No one knows why rocks are exploding from asteroid Bennu

Enlarge / A composite image of a short and long exposure photograph of Bennu showing the largest particle ejection on January 6, 2019. (credit: NASA | Goddard | University of Arizona | Lockheed Martin)

For the last year, NASA's OSIRIS-REx spacecraft has been circling a large asteroid named Bennu that regularly passes uncomfortably close to Earth. The spacecraft has been painstakingly mapping the asteroid's rocky surface using a suite of cameras and other instruments that will help it determine where to land next year. Once NASA selects a final landing site, OSIRIS-REx will kiss Bennu just long enough to scoop up a sample to bring back to Earth in 2023.

Many scientists expect the Bennu sample to revolutionize our understanding of asteroids, especially those that are near Earth and pose the greatest threat from space to life as we know it. But as detailed in a paper published this week in Science, NASA has already started making surprising discoveries around this alien world. Earlier this year, the OSIRIS-REx team witnessed particles exploding from the asteroid's surface-and the team's not sure why.

"No one has ever seen an active asteroid up close like this," says Carl Hergenrother, an astronomer at the University of Arizona and the scientist who proposed Bennu as the target for OSIRIS-REx. "It wasn't that long ago that the conventional wisdom was that asteroids are these dead bodies that didn't change very much."

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