Trajectory Estimation for Particles Observed in the Vicinity of (101955) Bennu

Abstract

We analyze the trajectories of 313 particles seen in the near‐Bennu environment between December 2018 and September 2019. Of these, 65% follow suborbital trajectories, 20% undergo more than one orbital revolution around the asteroid, and 15% directly escape on hyperbolic trajectories. The median lifetime of these particles is ∼6 hr. The trajectories are sensitive to Bennu's gravitational field, which allows us to reliably estimate the spherical harmonic coefficients through degree 8 and to resolve nonuniform mass distribution through degree 3. The particles are perturbed by solar radiation pressure, enabling effective area‐to‐mass ratios to be estimated. By assuming that particles are oblate ellipsoids of revolution, and incorporating photometric measurements, we find a median axis ratio of 0.27 and diameters for equivalent‐volume spheres ranging from 0.22–6.1 cm, with median 0.74 cm. Our size distribution agrees well with that predicted for fragmentation due to diurnal thermal cycling. Detailed models of known accelerations do not produce a match to the observed trajectories, so we also estimate empirical accelerations. These accelerations appear to be related to mismodeling of radiation pressure, but we cannot rule out contributions from mass loss. Most ejections take place at local solar times in the afternoon and evening (12:00–24:00), although they occur at any time of day. We independently identify ten ejection events, some of which have previously been reported. We document a case where a particle ricocheted off the surface, revealing a coefficient of restitution 0.57±0.01 and demonstrating that some apparent ejections are not related to surface processes.

Photometry of Particles Ejected From Active Asteroid (101955) Bennu

“…The new PSF fitting photometry results in a similar phase coefficient of 0.015 ± 0.006 mag/deg for P1 to P3 and agrees with Lauretta, Hergenrother, et al (2019). The larger number of particles in Chesley et al (2020, this collection) provided an expanded set of objects for phase angle analysis. While most particles have few photometric measurements or cover a small range of phase angles, we identified 15 “high‐quality” particles, including P1 to P3, having a positive phase coefficient (increasing astronomical magnitude with increasing phase angle) and 40 or more photometric measurements spanning a phase angle range of >20° (Figure 5 and Table 1).…”

“…We can also convert the photometry derived H V values from the phase functions into diameters using where D H is the particle diameter in centimeters and p V is the geometric albedo in the V band (Fowler & Chillemi, 1992; Pravec & Harris, 2007). The area‐to‐mass ratios ( η ) produced by Chesley et al (2020, this collection) provide another independent means to estimate particle size, where ρ is density of the particle (kg m −3 ). Both methods assume spherical particles.…”

“…Sequences of detections attributable to individual particles were designated as tracks. Orbit determination (OD) of tracks produced orbital solutions (Chesley et al, 2020; Lauretta, Hergenrother, et al, 2019, this collection; Leonard et al, 2020, this collection; Pelgrift et al, 2020, this collection). Some particles that remained in the Bennu environment for an extended period (days) were observed in multiple tracks.…”

“…This final manual adjustment step applied to about 15% of all detections in these six events. We did not apply the manual fix to those particle detections from the Chesley et al (2020, this collection) catalog. Finally, for all the particles with the best‐fit streaking parameter s found to be 0 (nonstreaked), we replaced the PSF model by a circular Gaussian and fit the model again.…”

“…More detailed evaluation of candidate mechanisms can be found in Bottke et al (2020, this collection) for meteoroid impacts, Rozitis et al (2020, this collection) for ice sublimation, and Molaro et al (2020, this collection) and Rozitis et al (2020, this collection) for thermal fracturing. An example of a particle ejected by means of a secondary surface impact was identified by Chesley et al (2020, this collection).…”