The University of Arizona’s mission to sample an asteroid many millions of miles from Earth is anything but a walk on the beach. In fact, Bennu “is not nearly the sandy beach we had hoped and expected,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate, during a September 24 media event. As the spacecraft approached and began sending back the first detailed images of Bennu’s surface, he surprised the mission team and the public by revealing a rocky surface dotted with stones the size of a house.
Since its arrival on December 3, 2018, the OSIRIS-REx spacecraft has spent its time flying around the asteroid while scanning, photographing, measuring and studying the dark rocky pile below – first from afar, then up close. Using its laser altimeter, or OLA, combined with data from images taken with the spacecraft’s PolyCam instrument, the mission has created maps with unprecedented detail, better than any planetary body visited by a spacecraft. The main sample of the mission, which is located in a crater called Nightingale, was selected based on these maps.
“We chose Nightingale because it has the finest material of all four sample candidates to date,” said Dante Laureta, the mission’s principal investigator and professor at UArizona’s Lunar and Planetary Laboratory, during the press conference. “We spent the beginning of 2020 making low-altitude reconnaissance passes over this site, eventually taking images of about one-eighth of an inch per pixel. In general, we have incredibly detailed images covering the entire crater, and we’ve counted all of these. rocks. “
Shortly before 11 a.m. in Arizona on October 20, the spacecraft’s thrusters will launch and gently push OSIRIS-REx out of its orbit around Bennu and direct it down to a solid surface. This burning will trigger a series of events that have been meticulously planned by the mission team.
What happens if everything goes according to plan? And if it doesn’t?
Once the spacecraft is on its way to its destination, it will rely on what the mission team calls a “hazard map” – a detailed presentation of areas within the test site that may pose a risk to the spacecraft due to the presence of large rocks or uneven terrain.
Just before it touches the surface, the spacecraft will compare images from one of its cameras with the hazard map stored in the spacecraft’s memory. If the descent path causes the spacecraft to touch a potentially dangerous location, the system will automatically trigger the spacecraft to retreat, a scenario that has a probability of less than 6% based on simulations.
If all goes well, the spacecraft will expand its Touch-and-Go or TAGSAM sampling mechanism, which hangs on top of an 11-foot-long arm. Reminiscent of an air filter used in an older car, it is designed to collect fine-grained materials, but is capable of absorbing material up to about three-quarters of an inch.
The sample will be collected during a “touch and move” maneuver or TAG, during which the test head will come into contact with the Bennu surface for about 10 seconds. When the spacecraft finds contact, it will launch one of the three bottles of nitrogen gas and, like a reverse vacuum cleaner, stir the surface material – called regolith – into the probe’s head before the spacecraft retreats.
As a backup, the sampling head includes a series of small discs designed to collect dust as sticky pads in case something goes wrong with the gas sampling process.
The team will examine images taken from the spacecraft’s sampling chamber, or SamCam, of the test head when it comes in contact with the surface. SamCam is one of three cameras aboard the spacecraft built in UArizona.
“We will be able to find out if we were tilted, if the gas blew from the side, if the material was stirred enough,” Loretta said. “We will also have a very good indication of the exact location in Nightingale where we made contact, and we can compare it with our sampling map to determine if we have touched an area where there is an abundance of sampling materials or one of the more rocky places. “
SamCam will also be able to take pictures of the test head after the spacecraft took off from Nightingale Crater and is at a safe distance from the asteroid. Because the test head is mounted on a wrist, the team will be able to examine it in a different orientation to the sun and the sampling chamber. The team will also see dust or materials on any other area of the TAGSAM, on the shoulder or on the blanket above the gas cylinders, Loretta explained.
“This will tell us if we’ve moved enough material around when we made contact, and maybe we’ll just be able to see some of the particles inside TAGSAM if the particles are in the right place in the head and if we get the right lighting conditions.” . “
After the TAG, the team will then spend a week assessing how much sample was collected. He will use several methods to estimate the amount of the sample, starting with the imaging of the sample collection head for visual inspection. The team will also inspect the spacecraft and instruments to see if this has led to degradation in either.
Pirouette in space
The spacecraft is then prepared to perform a maneuver designed to give Earth scientists approximately how much sample was collected. With its extended sampling arm, it will slowly rotate about an axis perpendicular to the TAGSAM to measure the change in mass attributed to the collected sample compared to a previous measurement made with an empty sampling head.
Due to uncertainty in the technique, the measurement result must exceed the required mass of the sample in order to have a high degree of confidence that a suitable sample is available.
“We will look for a 90% chance of having 60 actual grams or more,” Loretta said. “All under this, we will hold talks with NASA to assess the state of the spacecraft, its ability to enter for a second TAG, and decide whether we want to return with what we have or make a second attempt at a TAG. “
The spacecraft can make multiple attempts to sample, as it is equipped with three bottles of nitrogen gas. For example, if he had to land in a safe place but failed to come up with a good sample, the team developed contingency measures to ensure that the mission still met its main scientific goal: to collect at least 60 grams (little less than 2 ounces)) of the surface material and return it to Earth.
“If the decision is made, we need to re-enter, we need to put the spacecraft back in orbit and conduct a series of burns to sort out its position in orbit for the next marking attempt,” said Mike Moreau, deputy project manager NASA’s Goddard Space Flight Center in Maryland.
While the Nightingale has been identified as the best sampling site for the whole of Bennu, it still poses many challenges, Loretta said.
“Probably the most likely outcome we will have on October 20 is that we will connect to the surface and come up with a large sample that exceeds our requirements. But Bennu has already thrown us a few crooked balls, so we’re completely ready. to mark Osprey (the backup site) if necessary, ”he said.
Once a decision has been made to collect the sample, the team will proceed to place the head inside the sample return capsule and seal it to return to Earth in 2023. And when that time comes, the chances are that it will return even more than the minimum. of 60 grams, as TAGSAM is designed to capture at least 150 grams and under optimal conditions up to 4 kilograms – enough to keep generations of researchers in laboratories on Earth.
American probe to touch the asteroid Bennu on October 20
Provided by the University of Arizona
Quote: Why extracting a sample from an asteroid is more difficult than it seems (2020, October 16), extracted on October 17, 2020 from https://phys.org/news/2020-10-scooping-asteroid-sample -harder.html
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