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Researchers at CU Boulder have gotten front-row seats to one of the听closest encounters with an asteroid in history.

On Dec. 4, 2018, NASA鈥檚 Origins, Spectral Interpretation, Resource听Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft听zipped to within 4.5 miles of the asteroid Bennu. This space rock has an听orbit that carries it relatively near to Earth about once every six years.

It was the first in a series of planned meet-ups between OSIRIS-REx听and Bennu, and good practice for 2020. Next summer, the spacecraft听will dip just above the asteroid鈥檚 surface, using its retractable arm to听snag material from the top and then bring it back to Earth.

CU Boulder鈥檚 Daniel Scheeres leads the radio science team for OSIRIS-REx.听The overall mission is led by the University of Arizona. He said it鈥檚听an unprecedented opportunity to get a zoomed-in look at a class of听mysterious solar system residents.

鈥淲hen you鈥檙e going to a new world, you have some idea of what it听might look like,鈥� said Scheeres, a Distinguished Professor in the Ann听and H.J. Smead Department of Aerospace Engineering Sciences. 鈥淭hen听you actually go there, and you can start comparing what you听thought it听might look like versus reality.鈥�

In particular, his group has an eye on a simple-seeming but important听number: Bennu鈥檚 mass.

Scheeres and his colleagues are using OSIRIS-REx鈥檚 navigational听instruments to measure the minute pull that Bennu exerts on the听spacecraft鈥攊nformation that then allows them to map out the gravity at听its surface.

Scientists want to gather that kind of data for many reasons, said听Jay McMahon, an assistant professor in aerospace engineering at听CU Boulder.

Asteroids, for example, provide researchers with a rare window to look听back at the beginnings of the solar system.

鈥淥ne of the big draws for asteroids is that they鈥檙e leftovers from the听formation of the solar system,鈥� said McMahon, a co-investigator on听the mission. 鈥淏ennu is a building block of the planets that didn鈥檛 end听up in a planet.鈥�

The results have already started to roll in. In March 2019, the听researchers released their first estimates for the mass of Bennu:听a respectable 73 billion kilograms.

They鈥檝e also begun to get a closer look at the physics of this body鈥攑hysics that would boggle most earthlings.听

鈥淏ennu spins fast enough to create a competition between the gravity听that鈥檚 holding you down and the centrifugal acceleration, which is trying听to throw you off,鈥� Scheeres said.

And he isn鈥檛 stopping at Bennu, either. In June 2019, NASA picked a听mission led by Scheeres, called Janus: Reconnaissance Missions to听Binary Asteroids, as a finalist for its SIMPLEx small satellite program.

If the mission gets the final green light, Janus will send twin spacecraft听to rendezvous with binary asteroids. Scientists have yet to observe such听objects, in which two asteroids revolve around each other, up close.

鈥淭here are many theories of how binary asteroids form, but we haven鈥檛听had the proper measurements to sort through them all and see which听is correct,鈥� Scheeres said. 鈥淭he Janus mission will do this and also help听us better understand how primitive bodies in the solar system听have听formed and evolved over time.鈥�

In other words, the solar system鈥檚 asteroids are becoming a little less听puzzling thanks to CU Boulder.