Why OSIRIS-REx Will Not Land on the Bennu Asteroid


Without a doubt, the retrieval of a sample from an asteroid is the highlight of NASA’s OSIRIS-REx mission being launched Thursday. A pair of briefings held Tuesday afternoon at Kennedy Space Center only served to underscore that point: the spacecraft’s descent to near-Earth asteroid Bennu to collect some of that sweet meteor dust is “the culmination of the mission,” Dante Lauretta, OSIRIS-REx principal investigator from the University of Arizona, Tucson, told attendees.

But there’s one thing worth making clear about this step of the mission: when the OSIRIS-REx spacecraft actually does collect an asteroid sample in July 2020, it won’t land on Bennu. Instead, the whole maneuver will look something like a pogo-stick bounce lasting just a few seconds. In that short span, we could see either one of the most impressive space exploration achievements met, or utter failure.

Here’s the deal: after OSIRIS-REx launches on Thursday, it will spend a little less than two years orbiting the sun and getting a gravity assist from Earth before getting to Bennu in August 2018. The spacecraft will then spend about two more years orbiting Bennu and mapping it — and in the process, select 12 different candidate sites that might make for good sample retrieval locations. The instruments designed for such precision measurement are incredibly good. “We will be able to see an object the size of a penny on Bennu,” said Daniella DellaGiustina, OSIRIS-REx lead image processing scientist at the University of Arizona, Tucson.

Once a target is selected, the real fun begins. Rich Kuhns, OSIRIS-REx program manager for Lockheed Martin Space Systems in Denver, was part of the team to design TAGSAM — short for Touch-And-Go Sample Acquisition Mechanism. It’s basically a 10-foot arm with a pogo assembly that he and his team have been testing out for 10 years for this very moment.

OSIRIS-REx nearing the asteroid


When OSIRIS-REx descends to Bennu’s surface, TAGSAM will be used to make contact with the rock and slow movement down — like a pogo stick. The head of the instrument will then release a gas that helps move the asteroid rock and dust around and also creates a reverse vacuum that helps collect this material into TAGSAM’s head. The pogo mechanism moves back up, and OSIRIS-REx goes back into space and conducts a spin maneuver to verify it’s collected at least 60 grams worth of asteroid rock.

The descent of TAGSAM


Christina Richey, OSIRIS-REx deputy program scientist, described it as “a safe, smooth, slow high-five.” There is a potential to go back in case they’ve missed that 60-gram mark and try again, as long as there’s enough propellant to make that possible. But, “once we have that, we’re not going to touch back again,” said Richey.

Why not collect more? And why not land on Bennu itself like the ESA’s Rosetta mission did for comet 67P? Jason Dworkin, OSIRIS-REx project scientist at Goddard, summed NASA’s thoughts perfectly: “one [sample] is a lifetime of data anyway. When you bring a sample back to Earth, you can use laboratories the size of buildings” to study it in detail. “People not yet born … can test things in ways we haven’t even conceived of.”

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