Asteroid Bennu could be hollow and doomed to collapse

NASA’s OSIRIS-REx spacecraft has managed to pick up a bucket full of the asteroid Bennu, and it will soon begin its journey back to Earth. Scientists will learn a lot about space rock once they get their hands on this sample, but we are already learning surprising things based on the data collected by OSIRIS-REx. New analysis from the OSIRIS-REx team at the University of Colorado at Boulder suggests the Bennu is much less stable than expected. In fact, it could completely collapse in the eons to come.

OSIRIS-REx arrived in Bennu’s orbit in late 2018 and immediately set to work to find a suitable landing zone. NASA scientists noted that Bennu’s surface was much harder than expected, teeming with boulders and fields of uneven rock debris strewn across the surface. The Japanese Hayabusa2 mission recently encountered similar conditions on Ryugu, but the Japanese spacecraft and OSIRIS-REx were able to find suitable sampling sites.

Last month, OSIRIS-REx successfully mined Bennu’s surface to grab some regolith. While we wait for this sample to return to Earth, the team were puzzled by Bennu’s behavior. Since OSIRIS-REx arrived in orbit, scientists have noticed the way particles are lifted off the surface as Bennu spins (see below). This phenomenon, along with measurements of the spacecraft’s gravity field, helped the team make interesting observations on this space rock.

Bennu’s density varies widely – the less dense areas are near the bulging equator and internally around the center of mass, according to the new study. Previously, scientists expected Bennu’s core to be at least as dense as the outer layers. Instead, there may be a void in the middle of Bennu the size of several football fields. This empty center is likely a consequence of Bennu’s rotation, which increases in speed as he moves through space near Earth. As he spins faster, the loose block of material that makes up Bennu might continue to fly apart until he no longer has enough gravity to stay intact.

This work is helping to educate scientists on how scientists will test Bennu’s sample when he returns to Earth. For example, the team will analyze the cohesion between grains, which could help us understand the physical properties of asteroids on a larger scale. What is it that keeps some of them stuck together for billions of years and causes others to fly? Scientists can begin this important work when the OSIRIS-REx sample container returns to Earth in 2023.

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