The surface of asteroid Bennu is like a ball pit

Data collected in October 2020 during the sampling of asteroid Bennu by NASA’s OSIRIS-REx mission shows that the asteroid’s surface is composed of loosely cohesive fragments or rocksaccording to two new studies appearing this week in the journals Science s Advancement of Sciences.

The authors were surprised by the results: the spacecraft would have sunk Bennu if it hadn’t fired its thrusters to retreat immediately after picking up the dust and rocks from its surface.

The particulate matter that makes up this asteroid’s exterior is so loosely packed and so loosely bonded that if a person were to step on Bennu, they would feel very little resistance, like walking through a pool of plastic balls that children play in.

“If Bennu were completely compacted, that would imply that it would be almost solid rock, but we found a lot of empty space on the surface,” he says. Kevin Walshresearcher at the Southwest Research Institute (USA) and lead author of the article in advances in science.

Walsh and his team analyzed the forces experienced by the spacecraft and investigated the physical properties of the material down to a depth of 10 cm in the ground, finding that Bennu’s low gravity gave rise to the granular surface bed.

The material’s high porosity and low strength allow dust and other small particles to move within the asteroid’s subsurface. Spectral and thermal data collected during the mission suggest that these results apply to the entire asteroidnot just for the sampling site.

According to the study of Scienceled by the teacher Dante Lauratta from the University of Arizona (USA), some 250 grams of sample, which will be brought back to Earth in 2023 for laboratory analysis. Compared to the 5 grams taken by the Japanese spacecraft Hayabusa2 from the asteroid Ryugu, this is a little more, although both missions are considered important for studying these carbon-rich asteroids.

“We have a plan to understand the history of Bennu, from the beginning of the solar system to the origin of life on Earth. We are excited about the samples because they contain clay minerals, which are rich in water, and carbon in the form of carbonate minerals and organic molecules,” explains Lauretta to SINC.

We have a plan to understand Bennu’s history from the beginning of the solar system to the origin of life on Earth. The samples contain water-rich clay minerals and carbon in the form of carbonate minerals and organic molecules.

Dante Lauretta (University of Arizona)

The spacecraft’s Tactile Sample Acquisition Mechanism (TAGSAM) made contact and began to sink to the asteroid’s surface before releasing a jet of nitrogen gas that mobilized material underground and guided it to a collection chamber.

The professor offers more details about the sampling: “It lasted about five seconds, and there was another return flight over the surface of the original asteroid of about ten seconds.” The mission team decided to send the spacecraft back to take more pictures of Bennu’s surface “to see the size of the ‘disaster’ we made,” Lauretta says, referring to the crater created by the operation, which is larger than expected. , another surprise of the many that this asteroid has brought to the scientific community.

One surprise after another

The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) spacecraft spent about two years surveying Bennu, which is about 500 meters in diameter. After considering the best locations to collect a sample, the mission team chose a location within a 20-meter crater, dubbed the Nightingale.

Simulation of OSIRIS-REx hitting the surface of asteroid Bennu. /NASA/Goddard/University of Arizona

The asteroid presented its first surprise in December 2018, when this NASA spacecraft arrived at its destination, finding a boulder-strewn surface instead of the smooth, sandy beach they had come to expect based on observations from ground and space telescopes. Scientists also discovered that Bennu was ‘spitting’ rock particles into space.

“Our expectations of the asteroid’s surface were completely wrong,” admits Lauretta, and the same thing happened after the OSIRIS-REx spacecraft sampled and transmitted stunning, close-up images of the asteroid’s surface to Earth: “What we saw it was a huge wall of rubble radiating from the venue.”

New 9 meter crater

The researchers were intrigued by the abundance of scattered pebbles given how smoothly the craft hit the surface, and even more bizarre was that it left behind a large elliptical crater 8 to 9 meters long. “The times we tried to collect samples in the lab, we barely made a hole,” explains the professor, so they decided to send the spacecraft back to take more pictures of Bennu’s surface and confirm.

The authors analyzed the volume of debris visible in the images before and after the sampling site. They also examined acceleration data collected during the spacecraft’s landing to develop the studies. These data revealed that when OSIRIS-REx hit the asteroid, it experienced very little drag.

“When we fired our thrusters out of the surface, we were still diving into the asteroid,” he says. Rum Ballouz, researcher at the Johns Hopkins Applied Physics Laboratory (USA). His team ran hundreds of computer simulations to infer Bennu’s density and low cohesion from spacecraft images and acceleration information.

Now, this accurate information about Bennu’s surface can help scientists better interpret remote observations of other asteroids, which could be useful for design future missions them and develop methods to protect the Earth from collisions with these objects.

It’s possible that asteroids like Bennu – loosely held together by gravity or electrostatic force – could break apart in Earth’s atmosphere and therefore pose a different kind of hazard than solid asteroids.

“We are still at the beginning of understanding what these bodies are, because they behave in a very counterintuitive way,” concludes another of the authors, patrick michelOSIRIS-REx scientist at the Côte d’Azur Observatory (France).

View of asteroid Bennu ejecting particles from its surface on January 19, 2019. / NASA/Goddard/University of Arizona/Lockheed Martin