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WASHINGTON — NASA’s OSIRIS-REx spacecraft has secured material it collected from the asteroid Bennu into a sample return capsule, a process the mission accelerated after images showed material leaking into space.
At an Oct. 29 briefing, project officials said they had stowed a sample collection device called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) into a capsule on the spacecraft. A robotic arm moved the TAGSAM head into position within the capsule, which then sealed shut.
That TAGSAM head touched down on the surface of the asteroid Bennu Oct. 20 for several seconds, and appeared to capture a large volume of material. However, officials said Oct. 23 that the TAGSAM had gathered so much material a Mylar flap designed to seal the material into place had been wedged open by several large rocks, causing some material to leak out, as seen in images from the spacecraft.
NASA then decided to accelerate the process of stowing the TAGSAM head into the capsule that will return the material to Earth, skipping a maneuver to weigh the sample. Controllers spent about 36 hours on Oct. 26 and 27 to use the robotic arm to move the TAGSAM head into position in the sample return capsule and verify it was locked in place, and then sealed the capsule.
“Stow is an intense period of operations. Our flight team worked around the clock to accomplish these in a much shorter period of time than we had envisioned,” said Sandra Freund, OSIRIS-REx mission operations manager at Lockheed Martin. A further complication is the spacecraft’s distance from Earth, with a round-trip light travel time of more than a half-hour.
That stow process was originally scheduled for early November, so moving it up meant securing time on the Deep Space Network (DSN) from other users. “There was a lot of last-minute renegotiations with several missions,” said Lori Glaze, director of NASA’s planetary science division.
Dante Lauretta, the principal investigator for the mission at the University of Arizona, said he was confident that, despite the leaking material, there was still a large amount of material in the TAGSAM head when it was sealed inside the capsule. “We have continued to analyze the images and do estimate that tens of grams of samples probably escaped throughout the entire sequence of operations,” he said.
The accelerated stow process meant that the project skipped a maneuver to slowly spin the spacecraft, measuring the change in its moment of inertia from before the sampling attempt and, from that, estimating the mass of material collected from the asteroid. That maneuver could have caused even more material to escape.
However, Lauretta said he was confident that the spacecraft has retained far more material than its goal of 60 grams. He said images of the inside of the head showed at least 400 grams of material. He added those images only covered 17% of the volume of the TAGSAM head. “Everywhere we can see into the TAGSAM we can see abundant sample in there,” he said. “We’re probably in excess of a kilogram of material.”
There may be other ways to estimate the mass of material, such as how the robotic arm moved during the stowage process. “We would definitely like to have an estimated sample mass before 2023,” when the spacecraft returns to Earth, Freund said.
Scientists have been analyzing the images taken during and after the sampling effort to better understand the surface. The material leaking out of the TAGSAM, he said, were “flaky” in appearance. “It looks like you dumped a box of cornflakes out in space, and they’re fluttering around, kind of in random motion,” he said.
The surface of Bennu itself is quite weak. The TAGSAM head plunged as deep as half a meter into the surface during its brief encounter. “There’s basically almost no forces between the grains that keep them bound together. They are not sticking to each other in any way,” Lauretta said. If an astronaut stepped onto the surface, “she would sink to her knees or deeper, depending on how loose the soil was, until you hit a larger boulder or some kind of bedrock.”
The deep plunge, though, likely allowed OSIRIS-REx to collect material from well below the surface. Such material, he said, would be protected from sunlight and be richer in volatiles.
With the material collected form Bennu secured inside the sample return capsule, the mission is now focused on returning those samples to Earth. The window for the spacecraft’s maneuver to depart the vicinity of Bennu opens in early March 2021. Rich Burns, OSIRIS-REx project manager at NASA’s Goddard Space Flight Center, said the window for that maneuver extends through May. The departure maneuver would bring the spacecraft back to Earth in September 2023.