O-REX’s Bennu Sample Safely Tucked Away

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Samples of the asteroid Bennu collected by the OSIRIS-REx spacecraft are safely secure in their return canister for the more than 200 million mile trip back to Earth. So much material was obtained last week that it overfilled the container and some start drifting off into space. The government-academia-industry team quickly changed plans, skipping some steps and hastening others to make sure the bulk of it was stowed for the return trip home to scientists eagerly awaiting its arrival in 2023.

The Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx or O-REx) was launched in 2016. After arriving at Bennu in 2018, it spent almost two years orbiting the small asteroid to find the best place to grab a sample using its Touch-and-Go Sample Return Mechanism (TAGSAM). The goal was to collect at least 60 grams (2 ounces) and perhaps as much as 2 kilograms (4.4 pounds).

The asteroid was full of surprises, with a surface quite unlike what scientists expected.  Instead of a sandy beach, the surface is rugged with rocks. Finally, on October 20, the NASA-University of Arizona-Lockheed Martin team took the plunge, sending the spacecraft on the first of what could have been three attempts to touch the surface.

But the first time was the charm.  TAGSAM penetrated much further into the surface than anyone anticipated, at least 24 centimeters and perhaps twice that, completely filling the sample head with soil and rock — called regolith.  Using an onboard camera, mission controllers realized that some of the regolith was preventing a mylar flap on the spacecraft from closing and some of the particles were drifting away.

They had planned to spin the spacecraft to measure the change in its moment of inertia to calculate how much material was collected, but decided to forego that operation and an engine firing to put the spacecraft back into orbit around Bennu lest the motion liberate even more particles.

Instead, they focused intently on getting the sample head stowed in the Sample Return Canister (SRC), a slow, deliberate process. They planned to do it next week, but NASA project manager Rich Burns of Goddard Space Flight Center announced this afternoon that “we’ve successfully completed that operation.”

It was a slow, deliberate process. The spacecraft would execute a command, take images and send them back to controllers on Earth who would verify the step was completed properly, then send the next command and so forth.

NASA Planetary Science Division Director Lori Glaze and other team members today praised operators of NASA’s Deep Space Network (DSN) and the managers of other space missions that use the DSN for agreeing to change their plans to allow the DSN to support these unscheduled operations. The DSN consists of very large antennas in California, Spain and Australia specially designed to communicate with spacecraft far from Earth and every moment is allocated to specific tasks.  Glaze gave a shout out to Nataly Brandt, DSN scheduler at the Jet Propulsion Laboratory, for getting O-REx the time it needed.

Lockheed Martin’s Sandy Freund, O-REx mission operations manager, said today that was about 36 hours. Each step required roughly two hours in part because it simply takes a long time to communicate with O-REx, which is 330 million kilometers (207 million miles) away. The signal travel time to Earth is 18.5 minutes, or 37 minutes round trip.

Because they skipped the spin maneuver, scientists do not know exactly how much regolith is onboard. O-REx Principal Investigator (PI) Dante Lauretta of the University of Arizona believes it may be close to the maximum of 2 kilograms the SRC can hold.

Lauretta said today they can visually see 400 grams of material and that is in just 17 percent of the SRC’s volume visible to the onboard camera. He estimates only “10s of grams” were lost to space before the sample was stowed adding that while his “heart breaks” to lose any of the sample, it was a “cool science experiment” learning how the particles behaved in microgravity.

That is just the beginning. What they’ve learned so far, from millions of miles away, is that Bennu is unlike anything they expected. The regolith has no cohesion, no resistance. When TAGSAM touched the surface, the regolith “just flowed away like a fluid,” he said. TAGSAM was in contact with the surface for only 6 seconds, after which the spacecraft fired its thrusters to back away. Lauretta said if it were not for that maneuver, the spacecraft would have just kept going deeper into the asteroid.

“Luckily, we had those back away thrusters to reverse the direction of motion or we might have just flown all the way through the asteroid.” — Dante Lauretta

Bennu is a very small asteroid, a little taller than the Empire State Building.

O-REx’s primary task is asteroid science, but the results are also related to NASA’s planetary defense efforts through which the agency is responsible for finding, tracking and characterizing Near Earth Objects (NEOs) — asteroids and comets — that could pose a danger to Earth. Bennu is classified as a Potentially Hazardous Asteroid, but not for a very long time. It has a 1-in-2,700 chance of hitting Earth at the end of the 22nd Century. Asked what the findings mean for understanding how to mitigate threats posed by asteroids like Bennu, Lauretta said O-REx has provided a “phenomenal data set,” but mitigating asteroid threats is beyond the purview of this project and he will leave that to others.

Although O-REx completed its job on the first try and is drifting away from Bennu already, it will not fire its engines to begin the trip home until March 3, 2021 when Earth and Bennu are correctly aligned. Lauretta said they will relook at the orbital dynamics to see if there is an earlier opportunity, but no matter what, O-REx will return to Earth in September 2023. The SRC will separate from the rest of the spacecraft and descend under parachutes, landing at the Utah Test and Training Range (TTR) southwest of Salt Lake City nominally on September 24, 2023.

The capsule will undergo initial processing at the TTR before being flown to Johnson Space Center near Houston. NASA has a curation facility at JSC where all of its “astro materials” are kept. That includes the Moon rocks collected by Apollo crews, samples of a comet returned by NASA’s Stardust mission and of the solar wind collected by NASA’s Genesis mission, as well as particles from the asteroid Itokawa collected by Japan’s Hayabusa spacecraft. Hayabusa was the world’s first asteroid sample return mission.  Its follow-on, Hayabusa2, is on its way back to Earth right now after collecting samples from the asteroid Ryugu. It will land in Woomera, Australia on December 6, 2020. The Japan Aerospace Exploration Agency (JAXA) and NASA will exchange samples from Hayabusa2 and O-REx.

Lauretta hopes to get the O-REx canister opened “a few days” after it reaches JSC and then will spend the next six months cataloging what is inside so scientists can request samples to study.  He hopes the public will “join us in this great adventure” by watching through cameras in the curation facility as scientists find out just exactly what makes Bennu what it is.

Space Policy Online source|articles

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