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.
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.
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.
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.
