NASA’s OSIRIS-REx spacecraft leaves the surface of asteroid Bennu after taking a sample. Credit: NASA Goddard Space Flight Center/CI Lab/SVS
Scientists learned something astonishing after analyzing data collected when NASA’s OSIRIS-REx spacecraft sampled asteroid Bennu in October 2020. The spacecraft would have sunk into the asteroid if it hadn’t fired its engines to pull away immediately after grabbing his sample of dust and rock from Bennu’s surface.
“Our expectations of the asteroid’s surface were completely wrong.” —Dante Lauretta, Principal Investigator of OSIRIS-REx
Unexpectedly, it turns out that the particles that make up Bennu’s exterior are so loosely packed and loosely connected to each other that if a person stepped on the asteroid, they would feel very little resistance. It would be like stepping into a pit of plastic balls, which are popular places for children to play.
“If Bennu was completely packed, that would mean almost solid rock, but we found a lot of empty space on the surface,” said Kevin Walsh, a member of the OSIRIS-REx science team at the Southwest Research Institute, which is based in San Antonio.
Side-by-side images from NASA’s OSIRIS-REx spacecraft of the robotic arm as it descended toward the surface of asteroid Bennu (left) and as it tapped it to stir up dust and rocks for sampling (right). OSIRIS-REx landed on Bennu at 18:08 EDT on October 20, 2020. Photo: NASA 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 by Dante Lauretta, OSIRIS-REx principal investigator based at the University of Arizona, Tucson, and Kevin Walsh, respectively. These surprising results add to the intrigue that gripped scientists during the OSIRIS-REx mission, as Bennu has proven consistently unpredictable.
The first surprise the asteroid threw up was in December 2018, when NASA’s spacecraft arrived at Bennu. The OSIRIS-REx team found a rough, boulder-strewn surface instead of the smooth sandy beach they expected based on observations from ground-based and space-based telescopes. The researchers also discovered that Bennu is ejecting rock particles from its surface into space.
“Our expectations of the asteroid’s surface were completely wrong,” Lauretta said.
The latest clue that Bennu is not what it seems came after the OSIRIS-REx spacecraft sampled and beamed stunning close-up images of the asteroid’s surface back to Earth. “What we saw was a huge wall of debris radiating from the sample site,” Lauretta said. “We were like, ‘Holy cow!’
The near-Earth asteroid Bennu is a pile of rocks and stones left over from the formation of the Solar System. On October 20, 2020, NASA’s OSIRIS-REx spacecraft briefly touched down on Bennu and collected a sample for return to Earth. During this event, the spacecraft 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 revisit the sampling event and better understand how Bennu’s loose upper layers hold together. Credit: NASA Goddard Space Flight Center/CI Lab/SVS
Mission scientists were baffled by the abundance of pebbles scattered around, given how gently the spacecraft touches the surface. Even stranger, the spacecraft left a large crater that was 26 feet (8 meters) wide. “Every time we tested the sampling procedure in the lab, we barely made a deviation,” Lauretta said. The mission team decided to send the spacecraft back to take more pictures of Bennu’s surface “to see how big of a mess we made,” Lauretta said.
The researchers analyzed the volume of debris visible in before-and-after images of the test site, called Nightingale. They also looked at the acceleration data collected during the spacecraft’s touchdown. This data revealed that when OSIRIS-REx touched down on the asteroid, it experienced the same drag—very little—that a person would feel pushing the plunger of a French press coffee pot. “By the time we fired our thrusters to leave the surface, we were still diving into the asteroid,” said Ron Balluz, an OSIRIS-REx scientist based at the Johns Hopkins Laboratory for Applied Physics in Laurel, Maryland.
Ballouz and the research team ran hundreds of computer simulations to infer Bennu’s density and cohesion based on spacecraft images and acceleration information. The engineers varied the cohesive properties of the surface in each simulation until they found the one that most closely matched their real-life data.
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 applied, such as cropping and adjusting the brightness and contrast of each image. (Credit: NASA/Goddard/University of Arizona/Lockheed Martin)
Now, this precise information about Bennu’s surface can help scientists better interpret remote sensing observations of other asteroids, which could be useful in designing future asteroid missions and developing methods to protect Earth from asteroid collisions.
It’s possible that asteroids like Bennu—barely held together by gravity or electrostatic force—disintegrate in Earth’s atmosphere and thus pose a different type of hazard than solid asteroids. “I think we are still at the beginning of understanding what these bodies are because they behave in a very counterintuitive way,” said Patrick Michel, an OSIRIS-REx scientist and director of research at the Côte d’Azur National Center for Scientific Research. observatory in Nice, France.
References:
“Spacecraft Sample Collection and Subsurface 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, CY Drouet d’Aubigny, JP Emery, HL Enos, S. Freund Kasper, JB Garvin, K. Getzandanner, DR Golish, VE Hamilton, CW Hergenrother, HH Kaplan, LP Keller, EJ Lessac-Chenen, AJ Liounis, H. Ma , 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, 7 July 2022, Science.DOI: 10.1126/science .abm1018
“Near-Zero Cohesion and Loose Packing of Bennu Subsurface Revealed by Spacecraft Contact” by Kevin J. Walsh, Ronald-Louis Balluz, Erika R. Jauin, Hrisa Avdelidou, Olivier C. Barnouin, Carina A. Bennett, Edward B. Bierhaus, Brent J. Bos, Saverio Cambioni, Harold S. Connolly, Marco Delbo, Daniela N. Della Giustina, Joseph DeMartini, Joshua P. Emery, Datton R. Golish, Patrick S. Haas, Carl W. Hergenrother, Huikang Ma, Patrick Michel, Michael S .Nolan, Ryan Olds, Benjamin Rositis, Derek S. Richardson, Bashar Rizk, Andrew J. Ryan, Paul Sanchez, Daniel J. Scheeres, Stephen R. Schwartz, Sanford H. Selznick, Yun Zhang, and Dante S. Lauretta, 7 July 2022, Science Advances.DOI: 10.1126/sciadv.abm6229
NASA’s Goddard Space Flight Center provides overall mission management, systems engineering and safety and mission assurance for OSIRIS-REx. Dante Lauretta of the University of Arizona, Tucson, is the principal investigator. The university leads the science team and the mission’s science monitoring and data processing planning. Lockheed Martin Space in Littleton, Colo., built the spacecraft and provides flights. Goddard and KinetX Aerospace are responsible for the navigation of 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 Washington agency’s Science Mission Directorate.
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