Scientist Guesses Asteroid's Weight, Millions Of Miles Away
May 25, 2012

Scientist Guesses Asteroid’s Weight, Millions Of Miles Away

Lee Rannals for

A NASA scientist would be a shoe-in at any state or county fair if hosting the "guess your weight" game.

Steve Chesley of JPL's Near-Earth Object Program Office has accurately determined the mass of an asteroid from millions of miles away.

Chesley used data from the Goldstone Solar System Radar in the California desert, along with the orbiting Spitzer Space Telescope and the Arecibo Observatory in Puerto Rico, to make his findings.

He said he first needed to understand the orbit of the asteroid before being able to determine its mass.  He also needed to take into consideration everything that could affect the asteroid orbit, such as neighboring celestial bodies and any propulsive force the asteroid could generate.

Chesley calculated how far the asteroid deviated from its anticipated orbit by using observations made by astronomer Michael Nolan at Arecibo Observatory in September 2011, Arecibo and Goldstone radar observations made in 1999 and 2005, and the gravitational effects of the sun, moon, planets and other asteroids.

Nolan and his team measured the distance between the Arecibo Observatory and the asteroid to an accuracy of about a fifth of a mile when the asteroid was about 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.

The 1999 RQ36 asteroid deviated from the mathematical model about 100 miles in the past 12 years, according to Chesley's findings.  He said the only logical explanation for this orbital change was the rock generated a minute propulsive force known as the Yarkovsky effect.

This effect is named for the 19th-century Russian engineer who first proposed the idea that a small, rocky space object would be noticeably nudged in its orbit by the slight push created when it absorbed sunlight and re-emits that energy as heat.

"At its peak, when the asteroid is nearest the sun, the Yarkovsky force on 1999 RQ36 is only about a half ounce -- around the weight of three grapes," Chesley said.  "When you're talking about the force of three grapes pushing something with a mass of millions of tons, it takes a lot of high-precision measurements over a long time to see any orbital changes. Fortunately, the Arecibo Observatory provided a dozen years of great radar data, and we were able to see it."

Josh Emery of the University of Tennessee, Knoxville, who used NASA's Spitzer Space Telescope in 2007 to study the asteroid's thermal characteristics, was able to help Chesley out with the final piece of the puzzle.

Emery measured the infrared emissions from the asteroid, which helped Chesley determine the temperature of 1999 RQ36.

Using the newest piece of the puzzle, Chesley was able to determine the degree to which the asteroid is covered by an insulating blanket of fine material.

Considering the asteroid's orbit, size, thermal properties and propulsive force, he was able to determine that 1999 RQ36 is about 60 million metric tons at about 1,600 feet across.

"That means it has about the same density as water, so it's more than likely a very porous jumble of rocks and dust," Chesley said.

NASA is interested in this particular asteroid because it is the target of the agency's Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) mission, which is scheduled for launch in 2016.

The OSIRIS-REx will be visiting the asteroid, collecting samples from its surface, and then returning to Earth with the samples during the mission.

"In addition to the exciting Yarkovsky results, the low density shows that 1999 RQ36 is probably a loose aggregate of rocks--a so called rubble pile," Jason Dworkin, the mission's project scientist and Chief of Astrochemistry at NASA's Goddard Space Flight Center, said. "This makes it an ideal target for OSIRIS-REx to collect loose surface material."

Image 2 (below): 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. Image credit: NASA/JPL-Caltech