Team measures orbit of NASA’s target asteroid

Scientists have precisely measured the mass and orbit of an asteroid that is the target of a NASA mission scheduled for launch in 2016. Scientists have measured the orbit of the destination asteroid 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 objective of the mission, known as OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer), is to collect samples from the asteroid (1999 RQ36) and return them to Earth. The effort is lead by researchers at the University of Arizona. The new orbit for the half-kilometer (one-third mile) diameter 1999 RQ36 is the most precise asteroid orbit ever obtained, says OSIRIS-REx team member Steven Chesley of the NASA Jet Propulsion Laboratory. He presented the findings May 19 at the Asteroids, Comets and Meteors 2012 meeting in Niigata, Japan. Observations 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. 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 says. “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 University of Arizona, succeeded in an 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 explains. 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. Odds of an impact? 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 says. “The new results don’t really change what is qualitatively known about the probability of future impacts,” Chesley adds. “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 adds. “OSIRIS-REx science team members Steve Chesley and Mike Nolan have achieved a spectacular result with this investigation,” says Dante Lauretta, the mission’s principal investigator and professor of planetary science at the University of Arizona. “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 adds 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.” Calculate density 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,” says Chesley. “This means that it’s more than likely a very porous jumble of rocks and dust.” Finding the bulk density of a solitary space object by combining radar tracking and infrared observations might once have seemed almost science fiction, Chesley says. 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 says.