From: http://www.nytimes.com/2008/07/06/weekinreview/06revkin.html?ref=scienceJuly 6,
By ANDREW C. REVKIN
THE Earth is pockmarked with the evidence of ancient collisions — huge craters blasted into its surface by asteroids or comets. One such object, striking 65 million years ago in the Yucatán in Mexico, is believed by some experts to be linked to the demise of the dinosaurs.
For a decade, NASA has been busy trying to identify what else is headed this way, particularly those potential “civilization killers” of 1 kilometer (.62 miles) or more in diameter that have orbits coming within 30 million miles of the Earth’s — too close for comfort by space standards.
But the big ones are, in many ways, the easy part. Smaller rocks matter, too. Perhaps nowhere is that so evident as in central Siberia, where 100 years ago last week, something — presumably a meteoroid, most experts say — streaked across the sky and exploded at an estimated height of 28,000 feet with a force equivalent to 185 Hiroshima bombs, leveling some 800 square miles of forest. Simulations by the Sandia National Laboratories showed that object could have been just 90 feet across.
The explosion that lit up the Siberian sky in a fireball shortly after 7 a.m. on June 30, 1908, is known as the Tunguska event, after the river the flows through the damage zone, and is widely considered the modern-day warning about the dangers slinging through space.
NASA’s Jet Propulsion Laboratory in Pasadena, Calif., has estimated that a Tunguska-size asteroid will enter Earth’s atmosphere once every 300 years and says there may be 375,000 objects of such size out there.
Fortunately, the odds are good that the next one will fall over one of our oceans, which take up more than two-thirds of the Earth’s surface, or the planet’s still-vast stretches of uninhabited lands. How much in taxpayer dollars should be invested to pinpoint such hazards is one of the toughest risk-management exercises around.
Donald K. Yeomans, who manages the jet propulsion lab’s Near-Earth Object Program, said the Earth’s atmosphere is continually streaked by space stuff, ranging from the basketball-size (several a day) to the Volkswagen-size (twice a year). Almost everything burns up, though some may explode in the air, a phenomenon known as an airburst, with the potential of causing damage below. And then there are objects, like the meteorite that dug a 60-foot-wide crater in Peru last September. Perhaps no bigger than a basketball, the meteorite was a reminder of the destructive power of what is lurking out there.
“In fact, there was a daylight fireball event widely observed near Los Angeles two days ago,” Dr. Yeomans said in an e-mail message last Thursday. “I take these events as Mother Nature’s little reminders that we need to pay attention, find and track the large ones and then deflect the very few that threaten us. Tunguska was another reminder. Until recently, we humans did not pay heed to these shots across the bow but now, I think, there is more of a recognition of this low probability — but high consequence — type of event.”
NASA estimates that there are about 940 or so near-Earth space rocks a kilometer in diameter or larger. So far, according to Dr. Yeomans, NASA has identified 743. (Overall, NASA has pinpointed more than 5,500 near-Earth objects.)
Budget constraints have slowed NASA’s efforts to meet its goal of identifying 90 percent of those big objects by next year. As for rocks similar in size to the one that exploded over Siberia, a NASA search is probably a long way down the line.
Even so, one way or the other, many more potential incoming asteroids will be identified in the next decade, astronomers say. But awareness is just the first stage of grappling with the challenge, said Rusty Schweickart, the Apollo 9 astronaut who heads a foundation advocating more research on identifying near-Earth objects and developing unmanned spacecraft that could nudge them off track like a tugboat.
Mr. Schweickart said progress so far had come through constant pushing against resistant bureaucracies and politicians focused on whatever is the issue of the moment. And he said there was still no significant effort to devise an international agreement, let alone a deflection technique, for dealing with the inevitable earthbound asteroid or comet, large or small, when it is identified.
“It may be subtle,” he said, “but failure of the international decision process is the most likely reason that we’ll take a hit in the future.”