Now that the Cold War era has passed, the Star Wars technology developed in America could be used to destroy meteors big enough to threaten life on earth.

"Scientists have calculated that death by meteorite is as likely as being killed in a plane crash," says producer Tim Haines. "But the sort of rocks which kill a quarter of the world's population only arrive every few million years."

More than 150 large craters have been identified around the world, and evidence is growing to link these with incidents of mass extinction - of dinosaurs, for example. Alarmingly, too, satellites designed to monitor nuclear launches have been fooled by meteors detonating in the outer atmosphere. A false alarm raised in a politically sensitive part of the world could lead to an international disaster.

Horizon looks at the implications and politics behind the desire to save the world.

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