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The streak blazed so brightly through John Rogers' telescope that he had no doubt this stadium-size asteroid was moving perilously close to the Earth.

Other astronomers who had sighted this galactic intruder earlier believe it came within 280,000 miles of Earth last month - one of the closest calls on record for an object of its size.The asteroid was sighted for the first time when it was just four travel days away from Earth and packing more explosive power than an arsenal of nuclear bombs.

"If that one was headed toward the Earth, there would be nothing we could do to stop it," said Rogers, an amateur astronomer and aeronautical engineer for the Naval Air Warfare Center at Point Mugu, located 65 miles northwest of downtown Los Angeles.

That same night, Rogers aimed his telescope at another asteroid swinging past Earth - one that Jet Propulsion Laboratory astronomers in Pasadena, Calif., had discovered in their hunt for dangerous flying objects.

It was the fifth previously unknown asteroid that JPL's Near-Earth Asteroid Tracking system has discovered since its monthly observations began March 15.

Two of these flying chunks of cosmic debris are "potentially hazardous" to Earth, said NEAT lead astronomer Eleanor Helin.

"This is the size of the object we feel could be responsible for global catastrophe," she said. "It can cause such devastation and so much in the way of materials thrown up into the air" that sunlight could not get through.

It all might sound like bad science fiction or a Superman comic-book plot, but the Air Force Space Command in Colorado was not laughing when it launched its planetary defense study a year ago.

"As a country, as a government, there is no defense system against asteroids," Space Command chief scientist John Darrah said in an interview.

JPL has been allowed to use the Air Force's telescope in Hawaii, a spyglass typically used to probe the skies for objects of military significance. The JPL NEAT camera has turned the telescope into an asteroid hunter like no other in the world, Helin said.

Operating automatically, NEAT spots unidentified objects from the sky's mash of stars and planets, then beams the information to Pasadena, where Helin and her team study the results.

NEAT costs $300,000 a year.

Yet, for all of its sophistication, NEAT misses some near-Earth objects - for the simple reason that it can't search the entire sky. Other astronomers scan for near-Earth objects, but Helin said the Southern Hemisphere lacks coverage. Three or four more NEAT systems, positioned worldwide and coupled with existing telescopes, could do much to safeguard the planet from a cosmic attack, she said.

"If we are looking at the sky in a snapshot each night, we will know what is out there," Helin said. "It is a total planetary concern."

Even with such coverage, Helin and other scientists say no one can predict exactly which speeding asteroid or comet is heading toward Earth, or when it might hit.

"I am in a scientific community where it is not popular to preach doomsday and be an alarmist . . . (but) we simply don't know when one of these things will come shooting in - and we are at their mercy."

Scientists have identified about 250 asteroids larger than a kilometer in diameter that cross the Earth's orbit. None appears to be on a collision course with the Earth, but Helin said that any one of them could change orbit over time.

A panel commissioned by Congress concluded that the chance of a large-scale collision is one in a thousand over the average American's life. Other astronomers say it is more like one in several hundred million.

Helin cautioned that such statistics are just educated guesses.

"It might be one time in a million years (or in) hundreds of millions of years. We don't know where we are with that clock running," Helin said.

Craters and trenches on Earth and its moon are proof of past collisions with space debris. One popular theory has it that the dinosaurs met their demise because of a giant meteor that threw up so much dust that it cloaked the sun.

Meteor Crater in Arizona is 4,000 feet wide and 700 feet deep - the work of an asteroid perhaps no larger than 50 feet in diameter that hit more than 25,000 years ago, according to scientists' best estimates.

And in 1908, a small asteroid skipped along the atmosphere, creating a fireball in the Siberian wilderness that was seen as far away as London. The landscape was laid barren for 750 square miles, with scars still visible today.

Most recently, the power of a planetary collision was captured by an unmanned space probe that sent back pictures of Comet Shoemaker-Levy 9 hitting Jupiter's atmosphere, exploding in fiery showers.

George Friedman, retired as chief technical officer for the Northrop Corp., studies near-Earth objects for the American Institute of Aeronautics and Astronautics.

He said he loses no sleep over the threat of asteroids but believes more should be done to prepare for and protect against a possible collision.