Asteroid impact: beware the tsunami.

Pop quiz: Does an asteroid slamming into Earth do more damage if it lands on solid ground or if it plows into the middle of the ocean?

Give yourself a Bronx cheer if you chose the first answer. Surprising as it may seem, a watery landing holds the greater potential for destruction, says Jack G. Hills of Los Alamos (N.M.) National Laboratory.

An asteroid striking solid ground deposits its energy rapidly in a relatively small volume. A body striking the ocean creates waves that retain enormous energies as they travel great distances across the water's surface, Hills says. These waves, called tsunamis, can scour thousands of kilometers of coastline with debris and towering walls of water. In 1960, an earthquake-generated tsunami that originated in Chile spread halfway across the world, killing 200 people when it reached Japan.

When a tsunami hits a continental shelf, it slows down but grows higher. Hills and Charles Mader, retired from Los Alamos, calculate that a 5-kilometer-wide asteroid striking the mid-Atlantic would swamp the upper East Coast of the United States. The tsunami would also drown the coasts of France and Portugal. Hills presented the findings last month at a meeting of the American Astronomical Society in Washington, D.C.

A 5-km-wide asteroid is expected to hit Earth only once every 10 million years, but rocky bodies as little as 200 meters in diameter are much more common and can also spawn tsunamis. For example, an asteroid 400 in across would create a wall of water reaching 100 in high, Hills and Mader find.

Earlier calculations by Hills and M. Patrick Goda, then at Los Alamos, had revealed that an asteroid slamming into the ocean can pack a wallop. After the researchers published their preliminary findings in the March 1993 Astronomical Journal, Hills enlisted Mader, an expert in tsunamis, to help with detailed computer simulations.

Additional modeling over the next 3 years should reveal the damage caused by smaller asteroids, which is more difficult to calculate, Hills says. He notes that if an asteroid can be discovered before it strikes, a nuclear-powered rocket might be used to direct it away from Earth.