A SHOT ACROSS THE EARTH'S BOW

By LEON JAROFF

At the University of Arizona's Steward Observatory, Timothy Spahr, 26, peered through a stereoscopic microscope, shook his head and looked again. In the combined image of two telescopic photos he had shot 30 minutes apart a few nights earlier, a bright dot with a small tail stood out starkly against the background of fixed stars. "I was extremely excited, heart pounding and all that stuff," says Spahr, a graduate student from the University of Florida who was surveying the skies for undiscovered asteroids. He immediately shot and developed a second set of photos, and was shocked to see that in just two days the dot had doubled in size and speed. To Spahr, it seemed apparent that a large asteroid was barreling toward Earth.

"My own feelings went from disbelief to excitement to downright fear," says Spahr's survey partner, Carl Hergenrother, 23, an Arizona undergraduate who verified the find with a 90-in. telescope atop nearby Kitt Peak. "It was scary, because there was the possibility that we were confirming the demise of some city somewhere, or some state or small country."

Well, not quite. Early last week, four days after Spahr spotted it, his celestial interloper whizzed by Earth, missing the planet by 280,000 miles--a hairbreadth in astronomical terms. Perhaps a third of a mile across, it was the largest object ever observed to pass that close to Earth.

Duncan Steel, an Australian astronomer, has calculated that if the asteroid had struck Earth, it would have hit at some 58,000 m.p.h. The resulting explosion, scientists estimate, would have been in the 3,000-to-12,000-megaton range. That, says astronomer Eugene Shoemaker, a pioneer asteroid and comet hunter, "is like taking all of the U.S. and Soviet nuclear weapons, putting them in one pile and blowing them all up."

For many astronomers, last week's near-miss of asteroid 1996JA1, as it has been officially designated, was a kind of warning shot across Earth's bow. They have been trying to convince the world--with only modest success--that asteroids like this one represent a clear and present danger. To meet that threat, they have proposed a network of computer-monitored telescopes (equipped with sensitive electronic cameras) that would seek out threatening asteroids or comets in plenty of time to fend them off with appropriately designed long-range missiles.

As astronomer Steel dryly noted in an E-mail message sent to colleagues around the world, "It might be a useful point to make that this object was discovered only...days before closest approach, so that if it had been on a collision course with Earth, we would not have had time to do anything much about it."

It's also worth pointing out that 1996JA1 is hardly unique. Neighboring space teems with many more so-called Near Earth Objects, asteroids and comets with orbits that pass close to Earth's path around the sun. More than 100 NEOS big enough to cause the kind of worldwide disaster that wiped out the dinosaurs (six-tenths of a mile across or larger) have already been identified and charted. But Eleanor Helin, an astronomer at the Jet Propulsion Laboratory in Pasadena, California, notes that an estimated 2,000 more of these mountain-size hulks may be lurking undetected out there, to say nothing of a few hundred thousand smaller but still worrisome boulder-size objects.

By identifying most of these asteroids and plotting their orbits, astronomers insist, they could predict many years in advance if any were destined to strike Earth. Most of the large ones, they say, could be cataloged within a decade, given the necessary funding. But budgets are tight, congressional support is lacking, and NASA has opted to continue only its relatively modest $1 million annual support for three independent groups with telescopes specifically dedicated to NEO hunting. These teams are headed by Tom Gehrels at the University of Arizona, Helin at J.P.L. and Shoemaker at the Lowell Observatory in Flagstaff, Arizona.

Even with NASA's support, the NEO hunters have had to scrounge from industry and other donors for much of their equipment. "Times have been very lean in this past year," says Helin. "There've been times when I was receiving no salary at all." Despite the travails, however, Shoemaker is confident that improving technology will enable astronomers, even at the current level of funding, to identify perhaps 50% of the NEOs in the next decade.

And what if one of them is found to be on a collision course with Earth? Scientists at the Livermore and Los Alamos national laboratories have devised a number of ingenious plans that, given enough warning time, could protect Earth from a threatening NEO. Their defensive weapons of choice include long-distance missiles with conventional or, more likely, nuclear warheads that could be used either to nudge an asteroid into a safe orbit or blast it to smithereens.

Many people--including some astronomers--are understandably nervous about putting a standby squadron of nuclear-tipped missiles in place. Hence the latest strategy, which in some cases would obviate the need for a nuclear defense: propelling a fusillade of cannonball-size steel spheres at an approaching asteroid. In a high-velocity encounter with a speeding NEO, explains Gregory Canavan, a senior scientist at Los Alamos, "the kinetic energy of the balls would change into heat energy and blow the thing apart."

Some astronomers oppose any immediate defensive preparations, citing the high costs and low odds of a large object's striking Earth in the coming decades. But at the very least, Shoemaker contends, NEO detection should be accelerated. "There's this thing called the 'giggle factor' in Congress," he says. "People in Congress and also at the top level in NASA still don't take it seriously. But we should move ahead. It's a matter of prudence."

The world, however, still seems largely unconcerned with the danger posed by large bodies hurtling in from space, despite the spectacle two years ago of Comet Shoemaker-Levy 9 riddling the planet Jupiter with mammoth explosions. It remains to be seen whether last week's record near-miss has changed any minds.

--With reporting by Dan Cray/Pasadena

With reporting by Dan Cray/Pasadena