Earth is in a constant game of celestial bumper cars, colliding with—and obliterating—the relatively puny space rocks that dare cross its path. The planet is still standing after 4.6 billion years, but a modern collision could devastate cities, continents, and even life itself. (Just ask the dinosaurs.) NASA's Center for Near-Earth Object Studies keeps watch on more than 18,000 potential troublemakers, ranging from just 3 feet to more than 3,000 feet across. Meteorites smaller than 100 feet usually explode in midair, like one did over Chelyabinsk, Russia, in 2013. There wasn't enough shrapnel to leave a crater, but the sonic boom did blast out windows. So how much havoc could larger rocks wreak?