Up Close with an Active Asteroid

Near‐Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss in the form of ejection events emitting up to hundreds of millimeter‐ to centimeter‐scale particles. The close proximity of the Origins, Spectral Interpretations, Resource Identification, and Security–Regolith Explorer spacecraft enabled monitoring of particles for a 10‐month period encompassing Bennu's perihelion and aphelion. We found 18 multiparticle ejection events, with masses ranging from near zero to hundreds of grams (or thousands with uncertainties) and translational kinetic energies ranging from near zero to tens of millijoules (or hundreds with uncertainties). We estimate that Bennu ejects ~104 g per orbit. The largest event took place on 6 January 2019 and consisted of ~200 particles. The observed mass and translational kinetic energy of the event were between 459 and 528 g and 62 and 77 mJ, respectively. Hundreds of particles not associated with the multiparticle ejections were also observed. Photometry of the best‐observed particles, measured at phase angles between ~70° and 120°, was used to derive a linear phase coefficient of 0.013 ± 0.005 magnitudes per degree of phase angle. Ground‐based data back to 1999 show no evidence of past activity for Bennu; however, the currently observed activity is orders of magnitude lower than observed at other active asteroids and too low be observed remotely. There appears to be a gentle decrease in activity with distance from the Sun, suggestive of ejection processes such as meteoroid impacts and thermal fracturing, although observational bias may be a factor.

Temporal evolution of the dynamical environment around asteroid (101955) Bennu

Active asteroid (6478) Gault sheds mass independent of location along its orbit. Rotational instability is considered to induce the observed activities. If this is the case, because Gault’s breakup event has not been detected, surface failure is likely, implying that its surface materials are constantly ejected while its major body remains intact. Given this scenario, we first constrain Gault’s bulk cohesive strength. We then characterize heliocentric trajectories of ejected particles over thousands of years. The results show that Gault may be sensitive to structural failure at the current spin period (∼2.5 hr). Gault’s bulk density needs to be below 1.75 g cm−3 in order for particles on the equatorial surface to be shed owing to centrifugal forces. In this case, Gault requires cohesive strength of at least ∼200 Pa to maintain the structure at the center, whereas the surface strength needs to be less than ∼100 Pa to induce mass shedding. This suggests that Gault’s structure may consist of a weak surface layer atop a strong core. The trajectories of dust ejected from Gault depend on how efficiently they are accelerated by solar radiation pressure. Escaped particle clouds with sizes of < ∼100 μm could collide with Gault after ∼700–5300 yr with speeds of ∼0.2 km s−1. This implies a temporal increase in the impact flux and complex interactions between the ejected particles and their host body.

NASA sent the Double Asteroid Redirection Test (DART) mission to impact Dimorphos, the satellite of the asteroid binary system (65803) Didymos. DART will release LICIACube prior to impact to obtain high-resolution post-impact images. The impact will produce a crater and a large amount of material ejected at high speed (several tens of m s−1), producing an ejecta cone that will quickly disperse. We analysed an additional effect: the lofting of material at low velocity due to the generation of seismic waves that propagate inside Dimorphos, producing surface shaking far from the impact point. We divide the process into different stages: from the generation of impact-induced waves, the interaction of them with surface particles, the ejection of dust particles at velocities, and the prediction of the observability of the dust coma and trail. We anticipate the following observable effects: (i) generation of a dust cloud that will produce a hazy appearance of Dimorphos’ surface, detectable by LICIACube; (ii) brightness increase of the binary system due to enhancement of the cross-section produced by the dust cloud; (iii) generation of a dust trail, similar to those observed in some Active Asteroids, which can last for several weeks after impact. Numerical prediction of the detectability of these effects depends on the amount and size distribution of ejected particles, which are largely unknown. In case these effects are observable, an inversion method can be applied to compute the amount of ejected material and its velocity distribution, and discuss the relevance of the shaking process.

In early 2019, the OSIRIS‐REx spacecraft discovered small particles being ejected from the surface of the near‐Earth asteroid Bennu.sww Although they were seen to be ejected at slow speeds, on the order of tens of cm/s, a number of particles were surprisingly seen to orbit for multiple revolutions and days, which requires a dynamical mechanism to quickly and substantially modify the orbit to prevent re‐impact upon their first periapse passage. This paper demonstrates that, based on simulations constrained by the conditions of the observed events, the combined effects of gravity, solar radiation pressure, and thermal radiation pressure from Bennu can produce many sustained orbits for ejected particles. Furthermore, the simulated populations exhibit two interesting phenomena that could play an important role in the geophysical evolution of bodies such as Bennu. First, small particles (<1 cm radius) are preferentially removed from the system, which could lead to a deficit of such particles on the surface. Second, re‐impacting particles preferentially land near or on the equatorial bulge of Bennu. Over time, this can lead to crater in‐filling and growth of the equatorial radius without requiring landslides.

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.

