As the bumper sticker says, one nuclear bomb can ruin your whole day. But one asteroid can be far worse (just ask the dinosaurs).
For the world's Chicken Littles, scientists at the University of Arizona's Lunar and Planetary Laboratory have devised a handy Web-based impact calculator, at www.lpl.arizona.edu/impacteffects. Plug in a rock's size, speed, density, angle of approach and your distance from the impact, and the software tells you whether you'd be history.
For instance, while an asteroid 1,000 feet in diameter would be bad news for those at the impact site (the explosive force would equal 700 megatons of TNT), if you were 150 miles away, the calculator says, the immediate effects would be relatively slight - dish-rattling tremors, a dull roar and a smattering of dust.
But at the same distance, a faster-moving comet three miles in diameter would be a different story. Trees and clothing would ignite, and eventually everything would be buried in five feet of debris.
H. Jay Melosh, a professor of planetary science at Arizona, said he fielded inquiries about possible outcomes whenever there were reports of a potential asteroid impact. (That potential is almost always dismissed; only one known asteroid has even a slight chance of hitting the planet, on March 16, 2880.)
Melosh said that in response to such questions, he usually makes some quick calculations. "I've done this enough times that I thought I maybe could make it easier for myself by automating the process," he said.
So the idea for the Web site was born. He got help with the formulas from a postdoctoral researcher, Gareth Collins, and enlisted an undergraduate computer science student, Robert Marcus, to put the site together.
Melosh said the site made use of various sources, including nuclear weapons manuals, although some grislier details of blast effects were left out.
"The intent here is not to scare people," he said. On the contrary, Melosh said he thought the site might have the opposite effect. "People are worried about objects that are 100 meters in diameter," he said. While those at the impact site might be doomed, he added, "You don't have to be all that far away before you won't notice it."