Astronomers are stepping up the global effort to scan the skies for "near-Earth objects": asteroids and comets on a potential collision course with the planet and big enough to pack a deadly punch.

The International Astronomical Union said Thursday it has set up a special task force to broaden and sharpen its focus on impact threats.

Experts say there are an estimated 1,100 known objects that are 1 kilometer (about a half-mile) or wider across — large enough to not only take out a sizable European country but threaten the entire world.

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"The goal is to discover these killer asteroids before they discover us," said Nick Kaiser of the University of Hawaii's Institute for Astronomy, whose Pan-STARRS program will train four powerful digital cameras on the heavens to watch for would-be intruders.

NASA's Spaceguard Survey, which already has identified 800 of the larger objects and has 103 on an impact risk watchlist, wants to find 90 percent by the end of 2008.

The U.S. Congress has asked the space agency for a plan to comb the cosmos for faint objects as small as 140 meters (153 yards) across and log their position, speed and course by 2020.

Astronomers will have their work cut out for them: Experts say there are about 100,000 such objects hidden among the haze of stars, and as many as 1 million half that size.

One known as the Tunguska object slammed into remote central Siberia in 1908, unleashing energy equivalent to a 15-megaton nuclear bomb and wiped out 60 million trees over an 830-square-mile area. Had it hit a populated area, the loss of life would have been staggering.

Giovanni Valsecchi of Italy's National Institute of Astrophysics said the ultimate aim is a permanent warning system like those that now monitor the Pacific for tsunamis and keep tabs on volcanoes and earthquake zones.

The idea: Give the world enough lead time to come up with a workable response to a confirmed threat, such as sending up a rocket to deflect an Earth-bound object or a spacecraft to nudge it into a harmless orbit.

"Right now, unfortunately, there are no 'asteroid busters' or hotlines. Who ya gonna call?" said Andrea Milani Comparetti, a professor of mathematics at the University of Pisa.

The IAU offered some reassurance Thursday about 99942 Apophis, a smallish asteroid that will come within just 18,000 miles of Earth when it whizzes by in 2029. That's closer than many commercial satellites, and 220,000 miles nearer than the moon.

Last year, scientists were concerned Apophis could come even closer in another fly-by in 2036, with a 1-in-5,500 chance of striking Earth with enough energy to wipe out New York City and its suburbs.

However, Comparetti said the latest observations suggest it has only a 1-in-30,000 chance of hitting Earth, and that another initially alarming object, 2004 VD17, now has 1-in-24,000 odds.

"It's not particularly worrying," he said.

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.

But widening the search for threatening objects creates a problem: Discoveries of potential threats 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 IAU task force on impact threats.

To determine the odds of a collision, astronomers surround a target object with a swarm of 10,000 "virtual asteroids" and map out their preliminary orbits.

If one is found to be on a path toward Earth, it's given a 1-in-10,000 probability of impact, though — as it did in Apophis' case — fresh data can ease those odds considerably.

"Only in Hollywood do asteroids arbitrarily change orbits," Morrison said.

Ultimately, Valsecchi conceded, mankind may not be able to dodge every cosmic bullet. Earth's craters bear silent witness to what can happen.

"It's through collisions that planets are born," he said, "and through collisions that planets die."