NASA’s OSIRIS-REx Probe Was Nearly Swallowed by Asteroid Bennu

For most of human history, our experience with asteroids has been limited to the chunks of rock and metal that survive passage through the atmosphere before striking the surface — sometimes with disastrous results. It wasn’t until the last few years that we’ve been able to examine these space rocks up close before they smack into us. We are increasingly discovering that asteroids aren’t the monolithic objects we expected. New research from the NASA OSIRIS-REx team demonstrates that. According to the latest research, collecting a sample from the asteroid Bennu was like “punching a ball pit.”

NASA launched OSIRIS-REx in 2016, partnering with the University of Arizona for science operations. Two years later, the spacecraft arrived in orbit of the 500-meter-wide asteroid known as Bennu. The mission’s goal was to descend to the surface and scoop up a few bits of rock, but the team was immediately surprised to find a landscape strewn with debris when they were expecting something smooth. There were also small particles being flung from the surface. Following the sample run in October 2020, scientists saw a large cloud of regolith launched into space from the kiss of the sampling arm.

The surprises didn’t stop there — the new research, led by Dante Lauretta and Kevin Walsh from the University of Arizona, says that OSIRIS-REx could have been swallowed up by the loosely packed surface if it hadn’t been designed to back away so quickly. Before OSIRIS-REx got to Bennu, the team expected something like a gravel-covered road, but what they got was more like a ball pit, says the team. In just a few seconds, OSIRIS-REx’s arm sunk into the surface by about 1.5 feet (half a meter). The divot left in the surface was an astonishing 26 feet (8 meters) wide.

The team ran hundreds of simulations using data from the probe, eventually coming up with a model that matches reality. The surface of Bennu is most like “desert pavement” on Earth, which forms when fine dust is blown or washed away, leaving a bed of similarly sized pebbles. “It turns out that the particles making up Bennu’s exterior are so loosely packed and lightly bound to each other that they act more like a fluid than a solid,” Lauretta said. This might help explain how the probe ended up with much more material than expected — engineers hoped to collect at least 60 grams of material with the sampling mechanism, which used a burst of nitrogen gas to launch debris into the canister. NASA believes it got closer to two pounds, which will be a real boon once it gets back to Earth.

Knowing more about the surface of Bennu will help scientists interpret remote observations of other asteroids. That could change the way future asteroid missions are designed. If similar asteroids are little more than rubble piles, we might need to reevaluate how to address a killer asteroid on a collision course for Earth.