Nation/World

Chicken Little Right About Sky Scientists Say Earth Bombarded By Space Rocks

Killer rocks from outer space are finally getting some respect.

People laughed a few years back when Edward Teller, the father of the hydrogen bomb, suggested the need for an international arsenal of missiles powerful enough to fend off a doomsday asteroid.

Now, suddenly, the public imagination seems crowded with visions of celestial “incoming.” The scenario is proliferating in books, movies, magazine articles, television documentaries and the occasional miniseries, like one that begins tonight. Asteroid trackers are being hounded for interviews.

Is this mere millennial madness?

Actually, experts say the threat is real enough to merit more serious study than it’s getting, but not imminent enough to mobilize the nukes. At least not yet. They hope.

“In a sense, Chicken Little was right,” said planetary scientist Eleanor Helin of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The sky is falling.” And sooner or later, “we could become the dinosaurs.”

It would take an object at least a mile in diameter to cause global ecological damage and disrupt civilization. Scientists estimate that there are some 2,000 asteroids and comets larger than a kilometer in diameter (0.6 mile) in orbits that could someday intersect with Earth’s. But so far, only about a tenth that number has been located.

“We need to know where the other 90 percent is,” said Helin, who pioneered an early tracking effort 25 years ago and now is leader of the NEAT (Near-Earth Asteroid Tracking) system science team.

NEAT uses an electronic camera installed at a 39-inch telescope operated by the U.S. Air Force Space Command atop Maui’s Haleakala volcano. It is one of just two tiny search groups in the world scanning a total of just 2 percent of the sky each month in search of doomsday rocks.

The Earth is no stranger to encounters with space rocks. But happily for humanity, the big chunks of space rock are far outnumbered by smaller ones that swarm around Earth’s path. And those large enough to make it through the atmosphere tend to land in the vast oceans or in uninhabited lands.

In fact, every hour a ton of micrometeorite dust hits the Earth. Every few hours a baseball-sized lump survives intact all the way to Earth’s surface.

Some objects hit the upper atmosphere and bounce back out into space. Some tumble into view with unnerving suddenness - like the object at least 1,000 feet in diameter that appeared suddenly last May and took five days to cross the sky not much farther away than the moon’s orbit.

About once a month, recently declassified data from military satellites indicate, some extraterrestrial object detonates at high altitude with the force of a kiloton or more of TNT.

Based on what scientists know now, the odds that an object at least a mile in diameter will smash into Earth in the next century are slightly less than 1 in 1,000. The resulting damage would depend on the object’s size, velocity, the location of impact and other variables.

It could happen centuries from now, thousands of centuries from now or next month. There is no way to predict absolutely, even once scientists have completed their survey of detectable nearby objects, because interlopers from deep space could sweep in unexpectedly at any time.

Even so, scientists estimate that the probability that any individual will be killed by a doomsday rock is about the same as the chance of getting killed on a commercial air flight - just under one in a million per year, according to David Morrison, director of space at NASA’s Ames Research Center. The individual risk is that high only because the fatalities from just one such impact could number in the hundreds of millions or more.

“All this talk of probability is an expression of our ignorance,” Morrison said. “Either something will hit Earth in 1997 or it won’t. Right now, so little of the sky is being scanned that the probability of getting any warning of such an object is zero.”

Human awareness of the solar system as one big shooting gallery took a leap in 1994, with the unprecedented spectacle of shattered comet fragments ripping into Jupiter. The event was recorded by most of the world’s telescopes, covered on the evening news and discussed on the Internet.

But interest in approaching space rocks had been building among scientists for about 15 years, as they raked in new evidence of the important role played by collisions in the formation of planets and the evolution of life.

“Impact is the most fundamental of all geological processes,” said Eugene Shoemaker, of the U.S. Geological Survey, who co-discovered the comet that hit Jupiter. “It’s how the Earth was formed.”

The game of cosmic billiards has slowed its pace since the solar system formed some 4.5 billion years ago out of a primordial cloud of dust and gas. Some of the leftover rubble has been swept up by the planets, and much of the rest has sorted itself into fairly stable separate communities.



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