101955 Bennu is a carbonaceous asteroid in the Apollo group discovered by the LINEAR project on September 11, 1999. The space rock is a hazardous object listed on the Sentry Risk Table with the second-highest cumulative rating on the Palermo Technical Hazard Scale. It currently has a 1-in-2,700 chance of impacting Earth in the future, which could change following new observations.
Scientists working on NASA’s OSIRIS-REx mission used data gathered before the probe’s arrival to calculate Bennu’s rotation speed.
The asteroid was previously measured to rotate once every 4.3 hours.
However, new research has calculated it to be increasing by about one second every century.
Mike Nolan, lead author of the Lunar and Planetary Laboratory paper said: “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 kind of things we should be looking 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.”
The new research is not based on measurements from that probe, instead, it looks at data collected by two ground-based telescopes between 1999 and 2005 and by the Hubble Space Telescope in 2012.
The numbers caught scientists’ eyes because they did not line up with predictions astronomers had calculated with the ground-based data.
Dr Nolan added: “We couldn’t make all three of them fit quite right.
“That was when we came up with this idea that it had to be accelerating.”
Scientists do have a possible explanation for the phenomenon, though.
One could be that material moving around on the asteroid, or leaving it entirely, could be forcing its rotation rate to increase.
The other explanation is more complicated, the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect – the theory that sunlight could tweak the shape and spin rate.
The scientists behind the new research suspect it’s the YORP effect that Bennu is experiencing.