This week's meeting of the U.N. Security Council and its discussions of international political and economic crises are grabbing headlines, but astronauts are having a powwow of their own about another global concern: how to protect Earth from an asteroid or cometary impact.

That's right: continental or global disaster from the cosmos isn't just the stuff of blockbusters like Armageddon and Deep Impact—and as far as we know, Bruce Willis and Elijah Wood aren't part of this week's meeting of the Association of Space Explorers (ASE). Asteroid impacts have occurred before: a near-Earth object (NEO) became an on-Earth disaster 65 million years ago, wiping out dinosaurs and the majority of other species, and the 1908 Tunguska event, thought to have been caused by an exploding asteroid or comet, destroyed some 7,700 square miles (2,000 square kilometers) of Siberian forest.

The ASE's Panel on Asteroid Threat Mitigation will let us know how to prevent another catastrophe when they wrap up their four-day meeting this afternoon, and as Earth-dwellers, we're eager to hear what they have to say. Right now, we're wondering: Are we at risk of a catastrophic asteroid impact, and is there really anything Earthlings could do to prevent an asteroid from wiping out a town, a continent or the planet?

"It's real," says John Pike, director of Globalsecurity.org, an informational Web site focused on security issues, including space. "It's not a question of whether it's going to happen, it's just a question of when it's going to happen."

How bad it would be after any impact would depend not only on a space rock's size, but on its makeup (metallic or stony) and density; where it lands (water or land); and if it explodes, breaks up or remains in one piece. Impacts by larger objects would be major trouble. The probability of a small asteroid (tens of meters radius, or the size of a small building) making impact might be once in several decades.

A giant asteroid of multiple-meters radius might hit once in many millennia, Pike says. Most terrifying are ones over 0.6 mile (1 kilometer). Impacts from such large objects, although rare, would be catastrophic on a worldwide scale. Plunging through Earth's atmosphere and hitting with the explosive force of around 70,000 tons of TNT, they most likely would result in a major blow to human civilization. The damage would likely even be greater if it crashes into the ocean, because the impact could trigger a continental, coast-inundating tsunami that would dwarf the 2004 Indian Ocean event. The B612 Foundation, another asteroid watch group, estimates that "The probability of an unacceptable (meaning damage-causing) collision in this century is about 2 percent."

NASA told Congress last year that it didn’t have the money to meet its goal of identifying 90 percent of NEOs of 150 yards (137 meters) diameter or more by 2020. It's not clear what size asteroids the ASE panel is thinking about; besides the small and giant kinds, there are medium-size ones that are hundreds of meters across, or the size of a skyscraper.

"It is by far and away one of the biggest threats you can do something about—it's like hurricanes," Pike says. "With hurricanes you've got evacuation plans, weather forecasting. What have you got here? A few movies that have been made about it."
 
Speaking of which, Hollywood isn't so off in storylines about blasting apart an approaching asteroid. "If you're worried about blowing something out of sky, the answer is to send landers—robotic spacecraft—onto these objects to understand how they're put together," Pike says. "Popularly, asteroids are understood as a single object; maybe they're not. They could be boulders traveling together in close formation. Whether they're a rock or a rubble pile has significant impact about what do about the threat."

The B612 Foundation thinks a Bruce Willis-type solution is not desirable, and could actually cause more of a mess. It has offered its own plan for a space tug that will gently push any asteroid in an Earth-threatening orbit onto a new path. But early detection is necessary for such a scheme—or for any ameliorative attempts to stop an incoming disaster.

"For the detection of large objects, the answer is buy more telescopes," Pike says. "You'd figure out how large the object was, where it was going to hit and how much ejecta would get thrown out of it and tell people, 'this is how far away you're going to have to get.'"

(Image of Arizona meteor crater from the Smithsonian Scientific Series (1929)/U.S. Army Air Service)