Whew!

By LEON JAROFF

It scares me," said Jack Hills, an astronomer at the Los Alamos National Laboratory. "It really does." He and the rest of the world had good reason to be worried. Astronomer Brian Marsden at the Harvard-Smithsonian Center for Astrophysics had just announced that a newly discovered asteroid a mile wide was headed for Earth and might pass as close as 30,000 miles in the year 2028. "The chance of an actual collision is small," Marsden reported, "but not entirely out of the question."

An actual collision? With a mile-wide asteroid? It sounded like the stuff of science fiction and grade-B movies. But front-page stories and TV newscasts around the world soon made clear that the possibility of a direct hit and a global catastrophe well within the lifetime of most people on Earth today was all too real.

Then suddenly, the danger was gone. Barely a day later, new data and new calculations showed that the asteroid, dubbed 1997 XF11, presented no threat at all. It would miss Earth by 600,000 miles--closer than any previously observed asteroid of that size but a comfortable distance. Still, the incident focused attention once and for all on the largely ignored danger that asteroids and comets pose to life on Earth. As Los Alamos senior scientist Greg Canavan put it, paraphrasing Dr. Samuel Johnson, "Nothing so clears the mind as the sight of the gallows."

XF11 was discovered last Dec. 6 by astronomer Jim Scotti , a member of the University of Arizona's Spacewatch group, which scans the skies for undiscovered comets and asteroids. Using a 77-year-old telescope equipped with an electronic camera, he had recorded three sets of images, 30 min. apart, of a small sector of the night sky. The digitized images, fed into a computer programmed to look for objects moving against the background of fixed stars, revealed an asteroid that Scotti, in an E-mail to Marsden, described as standing out "like a sore thumb."

Marsden promptly posted Scotti's data on the Harvard Center's Website, making them available to other astronomers. In early March, those data and newer observations by two Japanese amateur astronomers and a Texas scientist were fed into the Harvard Center's number-crunching orbit predictor, which spat out the 30,000-mile "miss distance" that led Marsden to make his dramatic announcement.

For a brief but exciting 24 hours, the big asteroid commanded everyone's attention. Astronomer Hills calculated that an asteroid the size of XF11 colliding with Earth at more than 38,000 m.p.h. would explode with the energy of 300,000 megatons--nearly 20 million times the force of the bomb that leveled Hiroshima. If it hit in the ocean, he predicted, it would cause a tsunami (commonly called a tidal wave) hundreds of feet high, flooding the coastlines of surrounding continents. "Where cities stood," he said, "there would be only mudflats." A land hit, he calculated, would blast out a crater at least 30 miles across and throw up a blanket of dust and vapor that would blot out the sun "for weeks, if not months."

Inspired by the doomsday headlines, Internet posters and late-night monologuists made lame jokes about the futility of long-term insurance policies and the new significance of 30-year mortgages. The news was not lost on Washington, where Jim Kennedy, special adviser to the White House counsel, revealed that Clinton insiders had chosen their own name for the asteroid: "Ken Starr's backup plan."

Then, as abruptly as the asteroid mania began, it was over. Jet Propulsion Lab astronomer Eleanor Helin, rummaging through some photographic plates taken in 1990, found previously overlooked images of XF11. Combining the asteroid's position eight years ago with the current readings, three groups working independently arrived at the same conclusion: the miss distance was actually 600,000 miles, and the chance of XF11's hitting Earth in 2028 was zero, or as JPL astronomer Don Yeomans declared, "less than zero."

The worldwide sigh of relief was almost palpable. The threat of asteroid strikes, however, still looms over the planet, which has been hit many times in the past by large objects raining down from space. Evidence of these ancient impacts is everywhere: more than 150 craters pock Earth's surface, some clearly visible, some that can be seen only from aircraft or satellites, others long buried or on the ocean bottom.

By far the most notorious of these craters is the circular feature 120 miles in diameter discovered below the northern tip of Mexico's Yucatan Peninsula. This monster crater is believed to be the impact site of a six-to-eight-mile-wide comet or asteroid that struck 65 million years ago and wiped out the dinosaurs and some 70% of Earth's other species.

While these catastrophic events seem remote and unreal, there are plenty of more recent reminders that Earth's neighborhood in space is still teeming with mountain-size rocks and the occasional wayward comet. Arizona's spectacular Meteor Crater, for one, was gouged out only 50,000 years ago by an iron asteroid. The impact and explosion blasted a hole nearly three-quarters of a mile across and 700 ft. deep. Today it could destroy a city.

