NASA to launch new mission to better estimate asteroid impact hazard
Washington, February 8 : NASA is planning to launch a new mission in 2016 to find potentially hazardous asteroids and predict their impact threat to Earth.
According to NASA's Near-Earth Object (NEO) program, there are more than 1,300 "potentially hazardous asteroids" (PHAs) -- objects at least 150 yards (about 140 meters) across with a very small chance of impacting us someday because their orbital paths take them close to Earth's orbit.
The main difficulty is obtaining sufficient observations to be able to predict their orbits with enough certainty to find out if they could hit us at some point.
"When an asteroid makes a close pass to Earth, the gravitational pull from our planet changes the asteroid's orbit," said Edward Beshore of the University of Arizona, Tucson, deputy principal investigator for NASA's OSIRIS-REx asteroid sample return mission.
"However, how this change will affect the evolution of the asteroid's orbit is difficult for us to predict because there are also other small forces continuously acting on the asteroid to change its orbit. The most significant of these smaller forces is the Yarkovsky effect -- a minute push on an asteroid that happens when it is warmed up by the Sun and then later re-radiates this heat in a different direction as infrared radiation," he stated.
The Yarkovsky effect happens simply because it takes time for things to heat up and cool down. Objects tend to be coldest just before dawn and warmest at mid-afternoon, after hours of illumination by the high Sun.
NASA's OSIRIS-REx mission (Origins, Spectral Interpretation, Resource Identification, Security, and Regolith Explorer) will make the most precise measurements of the Yarkovsky effect to date by visiting a PHA called "1999 RQ36" or just "RQ36."
"For such a large object, it has one of the highest known probabilities of impacting Earth, a 1 in 2,400 chance late in the 22nd century, according to calculations by Steve Chesley, an astronomer at NASA's Jet Propulsion Laboratory," said Beshore. RQ36 is about 457 meters (500 yards) across.
The best measurements of the Yarkovsky effect are made when an asteroid's position is precisely known.
"If an asteroid comes very close, we can get radar observations on it," said Beshore.
"With radar measurements, we get very good data on its range and therefore can constrain one aspect of its orbit very well. If we can get that measurement a couple of times (or more) over a few years, it helps us understand its orbital behavior and we can start to make an estimate of the Yarkovsky effect. We estimate the position of the asteroid and what its orbit must be like by using Newtonian and relativistic physics. If we see a deviation from the estimated position, then it must be due to the sum of all these other small forces, and the greatest of these we believe to be the Yarkovsky effect," he explained.
Measurements like these enabled the team to estimate the very small force of the Yarkovsky effect on RQ36 -- about equal to the weight felt when holding three grapes, according to Beshore.
"Although very small, this force is constant and adds up over time to significantly change the asteroid's orbit," added Beshore.
Scheduled for launch in 2016, OSIRIS-REx will arrive at RQ36 in 2018 and orbit the asteroid until 2021. By communicating continuously with a spacecraft in orbit around RQ36, the team will get a much better idea of the asteroid's orbit.
"We expect OSIRIS-REx will enable us to make an estimate of the Yarkovsky force on RQ36 at least twice as precise as what's available now," said Jason Dworkin, OSIRIS-REx project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
The team will use what it learns about the Yarkovsky effect on RQ36 to help estimate the effects on other asteroids.
The key to all these strategies is to discover the asteroid well in advance of its impact date and attempt to deflect it early, added Beshore.