Today, NASA’s OSIRIS-REx team is readying to perform an early stow of a sample from asteroid Bennu, a process originally scheduled for November 2.
"The abundance of material we collected from Bennu made it possible to expedite our decision to stow,” OSIRIS-REx mission lead and University of Arizona planetary scientist Dante Lauretta said in a NASA press release on Monday. “The team is now working around the clock to accelerate the stowage timeline, so that we can protect as much of this material as possible for return to Earth."
On Tuesday, Oct. 20, NASA’s OSIRIS-REx spacecraft extended its 11-foot arm and touched Bennu, an asteroid about the size of the Empire State Building. From it, OSIRIS-REx autonomously collected a sample of rocks, dust, and debris.
Though collection didn’t require human oversight and input, stowing the sample is done in stages and will require aid from the team back on Earth. Scientists will send commands to the spacecraft to begin the process, and with each step of stowing, OSIRIS-REx will send the team data and images, waiting for a command to proceed.
Why stow early?
“We spent years preparing for this, analyzing every scenario,” Coralie Adams, the OSIRIS-REx TAG navigation manager, told NOVA about the sampling mission.
But last week’s scenario may be surprising even to the OSIRIS-REx team: The spacecraft may have collected too much of asteroid Bennu, causing the spacecraft’s collection container to jam. On Thursday, OSIRIS-REx scientists received images that showed some of the sample had leaked into outer space from the collection head, which wasn’t able to fully close.
“A substantial amount of sample is seen floating away,” Lauretta said on Friday.
The OSIRIS-REx team aimed to collect at least 60 grams (2.1 ounces) of rock, dirt, and dust from Bennu, an asteroid about 200 million miles away from Earth. Despite the distance, Bennu is considered a near-Earth object (NEO) with a 1 in 2,700 chance of striking our planet in the late 2100s. (Andrea Riley, NASA’s DART mission program executive, says “there’s no concern to be worried” given that chance.) Through collecting the sample and analyzing it back on Earth, scientists hope to better understand the threat other NEOs may pose—and whether asteroids can reveal the history of planetary formation in our solar system.
Collecting a big sample from Bennu was key, Hal Levinson of the Southwest Research Institute in Boulder, Colo., said in a NASA press conference on Monday, Oct. 19. “When I got into this a long time ago, we used to think the planets formed where we see them. What really happened is like someone picked up the solar system and shook it real hard. To really put the story together requires having a big sample” from Bennu and eventually other asteroids in our solar system, he explains.
To collect a sample from Bennu, a team at Lockheed Martin Space in Littleton, Colo., spent years developing OSIRIS-REx’s arm and its asteroid sampling method, collectively known as the Touch-and-Go Sample Acquisition Mechanism (TAGSAM). Rather than having the entire spacecraft touch down on Bennu, whose radius is a mere 861 feet, the team employed TAGSAM to reach out, touch, and ultimately collect material from the asteroid’s Nightingale Crater.
“The last 10 minutes, as the spacecraft was descending towards the surface of Bennu, was unreal,” Lauretta said. “It was this slow-motion thrill ride,” Beau Bierhaus, an OSIRIS-REx TAGSAM scientist added.
At TAGSAM’s end is a foot-wide collection head like an “air filter,” Lauretta explains. This blew high-pressure nitrogen gas as it settled down on Bennu on Tuesday, disturbing the collection site and sucking up as much rock, dirt, and dust as it possibly could in just 6 seconds on the surface.
The cameras reveal that OSIRIS-REx is carrying up to 2 kilograms (2.2 pounds) of sample, which is at its upper limit. “About 400 grams seems visible from the cameras. The collection lid has failed to close properly and remains wedged open by pieces that are up to three centimeters in size, creating a centimeter-wide gap for material to escape,” Neel V. Patel writes for MIT Technology Review. “It seems when OSIRIS-REx touched down on Bennu’s surface, the collection head went 24 to 48 centimeters deep, which would explain how it recovered so much material.”
Most of the escaped material—as much as 10 grams, Lauretta believes—got out because of movements of OSIRIS-REx’s arm on Thursday. Fortunately, the arm is now in a locked position, which should eliminate additional losses. Given the size of the sample, even with some of it having drifted into space, the team will not pursue a second sampling attempt. Nor will it proceed with its Sample Mass Measurement activity, in which the spacecraft was going to pirouette in a number of spin maneuvers, allowing scientists to estimate the mass of the sample. Realizing that such movement could result in more sample loss, the team decided to cancel the procedure.
OSIRIS-REx has been orbiting Bennu since December 2018. In March, the spacecraft will leave and make its way back to Earth. More than two years later, as it passes over the Utah desert, OSIRIS-REx will drop off a small capsule containing its bounty of rocks and dirt, which will parachute to a landing—and a team of eager scientists—on Sept. 24, 2023.