Introduction to the Special Issue: Exploration of the Activity of Asteroid (101955) Bennu

Abstract

Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss. The activity manifests itself in the form of ejection events emitting up to hundreds of millimeter‐ to centimeter‐scale particles. The Origins, Spectral Interpretation, Resource Identification, and Security‐Regolith Explorer spacecraft monitored particle activity for a 10‐month period that included Bennu's perihelion and aphelion. Novel and classical methods were utilized to detect the particles and characterize their orbital and physical properties. Roughly 30% of the observed particle mass escaped to heliocentric orbit. A majority of particles fell back onto the surface of Bennu after ejection, with the longest‐lived particle surviving for 6 days on a temporary orbit. Particle ejection events appear to preferentially take place in the afternoon and evening and from low latitudes, although they can occur at any time or latitude. The reaccumulation of material is biased toward low latitudes resulting in the possible in‐fill of craters and growth of Bennu's equatorial bulge. Of the potential mechanisms behind this activity that were investigated in focused studies, meteoroid impacts, thermal fracturing, and ricochet—but not water ice sublimation—were found to be consistent with observations. While phyllosilicate dehydration was not investigated with a focused study, it remains a possible mechanism. These mechanisms are not unique to Bennu, suggesting that many near‐Earth asteroids may exhibit activity that has gone undetected thus far. Spacecraft missions with wide‐field imagers are encouraged to further characterize this phenomenon.

Particle Size-Frequency Distributions of the OSIRIS-REx Candidate Sample Sites on Asteroid (101955) Bennu

“…Processes observed on Bennu include space weathering [58]; thermal fracturing [57]; impact cratering at large [52,59] and small [55] scales; mass movement due to surface slope changes, which likely results in abrasion and breakdown at large scales and equatorial deposition [60]; and particle ejection events [23,61]. Each of these physical mechanisms can result in particle fragmentation, thereby influencing the PSFD observed for particles at different size ranges.…”

Bennu's Natural Sample Delivery Mechanism: Estimating the Flux of Bennuid Meteors at Earth

“…At regular intervals, we compare all particle stream members to the current orbit of their parent body. These results are relative to a start date for particle production of 1780 though there is reason to suspect that particle production has been part of Bennu's history before this (Hergenrother, Adam, et al., 2020). In general, we will show how our simulation results compare with the initial investigations of Kováčová et al.…”

“…The process that is leading to mass loss on Bennu is probably not unique to Bennu (Hergenrother, Adam, et al., 2020). Thus, the discovery of particle ejection from Bennu opens up an avenue of investigation into meteors originating from the near‐Earth asteroid population.…”

“…Observations of the particle ejections show that particles are released most often during Bennu's local afternoon and evening (Hergenrother, Adam, et al., 2020). Though the energy added or removed from the particles' orbits due to ejection velocity is small compared to heliocentric motion, small differences in initial conditions can lead to large effects, especially for bodies like Bennu that have close encounters with major perturbers, such as Earth.…”

“…For example, active asteroid (3200) Phaethon has been identified as the parent body of the major Geminid meteor shower (Jenniskens, 2006). The near‐Earth asteroid target of NASA's OSIRIS‐REx asteroid sample return mission, (101955) Bennu, exhibits meteoroid‐generating activity (Hergenrother, Adam, et al., 2020; Hergenrother, Maleszewski, et al., 2020; Lauretta et al., 2019). With observed ejection events producing sometimes hundreds of centimeter‐sized particles at velocities of up to a few meters per second, and a small minimum orbit intersection distance (MOID) with Earth (Chesley et al., 2020), it is reasonable to ask if meteoroids ejected from Bennu could contribute to meteor activity at Earth.…”