Much more recently, in 1908, an asteroid or a chunk of a comet less than 200 ft. across roared into the atmosphere and exploded some five miles above the unpopulated Tunguska region of Siberia. The blast, estimated at tens of megatons, devastated an area of several hundred square miles, knocking down trees, starting fires and killing reindeer. Had it occurred over a large city, hundreds of thousands would have died.

And two years ago, an asteroid about 1,500 ft. across was discovered just four days before it sped by at 58,000 m.p.h., missing Earth by only 280,000 miles. If it had hit, the resulting explosion would have been in the 3,000-to-12,000-megaton range--equivalent, as the late astronomer Gene Shoemaker put it, to "taking all of the U.S. and Soviet nuclear weapons, putting them in a pile and blowing them up."

Scientists agree that it is only a matter of time before another celestial hulk hits home. "It's like a game of cosmic darts," said astronomer Clark Chapman on the PBS show Nova. "It could just as likely happen tomorrow as some day 300,000 years from now."

But what really worries astronomers is the devil they don't know. While they estimate that perhaps as many as 2,000 asteroids larger than a kilometer (six-tenths of a mile) across either cross or come close to Earth's orbit, they have discovered and tracked fewer than 200 of them. "We simply don't know where the other objects are," says JPL astronomer Helin. "But the ones that have been discovered," she warns, "certainly suggest that we could someday face a surprise encounter with a large, unseen object." The significance of the kilometer size? An impact of anything that large, scientists believe, would cause not just a regional but a global catastrophe.

Almost as worrisome are the estimated 300,000 asteroids larger than 300 ft. wide that also come perilously near or intersect Earth's orbit; each could inflict Tunguska-like damage over a large region. The number of Earth-crossing asteroids larger than 60 ft. across, says University of Arizona astronomer Tom Gehrels, could be as high as 100 million. A hit by any one of them could destroy a large city.

Gehrels heads Spacewatch, Scotti's astronomy group and one of two such teams dedicated exclusively to the discovery of threatening "near-Earth objects" (NEOs). The other group, called NEAT (for near-Earth asteroid tracking), is run by Helin and uses an Air Force telescope atop a mountain on Maui, Hawaii.

Strapped for funds--NASA contributes only $1.8 million annually to asteroid hunting--astronomers fear it will take decades to discover most of the larger objects. With only a few million more dollars a year, they say, and with access to the other two Air Force satellite-tracking telescopes, most of the kilometer-wide and larger asteroids could be identified and tracked within 10 years.

What if one or more of these asteroids are found to be a serious threat? Scientists generally agree on the best strategy for avoiding disaster: launch a rocket to intercept the intruder and, at the very least, change its orbit. If the asteroid is small and detected many years and orbits before its predicted impact, the solution would be straightforward. "You apply some modest impulse to the asteroid at its closest approach to the sun," says Los Alamos' Canavan. "The slight deflection that results will amplify during each orbit, ensuring that the asteroid misses Earth by a wide margin." That little push, he notes, could be provided by conventional high explosives.

For objects 300 ft. or larger and detected late in the game, however, nuclear weapons may well be the only answer. If XF11 had been discovered only 90 million miles away and on a beeline toward Earth, for example, the equivalent of a 1-megaton explosion would have been necessary to shove it into a safe orbit. Had it first been spotted at just a tenth of that distance, a 100-megaton blast would have been needed to turn it away.

If the incoming asteroid is composed largely of iron, a nearby or even a surface explosion would present no problem. But if the asteroid is rocky, a blast, particularly an ill-planned one, might well shatter it into chunks, each a potential danger to a terrestrial region or metropolitan area.

For that reason, Earth's defenders, if they have the luxury of time, would prefer to send a robot craft to rendezvous with a threatening asteroid and determine its composition and mechanical strength before dispatching a nuke to the scene. Physicist Edward Teller suggests that this is what we should do, just for practice, when XF11 passes far from Earth two years from now. Other defensive plans being bandied about at the Los Alamos and Lawrence Livermore national labs involve more exotic devices, such as neutron bombs or netlike arrays of interconnected tungsten balls.

Yet none of these defensive measures can be effective without adequate warning. And given the large numbers of undiscovered NEOs still out there, says David Morrison at NASA's Ames Research Center, an asteroid strike could take place with far less than a 30-year warning. Indeed, says Morrison ominously, "the most likely warning time would be zero."

--With reporting by Dan Cray/Los Angeles and Dick Thompson/Washington

With reporting by Dan Cray/Los Angeles and Dick Thompson/Washington