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Alert: Asteroid alarm

Every few years, astronomers who study asteroids are accused of crying wolf.

In 1998, one group predicted that an asteroid was headed toward a collision with Earth in 2028. A day later, another group said the estimate was based on faulty data and there was no chance of a disaster.

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In April 2002, astronomers announced that they'd found an asteroid a half-mile wide that has a 1-in-300 chance of hitting Earth. But it turned out that Asteroid 1950 DA, as it's formally known, won't arrive until March 16, 2880.

Hollywood has done its part, too. Movies such Deep Impact and Armageddon have entertained millions with tales of death-dealing rocks that are heading Earth's way.

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Experts say alarms like these are the price we pay for better surveillance of the heavens - and they're likely to continue as long as astronomers keep looking skyward.

"These asteroids were passing by before - it's just that we didn't have an ability to see them," said Clark Chapman, a planetary scientist at the Southwest Research Institute in Arizona.

Curiosity and concern

Asteroids are small celestial bodies that orbit the sun, mostly between Mars and Jupiter. Scientists believe they're made of the same rocks and metals that formed the planets, and they've long been objects of curiosity and concern.

NASA, for example, spends $3 million a year to search for asteroids that are potentially big enough to wipe out the planet - meaning bodies at least a kilometer (about 0.6 mile) in diameter.

About 100 scientists and researchers work on the asteroid search around the world, and they expect to have 90 percent of the dangerous rocks identified by 2008.

But at least one group of astronomers says that effort isn't enough.

"We're not alarmists. We're not worried about this happening tomorrow. We're just saying more attention should be paid to something that could really turn off the lights in a big way," said Thomas D. Jones, a former shuttle astronaut and leader of an effort to increase funding for asteroid searches.

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Wiped out the dinosaurs

According to the Southwest Research Institute's Chapman, the world's current asteroid fixation dates back to 1980, when Luis W. Alvarez hypothesized that a large asteroid had wiped out the dinosaurs by hitting Mexico's Yucatan Peninsula.

"If one killed off the dinosaurs, I guess it hit home that someday one could kill us off," Chapman said.

NASA began to focus on asteroids in 1990, when a series of highly publicized close calls piqued public interest and prompted Congress to appropriate funds to search for them. Spaceguard, a worldwide effort established in 1991, has so far found 650 asteroids at least a kilometer wide.

In a recent letter to Congress, Jones joined 10 astronomers, historians and other experts who argued that Spaceguard's efforts aren't enough.

They want the United States to increase spending almost sevenfold to build better telescopes and look for smaller asteroids. The smaller rocks, they note, hit more often - about once every thousand years. Their impact would have the force of a nuclear blast that could destroy major cities and perhaps entire countries.

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"One of the big questions facing us as a species is, how much stuff is out there that poses a danger to us, and how much of a danger is it?" said Lucy Ann McFadden, a University of Maryland astronomer who signed the letter. "We really don't know."

Jones, McFadden and others say the United States should hire more observers and build larger telescopes dedicated to the search for 200-meter asteroids, often too small to spot now but big enough to cause a major regional catastrophe on impact.

Near Earth Objects

Most asteroids are in a doughnut-shaped belt between Mars and Jupiter. They revolve in the same direction as Earth and take three to six years to complete an orbit. But others reside outside the belt, and any that approach within 30 million miles are classified as Near Earth Objects, or NEOs.

Comets can also be NEOs, but they generally travel much farther away than asteroids and are less likely to strike Earth.

Based on records of asteroid hits and the number of asteroids actually found by astronomers - about 100 a year - experts estimate that there are roughly 1,100 NEOs. But that's just a guess.

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"We really don't know how many there are," said Brian Marsden, who operates the Smithsonian's Minor Planet Center in Cambridge, Mass., where newly discovered asteroids are listed.

Hits and close calls

History is filled with asteroid hits and close calls. Many scientists credit asteroids with wiping out the dinosaurs 65 million years ago, forming the Chesapeake Bay 35 million years ago and creating a mile-wide crater in Arizona a mere 50,000 years ago. Researchers say asteroids also might have first brought water to the planet, allowing dinosaurs and other life forms to thrive.

In 1908, an asteroid 330 feet wide exploded over Siberia, creating the equivalent of a 10-megaton nuclear blast that felled trees over 40 square miles.

In January 2002, a 300-yard-wide asteroid that could have destroyed an area the size of New England missed Earth by 500,000 miles - about twice the distance to the moon.

Scientists agree that a strike by a kilometer-wide asteroid would cause global catastrophe, clouding skies, dropping temperatures and killing off most plants and animals. Even smaller asteroids that break up in the atmosphere can cause tremendous destruction. The 1908 Siberian asteroid generated enough blast force to kill thousands if it had struck an inhabited region.

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"If you happen to be in the area where a 10-megaton blast occurs, it's still going to cause serious damage, whether it leaves an impact crater or not," Jones said.

But not all astronomers are convinced that a search for smaller asteroids is necessary. "Identifying smaller asteroids may seem like they're worth doing, but the question is, are they worth doing in place of other programs?" Chapman said.

Then what do you do?

A key question: Is there anything we do about an Earth-bound asteroid once we know about it? Movies notwithstanding, technology to throw an asteroid off-course would take about two decades to develop, said David Morrison, who oversees NASA's asteroid efforts at the Ames Research Center at Moffett Field, Calif. The method would depend on the asteroid's composition, size and speed, but recent studies have focused on changing its orbit with rockets or solar deflectors planted on the surface.

Still, the chances that a kilometer-wide "doomsday asteroid" will hit us are minuscule - by NASA's estimate, once or twice every million years. But NASA says there are an additional million smaller asteroids at least 50 meters across out there.

"We are likely to be hit somewhere on Earth by one of those, with an energy equivalent to a large nuclear bomb, sometime in the next couple of centuries," Morrison warns on a NASA Web site.

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After the 1998 scare, the International Astronomical Union established protocols calling for peer review of asteroid discoveries that would check all calculations within 72 hours. But Morrison noted that there's nothing to prevent rushing to publication on the Internet:

"It's a very open process, but we're going to post the information [on Web sites] anyway and let the chips fall where they may. I'm sure there will be other controversies."


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