SAN FRANCISCO, CALIFORNIA—In December 2018, the scientists behind the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), NASA’s $800 million mission to grab a sample from the asteroid Bennu and return the rock to Earth, received a nasty shock. Rather than smooth plains of beachlike rubble, Bennu’s coal-dark surface was a jumble of large boulders, with few gaps that might allow the spacecraft’s sampling arm to shoot in and sample its surface grit, as designed.
After extensively mapping the half-kilometer-wide asteroid, including flying by a “final four” of candidate sites at just 1-kilometer above its surface, mission leaders today announced their primary sampling target: a 16-meter-wide site called Nightingale that sits within a crater on the asteroid’s northern flank. If all goes as planned, in late August 2020 OSIRIS-REx will undertake a risky pogo stick–like maneuver, briefly touching down on Bennu’s surface, sucking fine grit into a sampling arm, then pushing off.
Surveys suggest the crater is filled with fine-grained material that appears freshly exposed at the surface, according to Dante Lauretta, the mission’s principal investigator and a planetary scientist at the University of Arizona in Tucson who announced the decision here at a meeting of the American Geophysical Union. And the site’s poleward location keeps it cool, meaning it could preserve water and organic materials from the Solar System’s earliest days.
Hitting the target will be a challenge. The original mission plan called for a landing on a flat site 50 meters wide, not just 16. And the crater’s edge has a wall of tall rocks up to 7 meters high—one of which Lauretta has nicknamed “Mount Doom”—which will pose a risk to the spacecraft, which could slightly tip after it attempts to sample the material. “We recognize this does have some hazards around it,” Lauretta says. “We’re trying to get into a crater that’s a few parking spaces wide.”
To hit the narrower “bull’s-eye” demanded by the site, the mission’s team developed a new navigation technique that relies on images of the surface. “They’re going to push all the capabilities of the spacecraft and that team to bring down the size of the sampling area,” says Lori Glaze, director of NASA’s planetary science division in Washington, D.C.
The team has also selected a backup sampling site, dubbed Osprey. Located on Bennu’s equator, Osprey is less likely to preserve organics, and its rocks look slightly bulkier. But it is free of Mount Doom–like obstacles. And it could become OSIRIS-REx’s destination if it automatically aborts the approach to Nightingale after detecting a hazard.
This is the second asteroid-sampling mission to face this boulder-size conundrum. The Japanese spacecraft Hayabusa-2 also discovered its asteroid, Ryugu, to be strewn with unexpected boulders. The mission was able to navigate around these boulders and sample the surface; last month, Hayabusa-2 began its trip back to Earth.
Prior to arriving at Bennu, OSIRIS-REx’s science team had anticipated that scientific priorities would shape the debate over what would be a bounty of sampling sites. And although hazards ended up dominating the conversion, “What really drove the [final] decision for me was the scientific value,” Lauretta says. Team members who wanted to study the effect of cosmic radiation on surface dust favored Sandpiper, another candidate site. But the promise of preserved pristine materials expected at Nightingale was too great to ignore. Though, given the hazards the spacecraft faces, the team will be happy to collect any type of sample and return it safely back to the Utah desert in 2023.
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