Quasi-Hilda asteroid P/2010 H2 (Vales) underwent a spectacular photometric outburst by ≥7.5 mag (factor of ≥103) in 2010. Here, we present our optical observations of this event in the four month period from April 20 to August 10. The outburst, starting UT 2010 April 15.70, released dust particles of total cross-section 17,600 km2 (albedo 0.1 assumed) and mass ∼1.2 × 109 kg, this being about 10−4 of the mass of the nucleus, taken as a sphere of radius 1.5 km and density 500 kg m−3. While the rising phase of the outburst was very steep (brightness doubling time of hours), subsequent fading occurred slowly (fading timescales increasing from weeks to months), as large, low velocity particles drifted away from the nucleus. A simple model of the fading lightcurve indicates that the ejected particles occupied a broad range of sizes, from ∼1 μm to 6 cm, and followed a differential power-law distribution with index 3.6 ± 0.1 (similar to that in other comets). The fastest particles had speeds ≥210 m s−1, indicating gas-drag acceleration of small grains well coupled to the flow. Low-energy processes known to drive mass loss in active asteroids, including rotational disruption; thermal and desiccation stress cracking; and electrostatic repulsion, cannot generate the high particles speeds measured in P/Vales, and are discounted. Impact origin is unlikely given the short dynamical lifetimes of the quasi-Hildas and the low collision probabilities of these objects. The specific energy of the ejecta is estimated at 220 J kg−1. The outburst follows a series of encounters with Jupiter in the previous century, consistent with the delayed activation of buried supervolatiles (and/or the crystallization of subsurface amorphous ice) by conducted heat following an inward displacement of the perihelion. A potential origin in the debris cloud produced by avalanche is also considered.

Asteroids have been recognised as the parent bodies of most meteorites (Wylie, 1939), and, as a result of bombardment by micrometeorites and interplanetary dust particles, are believed to be surrounded by clouds of ejected surface dust particles (Szalay & Horányi, 2016).It has recently been postulated by Cohen et al. (2019) that the composition of asteroidal ejecta dust clouds may be determined by the interception of cosmic dust grains by an impact ionisation mass spectrometer aboard a spacecraft conducting a flyby mission (c.f. the Cosmic Dust Analyser aboard Cassini, Srama et al. 2004, or more modern instrumentation, such as the Surface Dust Analyser aboard Europa Clipper, Kempf et al. 2014). The geochemical compositions and abundances of different grains contribute a “fingerprint” identity of an asteroid body, from which the meteorite type most closely linked to the asteroid can be determined.The approach by Cohen et al. (2019) is generally applicable to asteroid flybys at high velocity regimes (approximately greater than 19 km/s), comparable to that of the planned DESTINY+ flyby mission to asteroid 3200 Phaethon. An extension to this technique, applicable to flyby speeds more typical of dedicated missions to the asteroid belt (e.g. 7-10 km/s), was subsequently proposed by Eckart et al. (2023).To test the efficacy of the proposed theories, we intend to generate and analyse ensembles of cosmic dust analogues from meteorite samples, which, following metal coating, will be electrostatically accelerated onto laboratory engineering models, or flight spares, of impact ionisation mass spectrometers, such as the Destiny Dust Analyser, to be aboard the DESTINY+ spacecraft (Krüger et al. 2019). The obtained mass spectral datasets will then be statistically analysed both individually and as a cohort in order to identify unique features, which can lead towards the linking of the mass spectra to the original meteorite type/class.Here we present the latest results and progress in obtaining, characterising, and preparing meteorite samples in order to simulate, within the laboratory, expected data from the impact ionisation mass spectrometry of asteroid dust-clouds aboard flyby spacecraft. The curation of a library of mass spectral data from meteorite mineral phases will largely facilitate the identification of unique meteorite samples and the subsequent tracing of these specimens back to their parent asteroids, providing a deeper insight into the evolution of our solar system. ReferencesEckart, L. M., Hillier, J. K., Postberg, F., Marchi, S., & Sternovsky, Z. (2023). Linking meteorites to their asteroid parent bodies: The capabilities of dust analyzer instruments during asteroid flybys. Meteoritics & Planetary Science, 58(10), 1449–1468.Kempf, S., Altobelli, N., Briois, C., Gru€n, E., Horanyi, M., Postberg, F., Schmidt, J., Srama, R., Sternovsky, Z., and Tobie, G. 2014. SUDA: A Dust Mass Spectrometer for Compositional Surface Mapping for a Mission to Europa International Workshop on Instrumentation for Planetary Missions, p. 7.Krüger, H., Strub, P., Srama, R., Kobayashi, M., Arai, T., Kimura, H., Hirai, T., Moragas-Klostermeyer, G., Altobelli, N., Sterken, V. J., Agarwal, J., Sommer, M., & Grün, E. (2019). Modelling DESTINY+ interplanetary and interstellar dust measurements en route to the active asteroid (3200) Phaethon. Planetary and Space Science, 172, 22–42.Srama, R., Ahrens, T. J., Altobelli, N., Auer, S., Bradley, J. G., Burton, M., Dikarev, V. V., et al. 2004. The Cassini Cosmic Dust Analyzer. In The Cassini-Huygens Mission: Orbiter In Situ Investigations, edited by C. T. Russell, 2nd ed., 465–518. Dordrecht: Springer.Szalay, J. R., and Horányi, M. 2016. The Impact Ejecta Environment of near Earth Asteroids. The Astrophysical Journal 830: L29.Wylie, C. C. 1939. Where Do Meteorites Come from? Science 90: 264–65.