Oh, hey look! A Bennu story. Unless you have been living under a rock – and frankly the way this year has gone, who could blame you? – you know that OSIRIS-REx snagged a sample of Bennu’s surface last month. Well, that sample is not the only work being done in regards to Bennu and its history.

Using all of those images and the resulting rock maps you helped with, scientists at Southwest Research Institute have taken detailed measurements of centimeter- to meter-sized craters in some of those bigger boulders we saw. The goal was to understand more about the age of Bennu.

A reminder for those new to our show: Bennu is a rubble pile asteroid. It’s held together by gravity (barely) and could be the remnant of a larger asteroid that was hit by a bigger object. It actually has a lot of impact craters, albeit smallish ones, scattered across the surface, and these are most easily seen in those boulders I mentioned earlier. They are not easy to see, however, as our citizen scientists found out the hard way.

So the team at the University of Arizona developed a mathematical method to calculate how much of an impact a boulder of a given size and strength could handle without being broken apart. The result? The teams “brought together an understanding of the number of craters, the strength of the materials impacted, and the numbers of impactors to help constrain the chronology of Bennu’s existence in the inner Solar System at 1.75 million years.”

Bennu hasn’t been in the inner solar system long at all. Of course, we’ll have to wait for that sample return to arrive in 2023 to gain any further understanding of Bennu’s age and composition. I suspect we’ll keep hearing about our favorite asteroid to hate as institutes attempt to keep the OSIRIS-REx mission in the public eye now that the sampling portion of the mission is done.

More Information

Southwest Research Institute press release 

“Bennu’s Near-Earth Lifetime of 1.75 Million Years Inferred from Craters on Its Boulders,” R.-L. Ballouz et al., 2020 Oct. 26, Nature