Out in the asteroid belt, 207 million miles (334 million km) from Earth, a little spacecraft briefly touched down on the surface of Asteroid Bennu today, attempting to collect samples of dust and rocks.
NASA’s OSIRIS-REx spacecraft (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) spent about 10 seconds on the ancient asteroid to collect samples, slated to come back to Earth in 2023. While mission scientists and engineers need to confirm that samples from the asteroid were collected, preliminary data show that every step of the procedure went as planned.
“This was transcendental. I can’t believe we actually pulled this off,” said mission Principal Investigator Dante Lauretta during a livestream from the Mission Support room at Lockheed Martin in Denver, Colorado. “The spacecraft did everything it was supposed to do. Even though we have some work ahead of us to determine the outcome of the event, this was a major accomplishment for the team. I look forward to analyzing the data to determine the mass of sample collected.”
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Since Bennu is so far away from Earth, there was an 18.5-minute delay for the signal to get from the spacecraft to the Earth. A member of the team called out the milestones that were achieved as the signal was received. The entire touchdown and collection sequence was autonomous, with software on board to navigate OSIRIS-REx safely to the surface, enable the collection and then a backaway burn to bring the spacecraft away from the asteroid’s surface.
The entire event, from deorbit burn to sample collection, took about four and a half hours.
“It will be four and a half hours of anxiousness,” Beth Buck, OSIRIS-REx mission operations manager at Lockheed Martin Space, said in a news conference on Monday, comparing this spacecraft’s descent to the “seven minutes of terror” when a spacecraft lands on Mars.
OSIRIS-Rex touched down in a clear spot in a crater on Bennu’s northern hemisphere that is about the size of a small parking lot. The site, called Nightingale, is one of the few relatively clear spots on this unexpectedly boulder-covered space rock.
Real-time data indicated the spacecraft’s sampling arm, known as the Touch-And-Go Sample Acquisition Mechanism (TAGSAM), TAGSAM successfully contacted the surface and fired a burst of nitrogen gas. The gas should have stirred up dust and pebbles on Bennu’s surface, some of which should have been captured in the TAGSAM sample collection head. OSIRIS-REx engineers also confirmed that shortly after the spacecraft made contact with the surface, it fired its thrusters and safely backed away from Bennu.
The science team wants to collect at least 60 grams of regolith; the sample container can hold as much as 2 kg.
OSIRIS-REx engineers and scientists will use several techniques to identify and measure the sample remotely. First, they’ll compare images of the Nightingale site before and after TAG to see how much surface material moved around in response to the burst of gas.
“Our first indication of whether we were successful in collecting a sample will come on October 21 when we downlink the back-away movie from the spacecraft,” said Michael Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center. “If TAG made a significant disturbance of the surface, we likely collected a lot of material.”
Next, the team will try to determine the amount of sample collected. One method involves taking pictures of the TAGSAM head with a camera known as SamCam, which is devoted to documenting the sample-collection process and determining whether dust and rocks made it into the collector head. One indirect indication will be the amount of dust found around the sample collector head. OSIRIS-REx engineers also will attempt to snap photos that could, given the right lighting conditions, show the inside of the head so engineers can look for evidence of sample inside of it.
A couple of days later, the spacecraft will attempt yet another method to measure the mass of the sample collected by determining the change in the spacecraft’s “moment of inertia,” a phrase that describes how mass is distributed and how it affects the rotation of the body around a central axis. This maneuver entails extending the TAGSAM arm out to the side of the spacecraft and slowly spinning the spacecraft about an axis perpendicular to the arm. This technique is analogous to a person spinning with one arm extended while holding a string with a ball attached to the end. The person can sense the mass of the ball by the tension in the string. Having performed this maneuver before TAG, and now after, engineers can measure the change in the mass of the collection head as a result of the sample inside.
“We will use the combination of data from TAG and the post-TAG images and mass measurement to assess our confidence that we have collected at least 60 grams of sample,” said Rich Burns, OSIRIS-REx project manager at Goddard. “If our confidence is high, we’ll make the decision to stow the sample on October 30.”
To store the sample, engineers will command the robotic arm to place the sample collector head into the Sample Return Capsule (SRC), located in the body of the spacecraft. The sample arm will then retract to the side of the spacecraft for the final time, the SRC will close, and the spacecraft will prepare for its departure from Bennu in March 2021 — this is the next time Bennu will be well aligned with Earth for the most fuel-efficient return flight.
“This was an incredible feat – and today we’ve advanced both science and engineering and our prospects for future missions to study these mysterious ancient storytellers of the solar system,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate at the agency’s headquarters in Washington. “A piece of primordial rock that has witnessed our solar system’s entire history may now be ready to come home for generations of scientific discovery, and we can’t wait to see what comes next.”
This is not the first time asteroid samples have been collected for return to Earth. Japan’s Hayabusa spacecraft brought back about a millionth of a gram of dust from asteroid Itokawa in 2010. And coming up on December 6, 2020, Hayabusa 2’s sample return container holding up to a gram of material from asteroid Ryugu is set to be delivered to the Woomera Prohibited Area in South Australia.