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Asteroid nudged by sunlight

Saturday, 26 May 2012

These series of radar images of asteroid 1999 RQ36 were obtained by NASA's Deep Space Network antenna in Goldstone, Calif., on Sept. 23, 1999. Credit: NASA/JPL-Caltech

Scientists on NASA's asteroid sample return mission, Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx), have measured the orbit of their destination asteroid, 1999 RQ36, with such accuracy they were able to directly measure 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," said OSIRIS-REx team member Steven Chesley of the NASA Jet Propulsion Laboratory, Pasadena, Calif. He presented the findings May 19 at the Asteroids, Comets and Meteors 2012 meeting in Niigata, Japan.

Observations that Michael Nolan at Arecibo Observatory in Puerto Rico made in September 2011, 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 nineteenth-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 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 and his team measured the distance between the Arecibo Observatory and 1999 RQ36 to an accuracy of 300 meters, or about a 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 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," 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.