Killer asteroids aren't just in the movies

May 21, 2006|By MICHAEL CABBAGE | MICHAEL CABBAGE,ORLANDO SENTINEL

CAPE CANAVERAL, Fla. -- Mark your calendar for Sunday, April 13, 2036. That's when a 1,000-foot-wide asteroid named Apophis might hit 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 kinds 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 astro-biologist 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 way 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 neighborings have been bombarded.

A glance at our moon shows the scars left by countless collisions with asteroids and comets. Some scientists believe the moon was created when part of 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 the 4,000-foot-wide Barringer Crater in northern Arizona, 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 4 to 6 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 crashes within 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 when the asteroid-disaster movie Armageddon was the top-grossing film 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 show the odds of a collision more remote.

Michael Cabbage writes for the Orlando Sentinel.

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