Asteroid
Scientists discovered that asteroid Bennu has been spinning faster over time. Pictured: In this handout from NASA, the giant asteroid Vesta is seen in an image taken from the NASA Dawn spacecraft about 3,200 miles above the surface July 24, 2011 in Space. Getty Images/NASA/JPL-Caltec

Asteroid Bennu's days are slowly shortening, and scientists have no idea why.

Scientists working on NASA's OSIRIS-REx mission to asteroid Bennu recently discovered one baffling thing about the space rock. Currently, Bennu spins once every 4.3 hours, but the researchers calculated, using data gathered before the NASA probe's arrival, that the asteroid's rotation rate has been speeding up over time.

Specifically, the asteroid has been spinning 1 second faster each century, according to a paper published in the journal Geophysical Research Letters.

Research lead author Mike Nolan, head of the OSIRIS-REx mission's science team and geophysicist at the Lunar and Planetary Laboratory at the University of Arizona, explained that asteroid Bennu should experience some changes due to its faster rotation. He and his team will focus on looking for what has changed in the asteroid over time, and "detecting this speed" has already given them a head start on what they should be observing for.

"We should be looking for evidence that something was different in the fairly recent past and it's conceivable things may be changing as we go," Nolan said in a statement released by the American Geophysical Union, which published the new study.

NASA's OSIRIS-REx probe arrived at asteroid Bennu on Dec. 3 last year and began its orbit around the 1,614-foot-wide (492 meters) space rock on New Year's Eve. The new study, however, isn't based on OSIRIS-REx's data, but instead uses measurements and observations gathered by two ground-based telescopes between 1999 and 2005 and by the NASA/ESA Hubble Space Telescope in 2012.

Hubble's data, in particular, didn't line up with predictions researchers calculated using the ground-based data, catching the attention of astronomers.

"You couldn't make all three of them fit quite right," Nolan said. "That was when we came up with this idea that it had to be accelerating."

The study offered one possible explanation as to why asteroid Bennu's rotation has been spinning faster. Material moving around on the space rock's surface or leaving it entirely might be causing the increase in the rock's rotation rate.

Another possible cause is the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect, which is caused by sunlight bouncing off the asteroid and slightly altering its spin rate. It could result in the asteroid spinning either faster or slower, depending on the shape of the space rock. The YORP effect can even tear asteroids apart if they are particularly weak.

The study's authors believe that the YORP effect is the more likely explanation for this new discovery. The scientists are hoping to use the measurements collected by OSIRIS-REx over the next two years to confirm the reason behind asteroid Bennu's faster spin rate.

Meanwhile, NASA previously announced that the OSIRIS-REx probe will start gathering samples from Bennu during the summer of 2020. The spacecraft will briefly touch the surface of the asteroid to collect samples it will bring back to Earth in September 2023.

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