The University of Arizona-led mission to sample an asteroid many millions of miles from Earth is anything but a stroll on the beach. As a matter of fact, Bennu “is nowhere near the sandy beach we had hoped for and expected,” said Thomas Zurbuchen, associate administrator for NASA’s science mission leadership, during a September 24 media event. Once the spacecraft approached and started sending the first detailed images of Bennu’s surface, it surprised the mission team and audience by revealing a rocky surface littered with house-sized boulders.
Since its arrival on December 3, 2018, the OSIRIS-REx spacecraft has spent its time flying around the asteroid as it scans, photographs, measures and studies the pile of dark and rocky debris below, first from afar, then up close. Using its laser altimeter instrument, or OLA, combined with image data taken with the spacecraft’s PolyCam instrument, the mission produced maps of unprecedented detail, better than those of any planetary body visited by a spacecraft. The mission’s main sample site, which is located inside a crater nicknamed Nightingale, was selected based on those maps.
“We selected Nightingale because, by far, it has the finest material of all four sample site candidates,” said Dante Lauretta, principal investigator of the mission and professor at the Arizona Lunar and Planetary Laboratory, at the press event. . “We spent the beginning of 2020 doing low-altitude reconnaissance passes on this site, eventually getting images at about an eighth of an inch per pixel. Basically we have incredibly detailed images that cover the entire crater and we counted all those rocks.” .
Shortly before 11am Arizona time on October 20, the spacecraft’s thrusters will fire and gently push OSIRIS-REx out of its orbit around Bennu and direct it toward the rugged surface. That burn will set in motion a sequence of events that has been meticulously planned by the mission team.
What if everything goes as planned? What if it doesn’t?
Once the spacecraft has set off on its descent to its target, it will rely on what the mission team calls a “hazard map,” a detailed representation of areas within the sample site that may present a risk to the spacecraft due to the presence of large rocks or uneven terrain.
Just before hitting the surface, the spacecraft will compare images from one of its cameras to the hazard map stored in the spacecraft’s memory. If the descent path led to the spacecraft landing at a potentially dangerous spot, the system would automatically trigger the spacecraft to back away, a scenario that has a probability of less than 6% based on simulations.
Hopefully, the spacecraft will extend its Touch-and-Go sample acquisition mechanism, or TAGSAM, which is suspended on the tip of an 11-foot-long arm. Reminiscent of an air filter used in an older car, it is designed to collect fine-grained material, but is capable of ingesting material up to about three-quarters of an inch.
The sample will be collected during a “touch and go”, or TAG, maneuver, during which the sampling head will make contact with the Bennu surface for approximately 10 seconds. When the probe detects contact, it fires one of three nitrogen cylinders and, just like a reverse vacuum cleaner, lifts the surface material, called regolith, inside the sampler head before the nacelle retracts.
As a backup, the sampling head is equipped with a series of small discs designed to collect dust as sticky pads, in case anything goes wrong with the gas-powered sampling process.
The team will examine footage captured by the probe’s sampling camera, or SamCam, of the sampling head as it makes contact with the surface. SamCam is one of three cameras aboard the spacecraft that were built in UArizona.
“We will be able to tell if we have been tilted, if the gas has come out to the side, if the material has been sufficiently agitated,” Lauretta said. “We will also have a great indication of the exact location in Nightingale where we made contact and can compare it with our sampling map, to assess whether we landed in an area where there is abundant sample material or in one of the more rocky locations. “
SamCam will also be able to capture images of the sampling head after the spacecraft has left Nightingale Crater and is at a safe distance from the asteroid. Since the sampling head is mounted on a wrist joint, the team will be able to examine it with different orientations than the sun and the sampling camera. The team will also see dust or material on any other area of the TAGSAM, on the arm or on the liner above the gas cylinders, Lauretta explained.
“This will tell us if we have moved enough material when we made contact, and maybe, just maybe, we will be able to see some of the particles inside TAGSAM, if the particles are in the right position inside the head and if we get the right lighting conditions. “
After the TAG, the team will spend a week evaluating the amount of sample collected. It will use several methods to estimate sample quantity, starting with imaging the sample collection head for visual inspection. The team will also carry out checks on the spacecraft and instruments to verify that they have not caused the degradation of either.
A pirouette in space
Next, the spacecraft is ready to perform a maneuver designed to give scientists on the ground an estimate of how much sample was collected. With the sampling arm extended, it will slowly rotate around an axis perpendicular to TAGSAM to measure the change in mass attributable to the collected sample compared to a previous measurement made with the sampling head empty.
Due to uncertainty in the technique, the measurement result must exceed the required sample mass to have a high degree of assurance that an adequate sample is present.
“We will be looking for a 90% chance of having an actual 60 grams or more,” said Lauretta. “Anything below that, we will have conversations with NASA to assess the spacecraft’s status, its ability to enter for a second TAG, and to decide whether we want to come back with what we have or make a second TAG attempt.”
The spacecraft can make multiple sampling attempts as it is equipped with three nitrogen gas cylinders. For example, if he were to land in a safe location but couldn’t find a good sample, the team developed emergency measures to ensure the mission still meets its primary scientific goal: to collect at least 60 grams (just under 2 ounces). ) of surface material and return it to the Earth.
“In case the decision is made, we have to re-enter again, we have to get the spacecraft back into orbit and conduct a series of burns to align its position in orbit for the next tag attempt,” said Mike Moreau, deputy manager. of the project at NASA’s Goddard Space Flight Center in Maryland.
While Nightingale has been identified as the best place to get a champion over all Bennu, it still presents a lot of challenges, Lauretta said.
“By far, the most likely outcome we will have on October 20 is that we will come into contact with the surface and come out with a large sample that exceeds our requirements. But Bennu has already thrown us a number of curved balls,” which is why we are completely ready to tag Osprey (the backup site) if necessary, “he said.
Once the decision is made to stow the sample, the team will proceed to place the head inside the sample return capsule and seal it for return to Earth in 2023. And when that time comes, it is likely to bring back even more than the minimum of 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 researchers busy in laboratories on Earth.
The US spacecraft will land on asteroid Bennu on October 20
Why Collecting an Asteroid Sample Is More Difficult Than It Looks (2020, Oct.16)
recovered on October 17, 2020
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