Robert Roy Britt - The history of Earth's encounters with asteroids remains largely mysterious to scientists. They can't even agree whether a huge space rock that hit Mexico's Yucatan Penninsula 65 million years ago killed off the dinosaurs or not. Nor can astronomers say when the next catastrophic impact will occur. They only know that it will happen, sooner or later.However, now anyone with a passing interest in the fate of the planet can remove some of the mystery regarding the effects of the next collision. A new University of Arizona web page allows visitors to plug in a hypothetical space rock's size, the visitor's distance from the impact site, and other parameters to generate an outline of devastation. But be warned: Removing the mystery invites a bit of terror over the hypothetical slams, bangs, fireballs, falling skies and rushing winds generated by a giant impact.Being somewhat of a voyeur when it comes to natural catastrophe, I couldn't resist running some scenarios through the new catastrophe calculator.If you read on, please keep in mind that the odds of a serious impact occurring in any year are extremely low. A civilization-ending impact, while possible, almost surely won't happen within our lifetimes (90 percent of all asteroids big enough and close enough to do the job will be found by 2008) and is extremely unlikely even over the next millennium.But hurling big virtual rocks at the planet is admittedly kind of fun. And in this case it's at least more scientifically meaningful than the average video game. I started by dropping a 9.3-mile-wide (15-kilometer) asteroid -- the estimated size of the suspected dinosaur killer -- on San Francisco.The Bay Area doesn't do so well. The resulting crater, at 113 miles (181 kilometers) wide, pretty much tells the story. The entire metropolis vanishes faster than you can say where you left your heart. What isn't consumed is knocked over in an earthquake of magnitude 10.2, bigger than any in recorded history. Heat from a scorching fireball would turn much of the state, and parts of others, into toast.
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