Avoiding asteroid Armageddon: How do you stop an asteroid from hitting Earth? Hollywood envisions nuclear weapons, but scientists favor a gentler approach. (science times).

Sooner or later, scientists say, it's bound to happen: Astronomers will discover an asteroid that has a significant chance of striking Earth.

Unlike several recently discovered asteroids that were given very long odds for a collision, in this imagined scenario precise orbital calculations won't eliminate the possibility. This one will be an asteroid "with our name on it," in the words of David Morrison, a scientist at the NASA Ames Research Center and one member of a small community of astronomers, physicists, engineers, and other scientists who think a lot about such an unthinkable event.

What would happen then is not dear, though Morrison and others are trying to awaken governments and the public to the need to at least think about developing a response. "Eventually we will discover something," Morrison says, though maybe not in this century or even this millennium. "Society should start planning for that unexpected but potentially tragic possibility."

A GENTLE NUDGE 

Many scientists--and Hollywood filmmakers--have long assumed that a nuclear weapon could best save the planet from a threatening asteroid. But this view has lost ground. Increasingly, those scientists who study asteroid hazards say that a subtler, quieter, slower approach might be called for.

A nuclear detonation, some scientists say, could break the asteroid into several large pieces, increasing rather than eliminating the threat. And a blast some distance from an asteroid, designed to shove it into a slightly different orbit, might not work either; the asteroid might soak up the energy like a sponge. "I'd say forget that," says Keith A. Holsapple, a professor at the University of Washington who studies the effects of simulated nuclear explosions.

By contrast, most of the alternative approaches would build up force gradually, gently nudging, rather than shoving, the asteroid. They would rely on the Newtonian principle that for every action there is an equal and opposite reaction. In these cases, tiny actions would create tiny opposite reactions that, given enough time, could shift an asteroid's orbit enough to change a hit into a close call.

Among the approaches being talked about: a magnetically powered conveyor belt (a "mass driver" in scientific language) that would be planted on an asteroid and hurl dirt from its surface; or a solar concentrator, a parabolic mirror that would orbit the body and heat up the surface, creating a plume of vaporized material.

Perhaps the most intriguing idea has been put forth by Joseph Spitale, a scientist at the University of Arizona. To move an asteroid, he says, just change its color.

This "paint it black" approach would change how much sunlight the asteroid absorbs, and how hot it gets. Heat radiating from an asteroid creates a small force in the opposite direction. Changing the amount of heat would change the force, affecting the orbit.

There are, of course, logistical problems with this and other alternatives. Getting buckets of paint to an asteroid, for instance, is no sure (or inexpensive) thing. Many scientists acknowledge that in some cases a nuclear weapon may be the only option.

IMPROVED DETECTION

Few scientists argue that society should be developing an asteroid-deflection system, given the extremely low odds of an impact anytime soon. Rather, most scientists say that any money available should go into detecting asteroids and investigating them to better understand the potential threat.

Improvements in detecting and understanding asteroids, in fact, are what is prompting the change of thinking toward a slow approach.

Several detection efforts are currently under way, trying to meet a federal mandate of finding 90 percent of near-Earth objects larger than a kilometer (0.6 miles) in diameter by 2008. Asteroids of this size are thought to strike Earth about once every million years. They are capable of producing destruction on a regional scale or worse, so they represent the biggest long-term risk to human life.

Scientists estimate that there are perhaps 1,100 of these large asteroids whose paths approach Earth's orbit; about half have been discovered and found to be harmless. The odds are extremely low that any of the remaining large asteroids will prove threatening.

If they do, they will cross Earth's orbit many times before a collision, so they would probably be detected decades in advance.

Scientists have no detection program for an estimated half million smaller asteroids capable of hitting Earth. (To penetrate Earth's atmosphere, an asteroid would need to measure about 50 meters in diameter--roughly the size of one that exploded over the Tunguska River region in Siberia, Russia, in 1908. It destroyed forests for hundreds of square miles). "So we would either very likely have a lot of warning or none at all," says Clark Chapman of the Southwest Research Institute.

No warning time means no options. A decade or two might leave a nuclear blast as the only choice. But with many decades of warning, a spacecraft could investigate the asteroid first, and then scientists could use a slow-acting method to divert it.

What makes some of these alternatives promising is what scientists have come to understand about asteroids: Many of them are rather loose agglomerations of stony fragments that have stuck together over time in the cosmic rock tumbler that is the solar system. They are more like giant popcorn balls than solid boulders.

PUSHING POPCORN

Such porous objects would be hard to obliterate or move with a nuclear blast, even one some distance from the surface, Holsapple says. "But pushing a little bit for a long time would work equally well, whether an asteroid is porous or not."

Porosity might prove to be a problem even for some of the alternative methods, however. A magnetically powered conveyer belt, for instance, would have to be firmly attached to an asteroid in order to work, as would a small rocket engine, another proposed method. It might not be possible to anchor such equipment to a popcorn-ball asteroid.

Spitale's idea--painting the asteroid--would get around that problem, but it would not be without other difficulties. For one thing, a lot of paint would be required. For another, small asteroids have very little gravity, so it is unclear that paint would stay in place.

Although they generally salute this kind of outside-the-box thinking, some scientists find Spitale's ideas impractical.

"I'll be the first to confess that this isn't the last word in asteroid hazard mitigation," Spitale says. Still, he adds, while it may not be easy, along with the nuclear option, it is the only approach that now appears technically feasible. "If we were faced with the problem today," he says, "this is one of maybe two approaches where we could say, `Well, we could do this.'"