But Chodas simulated 1,000 collisions with asteroids of various sizes and calculated how long after each object's discovery it would take astronomers with optical telescopes to warn of a 50 percent risk of collision.
All of the largest objects would reach that warning point before impact, most within five years of discovery.
"Including radar buys us about nine months more time," he said.
But for objects as small as 460 feet across, it would be 18 years before 75 percent reached the point at which scientists could determine a 50 percent risk of a strike. Radar cuts that to 15 years.
But 20 percent of those objects would strike Earth before they were discovered.
With enough warning, humans could come up with a scheme to deflect a dangerous comet or asteroid before it strikes. The first tactic would likely require smacking it with "impacters" or explosives to shove it off course.
The 'keyhole' factor
But the JPL's Donald K. Yeomans noted that an asteroid like Apophis might be deflected by an impacter onto a course that sends it (or fragments of it) through a "keyhole" in space that makes collision with Earth on a later orbit all but inevitable.
He and several colleagues proposed a "gravity tractor" - a spacecraft equipped with thrusters and a radar transponder to report its precise position.
Launched simultaneously with the asteroid impacter, the unmanned tractor would take a position just above the asteroid in, say 2028. Scientists on the ground would use its radar transponder to determine the asteroid's precise course.
All of this, according to Yeomans, is possible with current technology, and "it's not that hard."
If the impact sends the asteroid toward the keyhole for a collision with Earth in 2049, the tractor would go into action. Using only its thrusters and the weak gravitational attraction between itself and the asteroid - no cables or chains - the tractor would begin to tug the big rock onto another course.
The gravitational pull is "very tiny," Yeomans conceded, barely 0.16 inch per second for a 460-foot asteroid.
"But it's acceleration, and over 21 years it can amount to quite a bit."
The tractor doesn't have to move the asteroid a distance equal to the diameter of Earth to avoid a collision, only the width of the keyhole in space it's trying to avoid - about 2,000 feet wide in the case of Apophis.
The bottom line, said Yeomans: "In 200 days of tractoring the asteroid ... it could be moved completely off the 2049 keyhole, thus saving the world from imminent disaster."
frank.roylance@baltsun.com
