The leak’s source was easy to spot. Rock-sized pieces of regolith were bulging the head’s Mylar flap ring partially open in several places. The flap was meant to allow material in—but not out. Nothing like this had happened during testing, which had included simulations of near-zero-G conditions using regolith-like materials, says Beau Bierhaus, Lockheed Martin’s Tagsam lead scientist. The particles appearing to hold the flap open were the right size and shape for collection. “I can’t think of anything that would have prevented the particles from being collected [inside the Tagsam head], other than there was no more room left at the inn,” Bierhaus says. “Because there was no more room inside, it got stuck.”
How might the Tagsam head have become so full? Because Bennu’s surface was a mystery to scientists before OSIRIS-REx arrived to scope it up close, Bierhaus and other Lockheed engineers had to design their collector head to bounce off and suction up a range of surface types, from ones similar to a hard-packed gravel driveway to ones softer than a fine, sandy beach. Before the team saw Bennu up close, they modeled its surface based on the 25103 Itokawa asteroid, sampled in 2005 by the first Japanese Hayabusa mission. “We were hoping to, in essence, scoop up a big bucket of soft sand,” says Ed Beshore, the former deputy principal investigator of the mission, now retired from the University of Arizona. Instead, pictures of Bennu’s surface taken by OSIRIS-REx’s cameras before the touch-and-go appeared to show a minefield of sharp boulders and rocks.
But Bennu had more surprises in store. In fact, based on the Tagsam’s deep bounce, it seems the surface material was not hard. In the asteroid’s microgravity environment, it instead behaved like a viscous fluid—thousands of marbles bouncing and scattering in low gravity. “If you push into it, it displaces and moves in ways we could not have anticipated,” Bierhaus says.
The head penetrated the first few centimeters of surface without much resistance. This, Moreau says, “preloaded the center of the Tagsam head with material, and then when the gas blew, all that stuff went into the head immediately.” As the arm continued downward another half meter through the yielding surface, more regolith might have been jammed in. “By the time we backed away, the head would’ve been packed full,” he continues. Another possibility, given the surprisingly viscous surface material, is that the regolith’s soft, malleable rocks wedged into the Mylar flap opening and weren’t able to make it all the way into the head, Moreau says.
Still, at HQ, there was some good news. Twenty to 30 minutes after the spacecraft stopped moving its Tagsam arm, the leak of material appeared to have died down. “Every time we moved the arm, we were shaking stuff loose,” Moreau says. Now the team ordered the ship to quiet itself, point toward Earth for easy communication, and “park” its arm in place. The team also canceled the upcoming sample mass measurement maneuver, which required extending the Tagsam arm and spinning the spacecraft—an action that was likely to spray debris out of the head in 360 degrees.
Confident the Tagsam had gagged up only a portion of its enormous bite, the team moved on to the next question: Assuming the head had been crammed full of material when it bounced off Bennu, and that the leakage had been caused largely by movement of the arm, how much of the sample had been lost? Were there at least 60 grams left to stow away?
To answer those questions without the measurement maneuver, five teams set about making estimates using alternate techniques. One group analyzed high-resolution imagery of the landing zone, down to the individual rocks, to model how many grams should have been collected; they estimated it was likely hundreds. Another group pored over photos of the Tagsam after the touch and go, peering into its visible area (about 40 percent of the container) to estimate the volume of the debris inside. The obstruction of light seen in a screen ringing the outside of the container offered another clue that the capsule might be close to full. One team estimated that the rocky material jammed in the Mylar flap was in the tens of grams—not enough to make up the necessary sample on its own, but a sizable prize. Another team used new 3D imaging techniques to estimate the size and mass of hundreds of particles shown escaping during the 10-minute imaging session just after the movement of the Tagsam arm, and found loss in the tens of grams—a “decent amount,” says Coralie Adam, the mission’s lead optical navigation engineer, but “we probably lost the smallest material that could escape through those gaps.”