Less Danger From Falling Rocks

By Elizabeth Svoboda|Wednesday, October 01, 2003




The Wolf Creek Crater in Western Australia testifies to the destructive power of asteroids on land.
Photograph courtesy of Philip Bland

Have some prominent scientists inadvertently been playing Chicken Little? Working from computer models, several researchers warned that asteroid impacts could trigger devastating tsunamis with frightening frequency, at least once every 4,000 years. Never fear: Philip Bland, a planetary scientist at Imperial College London, says his improved calculations show that the actual rate is probably much lower.

The previous simulations portrayed incoming asteroids as continuous blobs—picture a ball of glue flattening and spreading as it hits a hard surface. Bland developed a more realistic model that allows asteroids to disintegrate on the way in. "We're looking at the forces acting on stone or iron fragments as an object breaks up upon entering the atmosphere," he says. Fragmentation greatly reduces the odds of a major tsunami. To generate a 16-foot-high tidal wave, an object must be about 720 feet wide when it hits the surface. According to Bland's model, few objects that large survive passage through the atmosphere. Asteroid fragments 15 feet wide, capable of forming 300-foot craters, strike Earth every 200 to 400 years, he finds. Chunks large enough to trigger a significant tsunami fall just once every 170,000 years.

Bland does not dismiss the danger of asteroids, however. "Even if an asteroid shatters in the atmosphere, it's a substantial threat to human life," he says. The still-mysterious Tunguska Event, an explosion in Siberia in 1908, was most likely caused by a comet or meteorite. Although the object disintegrated in the air, it still managed to flatten 800 square miles of terrain. And scientists still do not really understand how the atmosphere interacts with really large impactors, which will be the next focus of Bland's research. "That would let us see what kind of effect a big comet, for example, might have," he says.
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