Bennu’s unexpected moving particles

19 January · processed NavCam view — NASA/Goddard/University of Arizona/Lockheed Martin
Processed image. Separate 19 January 2019 event. Publisher composite of a 1.4 ms asteroid exposure and a 5 s particle exposure, cropped and adjusted for brightness and contrast. Released 19 March 2019. This is camera-based imagery, not a trajectory animation; it is not a raw single exposure.
NASA mission imagery used for factual editorial and informational purposes under its media guidelines, with credit and no endorsement implied. No third-party copyright restriction is identified on this image product; not a grant for advertising or merchandising.
· Original image source · Image licence

Compare two January camera composites showing particles near Bennu, then explore how investigators reconstructed their paths and tested possible causes.

World: Asteroids
Case date or period: January–February 2019

A camera view with two exposures

Look beside the bright asteroid rather than only at its surface: faint points around Bennu became a new part of the investigation. The published 19 January 2019 view combines a 1.4-millisecond NavCam 1 exposure for the asteroid with a five-second exposure that reveals particles. Cropping and brightness adjustments make both layers visible together. It contains actual camera observations, but the published composite is a processed presentation. A point or a trail in the longer exposure should not be read as a sharp silhouette or a directly measured particle diameter.

From unexpected points to moving particles

Lauretta, Hergenrother and colleagues describe how particles were initially recognised in navigation-camera images after OSIRIS-REx entered orbit. They followed individual detections across successive images to distinguish moving material from background stars. Their 2019 paper characterises major events on 6 January, 19 January and 11 February. Those dates refer to different ejections, rather than one long continuous eruption. Particle positions, timing and brightness support the analysis, while estimates of size and energy depend on assumptions about reflectivity and density. A visible dot does not independently supply all of those physical properties.

Paths that return and paths that escape

The December 2019 mission account reports several tracked outcomes: some particles briefly orbited Bennu and returned, while others escaped. The three major events occurred in local late afternoon, although their inferred source areas differed. This timing suggested a connection with the heating cycle, without establishing one cause for every event. A trajectory illustration explains fitted paths, but it is an animation of an interpretation, separate from the navigation-camera record.

Competing mechanisms remain testable

The same mission account discusses meteoroid impacts, thermal fracturing and water release as candidate mechanisms. A temperature cycle could crack surface rocks; an impact could then launch fragments. Heating water-bearing material might create another route. The researchers explicitly allowed mechanisms to act together. That keeps the investigation open to combinations rather than forcing each event into a single explanation. The image demonstrates particles near the asteroid; determining the process that detached them requires comparison with material properties, timing and models.

Later tracking adds another use for the particles

NASA’s 2020 account describes extended trajectory work by Steve Chesley and colleagues. Some particles stayed nearby for days and completed multiple revolutions. Their motion sampled Bennu’s irregular gravity at distances closer than the spacecraft could safely approach. The team used those trajectories to refine gravitational estimates. This is a scientific inference from tracked motion, not a direct picture of gravity. The same report notes that particle sizes fit thermal-fracturing expectations while ejection locations also fit modelled meteoroid impacts, leaving the cause unresolved.

Separate an observation from a reconstruction

The 2019 paper builds an event history from intermittent camera measurements. Fitted paths connect detections and help identify possible surface sources. Reflectivity assumptions affect estimated sizes; observing cadence limits what can be followed. The two images here depict separate events, not successive frames of one flight. Use them to compare appearances, while keeping the paper’s reconstructed trajectories separate from these processed camera observations.