Will A Killer Asteroid Hit The Earth?

By LEON JAROFF

When it comes to asteroids' wreaking disaster on Earth, the real question is not if, but when. Two hundred or so large craters and a geological record stretching over billions of years provide ample evidence that, time and again, explosive impacts by asteroids or comets have devastated large parts of the planet, wiped out species and threatened the very existence of terrestrial life. Astronomers are all too aware that more large hulks are out there, hurtling through space, some of them ultimately destined to collide with Earth.

As scary as this seems, disaster is not inevitable. For after nearly 4 billion years of life on Earth, a species has evolved that can prevent the next catastrophic encounter--if it has the will to do so. That species is us.

Why worry? After all, the most notorious impact of them all, the one that caused the extinction of the dinosaurs, occurred 65 million years ago. Really ancient history.

Yet if you want to get contemporary--on a geological scale, of course--it was only 49,000 years ago that an iron asteroid blasted out Arizona's three-quarter-mile-wide Meteor Crater, almost certainly killing any living creatures for hundreds of miles around. As recently as 1908, a small rocky asteroid or chunk of a comet exploded five miles above the Tunguska region of Siberia, felling trees, starting fires and killing wildlife over an area of more than 1,000 sq. mi. Had the blast, now estimated at tens of megatons, occurred over New York City or London, hundreds of thousands would have died.

And what about near misses? As recently as 1996, an asteroid about a third of a mile wide passed within 280,000 miles of Earth--a hairbreadth by astronomical standards. It was the largest object ever observed to pass that close and, had it hit, would have caused an explosion in the 5,000-to-12,000-megaton range. What was particularly unnerving about this flyby is that the asteroid was discovered only four days before it hurtled past Earth. All the more reason for a detection system that will discover asteroids early, plot their paths and predict, many years in advance, whether they will eventually threaten Earth.

The good news is that just such a detection system, after a slow start, is rapidly gearing up. Four small groups of dedicated astronomers in Arizona and California, totaling fewer than the number of employees at an average fast-food restaurant and using mostly off-the-shelf equipment for their telescopes, have been mapping the heavens and steadily adding to the number of known near-Earth objects (NEOs).

NEOs are asteroids or occasional comets that periodically intersect or come close to Earth's orbit. If a NEO cuts through our orbital path at the same time that Earth happens by, it's curtains for a metropolitan area, a region or even global civilization, depending on the size of the interloper.

In 1997 the asteroid-hunting pioneers were joined by a precocious upstart, a joint Air Force-M.I.T. Lincoln Laboratory group supported by generous Pentagon funding. Using an Air Force satellite-spotting telescope in New Mexico and a camera equipped with an advanced M.I.T.-designed charge-coupled device, the totally automated, computerized operation quickly began discovering more asteroids and comets, large and small, than all the other groups combined. Getting further into the spirit of the game, the Air Force has deployed a second asteroid-hunting scope, is lending another to astronomers and musing, unofficially, about launching a fleet of microsatellites for even better asteroid detection.

What to do if an Earth-bound comet or asteroid is discovered? Early detection, preferably many years in advance, would enable us to send out exploratory spacecraft to determine the nature of the interloper, much like the spacecraft near's current investigation of the asteroid Eros. Scientists at the Los Alamos and Lawrence Livermore national laboratories are already dreaming up a variety of ingenious defenses against an incoming asteroid. Depending on its mass and composition, they would use tailor-made nuclear explosions to pulverize a small asteroid or deflect a larger one. Given enough time, and under the proper circumstances, less drastic measures would be needed. Some schemes call for conventional explosives alone, or anchoring a rocket motor or a solar sail on an asteroid to alter its orbit enough to allow it to safely bypass Earth.

At the beginning of 2000, only about half the estimated 500 to 1,000 near-Earth asteroids six-tenths of a mile across or larger--big enough to cause a global catastrophe--had been detected. One of the unknowns could even now be on a collision course with Earth. The sudden appearance of long-period comets, usually larger and with better than twice the impact velocity of asteroids, presents an even greater menace. Such objects (comet Hale-Bopp was one) are usually not spotted until they begin to flare somewhere out near the orbit of Jupiter or closer, only a few to 18 months before they pass Earth's orbit. That doesn't leave much time for defensive measures. Then, too, only a tiny fraction of the more numerous and smaller NEOs, some of them potential city killers and tsunami producers, are yet known.

Someday in the foreseeable future, the first thing that strollers out for an evening walk might see would be a sudden glow on the horizon. Then, in short order, they would feel the ground shake, hear a thunderous roar and be incinerated by an onrushing blast of superheated air. All the more reason to identify and track every single near-Earth object and prevent a nasty surprise.