In October 2020, a small spacecraft briefly touched down on an asteroid to snag a piece of it to bring to Earth. Almost two years later, scientists have learned that if the OSIRIS-REx spacecraft had extended its stay even a tiny bit longer, it would have sunk right into the asteroid.
That’s because asteroid Bennu is nothing like scientists had predicted. Rather than being a solid, flying rock, Bennu is actually made up of small, pebble-like particles that are not strongly bound together, creating lots of space on its surface. It’s most comparable to a plastic ball pit, NASA writes in a new release. “Our expectations about the asteroid’s surface were completely wrong” Dante Lauretta, principal investigator of OSIRIS-REx and lead author of a recent paper detailing the findings, said in the release.
OSIRIS-REx arrived at the asteroid in December 2018 with a mission to retrieve a sample from Bennu and carry it to Earth for analysis. The spacecraft touched down on Bennu in October 2020, extending its robotic arm to scoop up a piece of the asteroid. OSIRIS-REx then immediately fired up its thrusters to back away from Bennu. The spacecraft’s sampling head touched Bennu’s surface for approximately 6 seconds before retreating. By stirring up some of the dust and pebbles on the asteroid, OSIRIS-REx was able to grab a couple ounces of material.
The brief rendezvous 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 sample pickup procedure in the lab, we barely made a divot,” Lauretta said. But after reviewing the footage from the real sample pick-up, the scientists were left confused. “What we saw was a huge wall of debris radiating out 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 from touching down on the asteroid as “a person would feel while squeezing the plunger on a French press coffee carafe,” NASA wrote in a statement. That is to say, the spacecraft met very little resistance, certainly not the type of resistance one would expect from landing on a rocky body. As the spacecraft fired its thrusters to depart, it was sinking into the asteroid.
“If Bennu was completely packed, that would imply nearly solid rock, but we found a lot of void space in 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, closeup images of Bennu revealed that its surface was filled with boulders, rather than the smooth sandy surface that had been predicted. The images also showed that Bennu was spitting out pebbles into space. “I think we’re still at the beginning of understanding what these bodies are, because they behave in very counterintuitive ways,” Patrick Michel, an OSIRIS-REx scientist, said in the NASA release.
Bennu has been full of surprises. One of the first was its odd shape, similar to a child’s spinning top.