Experts say there are about 1,100 comets and asteroids in the inner solar system that are at least a half-mile across, and that any one of them could unleash a global cataclysm capable of killing millions in a single blinding flash.
On Thursday, the International Astronomical Union said it has set up a special task force to sharpen its focus on threats from such "near-Earth objects."
"The goal is to discover these killer asteroids before they discover us," said Nick Kaiser of the University of Hawaii's Institute for Astronomy, which hopes to train four powerful digital cameras on the heavens to watch for would-be intruders.
A prototype of one of the four telescopes in the Hawaii project has already been set up on Haleakela on Maui and is scheduled to be operational by early 2007.
There are no asteroid busters now, but scientists believe that one day a defense could be devised, such as using spacecraft to divert a killer comet.
Congress has asked NASA for a plan to comb the cosmos for even smaller, more distant objects, including asteroids just 1½ football fields across. The space agency is to catalog their position, speed and course by 2020. Already, there are 103 objects on an "impact risk" watch list.
Scientists warn there are as many as 100,000 of these "smaller" heavenly bodies with the potential to take out entire cities or set off a tsunami like the killer wave that swept through the Indian Ocean in December 2004.
Earth's craters bear silent witness to what can happen even when a smallish asteroid slams home. In 1908, one struck remote central Siberia, unleashing as much energy as a 15-megaton nuclear bomb. Fortunately, it wiped out 60 million trees, not people. Had it hit a populated area, the loss of life would have been staggering.
There's some recent good news too: Earth's most pressing threat
the asteroid 99942 Apophis — appears to have eased. Scientists initially gave it a 1-in-5,500 chance of hitting the planet in 2036, with enough power to wipe out the New York City metro area. But experts said Thursday the latest observations suggest those odds have dwindled to 1-in-30,000.
They won't be sure until it makes an earlier pass in 2029, when it's expected to come within 18,640 miles of Earth. If that sounds comfortably distant, consider this: It's closer than many commercial satellites and a good deal nearer than the moon.
Although close encounters are unnerving, they give astronomers a unique opportunity to get a better glimpse of asteroids and comets
the leftover building materials of the universe — and gain a better understanding of the origins of the solar system.
Scientists say expanding their database of the objects crowding Earth's neighborhood could help produce a permanent warning system like those that now monitor the Pacific for tsunamis or keep tabs on volcanoes and earthquake zones.
Give the world a decade or so of lead time to deal with a specific threat, they say, and it stands a chance of getting out of harm's way — perhaps by sending up a spacecraft to nudge an asteroid off-course.
"Right now, unfortunately, there are no 'asteroid busters' or hot lines. Who ya gonna call?" said Andrea Milani Comparetti, a professor of mathematics at Italy's University of Pisa.
To be on the safe side, astronomers trying to determine the odds of one hitting Earth work with computer models that surround it with thousands of "virtual asteroids." Experts then map out the likely orbits for each one and factor those in to come up with the probability of an impact.
But widening the search for threatening objects creates a problem: Discoveries could become commonplace, either creating unnecessary panic and confusion or lulling the public into a false sense of complacency.
"We're now going to be finding such objects once a week instead of once a year," said David Morrison, a NASA scientist who will chair the new task force.
"Only in Hollywood do asteroids arbitrarily change orbits," he said. "But there is great potential for misunderstanding. Dealing with probability and risk is a problem for all of us, whether we're dealing with asteroid impacts or terrorist attacks."
Bottom line: Mankind may not be able to dodge every cosmic bullet.
"It's through collisions that planets are born," said Giovanni Valsecchi of Italy's National Institute of Astrophysics. "And it's through collisions that planets die."