Cape Canaveral, Fla. 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.
Objects this size are thought to hit Earth about once every 1,000 years, and, according to recent estimates, the risk of dying from a renegade space rock is comparable to the hazards posed by tornadoes and snakebites. Those kind of statistics have moved the once-far-fetched topic of killer asteroids from Hollywood movie sets to the halls of Congress.
"Certainly we had a major credibility problem at the beginning - a giggle factor," said David Morrison, an astrobiologist at NASA's Ames Research Center in Mountain View, Calif. "Now, many people are aware this is something we can actually deal with, mitigate and defend against."
In 1998, lawmakers formally directed NASA to identify by 2008 at least 90 percent of the asteroids more than a kilometer (0.6 mile) wide that orbit the sun and periodically cross Earth's path. That search is now more than three-quarters complete.
Last year, Congress directed the space agency to come up with options for deflecting potential threats. Ideas seriously discussed include lasers on the moon, futuristic "gravity tractors," spacecraft that ram incoming objects and Hollywood's old standby, nuclear weapons.
To help explore possible alternatives, former Apollo astronaut Rusty Schweickart has formed the B612 Foundation. The organization's goal is to be able to significantly alter the orbit of an asteroid in a controlled manner by 2015.
"You can watch all of the golf on television you want, but if you want to go out and break par, it's going to take a lot of playing," Schweickart said. "And you're going to learn a lot that you thought you knew, but you didn't."
Throughout their 4.5 billion-year history, Earth and its neighboring planets have been like sitting ducks in a cosmic shooting gallery.
A glance at our moon shows the scars left by countless collisions with asteroids and comets. In fact, the moon is thought to have been created when part of the early Earth was ripped away in a cosmic impact with an object the size of Mars.
Earth also has scars, but most have been hidden by vegetation or eroded by geologic processes such as rain and wind. About 170 major impact sites, including northern Arizona's 4,000-foot-wide Barringer Crater, have been identified around the globe.
Within the past century, an extraterrestrial chunk of rock about 200 feet wide is thought to have caused a 1908 blast near Tunguska, Siberia, that leveled 60 million trees in an area the size of Rhode Island. Researchers theorize the object exploded four to six miles above the ground with the force of 10 million to 15 million tons of TNT.