November 19, 2002

Armageddon Can Wait: Stopping Killer Asteroids

ooner or later, it's bound to happen.

Sooner or later, scientists who study Earth-crossing asteroids say, astronomers will find one that has a significant chance of striking the planet.

Unlike several recently discovered asteroids that were first given very long odds for a collision, this time more 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.

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

But it is becoming clear that a longtime assumption of many scientists — and of Hollywood filmmakers — that a nuclear weapon is the best way to save the planet from a threatening asteroid is no longer in such favor. Increasingly, those scientists who study asteroid hazards say that a subtler, quieter, slower approach might be called for. These scientists are turning T. S. Eliot on his head: it's not that the world will end with a whimper rather than a bang, they say. It's that it may not end that way.

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," said Dr. 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 same basic Newtonian principle — that for every action there is an equal and opposite reaction — only written small, with tiny actions creating 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 are some that have been the stuff of science fiction for years: a mass driver, a sort of electromagnetic conveyor belt 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 — and one that may not be as far-fetched as it sounds — has been put forth by Dr. 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 it absorbs, and how hot it gets. Heat radiating from an asteroid (in the form of thermal photons) creates a small force in the opposite direction — a phenomenon called the Yarkovsky effect, after I. O. Yarkovsky, a Russian engineer who first described it a century ago. Changing the amount of heat would change the force, affecting the orbit. The sun would move the asteroid, one photon at a time.

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

Few scientists are arguing that society should be developing an asteroid-deflection system, given the extremely low odds of an impact any time soon. "A major technological effort at this time is probably ill conceived because our children will be so much better at it," said Dr. Alan W. Harris of the Space Science Institute in Boulder, Colo.

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, which was exemplified by presentations at a NASA-sponsored workshop on asteroid hazards in September near Washington that "pretty much sent the nuclear weapon idea home packing," said Dr. Erik Asphaug, a professor at the University of California at Santa Cruz and one of the workshop's organizers.

There are several detection efforts under way, with a goal of meeting a Congressional mandate of finding 90 percent of objects larger than a kilometer in diameter by 2008. An asteroid of this size is thought to strike the Earth once every million years or so, but since it is capable of producing destruction on a regional scale or worse, in terms of potential loss of life over time it represents the biggest risk.

The best estimate is that there are perhaps 1,100 of these large asteroids, about half of which have been discovered and found to be harmless. The odds are extremely slight that any of the remaining large asteroids will prove threatening, either.

But any asteroid with a chance of hitting Earth would cross the planet's path many times before it actually hit, so it would probably be detected decades in advance.

There is no current detection program for smaller asteroids, of which there are perhaps half a million down to about 50 meters in diameter, the smallest size capable of penetrating Earth's atmosphere (and roughly the size of one that exploded over the Tunguska River in Siberia in 1908, destroying forests for hundreds of square miles). And there is no systematic survey for potentially hazardous comets, which come out of the astronomical equivalent of left field. "So we would either very likely have a lot of warning or none at all," said Dr. Clark Chapman of the Southwest Research Institute in Boulder.

No warning time means no options. A short amount, on the order of a decade or two, might leave a nuclear blast as the only choice. But with many decades of warning, there is room to investigate the asteroid first by sending a spacecraft to it, and then use a slow-acting method to divert it, one that wouldn't require launching a nuclear weapon. "We would want to seek out every alternative to a nuclear weapon before turning to that technology,'` Dr. Chapman said.

What makes some of these alternatives promising is what scientists have come to understand about asteroids. Many of them, the scientists say, 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 not giant solid boulders. "Maybe something like a popcorn ball is a better way to describe it," Dr. Holsapple said.

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

Porosity might prove to be a problem even for some of the alternative methods, however. A mass driver, 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.

Dr. Spitale's idea would get around that problem, but it would not be without other difficulties. For one thing, a lot of paint would be required. (He has also suggested dumping a thin layer of dirt on the asteroid to change its color, and has estimated that dozens of rocket loads would be needed.) For another, because they are so small, asteroids have very little gravity, so it is unclear that paint or dirt would stay in place. One solution to that problem and to the problem of transporting large amounts of material, he says, would be to pepper an asteroid with small explosives, to remove the top layer and expose material that might have different thermal characteristics.

While generally saluting this kind of outside-the-box thinking, some other asteroid experts find Dr. Spitale's ideas largely impractical. "I guess I consider that approach kind of quirky," Dr. Chapman said.

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