Astronomers Check the Movie

Terrific!" says David Morrison, NASA Ames Research Center.

"They got that part right" says Erik Asphaug, University of California at Santa Cruz.

OK, Siskel and Ebert they aren't. But when it comes to the film "Deep Impact," astronomers arebetter equipped than anyone to sit back, dip a hand

into the popcorn bag, and provide a reality check on one of the biggest box-office hits of the season.

Dr. Morrison, head of the space directorate at NASA's facility at Moffett Field, Calif., and an authority on comets and asteroids, describes the film as "very impressive, thoughtful, and even moving."

While he acknowledges that it's possible to pick nits, he notes that "Deep Impact" does a credible job of portraying the threat from a comet striking Earth. In the end, he says, the film could go a long way toward alerting millions of people to the hazard these and other orbiting objects pose.

End of story? Not quite. Others are willing to pick nits where Morrison generally declines to criticize.

"One of my objections to 'Deep Impact' is that they ultimately prevail in kind of, sort of, saving the world," says Alan Harris of the Earth and Space Sciences Division at CalTech's Jet Propulsion Laboratory in Pasadena, Calif. Off the movie set, "comets are the part of the impact problem that we can't do anything about."

Comets, he says, typically are discovered as they approach the sun and brighten to the point where telescopes can spot them. Some can appear two years before they pass Earth (see story at right). Other comets are not so generous.

Comet Hyakutake, which sped within 10 million miles of Earth on March 25, 1996, was discovered on Jan. 30 of that year. And, Dr. Harris adds, it can take up to a year of observations to accurately plot their orbits. Meanwhile, the comet continues to pick up speed under the tug of the sun's gravity. By the time astronomers can say with certainty that a comet will hit, he says, it's probably too close to divert.

What about a "Deep Impact" style mission to blow the comet apart - planned, built, and launched in one year after detection?

"Just fantasy," Harris says.

But fantasy is Hollywood's currency. So what about the mission's failure to blast the 6-mile-wide comet into another orbit or fragments with nuclear weapons?

That earns a thumbs up from Dr. Asphaug, a research associate in the Department of Earth Science at the University of California. He and colleagues at JPL, Washington State University, Iowa State University, and the University of Bern in Switzerland simulated impacts on a 1.6-kilometer peanut-shaped asteroid. They varied the asteroid's structure - from a porous rubble pile to solid rock. The impactor hit with energy equivalent to 17 kilotons of explosives - the size of the atomic bomb dropped on Hiroshima.

The results, published in today's issue of the journal Nature, point to "Deep Impact" like results on the porous asteroid: a minuscule nudge, some disruption of the structure, but no dispersal.

At root is a principle well known to a building-demolition expert. "You have to understand the structure of the object," Asphaug says, to achieve the desired result. That means at least one mission to the comet to survey it. But the "Deep Impact" comet left no time to do that. Little wonder, then, that six 1-megaton nuclear charges merely blow off a milewide chunk of the comet, which continues to hurtle earthward.

Even if the charges had been placed more strategically, the bang would have been too small. "A back-of-the-envelope calculation, assuming a 1 kilometer [0.62 miles] threat with a few months of lead time and some preparation time ahead of that... you'd need 100 megatons or more of explosive energy" to disperse or divert it, says David Crawford at the US Department of Energy Sandia National Laboratory in Albuquerque, N.M.

"That is the real fear we have," Asphaug adds, "breaking an incoming object into pieces" that fail to disperse.

Cut to the movie. Earth awaits two impactors. One is the milewide chunk, which will plunge into the sea off North Carolina. It's followed by the rest of the comet, which the president says is expected to land in Canada. Here, the president's descriptions of what would happen on impact are faithful in general terms, astronomers say, to their computer simulations of an impact.

So into the atmosphere comes the smaller piece at speeds near 44 miles a second, glowing white hot at temperatures approaching 5,000 to 10,000 degrees Celsius. Those sunlike temperatures would sear anything under the object's flight path. It plunges into the ocean, vaporizing seawater and burrowing into the relatively shallow sea floor. The good news is that an ensuing tsunami does not reach the Ohio Valley as in the movie, but stops at the Appalachian Mountains. The bad news is that the impact causes several tsunamis, not just one as in the movie, according to Jack Hills of Los Alamos National Laboratory, N.M.

Dr. Crawford has modeled the effects of an asteroid impact striking just south of Long Island in New York. The movie, he says, avoids: debris kicked upward, escaping Earth's gravity, and reentering the atmosphere thousands of miles away. The material kicked up is sufficient to blanket the atmosphere in dust and block the sun.

Only hours away from impact, the comet finally is blasted apart as the spacecraft burrows deep into a fissure in the comet and detonates 4 megatons of nuclear explosives.

Here, even Ames's Morrison acknowledges that 4 megatons probably isn't enough to shatter the larger part of the comet.

But give the movie its due for a spectacular ending - a meteor shower that spreads harmlessly across the sky.

Crawford adds a few crucial details. "That meteor shower would be so intense that you'd have continuous light from material as hot as the surface of the sun lasting from a few minutes to an hour or so," he says. For everything on the shower's side of Earth, "it would be like being in an oven set to broil."

If all this is enough to make you chuck the popcorn bag in the trash and make an early exit, take heart. "Earth being hit by a comet has a terribly low chance of occurring," says JPL's Harris. Encounters with asteroids in orbits that cross Earth are much more likely - and well within humanity's ability to counter, even with current technology.