The University-Arizona-led mission to test an asteroid many millions of miles from Earth is anything but a walk on the beach. In fact, Bennu is “not near the sandy beach we were hoping for and expecting,” Thomas Zurbuchen, associate director of NASA’s Science Mission Directorate, said during a Sept. 24 media event. As the spacecraft got closer and began sending the first detailed images of Bennus’ surface, it surprised both the mission team and the public by revealing a rocky surface filled with house-sized boulders.
Since arriving on December 3, 2018, the OSIRIS-REx spacecraft has spent its time flying around the asteroid, scanning, photographing, measuring and studying the dark, rocky rubble below – far from first and then close. Using its laser altimeter or OLA combined with data from images taken with the spacecraft’s PolyCam instrument, the mission has produced maps with unprecedented details, better than any planetary body visited by spacecraft. The mission’s primary test site, located inside a crater called Nightingale, was chosen based on these maps.
“We chose Nightingale because it has by far the most fine-grained material of all four test candidates,” said Dante Lauretta, the mission’s lead researcher and professor at the UArizona Lunar and Planetary Laboratory, during the press event. “We spent early 2020 with low-altitude reconnaissance over this location and eventually imaged about one-eighth of an inch per pixel. We basically have incredibly detailed images covering the entire crater, and we counted all those rocks.”
Shortly before 11 a.m. Arizona time on Oct. 20, thrusters on the spacecraft will shoot and gently push the OSIRIS-REx out of its orbit around Bennu and steer it down toward the rugged surface. This combustion will set in motion a series of events carefully planned by the mission team.
What happens if everything goes as planned? And if it does not?
Once the spacecraft has descended to its destination, it will rely on what the mission team calls a “danger map” – a detailed rendering of areas within the test area that may pose a risk to the spacecraft due to the presence of large rocks or uneven terrain. .
Just before touching the surface, the spacecraft compares images from one of its cameras with the hazard map stored in the spacecraft’s memory. If the descent path would result in the spacecraft moving down to a potentially unsafe location, the system would automatically trigger the spacecraft to return, a scenario that has a probability of less than 6% based on simulations.
If all goes well, the spacecraft expands its Touch-and-Go sampling mechanism or TAGSAM, which is suspended at the tip of an 11-foot-long arm. It is reminiscent of an air filter used in an older car and is designed to collect fine-grained material, but is capable of consuming material up to about three-quarters of an inch.
The sample is collected during a “touch-and-go” maneuver or TAG, where the sampling head comes into contact with the surface of the Bennus for about 10 seconds. When the spacecraft detects contact, it will fire one of three nitrogen gas cylinders, and much like an inverted vacuum cleaner, touching surface material – called regolith – inside the sampling head before the spacecraft smokes away.
As a backup, the sampling head has a series of small disks designed to pick up dust like sticky pads should something go wrong with the gas-powered sampling process.
The team will examine footage taken by the spacecraft’s sampling camera or SamCam of the sampling head as it comes in contact with the surface. SamCam is one of three cameras on board the spacecraft built in UArizona.
“We will be able to tell if we were tipped, if gas blew out to the side, if the material was stirred up sufficiently,” Lauretta said. “We also want a very good indication of the exact location in Nightingale where we contacted, and we can compare it with our sampling card to assess whether we touched in an area where there is ample sampling material or one of the more rocky places. “
SamCam will also be able to take pictures of the sampling head after the spacecraft has left the Nightingale crater and is at a safe distance from the asteroid. Because the sampling head is mounted on a wrist, the team can examine it in different directions relative to the sun and the sampling camera. The team will also see some dust or material on any other area of TAGSAM, on the arm or on the carpet above the gas cylinders, Lauretta explained.
“This will tell us if we moved enough material around when we got in touch, and maybe, maybe maybe, we will be able to see some of the particles inside of TAGSAM if the particles are in the right location inside. in the head and if we get the right lighting conditions. “
After TAG, the team then spends a week assessing how much sample was collected. It will use several methods to estimate the sample, starting with imaging the sample collection head for visual inspection. The team also performs checks on the spacecraft and the instruments to verify that it did not result in degradation to any of them.
A pirouette in the room
Next, the spacecraft is ready to perform a maneuver designed to give scientists on Earth an estimate of how much sample was collected. With the sampling arm extended, it will slowly rotate about an axis perpendicular to the TAGSAM to measure the change in mass attributable to the collected sample, compared to a previous measurement taken with the sampling head blank.
Due to uncertainty in the technique, the measurement result must exceed the required sample mass in order to have high confidence that a suitable sample is present.
“We will look for a 90% chance that we have 60 actual grams or more,” Lauretta said. “All in all, we want to have talks with NASA to assess the spacecraft’s status, its ability to go into another TAG and decide whether we want to return with what we have or go to another TAG. “
The spacecraft can perform several sampling tests as it is equipped with three bottles of nitrogen gas. For example, if it should touch a safe place but did not come up with a good sample, the team has developed emergency measures to ensure that the mission still meets its primary scientific goal: collect at least 60 grams (just under 2 ounces) of surface material and return it to Earth.
“If the decision is made, we have to go in again, we have to bring the spacecraft back into orbit and carry out a series of burns to line up its position in orbit for the next roof attempt,” said Mike Moreau, deputy project manager at NASA’s Goddard Space Flight Center in Maryland.
While Nightingale was identified as the best place to get a sample of the entire Bennu, it still offers many challenges, Lauretta said.
“By far the most likely result we will get on October 20 is that we will contact the surface and come away with a large sample that exceeds our requirements. But Bennu has already thrown us a number of basket balls, which is why we are fully prepared for to tag on the Osprey (backup page) if need be, ”he said.
Once the decision is made to store the sample, the team continues to place the head inside the sample return capsule and seal it for return to Earth in 2023. And when that time comes, the chances of it bringing even more back are at least 60 grams , as TAGSAM was designed to capture at least 150 grams and under optimal conditions up to 4 pounds – enough to keep generations of scientists busy in laboratories on earth.
US probe to touch the asteroid Bennu on October 20
Provided by the University of Arizona
Citation: Why getting an asteroid sample is harder than it looks (2020, October 16) Retrieved October 17, 2020 from https://phys.org/news/2020-10-scooping-asteroid-sample-harder.html
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