Image: EDWIN VIGILS, Los Alamos National Laboratory WASHOUT: This simulation shows a tsunami, generated by an asteroid impact in the Atlantic Ocean, inundating a section of the eastern U.S. The waves would move far inland to the foothills of the Appalachian Mountains. Norfolk, Va., the Delmarva Peninsula and Cape May, N.J., would be engulfed; Washington, D.C. would end up on a tiny island.

One if on land, two if at sea. That's the damage assessment for an asteroid hitting Earth. Indeed, although an impact on land would be catastrophic, a watery one would be doubly so. Reason being, towering waves, called tsunamis, would race outward and crash down upon the coastlines, which are home to more than half the world's population.

At least that is the conclusion drawn from recent computer simulations by researchers at the Energy Department's weapons labs. With real bombs falling out of fashion, these experts have kept their supercomputers humming by simulating enormous natural blasts. Last year, a group at Sandia National Laboratories tested a new computer by modeling the impact of an asteroid. Not to be outdone, a team at Los Alamos National Laboratory recently tackled the aftermath of such a hit. They revealed their findings at a meeting of the American Astronomical Society in early January.

The most recent simulation, created by Los Alamos astrophysicist Jack Hills and his colleague Charles Mader, assumes that an asteroid three miles across slams into the mid-Atlantic, thereby creating a series of waves that spreads out from the point of impact. Tsunamis are unlike other ocean waves. In the open sea, they are an almost undetectable swell, often barely inches high. But they can travel thousands of miles in a matter of hours.

COMET IMPACT. Scientists at Sandia National Laboratories modeled a comet striking the ocean.

When tsunami (from the Japanese characters for "harbor" and "wave") reach shallower coastal waters, their velocity is slowed, and they rear up into immense walls of water that slam into the shoreline and pour far inland. The Los Alamos model shows that the waves created by an asteroid impact would drown the entire upper East Coast of the U.S. inland to the foothills of the Appalachian Mountains. Delaware, Maryland, Virginia and Long Island would be inundated--and all the coastal cities completely destroyed. The waves would also swamp the coasts of France and Portugal.

Earth is likely to take a hit from an object that large only once every 10 million years or so. But the chance of a strike from a smaller asteroid is 2,000 or 3,000 times more likely, or once every few thousand years. The Los Alamos model shows that an asteroid a mere 1,300 feet in diameter would still cause enormous devastation, raking the coasts with tsunami more than 300 feet high.

Asteroids smaller than about 600 feet across burn away and lose most of their energy in the atmosphere. But even these smaller strikes are far from innocuous. It was an object of this type that exploded in the air near the Tunguska River in central Siberia in 1908. The Tunguska shock wave flattened 800 square miles of forest; the force of the detonation is estimated to have been 1,000 times greater than the Hiroshima bomb.

Although the accuracy of the Los Alamos models can be disputed, the devastating effects of tsunamis are well known, especially to coastal dwellers on the Pacific Ocean. There, sudden changes in the level of the seabed, triggered by undersea earthquakes or volcanic activity, cause these frequent marauders. Like the waves predicted for asteroid strikes, the potential for damage is spread over thousands of miles by the waves propagating through the water.