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In a swirl of rocks and dust, OSIRIS-REx probe touches an asteroid

Despite concerns that the surface of Bennu might be too rocky, the probe’s touchdown produced a dramatic shower of debris, opening the way for future insights into the mysteries of our solar system.

ByAlissa GreenbergNOVA NextNOVA Next

An animation of the OSIRIS-REx sampling arm touching down on the surface of asteroid Bennu to collect a sample of fine-grained material. Image Credit: NASA/Goddard/CI Lab

NASA may well have succeeded in its first attempt to sample an asteroid Tuesday when its OSIRIS-REx probe touched down for a few moments on the surface of the asteroid Bennu, kicking up a dramatic shower of rocks and dust. During its six seconds in contact with the surface, the probe emitted a puff of nitrogen to stir up a sample of regolith, or asteroid dust, to capture in its specialized head—then blasted off again.

Mission researchers, masked and in matching blue NASA shirts, cheered as they listened to mission operations narrate OSIRIS-REx’s descent to Bennu—“O-REx has descended below the five-meter mark; the hazard map is go for TAG,” and then, “Touchdown declared!”—from the Lockheed Martin Space facility in Denver, Colorado. 

“The OSIRIS-REx mission outperformed in every way,” said NASA Administrator Jim Bridenstine at a press conference.

Just the task of successfully touching down on Bennu was an achievement in accuracy and precision. After traveling some 207 million miles to the asteroid, the spacecraft attempted to descend safely to a spot the size of two parking spaces—and touched down within a meter of its target. And although the descent process took more than four hours, the touchdown itself had to be pre-programmed and carried out autonomously, since it takes almost 19 minutes for a signal to travel from Earth to OSIRIS-REx’s current location. 

Listening to OSIRIS-REx make contact with Bennu was a “transcendental” experience, said mission Principal Investigator Dante Lauretta moments afterward. “I can’t believe we actually pulled this off!”

OSIRIS-REx (whose name is an acronym that refers to its research goals) launched in September 2016, orbited the sun for a year, and then “arrived” at Bennu in 2018. Since then, its team has spent time getting familiar with this particular space rock, practicing flybys sometimes less than a mile away and mapping potential spots on the surface for sample collection—including the Nightingale site the probe touched down on Tuesday.

That task proved more difficult than expected. Based on long-range measurements of how quickly Bennu heats and cools with the sun’s rays, the OSIRIS-REx team had expected to find an asteroid whose surface looked more like a beach than a mountaintop. Instead, they struggled to find any area flat enough and free enough of obstacles to allow for a safe landing. And they worried that the probe’s mechanism for collecting samples—the puff of nitrogen designed to stir up dust and rocks and propel them into a kind of high-tech air filter to be brought back to Earth—wouldn’t work with a surface rockier and more solid than anticipated.

Those fears were resolved late Tuesday night, when images from OSIRIS-REx showed that its touch-and-go on Bennu “stirred up a swirl of material probably unlike anything Bennu has seen in quite a while,” NASA Science Associate Administrator Thomas Zerbuchen, said at the press conference. 

Images from the touchdown suggest the probe is likely to have successfully collected at least some material, Lauretta added at the same press conference, even crushing a fairly large rock with the probe’s sampling head. “We really did kind of make a mess on the surface of this asteroid,” he said, “but it’s a good mess, the kind of mess we were hoping for.” 

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But information about just how much material they picked up will have to wait until later this week, when OSIRIS-REx will use onboard cameras to inspect its own sampling head, then extend its arm and spin. The maneuver allows the team to compare the probe's "moment of inertia” (which refers to how distribution of mass affects the way something spins) from before and after touchdown, and thus infer the weight of its cargo. If it’s determined that it did not succeed in grabbing at least 60 grams of material, the team will have two more chances to touch down elsewhere on Bennu, starting in January. 

Two Japanese missions have attempted to capture material from asteroids in a similar way to OSIRIS-REx, but both ran into unexpected obstacles, and their sampling potential was much smaller than what NASA hopes this probe will bring home. Because its capacity for carrying asteroid dust is much larger than its predecessors, the scientists involved with this mission hope their very short time on Bennu’s surface will lead to a whole lot of information. 

Lauretta said NASA has already identified two distinct rock types on Bennu’s surface. The first, which he describes as a “dark, kind of hummocky rock” seems like it may be very weak in comparison to the rocks that we’re used to, and especially in comparison to the meteorites that usually survive their fall to Earth. The second is a little brighter, “shot through with bright white veins or white inclusions,” he said, a mineral carbonate that’s similar to the white crust that forms around a bathroom sink and is likely a bit stronger.  

Once the sample arrives home, the geology exploration will begin in earnest. “It's really amazing that these tiny specks of dust grains can tell you so much about how our universe formed, how our solar system formed, how asteroids like Bennu and Ryugu formed, and how Earth formed,” says Maitrayee Bose, a cosmochemist who studies the mineral makeup of space dust. “So we can basically trace all the processes, going from the really early part of the solar system to the present scenario where we have the planets, quite easily by doing very detailed characterization of these cosmic dust samples.” By examining their composition and the way they incorporate water into their structures, for example, Bose can learn what temperature and pressure conditions were like when the dust formed—and thus infer what the environment was like in our nebula at that time.

Bose is particularly excited to measure the water content of the Bennu regolith and, using that information, estimate how much water the asteroid and bodies of similar sizes might contain. That could provide key insight into where water on Earth and elsewhere in our solar system came from, and even give some context of how it formed. “I'm trying to map out what the predominant processes are that could change or alter the compositions of these materials,” she says. “Once I know that, then I can understand exoplanets, how they form. Can there be a similar situation like our solar system, where we could form a planet like Earth?”

Researchers will also use information gleaned from their brief visit to measure the Yarkovsky Effect on Bennu, or the way that heat emanating from an asteroid can change its path over time. That’s especially important because Bennu is considered one of the asteroids most likely to crash into Earth sometime next century. (Though that likelihood is still quite low.) Understanding more about its potential trajectory could help with future missions seeking to stop that kind of impact.

But OSIRIS-REx faces a long journey before that research can begin. “The line is tied and the sinker dropped, and we’re excited, but now we need to bring it in and see if we caught the fish,” Zerbuchen said. “And then we need to bring it home.” 

After touchdown and sampling, the spacecraft executed a “backaway burn,” in which it fired its thrusters and began its eventual return to orbit around Bennu. It will now cool its heels—having warmed significantly during its approach to the asteroid—while it waits for its team to assess their first-try sample and decide whether returning a second or third time might be necessary. 

The probe will stay with Bennu until the asteroid nears Earth again, starting its journey back to Earth in March 2021 and ultimately dropping its booty in the Utah desert in 2023. Then and only then can scientists begin the long process of unlocking the secrets it holds.

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