Asteroid Bennu's rotation period getting shorter, say scientists

Asteroid Bennu -- a target of NASA's sample return mission -- is spinning faster over time, an observation that may help understand the evolution of asteroids and their potential threat to Earth, scientist say. Bennu is located 110 million kilometers away from Earth. As it moves through space at about 101,000 kilometers per hour, it also spins, completing a full rotation every 4.3 hours. Last year, the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer (OSIRIS-REx) spacecraft arrived at Bennu, the asteroid it will be studying and sampling over the next several years.

The research, published in the journal Geophysical Research Letters, found that the asteroid's rotation is speeding up by about one second per century.

In other words, Bennu's rotation period is getting shorter by about one second every 100 years. While the increase in rotation might not seem like much, over a long period of time it can translate into dramatic changes in the space rock, researchers said.

As the asteroid spins faster and faster over millions of years, it could lose pieces of itself or blow itself apart, they said.

Detecting the increase in rotation helps scientists understand the types of changes that could have happened on Bennu, like landslides or other long-term changes, that the OSIRIS-REx mission will look for.

"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," said Mike Nolan, a senior research scientist at the Lunar and Planetary Laboratory at the University of Arizona in the US.

"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," said Nolan, who is the head of the OSIRIS-REx mission's science team.

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

Understanding Bennu's rotational change could help scientists figure out what asteroids can tell us about the origin of the solar system, how likely it is for asteroids to pose a threat to humans and if they could be mined for resources.

In order to understand Bennu's rotation, scientists studied data of the asteroid taken from Earth in 1999 and 2005, along with data taken 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 did not quite match their predictions based on the earlier data.

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, Nolan said.