Meteor Alert

By LEON JAROFF

It was about midnight when the astronomy buffs gathered atop Arizona's 7,000-ft. Kitt Peak spotted the first shooting stars streaking across the cloudless night sky. Then, slowly, the glowing trails began to multiply. Twenty an hour, then 30 and 40, until at 5 a.m. the sky erupted in a furious but eerily silent meteor storm that brightened the sky like a pyrotechnic grand finale. Some of the spectators instinctively shielded their faces, startled by the sensation of hurtling headlong into a cloud of flashing debris. An hour--and some 140,000 meteors--later, it was largely over; the storm waned and finally disappeared in the morning light.

That memorable 1966 display of the so-called Leonid meteors was visible across much of the Western U.S. and marked the century's greatest meteor storm to date. Now, after 32 years of relatively modest return visits, the Leonids are poised to stage another celestial spectacular on the nights of Nov. 17 and Nov. 18. How spectacular? Scientists forecast heavy meteor showers and, just possibly, a full-blown storm as dramatic as the one 32 years ago.

During those 32 years, however, something has changed. None of the handful of satellites orbiting the globe in 1966 was hit by a Leonid. But today the planet is circled by a bewildering variety of spacecraft--about 600 in all--that have become indispensable to modern society: relaying phone calls, e-mail and faxes; monitoring hurricanes, terrorist activities and crop yields. A collision with a meteor could damage or disable any one of them. That is why NASA, the Air Force and the Russian space agency are directing a wholesale reorientation of their fleets of orbiting spacecraft.

The Leonids, so named because they seem to radiate from the constellation Leo, are actually debris shed by comet Tempel-Tuttle. In an elongated, 33-year orbit of the sun, the comet travels as far out as Uranus, then back to within 91 million miles of the solar surface, passing close to Earth's orbit on both its way in and its way out. Like other comets, Tempel-Tuttle is, in effect, a dirty snowball that heats up as it approaches the sun and boils off some of its "dirt," which consists largely of particles, some pea size, a few the size of baseballs but most no larger than a grain of sand.

These particles, called meteoroids, remain in orbit and gradually disperse along the comet's orbital path, forming a giant, debris-laden stream in space. In its yearly travels around the sun, Earth intersects with or comes close to that stream every November, and sightings of the Leonids have been recorded in texts as far back as A.D. 902. The speeding meteoroids hurtle into Earth's atmosphere, are heated by friction and become blazing meteors that are incinerated in midair.

Not all Leonid showers are alike, however. As Donald Yeomans, an astronomer at the Jet Propulsion Laboratory, notes, "Most of the particles are following closely behind the parent comet. That's where the gathering is thickest, and it's only when the comet is in Earth's neighborhood that we get intense showers or storms."

Indeed, that is the situation this month. Tempel-Tuttle recently swept past Earth, swinging around the sun in February, and headed back toward the outer solar system. As a result, Earth will come within 700,000 miles of the center of the stream--a close shave by astronomical standards. And because Tempel-Tuttle orbits the sun in the opposite direction of Earth, the meteoroids will hurtle in at a closing speed of some 160,000 m.p.h.

At that velocity, says Guenter Riegler, a NASA senior scientist, a meteoroid as small as a dust particle could blast a hole nearly half an inch across in a solar panel or a layer of insulation. Equally threatening is the intense heat of impact, which would instantly vaporize the meteoroid and convert it to an ionized gas, or plasma, that would shock the spacecraft with an electrostatic charge. "If that charge got into some of your data circuitry," Riegler says, "it could wipe out data."

NASA is taking no chances. It will power down any threatened spacecraft to avoid short circuits and will temporarily orient each one, says Riegler, "so that its strongest side faces the incoming Leonids." Even the Hubble Space Telescope will turn its back to the meteoroids, to shield the aperture through which it scans the heavens. And the flat solar panels that energize most of the satellites will be turned edge on to the Leonid stream to minimize the possibility of impact.

Even though scientists expect this month's peak display of Leonids to occur over China, Japan and Southeast Asia--during daylight hours in the U.S.--J.P.L.'s Yeomans suggests that Americans who are curious should scan the early morning skies on both Nov. 17 and Nov. 18. They will certainly see some meteors, he says, and the vagaries of the meteor stream may just present them with a good show. Anyway, he says, celestial circumstances make it unlikely the Leonids will perform much in the next 100 years or so. "Do it now or next November," he urges, "or write it off for the next century."