Henry FOUNTAIN,
Armageddon Can Wait: Making Plans to Stop Killer Asteroids.

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.

But it is becoming clear that an assumption of many scientiste 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.

A nuclear detonation, so the 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 bit 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 - bas 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 creates a small 
force in the opposite direction. 

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 scientiste 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, Colorado.

Rather, most scientiste 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 objecte larger than a kilometer 
in diameter by 2008.

The best estimate is that there are perhaps 1100 of these large asteroids, 
about hall 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.