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.

Prepare to be scared

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.

The quick end to the Bay Area turns out to be a blessing compared to what Los Angeles residents face.

About 10 seconds after impact, radiation from the fireball sears Southern California, igniting clothing and even plywood. Within two minutes the ground under Hollywood begins to shake. Weak brick structures crumble. Concrete irrigation ditches are damaged. Frame houses not properly bolted to their foundations are knocked off. Even tree branches fall.

And then it gets nasty.

'Bad things happen'

Six minutes after impact, much of earth that used to be under San Francisco has soared high into the atmosphere and begins to fall on the City of Angels (and just about everywhere else). Ultimately, a blanket of ejected material nearly 18 feet (5.5 meters) deep is deposited in and around LA.

Within a half-hour of the initial cosmic impact, on comes a 66-mph (30 meters per second) wind to rake what's left of Los Angeles.

"If you're close to the site of a major impact, some pretty bad things happen," said Jay Melosh, an expert in impact cratering at the University of Arizona.

Melosh oversaw the computer program's development. It's intended to help scientists further their research and journalists to report on the risks of asteroid impacts. "We could have put in some worse stuff, but it started getting grisly," he told .

I continued with my West Coast disaster scenario by heading across the country. Denver, about a third of the way over, is coated with nearly a foot of ejected material, called ejecta. Winds reach 22 mph.

New York City residents are spared enough that they can at first sit comfortably and watch all this on TV (to the extent there are any video feeds). Still, about 13 minutes after impact, windows and doors rattle in the East. After about 21 minutes, stuff shot out from the world's largest new crater starts to rain down on the East, depositing a half-inch blanket across the city. About 3-1/2 hours later an 8-mph wind arrives.

Object as big as the one that presumably wiped out the dinosaurs -- some investigators aren't convinced it was the sole cause -- are extremely rare, slamming into the planet perhaps just once every 300 million years.

Presently there are no asteroids known to be on a collision course with Earth. And experts agree that one this big would almost surely come with decades or centuries of warning time, being fairly easy to spot well before it becomes an imminent threat. By then, experts hope, methods will be developed to divert or destroy such a rock.

More likely scenarios

But what about something the size of the asteroid that dug out Meteor Crater in Arizona? That 600-foot-deep (180-meter) hole in the ground, popular now with tourists, is less than 50,000 years old and was created by a space rock so small that another one like it might not be noticed until it hits.

The Arizona culprit was only about 66 feet (20 meters) wide. But instead of fragile stone, it was composed mostly of iron. Examples linked with the online impact-effect program show how to specify this harder, more destructive material. A rock this size (though not of this density) is thought to hit Earth every 158 years, on average.

I duplicated the Meteor Crater script in Newark, New Jersey, 10 miles (16 kilometers) from Manhattan as the crow flies.

Location, of course, is everything. A nearly mile-wide crater is dug in New Jersey. A moderate earthquake of magnitude 4.7 rattles the region. New York City has handled such temblors in the past and largely endures. No significant ejecta hits Manhattan, and winds reach an insignificant 3.7 mph.

So I cranked it up a bit.

An asteroid the size of two football fields, even if stony in nature, is another story. It carves a crater in New Jersey that's 2 miles wide (3.4 kilometers). The resulting earthquake is magnitude 6.4, enough to frighten the pants off New York City emergency planners. A rock this size hits the planet about every 14,000 years.

Just estimates

Melosh developed the program along with fellow researcher Gareth Collins and undergraduate Robert Marcus. Melosh said the program should prove valuable to scientists who otherwise would have to wade through complicated calculations to generate impact scenarios for their research.

It's important to point out that what actually happens when an asteroid crashes into the surface is not well known, because scientists have never actually witnessed an impact event of any scale. But they have used computer models to explore the probable effects of impacts, and they have some craters to study on Earth.

Researchers assume the blast of heat generated by a cosmic collision would be similar to nuclear bomb detonations. So for these thermal radiation effects, Melosh's team relied on a 1977 publication, "The Effect of Nuclear Weapons," by the U.S. Defense Department and the Department of Energy.

"We determine at a given distance what type of damage the radiation causes," said Marcus, who did most of the grunt work developing the computer program. "We have descriptions like when grass will ignite, when plywood or newspaper will ignite, when humans will suffer 2nd or 3rd degree burns."

For earthquake predictions, the researchers culled data from actual California events and knowledge of how earthquakes propagate like sound waves through the planet.

The global effects of an asteroid impact are harder to predict. Many scientists think the larger impacts throw so much dust into the atmosphere that a global winter would ensue, lasting for months or years. Melosh is skeptical about the dust.

"Nuclear explosions are good at raising dust that's already there, but they're not good at creating it," Melosh said. He figures most of the ejected material is melted and then hardens into tiny, hardened and almost microscopic spheres. "Most of the ejecta comes down within an hour." He calls the massive dust creation "a media myth."

More to come

There is much left out of the new online impact program. Melosh wanted it to be practical to use, and he's open to suggestions for improvement.

Important details of an impact -- exactly how it penetrates the atmosphere and the surface, are simplified. And one major effect is not considered: the tsunami that would result from an ocean impact (Earth's surface is two-thirds water). In addition, the results of a user's input do not mention that a rush of wind can't blow down a tree that's buried under ejecta, nor does either matter if the tree were already burned to a crisp.

Minor and major revisions to the program may be made based on input from other scientists, Melosh said. And in coming weeks, his group will post background information explaining the complex calculations behind the program.

The Impact Effects program