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.
Few outside scientific circles took the threat posed by near-Earth objects seriously until 1980. Then, Luis and Walter Alvarez published a study based on geologic evidence that concluded a cataclysmic asteroid or comet impact 65 million years ago caused the mass extinction of two-thirds of all plant and animal life on Earth — including the dinosaurs.
Dubbed the Great Exterminator, the colossal object was estimated at 7 miles in diameter and created a blast hundreds of millions of times more destructive than a nuclear weapon. Objects that size are thought to hit Earth about every 100 million years.
NASA scientists studying satellite photos bolstered the Alvarezes' theory with the discovery in 1991 of an impact crater 125 miles wide buried beneath the northwestern corner of Mexico's Yucatan Peninsula. Three years later, NASA photos of another sort drove home the potential for cosmic collisions in our part of the solar system.
Spectacular images from the Hubble Space Telescope of Comet Shoemaker-Levy's collision with Jupiter showed 21 comet fragments, some more than a mile wide, producing colossal fireballs that rose above the giant planet's cloud deck.
"I think the most important development for getting this (public awareness) going was the Alvarezes' research that the dinosaurs went extinct as the result of an impact," Morrison said. "We were faced with a real example where an impact had done terrible damage."
In 1998, a year in which the asteroid-disaster flick Armageddon was the top-grossing movie worldwide, Congress held hearings that led to the creation of a Near Earth Object Program office at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
That year marked the beginning of the Spaceguard Survey aimed at discovering 90 percent of near-Earth asteroids more than a kilometer wide.
Today, astronomers at five primary U.S. sites work on the survey, which NASA funds with about $4 million annually. Scientists estimate there are 1,100 near-Earth asteroids that are larger than a kilometer wide. With two years to go, they have found 834, or about 76 percent, of the estimated total.
Congress directed NASA in December to look at expanding the search to asteroids larger than 140 meters (460 feet) in diameter and completing the new survey by 2020. Objects that size are capable of destroying a city.
The more often an asteroid or comet is sighted, the more precisely its orbit can be calculated. Researchers hope that radar observations of Apophis taken last weekend by the Arecibo telescope in Puerto Rico could make the odds of a collision even more remote.
"I always use the analogy of a hurricane," said Don Yeomans, manager of the Near Earth Object Program. "When it first forms in the Caribbean, you have no idea where it's going to hit. If you continue to track the hurricane over days and weeks, the future path becomes more predictable."
That uncertainty led former astronaut Schweickart to send a letter to NASA Administrator Michael Griffin last June proposing to land a radio transponder on Apophis to better track its course. For now, the space agency plans to simply monitor the asteroid during passes this year and in 2013.
In 2029, seven years before the possible impact, the asteroid will come closer to our planet than the television and weather satellites that beam back signals from 22,300 miles above. Astronomers' big fear is that Apophis will pass through a gravitational "keyhole" that will put it on a collision course with Earth in 2036. "For all practical purposes, it (a mission) would have to be done before the 2029 flyby to take advantage of the leverage afforded by that encounter," said Steve Chesley, an astronomer in the Near Earth Object Program. "That means the 2036 impact needs to be addressed by 2026, 10 years earlier."
There is considerable debate about how to stop an asteroid or comet once astronomers have determined it will pass too close for comfort.
One idea would use a laser cannon on the moon or atop a spacecraft to shift the threatening object's course. Another involves slamming a spaceship into the object to nudge it away. A slight push a decade or so before a possible collision would translate into a wide miss years later.
Astronauts Ed Lu and Stanley Love published an idea last year for a "gravitational tractor" to change an asteroid's orbit. A nuclear-powered spacecraft would be launched toward the rock and hover near it, using gravity to slowly divert the intruder.
A fallback option using readily available technology involves detonating a nuclear weapon near the threat to shove it off course. It might be the only alternative if an object is discovered only a few months before impact.
Most experts agree the response will depend on the specific threat.
"You have to discover and know your enemy before you can even imagine what kind of mission or deflection you would do," Morrison said.
In recent months, some of the larger political questions are starting to be widely discussed.
If the Earth is threatened, NASA almost certainly would help lead the response. But who ultimately makes the decision on how to proceed? The United States? The United Nations? What about cases where deflecting an object away from an endangered region might move its course across another area? And how likely does a threat have to be to warrant taking action?
Schweickart is convinced those sorts of decisions should be made by the entire planet. He has begun work on a draft treaty he hopes to present to the United Nations by 2009.
As for Apophis, NASA scientists are confident the knowledge they've gained will prevent the asteroid from becoming the next cosmic catastrophe.
"Apophis is not going to hit the Earth. Period," Chesley said. "Whatever the impact probabilities that we compute right now are, we're not going to let it."