NASA monitors 127 asteroids with the potential to collide with the planet and cautions there may be 20,000 more yet to be discovered.

Among the identified threats is Apophis, a 140-metre piece of rock with the potential to strike in 2036. If the asteroid crashed into the ocean near the U.S. coast, the resulting tidal wave would cause $400-billion in damage, according to Mr. Schweickart.

"That's the equivalent of six or seven or eight [Hurricane] Katrinas," he said in a telephone interview.

According to Mr. Schweickart, there is only a slim chance Apophis will ever hit the planet. However, he warned the asteroid is representative of other near- Earth objects (NEOs) that could cause a "monstrous disaster."

Mr. Schweickart is co-founder of the b612 foundation, an organization dedicated to finding a way to alter the course of an asteroid by 2015. It is named after the rock where the Little Prince lived in the popular children's book by Antoine de Saint- Exupery.

Killer asteroids have long been popular features in pulp fiction and Hollywood movies. In 1998's Armageddon, Bruce Willis led a mission to destroy a giant asteroid by detonating a nuclear bomb at its core. But Mr. Schweickart said it is unlikely the planet's fate will ever fall into the hands of a ragtag group of misfits. He is pushing the United Nations to develop ways to assess asteroid threats and determine whether action is required to prevent them. "I am not proposing the UN should build a huge bureaucracy or mount its own space program," he said, "What I am suggesting is that it is the correct decision-making body to determine how problems should be addressed."

Scientists propose deflecting asteroids using unmanned space craft. Edward Lu, a former astronaut employed by the NASA Johnson Space Center, told the AAAS conference the gravity of a ship hovering close to the asteroid could exert a gentle tug on its trajectory, over time directing the rock away from the planet.

Mr. Schweickart said an international body should be charged with assessing potential threats because they will likely be identified years before it can be determined what country or countries asteroids will hit.

"You have to act very early to prevent one of these impacts. You can literally predict 100 years in the future," he said.

It will cost far less money to defend against asteroid strikes than is currently spent on airline safety, Mr. Schweickart said.

"You could never ensure that air travel is completely safe, but for a small fraction of the money spent on air safety, you could ensure that a space impact never happened again," Mr. Schweickart said.

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