Asteroid Bennu may be hollow and doomed to collapse

NASA’s OSIRIS-REx spacecraft has collected a bucket full of asteroid Bennu, and will soon begin its journey to Earth. Scientists will learn a lot about a space rock once they get their hands on that sample, but we actually learn some surprising things based on the data collected by OSIRIS-REx. A new analysis by the OSIRIS-REx team from the University of Colorado at Boulder indicates that Bennu is much less stable than expected. In fact, it can crumble completely in the coming eons.

OSIRIS-REx reached Bennu orbit in late 2018 and immediately proceeded to search for a suitable landing area. NASA scientists noted that the Bennu surface was more difficult than expected, teeming with rocks and fields of uneven rock debris strewn around the surface. The Japanese Hayabusa2 mission encountered similar conditions on Ryugu recently, but both the Japanese spacecraft and OSIRIS-REx were able to find suitable sampling sites.

Last month, OSIRIS-REx successfully tapped the Bennu surface to grab a little regolith. While we wait for that specimen to return to Earth, the team has been puzzled over Benno’s behavior. Since OSIRIS-REx has reached orbit, scientists have observed the way particles are ejected from the surface during Bennu’s rotation (see below). This phenomenon, along with measurements of the probe’s gravitational field, helped the team make some interesting observations about this space rock.

According to the new study, the density of Bennu differs greatly – the less dense regions lie near the bulging equator and internally around the center of mass. Previously, scientists expected the Bennu core to be at least as dense as the outer layers. Alternatively, there may be a void in the center of Pino the size of many football fields. This empty center is likely the result of the rotation of Bennu, which speeds up as it advances through space near Earth. As it rotates faster, the bulk of the material that makes up the Bennu can continue to fly away until it loses enough gravity to remain intact.

This work helps see how scientists tested the sample from Bennu upon its return to Earth. For example, the team will analyze the coherence between the grains, which could help us understand the physical properties of asteroids on a larger scale. What keeps some stuck together for billions of years and causes others to disintegrate? Scientists will be able to start this important work when the OSIRIS-REx sample container returns to Earth in 2023.

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