There's never a good time to get clobbered by an asteroid — something the  dinosaurs discovered in the worst way possible. It was 65.5 million years ago  that an asteroid measuring 6 mi. (10 km) across, slammed into the Earth just off  the coast of the Yucatan Peninsula, blasting out a 110 mi. (180 km) crater and  sending out a cloud of globe-girdling debris that cooled and darkened the world.  That spelled doom for species that had come to like things bright and warm.  Before long (in geological terms, at least) the dinos were gone and the mammals  arose.