In October 2020, a small spacecraft touched down briefly with an asteroid to grab a piece of it to bring to Earth. Almost two years later, scientists learned that if the OSIRIS-REx spacecraft had extended its stay just a little longer, it would have sunk straight into the asteroid.
That’s because asteroid Bennu doesn’t look like scientists predicted. Rather than being a solid, flying rock, Bennu is actually made up of small, pebble-like particles that are loosely bound together, creating a lot of space on its surface. This is most comparable to a plastic ball, NASA wrote in a new announcement. “Our expectations of the asteroid’s surface were completely wrong,” Dante Lauretta, OSIRIS-REx principal investigator and lead author of a recent paper describing the findings, said in the release.
OSIRIS-REx arrived at the asteroid in December 2018 on a mission to retrieve a sample from Bennu and bring it back to Earth for analysis. The spacecraft touched down at Bennu in October 2020, extending its robotic arm to pick up a piece of the asteroid. OSIRIS-REx then immediately fired its thrusters to pull away from Bennu. The spacecraft’s sampling head touched the surface of Bennu for approximately 6 seconds before retracting. By stirring up some of the asteroid’s dust and pebbles, OSIRIS-REx was able to grab a few ounces of material.
The OSIRIS-REx sample collection of asteroid Bennu: A view from TAGSAM’s SamCam
The brief encounter left quite an impression on Bennu, resulting in a chaotic explosion of pebbles and a crater 26 feet (8 meters) wide. “Every time we tested the sampling procedure in the lab, we barely made a deviation,” Lauretta said. But after reviewing the footage of the actual sampling, the scientists were left confused. “What we saw was a huge wall of debris radiating from the sample site,” Lauretta said. “We were like, ‘Holy cow!’
After analyzing the volume of debris seen in before-and-after images of the landing site, the scientists learned that OSIRIS-REx faced as much resistance when touching the asteroid as “one would feel while squeezing the plunger of a French press carafe for coffee,” NASA said in a statement. In other words, the spacecraft encountered very little drag, certainly not the drag one would expect when landing on a rocky body. As the spacecraft fired its engines to move away, it was sinking into the asteroid.
“If Bennu was completely packed, that would mean almost solid rock, but we found a lot of empty space on the surface,” Kevin Walsh, a member of the OSIRIS-REx science team and lead author of a second paper on Bennu’s composition, said in a statement.
When OSIRIS-REx first arrived at the asteroid, close-up pictures of Bennu revealed that its surface was full of rocks, rather than the smooth, sandy surface that had been predicted. The images also show Bennu spewing pebbles into space. “I think we’re still at the beginning of understanding what these bodies are because they behave in a very counterintuitive way,” OSIRIS-REx scientist Patrick Michel said in the NASA release.
Bennu was full of surprises. One of the first was its odd shape, similar to a child’s spinning top.
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