Asteroid Bennu could be empty and doomed to collapse

NASA’s OSIRIS-REx spacecraft has successfully collected a full bucket of the asteroid Bennu and will soon begin its journey back to Earth. Scientists will learn a lot about space rock once they get their hands on that sample, but we are already learning some amazing things based on data collected by OSIRIS-REx. A new analysis by the OSIRIS-REx team at the University of Colorado Boulder suggests that the Bennu is far less stable than expected. Indeed, it could completely fall apart in the next eons.

OSIRIS-REx arrived in Bennu’s orbit at the end of 2018 and immediately set to work finding a suitable landing zone. NASA scientists noticed that Bennu’s surface was much more challenging than expected, filled with boulders and irregular rocky debris fields scattered across the surface. Japan’s Hayabusa2 mission recently encountered similar conditions on Ryugu, but both the Japanese spacecraft and OSIRIS-REx were able to find suitable sampling locations.

Last month, OSIRIS-REx successfully exploited Bennu’s surface to grab some regolith. While we wait for that champion to return to Earth, the team questioned Bennu’s behavior. Since OSIRIS-REx first came into orbit, scientists have noticed how particles are lifted off the surface as Bennu rotates (see below). This phenomenon, along with measurements of the probe’s gravitational field, helped the team make some interesting observations on this space rock.

According to the new study, Bennu density varies considerably: the less dense areas are close to the protruding equator and internally around the center of mass. 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 is increasing in speed as it flows through space near Earth. As it spins faster, the clump of material that makes up Bennu may continue to fly away until it has enough gravity to remain intact.

This work is helping to inform 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 larger-scale asteroids. What keeps some of them stuck together for billions of years and causes others to drift apart? Scientists can begin this important work when the OSIRIS-REx sample container returns to Earth in 2023.

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