Mark your calendar for Sunday, April 13, 2036. That's when a 1,000-foot-wide asteroid named Apophis could hit the Earth with enough force to obliterate a small state.

The odds of a collision are 1-in-6,250. But while that's a long shot at the racetrack, the stakes are too high for astronomers to ignore.

For now, Apophis represents the most imminent threat from the worst type of natural disaster known, one reason NASA is spending millions to detect the threat from this and other asteroids.

A direct hit on an urban area could unleash more destruction than Hurricane Katrina, the 2004 Asian tsunami and the 1906 San Francisco earthquake combined. The blast would equal 880 million tons of TNT or 65,000 times the power of the atomic bomb dropped on Hiroshima.