Asteroid Bennu Is A Lot More Active Than Previously Thought

This mosaic image of asteroid Bennu is composed of 12 images collected from a range 24 kilometers. NASA/Goddard/University of Arizona

NASA’s OSIRIS-REx is getting ready to touchdown on asteroid Bennu next month and the team has just released some more insights into the asteroid. Researchers report that Bennu is losing pieces of rocks into space at a higher rate than expected.

In a series of papers published in the Journal of Geophysical Research: Planets, the OSIRIS-REx team studying Bennu describes the release of these particles, what causes them, and how they've been using them to understand Bennu in greater detail.

The team tracked hundreds of ejected particles and they have a bit of a mystery on their hands. The size of the particles matches what is expected to break away from an asteroid like Bennu due to thermal fracturing. The asteroid rotates on its axis in just over four hours, so its surface goes from hot to cold quickly, cracking its rocks. However, the location of the ejection events matches a model that sees these releases being down to the impacts of micrometeoroids on the surface of Bennu. Researchers are unsure which is the true cause. Maybe it's a combination of both.

But that's is not all. The particles have also helped the team study the extremely weak gravitational pull of the asteroid. They move far closer to Bennu than it would be safe for OSIRIS-REx so can provide a more accurate account of the asteroid’s irregular gravity.

"The particles were an unexpected gift for gravity science at Bennu since they allowed us to see tiny variations in the asteroid's gravity field that we would not have known about otherwise," said the lead author of one of the papers Steve Chesley, from NASA's Jet Propulsion Laboratory, in a statement.

These pieces of rocks are roughly 7 millimeters (0.25 inches) across. The team reports how some are thrown into space and come back down shortly, others stay around in orbit for a few days, others hop across the surface, and some are just lost into space.

"We thought that Bennu's boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us," said Dante Lauretta, the OSIRIS-REx principal investigator and a professor at the University of Arizona. "We've spent the last year investigating Bennu's active surface, and it's provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave."

OSIRIS-REx is expected to fly down to the surface of Bennu on October 20 to collect a sample. It will then return to Earth on September 24, 2023, making it the third spacecraft ever to bring a sample of an asteroid home.

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