One week from today an asteroid will swing within just 17,200 miles of Earth - closer than geosynchronous satellites that orbit the planet. While there is no chance of an impact event on February 15, there are other asteroids that could collide with the Earth sometime in the future.
To prepare for (and hopefully prevent) such a disaster, NASA has formed the Near-Earth Object (NEO) observations program, which finds and tracks potential celestial threats. The program estimates that there are over 1,300 "Potentially Hazardous Asteroids" (PHA) with a small chance of hitting the Earth someday.
Today, NASA outlined its next step in better understanding those objects to help researchers more accurately predict the probability of future impacts. In 2016 the agency will launch OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, and Regolith Explorer), a spacecraft designed to visit a PHA and measure its properties.
The spacecraft will arrive in orbit around an asteroid named 1999 RQ36 in the year 2018. The object is 457 meters across and is also one of most threatening PHAs yet found.
"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 Edward Beshore, deputy principal investigator for NASA's OSIRIS-REx asteroid sample return mission and a researcher at the University of Arizona.
The most important measurement the probe will make is of the Yarkovsky effect, which occurs as a result of asteroids heating and cooling.
"When an asteroid makes a close pass to Earth, the gravitational pull from our planet changes the asteroid's orbit," said Beshore. "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."
The magnitude of the effect is difficult to determine from Earth, since asteroids have different sizes, shapes, and compositions. Beshore and his colleagues expect OSIRIS-REx to provide an estimate of the Yarkovsky force on RQ36 twice as precise as current ones. The measurements should help researchers better estimate the effect on other asteroids.
If new estimates find RQ36 to be an imminent danger to Earth, researchers will have to come up with a way to alter the object's orbit.
"There are several mitigation strategies," said Beshore. "We could explode a small nuclear device close above the surface on one side of the asteroid. This could be very effective - it would vaporize the surface layer, which would then fly off at very high speed, causing a rocket thrust that would shove everything over by a few centimeters per second. This might be plenty to deflect the asteroid. Other strategies include kinetic impactors, where you strike an asteroid very hard with a heavy projectile moving at high speed. In 2005, NASA's Deep Impact mission hit comet Tempel 1 with a 370-kilogram (over 815-pound) copper slug at about five kilometers per second (over 11,000 miles per hour), not nearly enough to significantly alter the orbit of the five-kilometer-sized body, but a proof of the technology for this kind of mission. Another idea is to use a gravity tractor - station a spacecraft precisely enough near the asteroid which would gradually deflect it with only its gravitational pull."
(Image courtesy NASA Goddard Space Flight Center)