Today, for the first time, a scientist backed by decades of data is predicting that an asteroid similar in size to the one thought to have destroyed the dinosaurs 65 million years ago might well crash into Earth.
That's the good news.
This massive piece of space junk, after all, isn't scheduled to rendezvous with Earth for another 878 years. In the past few months, however, at least two smaller asteroids have buzzed Earth's orbit with almost no warning at all. In fact, one of the rocks big enough to turn Orlando, Fla., into a Disney-size divot wasn't seen until after it passed.
The two scenarios point out an emerging truth in humanity's effort to locate asteroids capable of widespread destruction: We're making good progress on finding the big ones that could wipe out civilizations, but the smaller asteroids remain largely a mystery.
Since the now-widely accepted theory of dinosaur extinction by asteroid was put forth in the early 1980s, researchers and politicians have spent time assessing the threat. The fascination has even permeated popular culture, through apocalyptic films such as "Armageddon" and "Deep Impact."
The results of this interest, as well as improved science, should hardly send anyone scurrying into underground bunkers: The chance of significant collision, scientists stress, remains extremely remote. Yet as astronomers press on to find more of the galactic rubble that could be headed for Earth, it is clear that most asteroids remain unaccounted for.
"We have improved during the past few years," says Brian Marsden of the Smithsonian Astrophysical Observatory in Cambridge, Mass. "But this is a problem that will have to be addressed progressively."
Of all the asteroids found so far big or small none has been considered a legitimate threat to Earth.
None,at least, until today.
The asteroid in question is big roughly 3,300 feet in diameter. Discovered in 1950 and seen again in 2000, the two observations give scientists the opportunity to track its actions in the past, and therefore, more accurately forecast its future. That future includes a 1 in 300 chance of colliding with Earth on March 16, 2880, says a team of researchers in today's issue of the journal Science.
For their part, the researchers are not overly concerned about a cataclysm on that day. There are too many uncertainties involved in guessing how this one piece of solar system detritus will act over hundreds of years, they say.
Yet the ability to calculate a 0.3 percent chance of collision in a science that often measures probability in trillionths of a percent represents a milestone. It shows how astronomers are gradually adding to their data about near-Earth asteroids and to their ability to predict.
"This pushes the boundary between knowledge and speculation," says team leader Jon Giorgini of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
For now, the focus is on finding and charting asteroids like Dr. Giorgini's those that are more than 3,000 feet in diameter and can cause as much damage as 1 million Hiroshima bombs. The scientific community has set a goal of finding 90 percent of such asteroids by 2008, and according to some estimates, astronomers are about halfway there.
In this net, smaller asteroids are found sometimes by accident, and often not at all. Earlier this year, an asteroid some 1,000 feet across shot by Earth at a distance only two times farther away than the moon. Scientists discovered it nine days before it passed. Asked what could have been done to prepare, one scientist said, "Not much."
On March 8, a 165-foot-diameter rock, similar in size to the one that razed 20 miles of Siberian forest in 1908, scooted by Earth at roughly the moon's distance. It wasn't seen until four days later: Coming from the sun's direction, it was hard to spot.
The fact is, near misses are not all that unusual, and tracking tens of thousands of small rocks floating randomly through the inner solar system is a monumental task. What's more, it's not a priority.
Few telescopes are devoted to tracking near-Earth asteroids, and those that do are relatively small. A task force in Britain recommended a broader array of telescopes last year, but nothing has been done. Australia stopped its program recently, meaning the Southern Hemisphere is currently not being watched.
Eventually, astronomers hope, more telescopes will be turned to the skies to look for incoming space rocks, widening the search to include smaller and smaller asteroids. But even now, with the charts of near-Earth asteroids still far from complete, scientists put the quest in context, acknowledging that risks remain slight, and they must be weighed against other needs.
"With all the other [issues] we face every day, it's very difficult to measure how much money we should put into this," says Dr. Marsden. "It's something we have to think about."