Howdy, neighbor. Launched two years ago, NASA’s OSIRIS-REx spacecraft fired its thrusters today and pulled alongside the 800-foot-wide (244-meters) asteroid Bennu. It’s fascinating to see the similarities between Ryugu, the target of the Japanese Hayabusa-2 mission, and Bennu — both are strewn with rocks and boulders, both have a similar octahedral (doube-pyramid) shape and both possess a giant boulder that sticks out like a sore thumb.
Lots of study lies ahead. The spacecraft will do an initial survey of the asteroid from 4.3 miles up to determine essentials like mass, shape and spin, then drop in close in January — between 0.9 and 1.2 miles — to enter orbit for a year-long close-up investigation. In mid-2020, it will descend to the surface, grab a sample and then return it to Earth in September 2023.
Video showing a time-lapse of Bennu rotating. It spins once every 4.3 hours
Bennu is a carbon-rich asteroid about about the same distance from the sun as the Earth and the smallest solar system object ever to be orbited by a spacecraft. Scientists are interested in this ball of loose rock, because it contains pristine carbon material from the early solar system, possibly including organic compounds — basic materials essential for life. We’ve also got our eye on Bennu because it’s a potentially hazardous asteroid (PHA) that has a 1-in-2,700 chance of impacting Earth between 2175–2199. Obviously, we’d like to get to know it better to become better acquainted with its future movements as well as destructive power should the worst happen.
So now we’ve got two spacecraft keeping watch on two small asteroids with a third, New Horizons, ready to fly by the remote, Kuiper Belt asteroid Ultima Thule on New Year’s Day 2019. Looks to me like we’re in for an asteroid Renaissance.
Bennu’s shape and texture reminds me of the kind of found marshmallow that would make your mother shout “DON’T EAT THAT!” She’d be right, too. Is the common shape of Bennu and Ryugu easily explained or a mystery? Are their rotation rates similar? Thanks for all the great work.
Hi Norman,
Ha — that’s a good one! Kind of burnt-up, right? Their rotation rates are similar: 7.6 hours for Ryugu. Their shapes possibly resulted from impacts that loosened overlying boulders. These would have preferentially rolled “downhill” toward their asteroids’ equators, widening their middles and narrowing their tops.