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Bennu asteroid keeps spinning faster and scientists aren't sure why

Victoria Bell, 03/18/2019

Days are getting shorter on the distant space rock being explored by a NASA probe  

A distant space rock called Bennu is spinning faster meaning its rotation period is getting shorter by about one second every 100 years - but scientists are still trying to figure out why.

NASA is observing the asteroid to help them understand the evolution of other similar objects, their potential threat to Earth, and if they could be mined for resources.

Scientists used data gathered during the OSIRIS-REx mission, before the probe's arrival, to calculate that Bennu's rotation rate is speeding up over time.

Bennu is 70 million miles (110m km) away from Earth. As it moves through space at about 63,000 miles per hour (101,000 km per hour), it also spins, completing a full rotation every 4.3 hours.

The increase in rotation might not seem like much but experts say that over a long period of time it can translate into dramatic changes in the space rock.

The probe looks at data collected by two ground-based telescopes between 1999 and 2005 and by the Hubble Space Telescope in 2012.

It was when they looked at the Hubble data that they noticed the rotation speed of the asteroid in 2012 didn't match their predictions based on the earlier data.

'You couldn't make all three of them fit quite right,' said Mike Nolan, lead author on the new research and a geophysicist at the Lunar and Planetary Laboratory at the University of Arizona, who is also head of the OSIRIS-REx mission's science team.

'That was when we came up with this idea that it had to be accelerating.'

According to the study's authors, as the asteroid spins faster and faster over millions of years, it could lose pieces of itself or blow itself apart.

'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,' Dr Nolan added in a written statement.

'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.'

'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 OSIRIS-REx mission is scheduled to bring a sample of Bennu to Earth in 2023.

Even at Bennu, the observations leave the mystery of what's causing it.

One possible explanation is that material moving around on the surface of Bennu or leaving the asteroid entirely could be allowing the rotation rate to speed up.

The idea that the rotation of asteroids could speed up over time was first predicted around 2000 and first detected in 2007.

To date, this acceleration has only been detected in a handful of asteroids.

The authors say that the change in Bennu's rotation could be due to a change in its shape, 'similar to how ice skaters speed up as they pull in their arms', an asteroid could speed up as it loses material.

Dr Nolan also suggested the reason for the increase in Bennu's rotation is more likely due to a phenomenon known the YORP effect.

This means that sunlight hitting the asteroid is reflected back into space. The change in the direction of the light coming in and going out pushes on the asteroid and can cause it to spin faster or slower, depending on its shape and rotation.

'The reason for the increase in Bennu's rotation is more likely due to a phenomenon known the Yarkovsky-O'Keefe-Radzievskii-Paddack effect,' Dr. Nolan said.

The OSIRIS-REx mission will determine Bennu's rotation rate independently this year, which will help scientists nail down the reason for the increase in rotation.


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