Bennu — the target of NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft mission — is a B-type asteroid with a 1,614-foot (492 m) diameter. It completes an orbit around the Sun every 436.6 days (1.2 years) and every 6 years comes very close to Earth, within 0.002 AU. As it moves through space at about 63,000 mph (101,000 km per hour), it also spins, completing a full rotation every 4.3 hours. According to a new study, published in the journal Geophysical Research Letters, Bennu’s rotation is speeding up by about 1 second per century.
“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 study lead author Dr. Mike Nolan, a senior research scientist at the Lunar and Planetary Laboratory at the University of Arizona and the head of the OSIRIS-REx mission’s science team.
“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.”
In order to understand Bennu’s rotation, Dr. Nolan and co-authors studied data of the asteroid taken from Earth in 1999 and 2005, along with data taken by the NASA/ESA 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 quite match their predictions based on the earlier data.
“You 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,” Dr. Nolan said.
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 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.
“The reason for the increase in Bennu’s rotation is more likely due to a phenomenon known the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect,” Dr. Nolan said.
“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.”
M.C. Nolan et al. Detection of Rotational Acceleration of Bennu Using HST Light Curve Observations. Geophysical Research Letters, published online January 31, 2019; doi: 10.1029/2018GL080658