We now know that the Earth is constantly bombarded by debris from space. Some 200 tons of ice, dust, and tiny meteorites fall into the atmosphere each day and are vaporized. Very large impacts that penetrate the atmosphere are less common. Scientists estimate that major impacts with global consequences occur every million years, and that midsize impacts, capable of wiping out a city, happen every century or so.

The thought that large comets and asteroids are a danger to the Earth was not widely accepted when Eugene Shoemaker first proposed the idea in 1960. He was a geologist studying a large crater in Winslow, Arizona, when he suddenly realized it had been created by a meteor. His subsequent work showed that the Earth experienced regular meteor and asteroid strikes, and would resemble the moon if not for our dynamic natural environment.

This theory was further strengthened in 1990, when scientists identified a giant hidden crater near the Yucatán Peninsula. It was created by the impact of a very large object, which probably caused the extinction of the dinosaurs some 65 million years ago.

Comet Shoemaker-Levy 9's fiery collision with Jupiter in July, 1994 proved there was reason to be concerned. If the comet had hit the Earth, instead of Jupiter, it would have destroyed human civilization. Just three days after the comet began to hammer Jupiter, Congress authorized NASA to develop a program to identify all potentially threatening asteroids and track comets entering the solar system.

Comets are rocky balls of ice that orbit the Sun, trailing a gas tail as they spin through space. Asteroids, and their smaller relatives, meteorites, are essentially rocks left behind from the formation of planets. They tend to cluster harmlessly in a belt between Mars and Jupiter, but collisions and gravitational shifts can propel them towards Earth. NASA estimates that there are some 160,000 asteroids with diameters greater than 100 meters (328 feet) in the solar system, and of these, approximately 1,000 objects larger than one kilometer (3,281 feet).


Today, hundreds of astronomers around the world scour the night skies. The majority of their work is coordinated by NASA, which has committed to finding and identifying 90% of all near-Earth objects larger than one kilometer in diameter by the year 2010.

"We've chosen objects larger than one kilometer because those are the most dangerous objects, and also because given the current state of our technology, it is a feasible goal," says Dr. Steve Pravdo, Project Manager for the Near-Earth Asteroid Tracking System at NASA's Jet Propulsion Laboratory, in California. Dr. Pravdo leads one of several teams scanning the sky. Others include the Linear Asteroid Detection Project in New Mexico, the Catalina Sky Survey in Arizona, the Loneos Program in New Mexico, and the Spacewatch Program at the University of Arizona.

Recent advances in electronic imaging and computer programs have created a revolution in astronomy. The system of photographic plates and visual identification used to identify comet Shoemaker-Levy 9 became outmoded about five years ago.

Today, most astronomers use advanced electronic imaging systems and sophisticated computers. These systems are ten times more sensitive than photographic film, and far more efficient than human labor.

"The computer and the camera work by themselves in Maui, Hawaii," says Dr. Pravdo. "They have baby-sitters, but the scientists are here at the Jet Propulsion Laboratory." Each night, the computers track moving objects across the sky and take about 1,000 separate images. This includes three or four images of each object taken at different times, which allows the astronomers to begin charting the object's orbit. When new objects are discovered, the details are passed on to the Minor Planet Center at the Harvard Smithsonian Astrophysical Observatory for confirmation and registration.


"We get information from about 150 different observers around the world," says Dr. Brian Marsden, director of the Minor Planet Center in Cambridge, Massachusetts, which catalogues all newly discovered near-Earth objects and tries to accurately determine their orbits.

"It usually takes two or three observations of an object, on separate nights, to gather enough information to figure out an orbit," says Dr. Marsden. Once an orbit has been determined, the object is given a number, and the discoverer is given credit. The spread of electronic detectors over the last few years has produced a dramatic increase in the number of objects being discovered. In 1998, the center listed 10,000 objects. Two years later, the number has risen to 15,000. Most are objects traveling harmlessly in the asteroid belts between Mars and Jupiter, and more are being discovered everyday.

About 1,700 listed objects are considered benign, since they travel in orbits that pass between five and 30 million miles from the Earth. A smaller subgroup, called Potentially Hazardous Asteroids, includes 249 objects that are one-tenth of one mile or larger, and pass within five million miles of the Earth. Last year, about 100 new objects were added to this list.

"Most of these objects are not a danger," says Dr. Marsden. "But in the last two years, there have been five objects with a significant impact possibility during the next half-century." Intense efforts are underway to determine the orbits of these objects. All but one has been ruled safe. This one object, which appeared in 1998, but vanished before an orbit could be determined, is listed as a potential threat to the Earth.

"The possibility that the Earth will be struck by a large asteroid or comet is not insignificant," says Dr. Marsden. "It's a low frequency event, but the destruction it could cause is quite tremendous. It's a serious question, given that 65 million years ago the dinosaurs and all the other life forms were extinguished by a large impact. It's not likely to happen soon, but you just don't know."

"We need to do the watching, so we can be aware if something is headed our way," says Dr. Marsden. "If we do discover something that will hit us, the more warning one has, the more possibility we have to actually do something about it. It might be possible to send a weapon out into space to deflect or blow up the object. As a responsible society, this is something we should bear in mind."