So, the overall picture is not pretty. A strike by a 0.6-mile-wide asteroid could cause "a very severe global impact" for several years, Bardeen said.
But a space rock would likely have to be about 10 times bigger to cause a mass extinction, he added. (The asteroid that's thought to have wiped out the dinosaurs, along with many other species, 65 million years ago was probably about 6 miles, or 10 km, wide.) [Wipe Out: History's Most Mysterious Extinctions]
Photos: Psychedelic Landscapes of Asteroid Vesta
Bardeen and his team modeled the aftermath of an asteroid strike on land. But it's more likely that a space rock would come down in water, since oceans cover about 70 percent of Earth's surface. What would happen then?
A 2010 modeling study by the late Elisabetta Pierazzo and her colleagues looked into this scenario, and determined that the effects on Earth's protective ozone layer would be dramatic.
An ocean strike by a 0.6-mile-wide asteroid, the team found, would loft enough salty water vapor to destroy huge quantities of ozone, causing the surface UV index to spike temporarily to 56. Such high radiation levels, which have never been experienced in human history, would probably force people to stay inside during the day, Pierazzo said when the study came out.