Who's afraid of the big, bad asteroid? A recent Earth- bashing "thriller" on TV was pretty scary. But it left a false impression that there's little we can do about this rare - albeit real - cosmic danger.
Actually, there's plenty we can do about it. Asteroids and comets present a unique type of natural hazard. It combines a potential for great destruction with a potential for great benefit. In its most extreme form, it's the only natural hazard that probably could destroy civilization all over the world. Yet its danger should be more predictable than that of any other natural disaster.
A modest monitoring effort could locate and track objects traveling on collision courses. This could give decades - even centuries - of warning time to find ways to limit damage or even avoid a hit. Moreover, as our space capabilities grow, mining asteroids could become a valuable source of minerals, as John Lewis, co-director of the NASA/University of Arizona Space Engineering Research Center in Tucson, has pointed out.
What happens when cosmic rocks ding our planet depends on their size and makeup. Meteors flash every day as atmospheric friction burns up thousands of sand grains and pebbles. In fact, the atmosphere shields us from most stony meteorites up to sizes about 10 meters (about 33 feet) in diameter, although some of their debris sometimes hits the surface. The rarer iron meteorites, which are denser and stronger, tend to survive atmospheric entry better.
As their size reaches the 100-meter range, however, asteroids and comets become dangerous regardless of their composition. In 1908, a 100-meter object flattened a forest when it exploded over the sparsely inhabited Tunguska region in Siberia. It is asteroids and comets in the range of 1 kilometer and larger that cause truly catastrophic damage.
Two years ago, a study led by David Morrison at the NASA Ames Research Center in Mountain View, Calif., concluded that debris from hits by objects larger than 1 to 2 kilometers across would darken the sky, causing widespread crop failures. Objects bigger than 5 kilometers could cause mass extinctions. That may be what did in the dinosaurs 65 million years ago.
Such a massive strike is a once-in-a-hundred-million-year event. Lesser impacts are more frequent. The Morrison group estimated that there are some 320,000 asteroids larger than 100 meters across and about 2,100 asteroids larger than 1 kilometer whizzing around in our part of the solar system. We need to locate them and identify possible threats. An Ames study also concluded that we could find virtually all potential threats within a decade with a systematic observing program costing $5 million a year.
In the case of a moderate-size object whose predicted impact would devastate a city or cause destructive tsunami waves if it hit an ocean, authorities could make evacuation plans. The best defense - and the only defense against larger objects - would be to deflect the asteroid. How to do this is a subject of ongoing low-level study in the US Department of Defense. While it's not yet clear what the best solution might be, experts such as Mr. Lewis say they think it's doable.
Meanwhile, most scientists interested in the subject give first priority to establishing an asteroid-comet monitoring system. The University of Arizona already has an asteroid spotting program. Australia has been running a companion effort. But the government in Canberra recently eliminated its funding as part of a budget cut. The US Army and Air Force are studying ways to establish a more effective watch. These all are piecemeal efforts. The kind of coordinated international survey the Ames study recommends has yet to to be mounted.
While the dangers of cosmic collisions are real, there's no need for fear. The need is for prudent watchfulness accompanied by research into ways to deflect the incoming missiles. And as we get to know asteroids better, we perhaps can look forward to turning this unique natural hazard into a natural resource.