NASA's asteroid sampler narrowly avoided sinking into surface of Bennu
In October 2020, NASA’s OSIRIS-REx spacecraft made history by touching down on the asteroid Bennu and grabbing a sample of rock and debris. New analysis of this encounter has revealed the probe was lucky to avoid sinking into the asteroid's surface, which mission scientists have found to be so loosely packed it resembles a pit of plastic balls.
This new perspective on the surface composition of Bennu comes via closeup images OSIRIS-REx collected as it snatched its precious sample. Prior laboratory testing had indicated the probe would barely make a divot in the asteroid throughout this process, but the images revealed pebbles had spread all around and that a massive crater measuring 26 feet (8 m) across had been created.
So, in April 2021, the scientists sent the spacecraft back for another inspection of the mess. This enabled them to analyze before and after images of the sampling site, and assess acceleration data the probe collected as it touched down. This revealed a very tiny amount of resistance from the surface, which the team compared to pressing down the plunger on a French Press coffee maker.
The team then ran computer simulations designed to calculate the density and cohesion of Bennu’s surface materials. These were based on spacecraft images and the acceleration data, with the team experimenting with various properties until the simulation married up with the mission data.
This revealed the particles making up the exterior of Bennu were so loosely packed that stepping onto the asteroid’s surface would be like stepping into a pit of plastic balls. So much so, the spacecraft would have sunk into the asteroid if it had’t immediately fired its thrusters to move away from the surface.
“By the time we fired our thrusters to leave the surface we were still plunging into the asteroid,” said Ron Ballouz, an OSIRIS-REx scientist based at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.
After analyzing data gathered when #OSIRISREx collected a sample from Bennu, scientists discovered the asteroid’s exterior is made of loosely packed & lightly bound rock. So, if you were to step onto Bennu, it would feel like stepping in a plastic ball pit https://t.co/7QCCN99aBv pic.twitter.com/3OZukbjUEj— Thomas Zurbuchen (@Dr_ThomasZ) July 7, 2022
The findings present another twist in our perspective of Bennu’s surface, which earlier mission data had revealed to be much more rugged and eruptive than Earth-based observations had suggested. The discovery also has implications for our understanding of other asteroids, and the dangers they may pose to our home planet.
“It’s possible that asteroids like Bennu – barely held together by gravity or electrostatic force – could break apart in Earth’s atmosphere and thus pose a different type of hazard than solid asteroids,” said Patrick Michel,” an OSIRIS-REx mission scientist. “I think we’re still at the beginning of understanding what these bodies are, because they behave in very counterintuitive ways.”