Asteroid Bennu Might Be Hollow and Doomed to Crumble

NASA’s OSIRIS-REx spacecraft has successfully scooped up a bucket-full of the asteroid Bennu, and it’ll begin its journey back to Earth soon. Scientists will learn a great deal about the space rock once they get their hands on that sample, but we’re already learning some surprising things based on data collected by OSIRIS-REx. A new analysis from the University of Colorado Boulder’s OSIRIS-REx team suggests the Bennu is much less stable than expected. In fact, it could completely go to pieces in the coming eons.

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

Last month, OSIRIS-REx successfully tapped the surface of Bennu to grab a bit of regolith. While we wait for that sample to get back to Earth, the team has been puzzling over the behavior of Bennu. Ever since OSIRIS-REx arrived in orbit, scientists have noted the way particles are being heaved off the surface as Bennu spins (see below). This phenomenon, along with the probe’s gravity field measurements, have helped the team make some interesting observations about this space rock.

According to the new study, Bennu’s density varies considerably — the least dense zones are near the bulging 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 careens through space near Earth. As it spins faster, the loose clump of material that makes up Bennu could continue flying apart until it no longer has enough gravity to remain intact.

This work is helping inform how scientists will test the sample from Bennu when it 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 keeps some of them stuck together for billions of years and causes others to fly apart? Scientists will be able to start this important work when the OSIRIS-REx sample container returns to Earth in 2023.