Asteroid (101955) Bennu is a near-Earth object and the target of NASA’s OSIRIS-REx sample return mission. In January 2019, navigational images returned by the spacecraft revealed that Bennu is active, or shedding mass, in the form of particles ejected from its surface in discrete events. The discovery sparked a groundswell of intellectual effort to characterize the properties and behavior of the ejected particles and understand the cause(s) and broader implications of the ejection events. I asked two of the conveners of the new special collection in Journal of Geophysical Research: Planets and Earth and Space Science– Carl Hergenrother, the OSIRIS-REx astronomy lead, who made the discovery, and Dante Lauretta, the OSIRIS-REx Principal Investigator – a few questions about Bennu’s activity and what it means for asteroid science.
What causes an asteroid to be “active”? Why would an asteroid shed mass in an ongoing way?
Not long ago, a small body in the solar system that was shedding mass would have been automatically called a comet. Comets formed in the outer solar system and have a large fraction of ice. The ice sublimates as they move into the inner solar system, producing activity. Asteroids, which either formed closer to the Sun or have been trapped in the inner solar system for a long time, were thought to be devolatilized, or completely dry. So, when it became clear in 1983 that Phaethon—an asteroid whose orbit does not extend anywhere near Jupiter—was the parent of the Geminids meteor shower, elaborate dynamical histories were invoked to explain how this object was really a comet. But over time, as asteroid surveys went digital, more objects that either formed in the inner solar system or had been there for billions of years were found to be active.
So a simpler explanation emerged: that there are mechanisms by which an inner solar system body can shed mass noticeably. Although hydration state can be a factor, it’s not a requirement for activity; for example, impacts or rapid rotation–driven landslides can create comae and tails, sometimes emerging in different directions from the same body.
It was suspected that Bennu might be active before the OSIRIS-REX mission began, yet the discovery of particle ejection came as a surprise. What makes the activity at Bennu, and the circumstances of its discovery, so different from what we might have expected?
Pre-encounter spectral observations indicated that Bennu is a B-type asteroid. Some members of this family of slightly blue (negative spectral slope) carbonaceous asteroids have a propensity toward comet-like activity. No such activity on Bennu was observed from the ground, but it was possible that we could see it at a closer distance, and every space mission must check for hazards. So, we performed a search designed to detect long-lived (stable) satellites and comet-like dust plumes as the OSIRIS-REx spacecraft approached Bennu in fall 2018 . We found nothing.
But unexplained off-body returns from the OSIRIS-REx Laser Altimeter in December 2018 raised suspicions that something might be afoot. In January 2019, a search for a public release–ready shot of Earth and the Moon in navigation images acquired from orbit ended up revealing what looked like a star cluster where no stars should be. It was not the expected dust plume but what turned out to be a spray of discrete, millimeter- to centimeter-scale particles.
More particles were subsequently observed emanating periodically from different locations on the asteroid’s surface [Hergenrother et al., 2020; Liounis et al., 2020; Pelgrift et al., 2020; Leonard et al., 2020]. The observed particles’ orbits lasted for days at most [Chesley et al., 2020], although simulations suggest longer orbits are possible [McMahon et al., 2020].
The animation below (created by M. Brozovic / JPL / Caltech / NASA, with data from Chesley et al. ) shows the four largest particle ejection events detected at Bennu from December 2018 through September 2019.
While it was not a surprise that Bennu was active, ground-based observations had not prepared us to expect this particular expression of activity, which could only have been detected by a spacecraft in close proximity [Hergenrother et al., 2020].
What does Bennu’s activity say about its past, its future?
The answer to this question is related to the plausible causes of Bennu’s observed activity.
Meteoroid impacts [Bottke et al., 2020], thermally driven rock fracture [Molaro et al., 2020; Rozitis et al., 2020], volatile release by dehydration of phyllosilicate rocks, and the less likely (in Bennu’s case) water ice sublimation [Rozitis et al., 2020] all correspond to distance from the Sun—and thus all should drive an increase in activity at Bennu’s perihelion of 0.89 AU, making it difficult to disentangle these candidate mechanisms.
Further complicating things is that Bennu’s aphelion is closer to the Sun than most comets are, so there might be no point in its orbit where activity driven by these mechanisms would drop off completely. We might see just a modest increase at perihelion, as indeed appears to be the case [Hergenrother et al., 2020], although controlling for observational bias is still important work to go.
During Bennu’s early history in the main belt, before its transit to the inner solar system, its surface would not have been hot enough to enable thermal fracturing or volatile outgassing. Whether it would have experienced more or fewer meteoroid impacts like those that may be driving particle ejection today is unclear.
Whatever the mechanism, Bennu is shedding mass at a very low rate. Unless particle ejection is happening in a preferential direction not yet observed, it is unlikely to contribute meaningfully to Bennu’s orbital drift or spin state [Scheeres et al., 2020]. The potential loss via electrostatic lofting of particles smaller than those observed in the ejection events is an ongoing area of study.
Given enough time, mass shedding could conceivably reduce the asteroid down to nothing, but given the observations, it is far more likely that Bennu will collide with a larger object or fall into the Sun first.
What are the next steps for the field? Where do we go with this discovery?
Bennu’s activity poses the immediate question of whether this is a common phenomenon for asteroids in the inner solar system. If impacts are the cause, or a cause, every near-Earth asteroid should be shedding mass similarly to Bennu—meaning there should be particles from all these asteroids floating through near-Earth space. This could be explored with a systematic campaign to collect interplanetary dust particles via stratospheric flights.
The OSIRIS-REx mission was not designed to detect Bennu’s kind of activity, and its primary focus is sample collection, so particle observations have been on a best-effort basis. Now that we know this kind of activity is possible, future small-body missions (to Bennu or elsewhere) could include imagers and spectrometers expressly designed for observing the particle ejection phenomenon.
We can also investigate from the ground by improving Earth-based meteor and fireball monitoring capabilities to try to detect meteor showers from Bennu and other near-Earth objects. The OSIRIS-REx mission funded nine new camera systems in the southern hemisphere for CAMS (Cameras for Allsky Meteor Surveillance) toward this end.
—Catherine W. V. Wolner ([email protected]), OSIRIS-REx chief editor and JGR Planets guest associate editor; Carl W. Hergenrother ( 0000-0002-9120-7212), OSIRIS-REx astronomy lead; and Dante S. Lauretta ( 0000-0002-2597-5950), OSIRIS-REx PI, Lunar and Planetary Laboratory, University of Arizona, USA