“These things have hit the earth in the past, and they will hit the earth in the future,” warned Eugene Shoemaker, the space-probing geologist who first alerted the world to the danger of near-earth asteroids (NEAs) before he died in 1997. “The catastrophe will exceed other natural disasters by a long shot.”

The asteroid that ended the age of dinosaurs was at least six miles (10 kilometers) wide, but smaller asteroids can still be devastating. Scientists estimate that the impact of an asteroid with a diameter of one kilometer (0.6 miles) or more could kill at least a quarter of the world’s human population, as well as many other life forms. Less than a century ago a space rock only 330 feet (100 meters) wide exploded over Siberia. It leveled more than a half million acres (2,000 square kilometers) of forest.

However remote, the possibility of the end of life as we know it has energized the astronomical community. The search for new asteroids, once considered the realm of space fanatics, has become serious science.

OUT THERE

Images of the night sky, as seen through powerful telescopes at California’s Palomar Observatory and elsewhere around the world, are systematically sifted for evidence of yet undiscovered threats. Each object that doesn’t look like an asteroid is carefully removed. The process is painstaking.

So far only about half of the estimated 1,100 asteroids with a potential for a catastrophic impact with Earth have been discovered. British astronomers recently urged their government to become more actively involved in the effort.

The United States, faced with the possibility of having to head off a disastrous collision someday, is in the midst of NEAR (Shoemaker-Near Earth Asteroid Rendezvous), a year-long project to orbit Eros, the second-largest NEA, in order to better understand the makeup and physical dynamics of asteroids.

Close-up images of potato-shaped Eros show an asteroid about the size of Manhattan that has been bombarded many times. Some 100,000 craters more than 50 feet (15 meters) wide pock its surface. More than a million boulders the size of houses or larger litter the surface of Eros. The asteroid’s consistent color suggests a uniform composition.

This information may be critical should it become necessary to explode an asteroid or deflect its orbit to prevent it from striking Earth someday. NEAR, a joint project of the National Aeronautics and Space Administration (NASA) and The Johns Hopkins Applied Physics Laboratory, will conclude in February.

ESCAPEES FROM THE BELT

Earth-threatening asteroids are strays from what is known as the “main belt,” an elliptical ring consisting of tens of thousands of space rocks of various shapes and sizes in orbit around the sun between Mars and Jupiter. Smaller asteroids—less than 12.5 miles (20 kilometers) wide—sometimes migrate to unstable areas of the main belt, known as resonances.

Once within the resonances, asteroids are vulnerable to the gravitational pull of nearby planets—Mars, Jupiter or Saturn—which can elongate an asteroid’s orbit. The change is sometimes enough to swing the asteroid onto a path that crosses Earth’s orbit, setting up the possibility of a future collision.

Uneven warming by the sun may play a role in moving asteroids into the resonances. Energy reradiating from the warmed side of the asteroid delivers a kick in the opposite direction, similar to the recoil of a rocket spewing gas. Kicks over millions of years can move an asteroid’s orbit into an unstable area.

Only a small fraction of asteroids leave the main belt, and of those only a small fraction moves into a potential collision course with Earth. However, over billions of years many asteroid objects have collided with Earth and left their marks with craters that can still be seen. Arizona’s Meteor Crater, with its uplifted rock walls and scattered beads of glass, is an example of an asteroid impact that occurred 50,000 years ago.

However, the history of asteroid impacts isn’t all bad news. One theory suggests that chemical components of life, including much of Earth’s water, arrived with asteroids and comets that bombarded the planet in its youth.

Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at http://www.earth-info.org.