Bennu. Our findings indicate that there is a continuum of mass loss event magnitudes among active asteroids. Schematic diagram of orbit determination model output for the 19 January 2019 particle ejection event from asteroid Bennu observed by the OSIRIS-REx spacecraft. Bennu is depicted in gray and has a diameter of ~500 m. OSIRIS-REx is indicated with the brown dot, ~2 km from Bennu’s center of mass; the cone represents the viewing angle. Blue arcs are particle trajectories, ending or with gaps where the trajectories pass into shadow. The Sun–angular momentum frame coordinates are shown at bottom right: x, solar vector; y, Bennu orbital direction; z, Bennu north. Active asteroids are those that show evidence of ongoing mass loss. We report repeated instances of particle ejection from the surface of (101955) Bennu, demonstrating that it is an active asteroid. The ejection events were imaged by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft. For the three largest observed events, we estimated the ejected particle velocities and sizes, event times, source regions, and energies. We also determined the trajectories and photometric properties of several gravitationally bound particles that orbited temporarily in the Bennu environment. We consider multiple hypotheses for the mechanisms that lead to particle ejection for the largest events, including rotational disruption, electrostatic lofting, ice sublimation, phyllosilicate dehydration, meteoroid impacts, thermal stress fracturing, and secondary impacts.