Bits of big asteroid found on small one

Scientists poring over images from the asteroid 101955 Bennu, now being orbited by NASA’s OSIRIS-REx spacecraft, have found boulders that appear to be pieces blown off the asteroid Vesta billions of years ago.

Vesta is the second-largest asteroid in the Solar System, and lies well out in the heart of the Asteroid Belt. It measures 525 kilometres across, and while it is largely intact, it is known to have taken a pummelling from ancient impacts.

Bennu is only 490 metres across and is on an orbit that comes close enough to Earth that there is a small chance that it might hit us sometime late next century.

Figuring out how pieces of the one asteroid got to the other, scientists say, is an important clue to unravelling the history of Bennu, as well as probing the behaviour and evolution of asteroids during the history of the Solar System.

The boulders first attracted attention because they are much brighter than other materials on Bennu’s surface, Daniella DellaGiustina of the University of Arizona will tell this week’s virtual meeting of the American Astronomical Society’s Division of Planetary Sciences (DPS).

Even on the spacecraft’s early images of the asteroid, she says, these boulders stood out as bright spots, 1.5 to four metres in diameter. “There were six of them,” she says. “The brightest is almost 30 sigma [standard deviations] brighter than the mean [for Bennu’s surface].”

Or, to put it less technically, these boulders were about three to six times brighter than the bulk of Bennu’s surface (which is about the colour of charcoal).

But that proved to be just the beginning. “We have now identified about 77 [such] boulders and analysed 44 of them,” says Humberto Campins, a planetary scientist from the University of Central Florida, who is also part of the study team.

The analysis, he adds, involves using spectroscopic techniques to see how these boulders reflect different wavelengths of sunlight.

The OSIRIS-REx sampling arm TAGSAM poised above the surface of Bennu. Credit: NASA / Goddard / University of Arizona

The scientists have found that they contain the mineral pyroxene, a known component of Vesta – but the boulders probably didn’t come from chips of Vesta hitting Bennu itself. More likely, they are remnants of a larger piece that hit Bennu’s parent body – possibly even the one that smashed it into the bits – some of which later coalesced to form Bennu.

It’s an exciting find, he says, because until recently it had been believed that when an asteroid is hit hard enough to break it up, the impactor would be vaporised and leave no traces.

Now, that appears not to be the case, meaning that it may be possible to use this information to help figure out the process by which Bennu’s parent asteroid was destroyed.

Better yet, he says, it’s likely that Bennu is strewn with smaller chips of the same type of rocks. If so, it’s possible that when OSIRIS-REx briefly touched down on Bennu last week to snag a sample for return to Earth, it may have gotten a few of them.

If so, Campins says, the sample return mission might turn out to be an unexpected twofer: in effect, “a sample return mission to both Bennu and Vesta”.

Japan’s Hayabusa2 mission to asteroid 162173 Ryugu may have the same unexpected bonus. Ryugu also has bright boulders, Eri Tatsumi of the Instituto de Astrofísica de Canarias, Tenerife, Spain will tell the DPS meeting – though these appear to have come from a source other than Vesta.

For these twofers to occur, however, the samples have to get safely back to Earth. Japan’s is already en route, expected to parachute down in Woomera, South Australia, on 6 December.

NASA’s won’t arrive until 2023. But the agency says that it has collected so much material that a flap intended to seal its sample collection chamber after the sample was collected can’t close, causing it to expedite its process of stowing the sample away onboard, before too much precious material escapes into space.

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