NASA’s OSIRIS-REx spacecraft leaves the surface of asteroid Bennu after collecting a sample. Credit: NASA/CI Lab/SVS Goddard Space Flight Center
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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.
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!’”
Near-Earth asteroid Bennu is a pile of rubble of rocks and boulders left over from the formation of the solar system. On October 20, 2020, NASA’s OSIRIS-REx spacecraft briefly landed 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 OSIRIS-REx data to revisit the sample collection event and better understand how the loose upper layers of Bennu are held together. Credit: NASA/CI Lab/SVS Goddard Space Flight Center
Mission scientists were puzzled by the abundance of scattered pebbles, given how gently the spacecraft tapped the surface. Even more bizarre, the spacecraft left a large crater 26 feet (8 meters) wide. “Whenever we tested the sample collection procedure in the lab, we barely did a divot,” Lauretta said. The mission team decided to send the spacecraft back to take more photos of Bennu’s surface “to see what a mess we’ve made,” Lauretta said.
Researchers analyzed the volume of debris visible in before and after images of the sampling site, dubbed “Nightingale”. They also looked at acceleration data collected when the spacecraft landed. This data revealed that when OSIRIS-REx hit the asteroid, it experienced the same amount of resistance – very little – that a person would feel pressing the plunger of a French-press coffee carafe. “At 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 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 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 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 observations of other asteroids, which could be useful in designing future asteroid missions and in developing methods to protect Earth from collisions. of asteroids.
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. “I think we are still at the beginning of understanding what these bodies are, because they behave in a very counter-intuitive way,” said Patrick Michel, OSIRIS-REx scientist and research director at the National Center for Scientific research of Côte d’Azur. ‘Observatoire d’Azur in Nice, France.
References:
“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, 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, July 7, 2022, Science.
DOI: 10.1126/science.abm1018
“Near-Zero Cohesion and Loose Packing of Bennu’s Near-Subsurface 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. Laurette, July 7, 2022, Scientists progress.
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 planning of the scientific observation and data processing of the mission. 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 in Washington.
