The U.S. space agency, NASA, says it doesn't have the money to track the asteroids and comets that could potentially hit the Earth, even though it has the technical ability to do it. The announcement followed a recent symposium at the annual meeting of the American Association for the Advancement of Science where experts described how a killer asteroid could be deflected ? if it's found in time.

An asteroid striking Earth is more than just a theoretical worry or, for that matter, the plot of a Bruce Willis movie. Asteroids have hit our home planet repeatedly. A century ago, an asteroid flattened 2,000 square kilometers of a sparsely-populated area in Siberia, a relatively local event. But 65 million years ago, an asteroid estimated at just 10 to 15 kilometers across slammed into what is now Mexico. Then, it killed off the dinosaurs. Today, it would wipe out human civilization.

A systematic survey of the skies to identify threatening objects that cross Earth's orbit could cost a billion dollars, money NASA plans to spend on other projects.

But even without a systematic survey of the sky, astronomers have occasionally stumbled on objects heading our way. One that looked particularly dangerous is an asteroid known as Apophis.

"Apophis was discovered a few years ago. It's due to have a very close approach to the Earth in 2029, closer than the altitude of geosynchronous satellites, so close, in fact, that something like this is expected to happen only every 1,500 years or so," says Steven Chesley of the Jet Propulsion Laboratory in California. He notes that as astronomers continued to track Apophis, they concluded that it would miss the Earth in 2029, but they determined that, if the asteroid flies through one particular area, "what we call a keyhole, a narrow slot, through which, if the asteroid passes, it will be deflected onto a course to rendezvous -- in fact, collide -- with the Earth seven years later."

So they'll continue to keep an eye on Apophis. Meanwhile, NASA has identified about 130 asteroids that might hit the Earth. But Chesley says there are an estimated 20,000 yet to be discovered.

So what happens if, one day, astronomers announce that they've discovered a distant asteroid heading right for Earth? Over the years, scientists -- and science fiction writers -- have come up with various solutions. You've probably seen them in the movies. But a more elegant proposal comes from astronaut and physicist Ed Lu, who explains his idea for a gravity tractor to nudge an asteroid out of a collision orbit.

"What it is, is a spacecraft that would hover near an asteroid," Lu explained. "And what it would do is fire its engines to hold its position, and using the gravitational pull between them would, therefore, slowly tow the asteroid. Now it's a very small force, but you can hover there for a very long time. And the combined effect of a small force over a very long time can be enough to deflect an asteroid."

And when Ed Lu says a very long time, he's talking about years, or even decades.

Still, when an asteroid is identified as bearing down on us, it will be important to act as soon as possible. Former Apollo astronaut Rusty Schweickart stresses that we can and should be prepared. "With the early warning systems, and the technology, which as humans we have developed, we can prevent this. We can't prevent a hurricane. We can't prevent a tornado. But we can prevent an asteroid impact. Now, if we don't do that, we're not that far past the dinosaurs," he said.

NASA's David Morrison, the senior scientist at the space agency's Astrobiology Institute, says today's list of potentially threatening asteroids is a big step from the generic concern and mathematical probabilities that were cited only a few years ago. "Notice we're not doing that anymore. We're talking about real asteroids, a real survey, real circumstances. This has gone from being an esoteric, statistical argument to talking about real events," Morrison said.

Which is why scientists stress the need to search the skies for asteroids that might be halfway across the solar system now, but could pose a civilization-ending threat to Earth in the decades to come.