PASADENA, Calif. A small, newly discovered asteroid has a one-in-500 chance of striking the Earth in 2030, scientists said, stressing that while the odds are slim, theyve never been worse.

"This is the first time we have ever had a prediction of an impact at this high a probability level, but its still very unlikely," said David Morrison, chairman of the International Astronomical Unions Working Group on Near-Earth Objects.

An asteroid just 80 feet (24 meters) across created Meteor Crater in Arizona 20,000 to 50,000 years ago.

However, until further observations can be made, astronomers are still uncertain whether the object even is an asteroid.

"We havent ruled out the possibility its actually an Apollo launch stage put into heliocentric orbit in early 1970s," said Don Yeomans, manager of the Near-Earth Object office at JPL. The leading candidate would be the 4B stage of a Saturn rocket which would pose no risk to the Earth.

Astronomers discovered the object, dubbed 2000 SG 344, on September 29, using the 3.6-meter Canada-France-Hawaii telescope on the island of Hawaii. At the present, the object lies at a distance of 8.4 million miles (13.4 million kilometers) and is moving away from Earth.

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Initial analysis shows the asteroid should miss the Earth in 2030 by the equivalent of 15 times the distance separating our planet from the Moon, or 3.6 million miles (5.8 million kilometers).

However, there are enough uncertainties in the understanding of its current orbit, and how close it will bring it to Earth's orbit, that could allow for a much closer approach. As such, the small rock zooms to the top of the list of about 300 potentially hazardous asteroids.

The objects orbital period is 354 days, thus it moves around the Sun faster than does Earth. Because of that, the object is slowly drifting away from Earth and is not expected to return to our vicinity for another three decades. Indeed, it was last in our planet's neighborhood in 1971.

Fridays word of a possible encounter marked the first time the International Astronomical Union (IAU) has employed a new procedure to handle such announcements. The new regime was prompted by a rushed March 1998 claim that another asteroid -- 1997 XF 11 -- could strike Earth.

The announcement was recanted the next day, after further analysis by the astronomical community proved it wrong. Now, the IAUs Morrison said, exhaustive orbital analyses including by American, Italian and Finnish astronomers in the case of 2000 SG 344 are done before any discovery is made public.

"If its wrong that means a half-dozen of the best orbit calculators are wrong," Morrison said of the newly discovered object.

To assuage public fears, astronomers have since developed what is known as the Torino scale to measure the risk of an asteroid hitting Earth. A "0" or "1" on the scale indicates a low probability that any particular space rock will hit Earth, while a "10" practically guarantees global catastrophe.

The asteroid 2000 SG 344 would register a "0" on the scale if its at the lower end of its size estimate; at a mere "1" if it lies at the upper end.

If it reaches "1," it would be the first known Earth-threatening asteroid to do so, JPLs Chodas said.

Morrison stressed no matter what, the asteroid will not endanger this planet. If it struck Earth, it would do so with an estimated 10 percent of the energy of the space rock that carved Arizonas Meteor Crater some 50,000 years ago or that exploded over Siberia in 1908.

To further calm fear, Yeomans said the risk of the unknown is still greater, even with 2000 SG 344s 500-to-1 odds.

"The likelihood of the Earth being hit in 2030 by an object of comparable size that we dont know about is actually greater," he said.