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Surprise – again! NASA probe reveals asteroid Bennu is not what it seemed

OSIRIS REx spacecraft leaves the Bennu surface

NASA’s OSIRIS-REx spacecraft exits the surface of asteroid Bennu after taking a sample. Photo credit: Goddard Space Flight Center/NASA CI Lab/SVS

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“Our expectations about the asteroid’s surface were completely wrong.” — Dante Lauretta, principal investigator of OSIRIS-REx

Unexpectedly, it turns out that the particles making up Bennu’s exterior are so loosely packed and lightly bound to each other that if a person were to step onto the asteroid they would feel very little resistance. It would be like stepping into a pit of plastic balls that are popular play areas for kids.

“If Bennu was completely packed, that would imply nearly solid rock, but we found a lot of void space in the surface,” said Kevin Walsh, a member of the OSIRIS-REx science team from Southwest Research Institute, which is based in San Antonio.

NASA's OSIRIS REx Spacecraft Surface of Asteroid Bennu

Side-by-side images from NASA’s OSIRIS-REx spacecraft of the robotic arm as it descended towards the surface of asteroid Bennu (left) and as it tapped it to stir up dust and rock for sample collection (right). OSIRIS-REx touched down on Bennu at 6:08 pm EDT on October 20, 2020. Credit: NASA’s Goddard Space Flight Center

The latest findings about Bennu’s surface were published on July 7, 2022, in a pair of papers in the journals Science and Science Advances, led respectively by Dante Lauretta, principal investigator of OSIRIS-REx, based at University of Arizona, Tucson, and Kevin Walsh. These surprising results add to the intrigue that has gripped scientists throughout the OSIRIS-REx mission, as Bennu has proved consistently unpredictable.

The first surprise the asteroid presented was in December 2018, when NASA’s spacecraft arrived at Bennu. The OSIRIS-REx team found a rough surface littered with boulders instead of the smooth, sandy beach they had expected based on observations from Earth- and space-based telescopes. Reasearchers also discovered that Bennu was ejecting particles of rock from its surface into space.

“Our expectations about the asteroid’s surface were completely wrong,” said Lauretta.

The latest clue that Bennu was not what it seemed came after the OSIRIS-REx spacecraft picked up a sample and beamed stunning, close-up images of the asteroid’s surface to Earth. “What we saw was a huge wall of debris radiating out from the sample site,” Lauretta said. “We were like, ‘Holy cow!’”

The near-Earth asteroid Bennu is a debris pile of rocks and debris left over from the formation of the solar system. On October 20, 2020, NASA’s OSIRIS-REx spacecraft touched down briefly on Bennu and collected a sample for return to Earth. During this event, the spacecraft’s arm sank much deeper into the asteroid than expected, confirming that Bennu’s surface is loosely bound. Now scientists have used data from OSIRIS-REx to double-check the sampling event and better understand how Bennu’s loose upper layers are held together. Photo credit: Goddard Space Flight Center/NASA CI Lab/SVS

Mission scientists were stunned by the abundance of scattered pebbles, considering how gently the spacecraft tapped the surface. Even more bizarre was that the spacecraft left a large crater 8 meters wide. “Every time we’ve tested the sampling procedure in the lab, we’ve barely made a divot,” Lauretta said. The mission team decided to send the spacecraft back to take more photos of Bennu’s surface “to see how big a mess we made,” Lauretta said.

Researchers analyzed the volume of debris visible in before-and-after images of the sample site, nicknamed “Nightingale.” They also examined acceleration data collected during the spacecraft’s landing. This data showed that when OSIRIS-REx touched the asteroid, it experienced the same resistance – very little – that a person would feel when squeezing the plunger of a French press coffee carafe. “When we fired our thrusters to leave the surface, we were still diving into the asteroid,” said Ron Ballouz, an OSIRIS-REx scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.

Ballouz and the research team ran hundreds of computer simulations to infer Bennu’s density and cohesion based on spacecraft imagery and acceleration information. Engineers varied the surface cohesion properties in each simulation until they found the one that most closely matched their real data.

Asteroid Bennu Particles

This view of asteroid Bennu ejecting particles from its surface on January 19, 2019 was created by combining two images taken aboard NASA’s OSIRIS-REx spacecraft. Other image processing techniques were also used, such as cropping and adjusting the brightness and contrast of each image. (Source: NASA/Goddard/University of Arizona/Lockheed Martin)

Now, this precise information about Bennu’s surface can help scientists better interpret remote observations of other asteroids, which could be useful in planning future asteroid missions and in developing methods to protect Earth from asteroid collisions.

It’s possible that asteroids like Bennu, which are 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. “I think we’re still at the beginning of understanding these bodies because they behave in a very counterintuitive manner,” said Patrick Michel, an OSIRIS-REx scientist and research director at the Center National de la Recherche Scientifique at the French Riviera Observatory in Nice, France.


“Spacecraft Sample Collection and Underground Excavation of Asteroid (101955) Bennu” by DS Lauretta, CD Adam, AJ Allen, R.-L. Ballouz, OS Barnouin, KJ Becker, T Becker, CA Bennett, EB Bierhaus, BJ Bos, RD Burns, H Campins, Y Cho, PR Christensen, ECA Church, BE Clark, HC Connolly, MG Daly, DN DellaGiustina, Drouet d’Aubigny CY, Emery JP, Enos HL, Freund Kasper S, Garvin JB, Getzandanner K, Golish DR, Hamilton VE, Hergenrother CW, Kaplan HH, Keller LP, Lessac-Chenen EJ, Liounis AJ, Ma H , LK McCarthy, BD Miller, MC Moreau, T Morota, DS Nelson, JO Nolau, R Olds, M Pajola, JY Pelgrift, AT Polit, MA Ravine, DC Reuter, B Rizk, B Rozitis, AJ Ryan , EM Sahr, N Sakatani, JA Seabrook, SH Selznick, MA Skeen, AA Simon, S Sugita, KJ Walsh, MM Westermann, CWV Wolner, and K Yumoto, July 7, 2022, Science.
DOI: 10.1126/science.abm1018

“Near-Zero Cohesion and Loose Packing of Bennu’s Near Ground Revealed by Spacecraft Contact” by Kevin J. Walsh, Ronald-Louis Ballouz, Erica R. Jawin, Chrysa Avdellidou, Olivier S. Barnouin, Carina A. Bennett, Edward B. Bierhaus, Brent J. Bos, Saverio Cambioni, Harold C. Connolly, Marco Delbo, Daniella N. DellaGiustina, Joseph DeMartini, Joshua P. Emery, Dathon R. Golish, Patrick C. Haas, Carl W. Hergenrother, Huikang Ma , Patrick Michel, Michael C. Nolan, Ryan Olds, Benjamin Rozitis, Derek C. Richardson, Bashar Rizk, Andrew J. Ryan, Paul Sánchez, Daniel J. Scheeres, Stephen R. Schwartz, Sanford H. Selznick, Yun Zhang, and Dante S. Lauretta, July 7, 2022 scientific advances.
DOI: 10.1126/sciadv.abm6229

NASA’s Goddard Space Flight Center provides all mission management, systems engineering, safety and mission assurance for OSIRIS-REx. Dante Lauretta from the University of Arizona, Tucson is the principal investigator. The university directs the science team and the mission’s scientific observation planning and data processing. Lockheed Martin Space in Littleton, Colorado built the spacecraft and provides flight operations. Goddard and KinetX Aerospace are responsible for navigating the OSIRIS-REx spacecraft. OSIRIS-REx is the third mission in NASA’s New Frontiers Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate Washington.