So, what can be done? The first thought, dramatically depicted in the 1998 movies "Deep Impact" and "Armageddon," is to nuke the intruder into small pieces so it will burn up in Earth's atmosphere.
Many scientists say, however, that this is unacceptably sloppy -- instead of obliterating the target, the bomb could break the asteroid into large radioactive chunks capable of transforming huge stretches of Earth into wasteland.
Or the explosion could deflect but not destroy the asteroid, putting it on a future collision course. A nuclear strategy would also forever require a stockpile of doomsday weapons.
"The cure's worse than the disease," said former Apollo astronaut Russell L. "Rusty" Schweickart. He is a board member of the B612 Foundation, a group of experts promoting a space mission by 2015 to send a "tugboat" spacecraft to a near-Earth object, dock with it and gently alter its speed enough to change its orbit -- to show that it can be done. (B612 is the name of the asteroid home of "The Little Prince," in the Antoine de Saint-Exupery story.) "You want to delay or speed up the asteroid a little," said Berlin-based Alan Harris, chairman of the European Space Agency's Near-Earth Object Mission Advisory Panel. "What kind of surface do you have: Is it rocky? Dusty? Rubbly? How much force can I apply? I don't want to break it up -- unless I really break it up."
B612 has a design but little money, while ESA has spent only a nominal amount to study the feasibility of a reconnaissance mission to an asteroid. NASA, at $4 million a year, is currently the big spender for near-Earth object research.
With this, NASA maintains a database at JPL to plot and record orbits for all known near-Earth objects, and contributes money to the Minor Planet Center and to sky surveys underway at telescopes in Arizona, California, Hawaii, New Mexico and Australia.
The money was authorized after a push from Congress led by Rep. Dana Rohrabacher (R-Calif.), a conservative, and former House Science Committee chairman George E. Brown Jr. (D-Calif.), known as one of Congress's most liberal members before his death in 1999. "I have a vision of something terrible happening, and I feel compelled to see that it doesn't happen," Rohrabacher said.
NASA's task -- which Congress imposed in 1998 -- is to find 90 percent of the estimated 1,100 near-Earth objects that are one kilometer (0.6 miles) or greater in diameter by 2008. As of mid-March, JPL's database included 762 of these.
On March 1, Rohrabacher introduced the George E. Brown Jr. Near-Earth Object Survey Act, mandating $40 million for a two-year start-up to survey every object 100 meters (328 feet) across or larger, of which there may be 300,000. To date, Marsden has registered 3,265 near-Earth objects of all sizes.
Tholen, of the University of Hawaii, is a frequent contributor in the search for threatening objects. He specializes in "Atens," a subspecies that orbit mostly between the Earth and the sun and are difficult to see in the glare of the sun. To spot Atens, astronomers must work at dawn or dusk.
Tholen's team, on a field trip to the University of Arizona's Steward Observatory, had booked an hour on the evenings of June 19, 20, 23 and 24, 2004. They found a new Aten on the first evening and saw it again on the second evening. It was about 106 million miles away.
The team recorded the sightings and sent them electronically to Marsden, who published the object's position, which he named 2004 MN4 in accordance with a complicated coding system based on the date of discovery.
Tholen waited for another opportunity, but rain clouds cloaked the sky. When the storm passed, the moon was squatting right where the team wanted to look. For the next six months, nobody looked for it.
Then, on Dec. 18, astronomer Gordon Garradd, working at the Siding Springs telescope in Coonabarabran, Australia, 240 miles northwest of Sydney, spotted what he thought was a new near-Earth object, "brightly lit and traveling fast," he recalled. He took four images in his first set, then followed up with two more sets.