Scientists on NASA's asteroid sample return mission, OSIRIS-REx, have been able to measure the orbit of their target asteroid so precisely that they can see the effects of sunlight on its motion.
What the team's observed is the Yarkovsky effect – the slight push created when the asteroid absorbs sunlight and re-emits that energy as heat.
"The new orbit for the half-kilometer (one-third mile) diameter 1999 RQ36 is the most precise asteroid orbit ever obtained," says team member Steven Chesley of NASA's Jet Propulsion Laboratory.
Observations made at Arecibo Observatory in Puerto Rico in September 2011, along with Arecibo and Goldstone radar observations made in 1999 and 2005, show that the asteroid has deviated from its gravity-ruled orbit by roughly 100 in the last 12 years, thanks to the Yarkovsky effect.
The Yarkovsky force on 1999 RQ36 at its peak, when the asteroid is nearest the sun, is only about a half ounce - while the mass of the asteroid is estimated at about 68 million tons.
"You need extremely precise measurements over a fairly long time span to see something so slight acting on something so huge," says Chesley.
The team measured the distance between the Arecibo Observatory and 1999 RQ36 to an accuracy of about a fifth of a mile, when the asteroid was 20 million miles from Earth.
"That's like measuring the distance between New York City and Los Angeles to an accuracy of two inches, and fine enough that we have to take the size of the asteroid and of Arecibo Observatory into account when making the measurements," says Arecibo's Michael Nolan.
The team's calculated that the asteroid has made will make 11 approaches closer to Earth than 4.6 million miles between 1654 and 2135. It appears to have a low density - about the same as water - and is probably made up of a loose aggregate of rocks.
"This information is critical for assessing the likelihood of an impact from our target asteroid and provides important constraints on its mass and density, allowing us to substantially improve our mission design," says Dante Lauretta, the mission's principal investigator and professor of planetary science at the University of Arizona.
The OSIRIS-REx spacecraft is to launch in 2016, reach the asteroid in 2019, examine it up close during a 505-day rendezvous, then return at least 60 grams of it to Earth in 2023.