NASA and the Air Force are studying ways to ward off a medium-sized asteroid that will streak within 18,000 miles of Earth in 2029 and has an extremely slight chance of crashing into our planet in 2036.
Ideas discussed this week at a Planetary Defense Conference in Washington include a "gravity tug" or "space tractor" that would hover near the space rock and tow it into a safe orbit. Other possibilities include a head-on collision with an unmanned spaceship or a nuclear explosion.
In the past eight years, 754 asteroids bigger than 1 kilometer (about six-tenths of a mile, or 3,280 feet) across have been detected orbiting near Earth. But none is expected to come as close as a smaller one called Apophis, which was discovered just before Christmas in 2004.
Named after an ancient Egyptian god of evil, Apophis is about 900 feet long - three times the length of a football field - and is traveling at a speed of 12,000 mph.
If an object that size hit Earth, it would "destroy England or Northern California," said Steven Chesley, an asteroid expert at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Even if it landed in the Pacific Ocean, it would cause a tsunami as big as the one that devastated Indonesia and neighboring countries in 2004, according to Clark Chapman, a planetary scientist at the Southwest Research Institute in Boulder, Colo.
Simon "Pete" Worden, the director of NASA's Ames Research Center in Mountain View, Calif., told the conference that the United States has the technology needed to send a mission to deflect an asteroid such as Apophis, but the Bush administration hasn't requested money to pay for it.
"We don't yet have the resources to do much about this," Worden said. NASA's budget includes $4 million a year to study the asteroid threat.
Edward Lu, a former astronaut at the Johnson Space Center in Houston, estimated that the cost of a gravity tug would be about $300 million. He said the gravitational force of a 1-ton robotic spacecraft orbiting just ahead of the asteroid would gradually pull the space rock out of Earth's way.
Other methods, such as bombing the asteroid, might break it into smaller pieces that could be even more dangerous to our planet, Lu said.
Apophis gave NASA a brief scare at first, when astronomers thought it had a 3 percent chance of hitting Earth. Additional evidence, however, soon eliminated any danger of a collision in 2029 and reduced the odds of an impact in 2036 to 1 in 45,000.
"Apophis is not really a threat," said Don Yeomans, the manager of NASA's NEO (Near Earth Object) office in Pasadena, Calif.
However, he said, the passage of the big rock across the sky will be visible to the naked eye in Europe and North Africa.
"It's going to be quite a sight," Yeomans said.
About 70 percent of the estimated 1,090 asteroids bigger than 1 kilometer across have been detected and their orbits identified. Now NASA is under congressional orders to find 90 percent of the much more numerous small asteroids - those at least 140 meters (459 feet) across - by 2020.
"They run the gamut from wimpy ex-comets to slabs of solid iron," Yeomans said. "Our goal is to eliminate 90 percent of the risk from these smaller objects."
"We can't prevent hurricanes or tornadoes," said Russell "Rusty" Schweikert, another former astronaut, "but we can prevent this asteroid impact."
To accomplish the goal, the Air Force is financing a system of ground-based telescopes in Hawaii called Pan-Starrs that will start searching in 2010 for asteroids or comets that are on a collision course with Earth.
The National Science Foundation is building a Large Synoptic Survey Telescope in Chile that will scan the sky every three days for faint objects - including asteroids - starting in 2014.
Although almost all possibly dangerous asteroids can be detected, the risk can't be reduced to zero.
"There's always a tiny chance that something is hiding behind the sun," said Alan Harris, a member of the Space Science Institute in Boulder, Colo.
To see images of Apophis moving against background of stars, go to en.wikipedia.org/wiki/Image:2004MN4_Sor mano.gif.