Ask most people what an asteroid is and they’ll probably say it’s a small space rock which is only dangerous if it hits Earth. Ask a businessperson the same question and they might say it’s a dead space rock full of rare minerals that belong to no one so they’re just waiting to be mined and brought back. Tell that to a NASA scientist working on the OSIRIS-REx mission which is currently orbiting the asteroid Bennu and they might say “Not so fast – this one is mysteriously exploding and firing rocks at us!” Is there some Little Prince on Bennu protecting his stuff? (Who? Google it.)
“No one has ever seen an active asteroid up close like this. It wasn’t that long ago that the conventional wisdom was that asteroids are these dead bodies that didn’t change very much.”
Well, Carl Hergenrother, an astronomer at the University of Arizona and the scientist who proposed Bennu as the target for OSIRIS-REx, doesn’t believe THAT anymore. In a new paper published in the journal Science and reviewed in Wired, Hergenrother tells us that Bennu has been observed regularly exploding tiny pieces of rocks off of its surface – some high enough that they enter temporary orbits around it. This was seen as soon as OSIRIS-REx itself entered orbit around Bennu in December 2018. Since a few other asteroids have been observed doing the same thing, astronomers assumed Bennu was either flinging rocks into space with its centrifugal force or steaming them there as ice particles quickly turned to gas. While that’s the case for other asteroids, neither fit Bennu.
“Our observations classify Bennu as an active asteroid. Active asteroids are commonly identified by major mass loss events observable with telescopes, on scales much greater than we observed at Bennu.”
“Active asteroids” are dead rocks that are still acting alive – space zombies. However, OSIRIS-REx’s close proximity has found that Bennu is not made up like other active asteroids. So, without ice or centrifugal force, why is it exploding rocks off of its surface?
“Plausible mechanisms for the large ejection events include thermal fracturing, volatile release through dehydration of phyllosilicates, and meteoroid impacts.”
Hergenrother and the other astronomers in the study noticed that the explosions generally occurred in what would be considered the late afternoon of a Bennu day. That’s when the surface, which has been baking in a non-atmosphere-blocking sunlight all day reaches the temperature when its rocks fracture. Phyllosilicates are parallel sheets of silicate minerals that are also affected by the extreme heat and facilitate the rocks breaking via shrinkage. Rocks flinging off the surface during other times are probably caused by tiny micrometeoroids hitting the surface of Bennu hard enough to knock other tiny rocks into space, only to have them fall back down and possibly knock more loose, creating a cycle of ever-decreasing spitting events.
Whatever is causing Bennu’s rock explosion, it’s not enough to abort the mission whose main purpose is for OSIRIS-REx itself to bang into its surface and extract rock and soil samples, then return then to Earth in 2023. Any further studies of the zombie asteroid effect will have to wait for another future expedition.
One thing is for certain – there’s no Little Prince on Bennu tending to its volcanoes, because there are no volcanoes on Bennu. If OSIRIS-REx spots a little man appears who laughs, who has golden hair and who refuses to answer questions, it’s someone else. (What? Google it.)