Thursday, January 28, 2021

How NASA scrambled to save OSIRIS-REx from a disaster caused by the leak

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The source of the leak was easy to spot. Rock-sized chunks of regolith swelled the partially open Mylar flap ring of the head in several places. The shutter was intended to allow material to enter, but not to exit. Nothing like this happened during testing, which included simulations of near-zero conditions using regolith-like materials, says Beau Bierhaus, senior scientist at Lockheed Martin’s Tagsam. The particles appearing to hold the flap open were the correct size and shape for collection. “I can’t think of anything that would have prevented the collection of particles [inside the Tagsam head]except that there was no more room at the inn, ”says Bierhaus. “Because there was no more room inside, he got stuck.”

How did Tagsam’s head get so full? Because Bennu’s surface was a mystery to scientists before OSIRIS-REx came to study it closely, Bierhaus and other Lockheed engineers had to design their manifold head to bounce and suck a range of surface types, from those similar to gravel driveways to those softer than a sandy beach. Before the team saw Bennu up close, they modeled his surface based on asteroid 25103 Itokawa, sampled in 2005. by the first Japanese Hayabusa Mission. “We were basically hoping to pick up a big bucket of soft sand,” says Ed Beshore, the mission’s former deputy principal investigator, now retired from the University of Arizona. Instead, photos of Bennu’s surface taken by OSIRIS-REx’s cameras before he touch and go appeared to show a minefield of sharp boulders and boulders.

But Bennu had other surprises in store. In fact, based on the Tagsam’s deep bounce, it looks like the surface material was not hard. In the asteroid’s microgravity environment, it behaved more like a viscous fluid – thousands of balls bouncing and scattering in low gravity. “If you sink into it, it moves and shifts in a way we couldn’t have anticipated,” Bierhaus says.

The head has penetrated the first few centimeters of the surface without too much resistance. This, says Moreau, “preloaded the center of Tagsam’s head with material, and then when the gas blew out, all that stuff immediately went into the head.” As the arm continued to descend half a meter through the elastic surface, more regolith could have been stuck. “By the time we stepped back, the head would have been full,” he continues. Another possibility, given the surprisingly viscous surface material, is that the soft, malleable rocks of the regolith got stuck in the opening of the Mylar shutter and were not able to fully penetrate the head, says Moreau. .

Yet at headquarters there was good news. Twenty to 30 minutes after the spacecraft stopped moving its Tagsam arm, the material leak appeared to have subsided. “Every time we moved our arm, we were shaking things,” says Moreau. Now the team has ordered the ship to calm down, point to Earth for easy communication, and “park” its arm in place. The team also canceled the next sample mass-measuring maneuver, which required extending Tagsam’s arm and spinning the spacecraft – an action that was likely to throw debris out of the head to 360 degrees.

Confident that the Tagsam had gagged only part of their huge bite, the team moved on to the next question: assuming the head was full of material when it bounced off Bennu, and the leak had been caused largely by arm movement, how much of the sample was lost? Was there at least 60 grams left to store?

To answer these questions without the measurement maneuver, five teams set about making estimates using alternative techniques. One group analyzed high-resolution images of the landing zone, down to individual rocks, to model the number of grams that should have been collected; they estimated it was probably hundreds. Another group looked at photos of the Tagsam after touch-and-go, scanning its visible area (about 40% of the container) to estimate the volume of debris inside. The obstruction of light seen in a sounding screen outside the container offered another clue that the capsule may be nearly full. One team estimated that the rock material stuck in the Mylar shutter weighed a few dozen grams – not enough to make the necessary sample on its own, but a hefty price tag. Another team used new 3D imaging techniques to estimate the size and mass of hundreds of particles that escaped during the 10-minute imaging session just after the Tagsam arm movement, and found a loss of the order of tens of grams – a “decent amount”. said Coralie Adam, the mission’s senior optical navigation engineer, but “we probably lost the smallest piece of equipment that could escape through these gaps.”

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