Asteroid Impact.

The last asteroid (large space rock) to slam into Earth and cause massive devastation happened long before you were born--about 65 million years ago, to be exact. Scientists think the asteroid smashed into Mexico's Yucatan Peninsula and wiped out half of Earth's species, including dinosaurs. Could it happen again? Is there any way to warn Earthlings?

"It's going to happen again," says Richard Binzel, an atmospheric scientist at the Massachusetts Institute of Technology. But such a clobbering could be millions of years away.

He and other scientists met last June in Torino, Italy, to devise a strategy to predict possible asteroid or comet (icy space rock) collisions with Earth. Result: the Torino Impact Hazard Scale, which measures both the energy of a speeding asteroid and the probability it will strike Earth. Unlike the Richter Scale, which gauges earthquakes after they occur, the Torino Scale measures the probability of an asteroid attack in advance.

The scale runs from 0, for a complete miss, to 10, for global devastation. It takes into account an asteroid's kinetic energy--a measure of its size and speed--and the percentage chance of its smashing into Earth. Astronomers track virtually every known asteroid in space.

The Torino scale has assigned a level-8 rating to an asteroid that struck 91 years ago in a remote corner of Siberia. Had the asteroid smashed into a major city, millions of people might have been killed.

Ever since the release of sci-fi thrillers like Armageddon and Deep Impact, and the 1994 collision of comet Shoemaker-Levy with Jupiter, "it's become difficult to inform people about newly discovered objects without being sensational," Binzel says. "The Torino scale puts asteroid impact in perspective.".