Bennu, the asteroid targeted by NASA for its OSIRIS-Rex mission, is spinning at increasing rate and scientists aren’t quite sure why.
The asteroid, shaped somewhat like a tabletop spinner, is rotating faster and faster, taking one fewer second to rotate per hundred years. It may not sound like much, but given enough time, the whole asteroid might tear itself apart, according to a team of astronomers.
But don’t worry, NASA's OSIRIS-REx spacecraft stalking and surveying the asteroid, with the aim to extract and analyze a sample from its surface, won’t be in any danger any time soon. It’ll take millions of years before anything dramatic happens.
Bennu, measuring 510 metres from top to bottom, completes a full rotation every 4.3 hours while whipping around the Sun at an average speed of 63,000 miles per hour. It's, on average, 105 million miles from the Sun, close to Earth's 93-million-mile orbit.
The researchers dug through records of observations of Bennu, studying its motion at three different times: in 1999, 2005, and 2012. They found a discrepancy when they looked at the asteroid’s rotation speed.
“You couldn’t make all three of them fit quite right,” said Mike Nolan, a senior research scientist at the University of Arizona and first author of the paper published in Geophysical Research Letters, this week. “That was when we came up with this idea that it had to be accelerating.”
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It’s not unknown to find chunks of space rocks speeding up over time, this behavior has been spotted before but only in a very few asteroids and the researchers aren’t quite sure what could have caused Bennu to start spinning faster. They reckon it’s either because its shape has changed, or its down to the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack effect (YORP).
YORP describes how sunlight can exert a small kick to an asteroid making it spin faster or slower. The photons from the Sun’s rays can be absorbed or reflected back out to space and since the photons carry some momentum, the change in direction of incoming and outgoing light pushes the asteroid.
The team hopes that observations gathered by the OSIRIS-REx spacecraft will help them work out why Bennu is speeding up over time.
“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 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." ®