Bennu is now one of about a dozen active asteroids that share at least some characteristics with comets. The OSIRIS-REx team doesn't know what's causing Bennu particles to suddenly jump off of the surface. The first step in understanding the process is to trace the flying particles’ paths back to their sources.

The discovery was a surprise because these ejection events weren’t detected from Earth, though there was at least some speculation ahead of OSIRIS-REx's arrival that Bennu might be an active asteroid. Upon learning that Bennu was active, NASA performed an analysis using orbital debris models for Earth orbit to see if the particles posed any danger to OSIRIS-REx, and determined the spacecraft was safe.

Another interesting bit of trivia is that the particle ejections could, in theory, be associated with an annual meteor shower here on Earth; such a shower would happen in September, in the southern hemisphere.

Rotation rate

One of the mission's major goals was to measure the Yarkovsky effect, a force caused by the asteroid's continual absorption of solar radiation and re-radiation of heat as it rotates in and out of sunlight. The Yarkovsky effect is one part of a larger effect called YORP that can alter an asteroid's trajectory, which is important for tracking potentially hazardous near-Earth asteroids.

Bennu's rotation rate has been increasing since 1999, and now, OSIRIS-REx observations have concluded that this increase is caused by the YORP effect. Just how much of an impact does this force have on Bennu? Every 1.5 million years, its rotation rate will double! Scientists believe YORP can eventually spin asteroids fast enough to break them apart, and it could be a contributing factor behind Bennu's particle ejections, though Lauretta cautioned it's too early to tell.

Sample collection

As we've previously noted, Bennu is much more rugged than expected. Ground-based radio astronomy led the OSIRIS-REx team to believe the surface was mostly covered in centimeter-size particles, which informed the design of TAGSAM sample collector. The team now knows Bennu is covered in boulders, 200 of which have been found to have diameters greater than 10 meters.