Scientists with the University of Arizona-led asteroid sample return mission OSIRIS-REx have measured the orbit of their destination asteroid, 1999 RQ36, with such accuracy they were able to directly determine the drift resulting from a subtle but important force called 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, OSIRIS-REx team member Steven Chesley of the NASA Jet Propulsion Laboratory said. He presented the findings May 19 at the Asteroids, Comets and Meteors 2012 meeting in Niigata, Japan.

Remarkable observations that Michael Nolan at Arecibo Observatory in Puerto Rico made in September, along with Arecibo and Goldstone radar observations made in 1999 and 2005, when 1999 RQ36 passed much closer to Earth, show that the asteroid has deviated from its gravity-ruled orbit by roughly 100 miles, or 160 kilometers, in the last 12 years, a deviation caused by the Yarkovsky effect.

The Yarkovsky effect is named for the 19th-century Russian engineer who first proposed the idea that a small rocky space object would, over long periods of time, be noticeably nudged in its orbit by the slight push created when it absorbs sunlight and then re-emits that energy as heat.

The effect is difficult to measure because it's so infinitesimally small, Chesley said.

"The Yarkovsky force on 1999 RQ36 at its peak, when the asteroid is nearest the Sun, is only about a half-ounce - about the weight of three grapes on Earth. Meanwhile, the mass of the asteroid is estimated to be 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."

Nolan, who obtained his doctorate at the UA, succeeded in a heroic effort to get a 16-ton power supply for the transmitter from Pennsylvania to Puerto Rico in six days in time for the observations, which he made on three separate nights last September. Nolan and his team measured the distance between the Arecibo Observatory and 1999 RQ36 to an accuracy of 300 meters, or about one-fifth of a mile, when the asteroid was 30 million kilometers, or 20 million miles, from Earth.

"That's like measuring the distance between New York City and Los Angeles to an accuracy of 2 inches, and fine enough that we have to take the size of the asteroid and of Arecibo Observatory into account when making the measurements," Nolan said.

Chesley and his colleagues used the new Arecibo measurements to calculate a series of 1999 RQ36 approaches closer to Earth than 7.5 million kilometers (4.6 million miles) from the years 1654 to 2135. There turned out to be 11 such encounters.

In 2135, the 500-meter (1,640-foot) diameter asteroid will swing by Earth at around 350,000 kilometers (220,000 miles), its closest approach over the 481-year time span. That's closer than the Moon, which orbits about 390,000 kilometers (240,000 miles) from Earth. At such close distances, the asteroid's subsequent trajectory becomes impossible to accurately predict so close approaches can only be studied statistically, Chesley said.

"The new results don't really change what is qualitatively known about the probability of future impacts," Chesley said. "The odds of this potentially hazardous asteroid colliding with Earth late in the 22nd century are still calculated to be about one in a few thousand."

But the new results do sharpen the picture of how potentially hazardous 1999 RQ36 could be farther into the future. Scientists now have identified many low-probability potential impacts in the 2170s through the 2190s while ruling out others, Chesley said.

"OSIRIS-REx science team members Steve Chesley and Mike Nolan have achieved a spectacular result with this investigation," said Dante Lauretta, the mission's principal investigator and professor of planetary science at the UA. "This study is an important step in better understanding the Yarkovsky effect - a subtle force that contributes to the orbital evolution of new near-Earth objects."

Lauretta added that "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."

The final piece to the puzzle was provided by the University of Tennessee's Josh Emery, who used NASA's Spitzer Space Telescope in 2007 to study the space rock's thermal characteristics. Emery's measurements of the infrared emissions from 1999 RQ36 allowed him to derive the object's temperatures.

From there he was able to determine the degree to which the asteroid is covered by an insulating blanket of fine material, which is a key factor for the Yarkovsky effect.

With the space rock's orbit, size, thermal properties and propulsive force (Yarkovsky effect) understood, Chesley could perform the space rock scientist equivalent of solving for "x" and calculate its bulk density.

"1999 RQ36 has about the same density as water, and so it's very light for its size," said Chesley. "This means that it's more than likely a very porous jumble of rocks and dust."

Asteroid 1999 RQ36 is of particular interest to NASA as it is the target of the agency's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) mission. Scheduled for launch in 2016, ORIRIS-Rex will visit 1999 RQ36, collect samples from the asteroid and return them to Earth.

NASA detects, tracks and characterizes asteroids and comets passing close to Earth using both ground and space-based telescopes. The Near-Earth Object Observations Program, commonly called Spaceguard, discovers these objects, characterizes a subset of them, and establishes their orbits to determine if any could be potentially hazardous to our planet.

Finding the bulk density of a solitary space object by combining radar tracking and infrared observations might once have seemed almost science fiction, Chesley said.

What OSIRIS-REx scientists are beginning to learn about Yarkovsky drift strengthens the idea that "the Yarkovsky effect can be used to probe the physical properties of asteroids that we can't visit with spacecraft," he said.

OSIRIS-REx stands for "Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer." The OSIRIS-REx spacecraft is to launch in 2016, reach asteroid (101955) 1999 RQ36 in 2019, examine it up close during a 505-day rendezvous, then return at least 60 grams of it to Earth in 2023. More information can be found on the mission's website.