Scientists have learned something surprising after analyzing data collected when NASA’s OSIRIS-REx spacecraft collected a sample from asteroid Bennu in October 2020. The spacecraft would have plunged into the asteroid had it not fired its thrusters to retreat immediately after grabbed his sample of dust and rock from Bennu’s surface. “Our expectations of the asteroid’s surface were completely wrong.” — Dante Lauretta, OSIRIS-REx Principal Investigator Unexpectedly, it turns out that the particles that make up Bennu’s exterior are so loosely packed and lightly bound together that if someone stepped on 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 children. “If Bennu was completely filled, 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 from the Southwest Research Institute, 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 struck it to pick up dust and rock for sample collection (right). OSIRIS-REx touched down on Bennu at 6:08 p.m. EDT on October 20, 2020. Credit: NASA’s Goddard Space Flight Center The latest findings on 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 the 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 proven 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 filled with boulders instead of the smooth, sandy beach they expected based on observations from Earth-based and space-based telescopes. The researchers also discovered that Bennu was ejecting rock particles from its surface into space. “Our expectations of the asteroid’s surface were completely wrong,” Lauretta said. The latest indication that Bennu was not what it seemed came after the OSIRIS-REx spacecraft took a sample and sent stunning, close-up images of the asteroid’s surface back 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 rubble pile of rocks and boulders left over from the formation of the solar system. On October 20, 2020, NASA’s OSIRIS-REx spacecraft briefly approached 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 re-examine the sample collection event and better understand how Bennu’s loose upper layers are held together. Credit: NASA Goddard Space Flight Center/CI Lab/SVS Mission scientists were puzzled by the abundance of pebbles strewn around, given how gently the spacecraft hit the surface. Even stranger was that the spacecraft left a large crater 26 feet (8 meters) wide. “Every time we tried the sample pickup process in the lab, we barely made a difference,” Lauretta said. The mission team decided to send the spacecraft back to take more pictures of Bennu’s surface “to see how much of a mess we made,” Lauretta said. The researchers analyzed the amount of debris visible in the images before and after the sampling, nicknamed ‘Nightingale’. They also looked at acceleration data collected during the spacecraft’s descent. This data revealed that as OSIRIS-REx touched down on the asteroid, it encountered the same resistance – very little – that a person would feel when squeezing the plunger on a carafe of French-style coffee. “As we fired our thrusters to leave the surface, we were still sinking into the asteroid,” said Ron Ballouz, an OSIRIS-REx scientist based at the Johns Hopkins Laboratory for Applied Physics in Laurel, Maryland. Balouz and the research team ran hundreds of computer simulations to infer Bennu’s density and consistency based on spacecraft images and acceleration information. The engineers varied the surface cohesion properties in each simulation until they found the one that best matched their real data. This view of asteroid Bennu ejecting particles from its surface on January 19, 2019, was created by combining two images taken by 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 observations of other asteroids, which could be useful in planning future asteroid missions and developing methods to protect Earth from asteroid impacts. It’s possible that asteroids like Bennu—barely held together by gravity or electrostatic force—could break up in Earth’s atmosphere and thus pose a different kind of hazard than solid asteroids. “I think we’re still at the beginning of understanding what these bodies are, because they behave in very counterintuitive ways,” said Patrick Michel, an OSIRIS-REx scientist and director of research at the Center National de la Recherche Scientifique in Côte d’Ivoire. “Azur Observatory in Nice, France. Bibliographical 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.abm1 by Kevin J. Walsh, Ronald-Louis Ballouz, Erica R. Jawin, Chrysa Avdellidou, Olivier S. Barnouin, Carina A. Bennett, and Edward B. Ballouz. 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 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 scientific observation and data processing design. Lockheed Martin Space in Littleton, Colorado, built the spacecraft and provides flight operations. Goddard and KinetX Aerospace are responsible for piloting 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 in Washington.
