As NASA's OSIRIS-REx probe starts to really close in on its target, scientists here on Earth have made a surprising discovery about the asteroid Bennu. Newly published research describes how the asteroid is rotating as you might expect, but these rotations appear to be speeding up over time, a new clue for scientists working to understand the composition and behavior of the asteroid and others like it.

Though NASA's OSIRIS-REx spacecraft performed an eight-second burn to enter a history-making orbit around Bennu in January, the new discovery was actually made through observations from ground-based telescopes and the Hubble Space Telescope. This involved looking at data from the year Bennu was discovered in 1999, from the year 2005 and then again from 2012.

Bennu hurtles through space at around 101,000 km/h (63,000 mph) at an average orbital distance of around 168 million km ( 105 million mi) from the Sun. As it does so, it makes one full rotation on its axis every 4.3 hours. But looking at the historical data across that 13-year span, researchers on the OSIRIS-REx mission science team found that the speed of the rotations didn't seem to add up.

"You couldn't make all three of them fit quite right," says Mike Nolan, lead author of the new paper and the head of the OSIRIS-REx mission's science team. "That was when we came up with this idea that it had to be accelerating."

Closer examination of the data led the scientists to conclude that Bennu's rotation is speeding up by around one second each century. These accelerating rotations might sound trivial, but Bennu is believed to be around 4.5 billion years old, and small changes in its rotation speed over large timespans could mean big things for the 500-meter-wide (1,600-ft) piece of space rock.

As its rotations get faster and faster, the researchers point out, the asteroid could be shedding some of its mass or even blow itself apart. And learning more about how quickly things are speeding up can help scientists form theories about the formation of surface features, such as landslides.

"As it speeds up, things ought to change, and so we're going to be looking for those things and detecting this speed up gives us some clues as to the kinds of things we should be looking for," says Nolan. "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."

As for what might be causing the increase in rotation rate, the scientists have a few ideas. One possibility is it is speeding up due to changes in its shape as it loses material, not unlike the way an ice skater speeds up by tucking in their arms. But more likely, they believe, is that sunlight bounces off the asteroid at different angles and causes it to spin faster or slower depending on the shape, something known as the YORP effect.

Though increasing rotation rates have been detected in a small amount of asteroids before, the OSIRIS-REx science team is well placed to study this one in close detail. After completing a series of flybys in the coming months, the OSIRIS-REx spacecraft will then determine Bennu's rotation rate on its own, independently of the telescope data. It is then hoped to return a sample of the asteroid to Earth for further study in 2023, all going to plan, which may uncover some secrets of the early solar system.

The research was published in the journal Geophysical Research Letters.

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