While only a small chance exists that asteroids could strike Earth today, asteroids and comets may have destroyed virtually all life on this planet almost 4 billion years ago.

These stellar objects could have evaporated the planet's oceans and had a major impact on the origin of life on the planet, said James F. Kasting, associate professor of geosciences.

"If life did get started early on and one of these asteroids struck, that life would have been destroyed," he said.

With co-researchers Norman H. Sleep of Stanford University, Kevin J. Zahnle of NASA Ames Research Center and Harold Morowitz of George Mason University, Kasting studied the lunar cratering record to determine the size and probable effects of asteroids.

They detailed their research at the recent American Association for the Advancement of Science's annual meeting in New Orleans.

A 440-kilometer diameter asteroid could evaporate Earth's oceans if it struck at the speed of present-day asteroids. A photic zone evaporator could be only 190 kilometers in diameter, and smaller objects could do an equivalent amount of damage if they came in faster, Kasting said.

"The earth was just lucky that nothing super-big hit," Sleep said.

Sleep said fossil evidence suggests life was present 3.5 billion years ago, and sediments containing organic carbon, without fossils, date back 3.8 billion years.

The asteroids' impact also would have changed the chemical composition of the planet's atmosphere, Kasting said. Earth's early atmosphere is believed to have been dense carbon dioxide, but the asteroids would have brought in organic carbon, which would have reacted in the impact plume to form carbon monoxide, he said.

"If you want to form life near the surface, you need carbon monoxide," he said. "The impacts, then, are sort of a two-edged sword. The big ones wipe out all life, but the sum total would create a reducing atmosphere, which is better suited for originating life."

Kasting said a very small chance exists that such asteroids and comets could strike Earth today. Three asteroids in the asteroid belt -- Ceres, Vesta, and Palas -- are large enough to vaporize the oceans, but they are in stable orbits. However, the comet Chiron, in a Saturn orbit, is large enough to evaporate the photic zone and could cross Earth's orbit someday, he said.

"There's almost no way humanity could survive such an occurrence," he said.

Sometime in the next century, when cost effective, a space system could be constructed to shield the planet from such an event, he said.