Data from the University of Arizona-led OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security–Regolith Explorer) mission has regularly defied expectations.

The latest findings, published in the journals Science and Science Advances, offer the biggest surprises yet.

"As soon as I saw the first image coming down from the spacecraft after we made contact, I knew something interesting was happening, because the surface just crushed as we just pushed into it and barely registered on the spacecraft accelerometer," said project lead and planetary scientist Dante Lauretta of UA.

Although the 250-gram sample — about enough to fill a pint glass — from the asteroid Bennu won't reach Earth until next year, scientists were able to analyze how the surface reacted to the touch-and-go sampling process itself.

Rather than clumping together cohesively, as expected from certain surface features, Bennu's subsurface is packed as loosely as a dust bunny.

"The fact that there was no resistance to the downward motion of the spacecraft meant that the surface just kind of flowed away. I use the analogy of one of those plastic ball pits in a kid's playground, right? You kind of jump into that, and you sink," said Lauretta. "If we hadn't fired the back-away thrusters, I think the spacecraft would have just sunk into the asteroid and disappeared, like in a pit of quicksand."

NASA

An artist rendering of OSIRIS-REx contacting the asteroid Bennu with the Touch-And-Go Sample Arm Mechanism.

The first hint of the unexpected came during the October 2020 sample collection at the Nightingale site in Hokioi crater, when the pogo-stick-like TAGSAM (Touch-and-Go Sample Acquisition Mechanism)kicked up a huge debris wall and left a crater 20 times larger than predicted.

"That is not at all what we expected. We thought the surface would be pretty tough, and that we would dig a little hole maybe 30 centimeters across, like the size of a dinner plate," said Lauretta.

Partly, that's because OSIRIS-REx's design process required scientists to make educated guesses about many of the asteroid's characteristics, including cohesion.

"The engineers came to the scientists and said, 'This is a really important number: What is the cohesion of the surface going to be?' And we all sat around the room and kind of looked at each other and shrugged our shoulders and were like, 'We have no idea. We have never measured this before.'"

The team was previously surprised to find that Bennu was covered in boulders, instead of offering the sandy surface they had expected.

"We measured its thermal properties — the rate at which it heated up and the rate at which it cooled off — and it still behaves like a beach. That part wasn't wrong; it was our interpretation. And so what we think is the boulders themselves are highly porous and very weak and fragile," said Lauretta.

Indeed, some of the fine material kicked up during the touchdown, nitrogen gas injection and reverse thrusting might have come from pulverized boulders.

The OSIRIS-REx mission explores the origins of life on Earth by sampling an ancient, carbon-rich asteroid, a gravity-bound rubble pile built from the remains of a parent body that likely was smashed apart in an impact. Unlike bits of space rock that fall to Earth, samples taken directly from such asteroids reflect the conditions present in the space environs through which they have traveled rather than the transformative consequences of burning through Earth's atmosphere.

Scientists also study Bennu because the 500-meter-wide (1,640-foot) asteroid has a 0.05% chance of striking Earth in the next 300 years. How this new data changes the amount and type of danger Bennu would pose in that unlikely event requires further study.