A UH doctoral student has determined that the asteroid believed to have wiped out the dinosaurs 65 million years ago was much smaller than thought.

Francois Paquay says the asteroid considered responsible for the Chicxulub Crater under Mexico's Yucatan Peninsula was only about half as big of the previous estimate of 9.3 to 12 miles in diameter. The study, reported in the journal Science, says the asteroid was 2.5 to 3.7 miles in diameter.

Paquay used a different method to determine the size of meteorites that have hit the Earth. He relied on osmium isotopes in ocean sediments.

Osmium is found in high concentrations in meteorites. When a meteorite hits the Earth, it vaporizes, and everything in the vapor falls onto the land and sea, he said.

During the impact events he studied, Paquay said osmium levels spiked. "Based on this, you know using simple math what was the size of the impactor," he said.

Paquay's adviser Gregory Ravizza said the previous measurement method relied on the element iridium and was labor intensive. Paquay's team analyzed more than 130 samples from two Ocean Drilling Program sites: one in the equatorial Pacific and the other off the tip of South Africa.

They looked at two geologic time periods and studied the deep ocean sediments during and after the impacts.

"These events are fascinating to me because of their magnitude," Ravizza said. "They represent huge explosions."

A giant meteorite is one of several theories, including global warming, advanced for the disappearance of the dinosaurs and other species.

Tarun Dalai of the Indian Institute of Technology and Bernhard Peucker-Ehrenbrink of Woods Hole Oceanographic Institute collaborated with Paquay on the study.