NASA’s Osiris-Rex Spacecraft Just Arrived At Asteroid Bennu

We side one hemisphere of Bennu in this photo taken by NASA’s OSIRIS-REx probe today from a distance of 50 miles (80 km). The asteroid appears to be a conglomerate of rocks. Notice the big boulder sticking out along the lower left side. Click the image to watch the asteroid rotate. NASA/Goddard/University of Arizona

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.

In this view, Bennu has rotated so the large boulder is off the lower right now. Notice the interesting contrast between the more reflective rocks and the dark ones. I’m very curious what causes the difference in color and suspect it’s either composition, exposure to the solar UV light or both. NASA/Goddard/University of Arizona

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.

The Japanese probe Hayabusa-2 photographed asteroid 162173 Ryugu on June 26, 2018 from a distance of 12 miles (20 km). JAXA

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.

2 Responses

  1. Norman Sanker

    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.

    1. astrobob

      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.

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