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Research led by CU Boulder is revealing the Alice in Wonderland-like physics that govern gravity near the surface of the asteroid Bennu.听

The new findings are part of a suite of papers published today by the team behind NASA鈥檚 (OSIRIS-REx) mission. And they come just three months after OSIRIS-REx first encountered Bennu on Dec. 3, 2018.听

Since then, the spacecraft has completed a few dozen laps around the asteroid, which is roughly as tall as the Empire State Building, circling Bennu from a distance of about a mile. And those early circuits are giving scientists a whole new look at this mysterious rock, said CU Boulder鈥檚 Daniel Scheeres, who leads the mission鈥檚 radio science team.

In research appearing in Nature Astronomy, for example, his team reports the mass of that asteroid: a respectable 73 billion kilograms. 听

But Scheeres and his colleagues are also working to develop a map of the asteroid鈥檚 gravitational pull. Their findings suggest that Bennu exists in a delicate balance between two competing forces, the result of the asteroid鈥檚 wild spin. Bennu completes a full revolution about once every four hours. 听听And, Scheeres said, those forces could play an important role in the asteroid鈥檚 long-term evolution鈥攁nd potential demise.听

鈥淲hen you spin this guy up, you create a competition between the gravity that鈥檚 holding you down and the centrifugal acceleration, which is trying to throw you off,鈥� said Scheeres, distinguished professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences who leads the mission鈥檚 radio science team.

Banana peels

To study those forces, Scheeres and his colleagues use OSIRIS-REx鈥檚 navigational instruments to measure the minute tug that the asteroid exerts on the spacecraft.听

And they dug up more than they expected. Based on the group鈥檚 calculations, the region around Bennu鈥檚 equator is trapped within a gravitational feature called a rotational Roche lobe--something that scientists had not yet clearly observed on an asteroid.

In practice, that feature gets weird. If you were standing inside the boundaries of Bennu鈥檚 Roche lobe and slipped on a banana peel, for example, not much would happen鈥攜ou鈥檇 be captured by the lobe and fall back to the surface.

鈥淏ut if you were outside of the Roche lobe and slipped on a banana peel, you would roll toward the equator,鈥� Scheeres said. 鈥淎nd you could gain enough energy so that you鈥檇 roll off the equator and maybe up into orbit and then out into space.鈥�

It sounds like the sort of environment that Lewis Carroll could appreciate. But it also matters for the lifespan of Bennu, he added.

Spinning to death

That鈥檚 because radiation from the sun is causing Bennu to spin faster and faster over time. And as the asteroid鈥檚 whirling builds up speed, its Roche lobe might also be shrinking, along with the forces that are holding it together.听

鈥淎s that Roche lobe narrows further and further around the equator, it becomes easier and easier for this asteroid to lose material,鈥� Scheeres said. 鈥淪o far, that material has been trapped by gravity, but at some point, if the asteroid keeps spinning faster, then you fall off the cliff.鈥澨�

In other words, Bennu could be in the process of spinning itself into oblivion.

Understanding those physics matters for advance OSIRIS-REx鈥檚 scientific mission, too, said Jay McMahon, an assistant professor in aerospace engineering and a co-author of the new study.

He explained that in 2020, mission scientists will nudge OSIRIS-REx to within a few feet of Bennu, using the spacecraft鈥檚 retractable arm to collect a sample of material from the surface. And to do that safely, they will need to know the rock鈥檚 physics inside and out.听

鈥淵ou need to know the gravitational field for spacecraft operations, really to enable all the other science,鈥� McMahon said.听

鈥淲hen you鈥檙e going to a new world, you have some idea of what it might look like,鈥� Scheeres said. 鈥淭hen you actually go there, and you can start comparing what you thought it might look like versus reality.鈥�

The University of Arizona leads science operations for OSIRIS-REx, which was built by the Colorado-based Lockheed Martin Space. NASA鈥檚 Goddard Space Flight Center in Maryland manages the overall mission.

Coauthors on the new study include CU Boulder graduate students Andrew French and Daniel Brack and Paul S谩nchez, senior research associate at CU Boulder鈥檚 Colorado Center for Astrodynamics Research. Former CU Boulder graduate students Masatoshi Hirabayashi, Stefaan Van wal, Jason Leonard, Yu Takahashi, Jeroen Geeraert, Pete Antreasian, Dolan Highsmith, Mike Moreau and E. Beau Bierhaus also contributed to the paper.