People made fun of Chicken Little when he ran around shouting that the sky was falling. But in the past year, respected scientists have come to the realization that the end of civilization may come from the heavens. Humanity may be in as much danger from the fate of the cosmos as from the error of our ways.

The danger is a comet or asteroid strike on the Earth. Thousands of chunks of cosmic rock enter the atmosphere of our planet every year. The vast majority burn up in the atmosphere, creating spectacular display for young lovers on lazy summer nights.

Almost all of the cosmic matter that survives to hit the Earth is nothing more than a curiosity. A mysterious lump of iron is found in a cornfield; a twenty-two pound rock appears amid the demolished trunk of a car in Peekskill, New York just last year. The Harvard Geological Museum contains dozens of examples, carefully classified by composition, ranging from a few ounces to over a hundred pounds.

It's the monsters that we need to worry about are the monsters. One of these massive asteroids intersected the orbit of the Earth on June 30th 1908. Six miles above Tunguska, Siberia, it exploded with a force 1,000 times more powerful than the nuclear weapon dropped on Hiroshima. Almost 500,000 acres of forest were destroyed. The shockwave was strong enough to be detected in America.

Dr. Brian G. Marsden, associate director for planetary sciences at the Harvard-Smithsonian Center for Astrophysics, rates the chance of a Tunguska-sized asteroid hitting the Earth during the next 100 years as "excellent," and gives smaller but significant odds to an even larger asteroid strike.

Measuring only 60 meters across, the Tunguska asteroid pales in size compared to the one kilometer asteroids which could cause a global climate change, or the 10 kilometer mass to which most scientists attribute the extinction of the dinosaurs.

Scientists believe this mother of all asteroids formed a crater 180 kilometers across when it struck the Yucatan Peninsula. Earthquakes occurred worldwide, and billions of tons of dust and molten rock burned up, turning the atmosphere into a furnace and setting off forest fires worldwide.

To make matters worse, the impact site was covered in limestone, which vaporized into trillions of tons of carbon dioxide. It seems likely that the dinosaurs died from the global warming that resulted from this release.

A stronomers have already identified more than 100 kilometer-wide asteroids and comets whose orbits may cross the Earth's--Marsden and others believe these sighted projectiles represent only a tiny portion of the objects that threaten us.

Geological evidence, as well as studies of the surfaces of other planets, lead scientists to believe that one of these asteroids strikes the Earth every few hundred thousand years. A report to Congress last year estimated that one quarter of the world's population would die if an impact of this magnitude occurred.

According to the numbers, each of us has a lifetime risk of one in 30,000 of death by asteroid--roughly equivalent to our chances of dying in a plane crash.

A few observers, mainly underemployed scientists in the nuclear weapons industry, have suggested the immediate development of nukes to destroy incoming asteroids--weapons thousands of times more powerful than those we already have. An interesting idea, but likely laced with more than a little self-interest.

A more practical idea is the proposed "Spaceguard" program, which would cost $10 million per year--pocket change compared to the amount the government spends on the space shuttle and other forms of interplanetary pork.

In 20 to 25 years, the Spaceguard program could find over 90 percent of the asteroids and comets that may endanger us. Its relatively small price could be shared among several developed countries.

Once scientists find the asteriods and comets, they can calculate orbits and forecast any possible impacts. The more warning we have, the easier it will be to do something about the danger.

With 20 years warning, a neutron bomb could turn a direct hit into a near miss. Two years warning would make a deflection more difficult, but still possible. If we only have two months, we're in big trouble.

Marsden says that if we didn't specifically search for it, we probably wouldn't detect an incoming asteroid or comet until radar picked it up at the range of the moon.

At that point, we could do nothing more than prepare, in a few hours, for the end of the world.

We worry about earthquakes, hurricanes, tornadoes and other natural disasters because they frequently affect populated areas. But we underestimate the danger of asteroid and comet strikes because they often occur in the ocean.

An example is the alleged Israeli-South African nuclear test in the Indian Ocean in 1978, which now is believed to have been an asteroid strike.

Marsden pessimistically concludes that "our scientific and technological capabilities are infinitely greater than were those of the dinosaurs, [but] I don't know how much more sensible we are."

Perhaps Chicken Little had a point after all.