Scientists look into an asteroid – is Bennu going to spin...

This series of images recorded by the OSIRIS REx spaceship shows Bennu in a full turn from a distance of about 80 km. The spacecraft’s PolyCam camera received the thirty-six 2.2 millisecond images over a period of four hours and 18 minutes. Photo credit: NASA’s Goddard Space Flight Center / University of Arizona

New findings from NASA’s OSIRIS-REx mission suggest that the interior of the asteroid Bennu may be weaker and less dense than its outer layers – like a cream-filled chocolate egg flying through space.

The results appear in a study published in the journal Advances in science and run by the University of Colorado Boulder’s OSIRIS-REx Team, including Professors Daniel Scheeres and Jay McMahon. The results could give scientists new insights into the evolution of the solar system’s asteroids – how bodies like Bennu transform over millions of years or longer.

In late 2018, OSIRIS-REx met with Bennu, an asteroid orbiting the Sun more than 200 million miles from Earth. Since then, the Colorado-built spaceship, built by Lockheed Martin, has examined the object more closely than any other asteroid in the history of space exploration.

So far, however, one question is difficult to answer: How is Bennu inside?

Diagram of Bennu’s orbit in relation to the Earth and other planets. Photo credit: NASA / Goddard / University of Arizona / Lockheed Martin

Scheeres, McMahon, and their colleagues on the mission’s radio science team now believe they have an answer – or at least part of it. Using OSIRIS-REx’s own navigation instruments and other tools, the group spent almost two years recording the ebb and flow of Bennus Schwerefeld. Imagine taking an X-ray of a piece of space debris an average width of about the height of the Empire State Building.

“If you can measure the gravitational field with sufficient accuracy, the position of the mass is severely restricted, even if you cannot see it directly,” said Andrew French, co-author of the new study and former graduate student at CU Boulder, now at NASAJet Propulsion Laboratory (JPL).

What the team found can mean trouble for Bennu too. The asteroid’s core appears to be weaker than its exterior, a fact that could threaten its survival in the not-too-distant future.

“You could imagine that in a million years or less, the whole thing would fall apart,” said Scheeres, a distinguished professor in the Ann and HJ Smead Department of Aerospace Engineering.

Evolution of the asteroids

That’s part of the fun of studying asteroids, of course. Scheeres explained that Bennu belongs to a class of smaller bodies that scientists refer to as “debris pile” asteroids – which, as the name suggests, resemble loosely held piles of debris.

Asteroids also change more than you think over time.

“None of them have been sitting out there unchanged since the beginning of the solar system,” said Scheeres. “They are changed by things like sunlight, which affects their rotation and collisions with other asteroids.”

To study how Bennu and other similar asteroids might change, he and his colleagues had to peek inside.

On January 19, 2019, OSIRIS-REx observed small pieces of material that jumped from the surface of the asteroid Bennu. Photo credit: NASA / Goddard / University of Arizona / Lockheed Martin

The team was lucky here. When OSIRIS-REx arrived in Bennu for the first time, the spaceship discovered something unusual: Tiny pieces of material, some only the size of marbles, seemed to jump from the asteroid into space. In many cases, these particles orbited Bennu before falling back to the surface. Members of the mission’s radio science team at JPL had a firsthand experience of the body’s gravity – a bit like the apocryphal story of Isaac Newton, who suggested the existence of gravity after observing it Apple fell on his head.

“It was a bit like someone was on the surface of the asteroid throwing up these marbles so they could be tracked,” Scheeres said. “Our colleagues were able to deduce the gravity field in the trajectories of these particles.”

Squishy center

In the new study, Scheeres and his colleagues combined these recordings of Bennus’s gravity while working with data from OSIRIS-REx himself – precise measurements of how the asteroid dragged on the spaceship over a period of months. They discovered something surprising: Before the mission began, many scientists had assumed that Bennu would have a homogeneous interior. As Scheeres put it: “A pile of stones is a pile of stones.”

But the gravitational field measurements indicated something else. To explain these patterns, certain parts of Bennu’s interior would likely need to be packed closer together than others. And some of the least dense spots in the asteroid appeared to be around the pronounced bulge on its equator and in its core.

“It’s like there’s a gap in the middle that a couple of soccer fields could fit into,” said Scheeres.


Thanks to laser altimetry data and high-resolution images from OSIRIS-REx, we can now take a tour of Bennu’s remarkable terrain. Photo credit: NASA’s Goddard Space Flight Center

The asteroid’s spin may be responsible for this void. Scientists know that the asteroid is spinning faster and faster over time. According to Scheeres, these structural dynamics could slowly push the material away from the center of the asteroid and towards its surface. In other words, Bennu may be spinning himself to pieces.

“If its core has a low density, it will be easier to pull the entire asteroid apart,” said Scheeres.

For the scientist, the new findings are bittersweet: After Bennus Schwerefeld’s measurements, Scheeres and his team have largely completed their work on the OSIRIS REx mission.

Their results have contributed to the mission’s sample analysis plan, which is currently under development. The returned sample is analyzed to determine the cohesion between the grains – an important physical property that affects the mass distribution observed in their study.

“We were hoping to find out what happened to this asteroid over time, which may give us a better understanding of how all these small asteroids change over millions, hundreds of millions, or even billions of years,” Scheeres said. “Our results have exceeded our expectations.”

Read Asteroid Bennu Secrets Unlocked from NASA’s OSIRIS-REx Ahead of Historic Heist to learn more about this and related research.

“Heterogeneous Mass Distribution of Trümmerhaufen Asteroids (101955) Bennu” by DJ Scheeres, AS French, P. Tricarico, SR Chesley, Y. Takahashi, D. Farnocchia, JW McMahon, DN Brack, AB Davis, R.-L. . Ballouz, ER Jawin, B. Rozitis, JP Emery, AJ Ryan, RS Park, BP Rush, N. Mastrodemos, BM Kennedy, J. Bellerose, DP Lubey, D. Velez, AT Vaughan, JM Leonard, J. Geeraert, B Page, P. Antreasian, E. Mazarico, K. Getzandanner, D. Rowlands, MC Moreau, J. Small, DE Highsmith, S. Goossens, EE Palmer, JR Weirich, RW Gaskell, OS Barnouin, MG Daly, JA Seabrook MM Al Asad, LC Philpott, CL Johnson, CM Hartzell, VE Hamilton, P. Michel, KJ Walsh, MC Nolan and DS Lauretta, October 8, 2020, Advances in science.
DOI: 10.1126 / sciadv.abc3350

The University of Arizona leads scientific operations for OSIRIS-REx. NASA’s Goddard Space Flight Center in Maryland manages the overall mission.

Other co-authors of the new study are researchers from the Jet Propulsion Laboratory, the Smithsonian Institution, the Open University, Northern Arizona University, KinetX Aerospace, Inc., NASA’s Goddard Space Flight Center, the University of Maryland, Johns Hopkins University, from York University and the University of Maryland British Columbia, Southwest Research Institute, Université Côte d’Azur and University of Arizona.

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