Science: Asteroids' tidal wave threat

The impact of a wayward asteroid on one of the earth's oceans would spell disaster for coastal cities, a US scientist says. Such an impact would trigger massive tidal waves, or "tsunamis" capable of travelling thousands of kilometres and devastating entire coastlines - though Britain, it seems, would be spared the worst effects.

Jack Hills, an astrophysicist at Los Alamos National Laboratory in New Mexico, used a computer program to make the first detailed simulations of cosmic impacts on our planet's oceans. He presented the results yesterday at the winter meeting of the American Astronomical Society.

Even a comparatively small asteroid, with a diameter of 200m, would "cause a large-scale tsunami on all of the coastal areas on the particular ocean that it hits", Mr Hills said. Such an impact is expected every few thousand years. The resulting ocean waves would be some 30m in height, and would grow even larger as they approached the shore. They would also be unstoppable.

The worst-case scenario Mr Hills investigated was the impact of an object five kilometres across - an event expected once in about 10 million years. The simulation looked at a strike in the north of the Atlantic Ocean. The model predicted waves powerful enough to swamp the eastern seaboard of the United States right up to the Appalachian Mountains.

Though parts of Europe would also be devastated by such a mighty splash, a quirk of geography acts in Britain's favour: the ocean floor off south- west England has a very gradual slope. This shallow incline, Mr Hills explains, would reflect some of the tsunami's energy back into the ocean. "The damage in England is much smaller than one would think," he said.

Ireland would fare slightly worse, while France and Portugal would take the brunt of devastation on the eastern side of the Atlantic.

Mr Hills said the first step in dealing with the threat of cosmic collisions is a programme to identify the thousands of objects that could potentially hit our planet, along with better evacuation planning for coastal areas.