Photometry of Particles Ejected From Active Asteroid (101955) Bennu

Hergenrother, C. W.; Maleszewski, C.; Li, J.‐Y.; Pajola, M.; Chesley, S. R.; French, A. S.; Davis, A. B.; Pelgrift, John; Leonard, Jason M.; Shelly, F. C.; Liounis, Andrew J.; Becker, K. J.; Balram‐Knutson, S. S.; García, R.; Kareta, Theodore; Adam, Coralie D.; Alkiek, Khaled; Bos, Brent J.; Brozović, M.; Burke, K. N.; Christensen, E.; Clark, B. E.; DellaGiustina, D. N.; d’Aubigny, C. Drouet; Farnocchia, Davide; Howell, E. S.; Jacobson, R. A.; Kidd, John; Lessac-Chenen, Erik J.; Melikyan, Robert; Nolan, M. C.; Park, R. S.; Selznick, Sanford; Rizk, B.; Лауретта, Д. С.

doi:10.1029/2020je006381

Cited by 27 publications

(95 citation statements)

References 82 publications

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“…The Yarkovsky effect depends nonlinearly on the rotation rate of the body in question, and yet our data provide no direct constraint on the particle rotation periods (Hergenrother et al., 2020). We do presume, however, that the partition between translational and rotational kinetic energy is not extreme.…”

Section: Force Modelsmentioning

confidence: 59%

“…To this end, we also define the diameter D H derived from absolute magnitude H , again assuming spherical particles, as

D_{H} = 1329 \times 1 0^{5} cm \cdot \frac{1 0^{- H / 5}}{\sqrt{p_{V}}} .

Hergenrother et al. (2020) shows that assuming the Bennu albedo of 4.4% leads to a discrepancy in D H compared to D η , with the absolute magnitudes pointing to larger particles than the area‐to‐mass ratios by a median factor 1.5×. Hergenrother et al.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2020

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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.

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Section: Force Modelsmentioning

confidence: 59%

“…To this end, we also define the diameter D H derived from absolute magnitude H , again assuming spherical particles, as

D_{H} = 1329 \times 1 0^{5} cm \cdot \frac{1 0^{- H / 5}}{\sqrt{p_{V}}} .

Section: Resultsmentioning

confidence: 99%

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

et al. 2020

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“…Appendix B: Summary of Observational Data Table A1 summarizes some of the relevant data from seventeen particle ejection events observed between 6 January and 14 September 2019. These data are compiled from Lauretta, Hergenrother, et al (2019), Hergenrother et al (2020), Chesley et al (2020), Leonard et al (2020), and Pelgrift et al (2020) for use in Figures 8 and 9, Table 2, and the text in section 5. Table 1 reports the number of observed particles during each ejection event, which may differ from the number of analyzed particles in the aforementioned studies.…”

Section: 1029/2019je006325mentioning

confidence: 99%

“…Multiple particle ejection events were observed starting in January 2019, shortly after the OSIRIS‐REx spacecraft entered orbit, characterized as bursts of centimeter‐scale and smaller particles leaving the asteroid surface. The two largest events occurred near asteroid perihelion, with numerous smaller events observed in the months following (Hergenrother et al, 2020; Lauretta, Hergenrother, et al, 2019; Leonard et al, 2020; Pelgrift et al, 2020). Several mechanisms have been suggested to drive these ejection events, including electrostatic lofting of particles, meteoroid impacts (Bottke et al, 2020), phyllosilicate dehydration, and thermal fracturing (Lauretta, Hergenrother, et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

et al. 2020

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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.

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“…An additional longer, dedicated particle monitoring campaign was added to the mission plan: Orbital C, a frozen orbit very similar to Orbital A, with a particle monitoring observation frequency of every 13 min, between 6 August and 16 September 2019. During this campaign, six ejection events were detected (Hergenrother et al, 2020).…”

Section: The Osiris‐rex Mission At Bennumentioning

confidence: 99%

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

et al. 2020

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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.

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Photometry of Particles Ejected From Active Asteroid (101955) Bennu

Cited by 27 publications

References 82 publications

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

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

Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

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

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