An asteroid hurtles toward Earth, threatening devastation. A team of attractive young scientists and engineers launches a rocket that crashes into the asteroid and knocks it off course - just in the nick of time.
But wait. The crash pushes the giant space rock toward a "keyhole" in space: a tiny window that guarantees that the asteroid will come back and obliterate some hapless city in the future. What to do?
A scruffy grad student raises his hand. How about a "gravity tractor" to tow it off course?
Real-life scientists are actually exploring the gravity tractor, the keyhole problem and other issues surrounding the very real, if remote, danger that a comet or asteroid will some day cross Earth's orbital path at precisely the wrong time.
A crowd of them met this week in Baltimore as part of the "Asteroids, Comets, Meteors 2008" conference sponsored by the Johns Hopkins University's Applied Physics Laboratory.
It's not an idle concern. On June 30, 1908, astronomers say, a comet exploded over a remote area of Russia with the force of a 10-megaton nuclear bomb. The "Tunguska" impact, as it's known, flattened 80 million trees over 830 square miles - that's enough destructive power to devastate a city of millions, had it struck one.
Since 1950, geologists have identified more than 160 prehistoric meteor impact craters on Earth, and several new ones are found each year, according to the Earth Impact Database maintained by the University of New Brunswick in Canada.
Scientists presume that far more have been obliterated by erosion and plate tectonics. And most objects that strike Earth probably fall into the oceans.
NASA supports four full-time sky surveys to find and track every space rock orbiting the sun that could threaten Earth. So far, scientists have found close to 1,000 at least a kilometer in size -big enough to cause global problems. But they think that may be only 90 percent of the total.
There are also likely to be tens of thousands more rocks big enough to threaten whole cities and millions of human lives. The search programs estimate they've found only a small percentage of those.
One key member of Congress says the U.S. is not doing enough to understand and plan for such threats. Republican Dana Rohrabacher, who represents Southern California's conservative 46th District, is a former chairman of the House Subcommittee on Space and Aeronautics and a senior member of the House Committee on Science.
"It doesn't take a genius to recognize there is a potential threat to our planet that could well cause ... the loss of millions and millions of people," he told the scientists gathered in Baltimore. "We are not prepared, and it's not something that would cost a gazillion dollars to be prepared for."
He urged the scientists to lobby Congress for funds to help international efforts in the field and to prevent proposed budget cuts in NASA's own search programs.
In particular, he argued for a rescue of the Arecibo Observatory in Puerto Rico. Radar astronomy from Arecibo enhances the accuracy of optical tracking of asteroids. But the National Science Foundation has recommended cuts that could lead to its closure.
"We know what can happen if we just sit by and wait, and pray," Rohrabacher said.
So far, scientists haven't found any dangerous space objects on a path toward Earth. But they are closely watching an asteroid called 99942 Apophis, a rock with a diameter of 700 to 1,000 feet, discovered in 2004.
Apophis caused a brief sensation when initial orbit calculations gave it a 2.7 percent chance of smashing into Earth in 2029 - by far the most serious threat ever identified. Subsequent observations of its orbit eliminated the possibility of a 2029 collision, but scientists still rate the chances of an impact at 1 in 45,000 when the asteroid returns in 2036.
At this week's conference in Baltimore, Jon D. Giorgini, a senior analyst with the Solar System Dynamics group at NASA's Jet Propulsion Lab, said 1 in 45,000 is still regarded as a "historically high" risk, a once-in-800-years event.
The problem, he said, is that precise measurements of the threat from Apophis won't be possible for years. Too many factors have to be understood and evaluated for a precise fix on the asteroid's path.
Solar wind alone can deflect the asteroid by up to 18.6 million miles between now and 2036, Giorgini said. And the tiny gravitational influence of other asteroids could move it by twice the width of Earth, according to a paper Giorgini published this year in the journal Icarus.
Scientists will need new optical observations in 2011 and radar observations from Arecibo in 2012 and 2013 to refine their Apophis calculations.
The Jet Propulsion Lab's Paul W. Chodas reported on work he has done to estimate how much warning we can expect of an asteroid headed for a collision with Earth. The majority of objects miss Earth entirely, of course.
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."