Once every 40,000 years or so, to borrow Thomas Pynchon's phrase, a screaming comes across the sky.

With a flash of light brighter than the sun, a jolt greater than any earthquake and a roar that might literally be heard around the world, an asteroid or comet larger than a quarter-mile in diameter slams into the fragile blue-and-green oasis we call Earth.

Two new films about these killer collisions are due out this season -- "Deep Impact" tomorrow and "Armageddon" on July 1. As science fiction movies go, they are unusual.

Hollywood typically loves to conjure up aliens, from the beatific ET to the ruthless insectoids of "Independence Day," as exotic life forms with big brains.

But the nonhuman stars of these new films are based on real-life characters, and they're as common as dirt, as dumb as rocks.

In fact, they are rocks, or chunks of ice. But they're the only extra-terrestrials known to have visited Earth in the past. They're the only ones certain to do so in the future.

And they never come in peace.

If and when Earth gets hit again, what will it be like?

"That's very, very hard for humans to understand," says Bruce Marsh, a professor of earth and planetary science at Johns Hopkins University.

There is no firsthand account of an asteroid impact in recorded history. Other natural disasters can only suggest the scale of these events.

"Mount St. Helens is a baby compared to one of these things," Marsh says.

He will travel soon to the site of one of the largest known craters: Sudbury, in Ontario, Canada. That's where, 1.85 billion years ago, a huge asteroid or comet hit Earth and dug a hole about 100 miles wide.

It happens very fast, Marsh says. "It's almost unfathomable how rapidly this process happens, and how dramatically."

As a medium-size asteroid penetrated the atmosphere at 45 miles a second, it would instantly generate a mammoth shock wave and fireball. If it hit the deep ocean, it would throw up a curtain of water that could snare an airliner cruising at 35,000 feet. Thousands of square miles of ocean would almost instantly turn to steam.

Bedrock wouldn't fare much better.

"When you hit the Earth with something like an asteroid, the Earth in many ways acts like a viscous fluid," Marsh says. "Pressures near the impact are on the scale of pressures you'd find at the center of the Earth. Millions of atmospheres."

Rocks start to melt at about 2,200 degrees Fahrenheit; in Sudbury, temperatures at the point of impact approached 4,500 degrees.

"It's so far beyond their melting range, rocks start to vaporize," Marsh says.

As it formed, the impact crater would resemble the shape created when a drop of milk hits the surface of a glass of milk. At the crater's edges, molten "droplets" of bedrock would splash off the rim.

Energy would pulse through the planet toward the core, then back again to the impact site. On the rebound, at the center of the bull's eye, the now-fluid rock would spurt skyward in a hellish geyser.

Everything in the object's path would be obliterated, of course. But that wouldn't be the limit of the mayhem. Houses and cars, forests and fields, factories and fast-food stands within 1,000 miles? Forget about them. Swept away by the shock wave, incinerated in the heat.

At Sudbury, the sheer force of the impact excavated a crater about 25 miles deep within a minute or so. But the surface remained superheated, and the crater quickly filled with molten rock, to within about a mile of the surface.

The air temperature probably hovered at about 1,000 degrees Fahrenheit.

"A lot of the material in the air would be incandescent," Marsh says. "In a lot of ways it would be like being inside a light bulb."

All around, there was chaos.

"There's stuff falling back to Earth through the air," Marsh says. -- "There's all kinds of molten gobs of rock dumplings, chicken-and-dumplings-like stuff falling back, and they're making secondary craters."

On a global scale, there would likely be secondary earthquakes, tsunamis, continent-wide forest fires, and a planet-smothering cloud of soot and debris that would remain airborne for years. If an object about one-third of a mile in diameter were to hit Earth today, one out of four humans probably would die.

Earthlings haven't always appreciated how directly their planet sits in the line of cosmic fire.

Until about 50 years ago, terrestrial craters were generally considered to be extinct volcanoes. Then, in the late 1940s and ** early 1950s, the craters left by atomic-bomb tests in America's southwest reminded scientists of some of these supposed "volcanoes." Gradually, scientists realized that impact craters were everywhere.

In 1994 Wylie Poag of the U.S. Geological Survey and some colleagues presented the first evidence that the basin that would eventually become the Chesapeake Bay was created by an asteroid or comet collision 35 million years ago.

That impact left a crater 50 miles across, with ground zero at a site that is now Cape Charles, on Virginia's portion of the Eastern Shore.

Last year, Richard Grieve of the Geological Survey of Canada and a colleague suggested that the bay impact occurred at roughly the same time as another one that created the 60-mile-wide Popigai crater in Siberia.

"We had two of them striking simultaneously or within a few tens of thousands of years, which is the same thing in geology," Poag says.

As geologists hunt for craters, astronomers are searching for the asteroids that make them.

Back in March, one of the world's experts on asteroids said that a hurtling object named 1977 XF11 had a small but real chance of striking Earth on Oct. 26, 2028. More calculations were made, more data uncovered, and that small chance quickly became no chance at all.

But just because the sky won't fall in 30 years doesn't mean it never will. An asteroid or comet may have already claimed more than one human culture, including that of the Mycenaeans, who dominated the eastern Mediterranean but vanished around 1,100 B.C.

Some scholars suspect that Plato was referring to the hapless Mycenaeans in his "Timaeus," when he reported about an ancient nation wiped out by a "great conflagration" caused by heavenly bodies. One British astronomer has suggested that the gods of ancient cultures were inspired by asteroid impacts.

A team of scientists including James Garvin, a planetary geologist at NASA's Goddard Space Flight Center, want to dig for evidence of these recent catastrophes. They have proposed a global effort to catalog, survey and drill every relatively young crater they can find. Then they would look at pollen, drill core and other evidence to figure out what kind of effect on global climate these Earth-shattering impacts really have.

"We've proposed to exhaustively examine the impact record for the Earth over just the past few million years," Garvin says.

So far, scientists have found only about 150 of the 1,000 impact craters that are likely to be found in Earth's rocks. And they've barely started trying to figure out how they affected plant and animal life.

"We're just starting to see the interconnectedness," says Marsh. "These events are so rapid and so short term, they're not the kind of events geologists are used to looking at."

Pub Date: 5/07/98