Forecast:

By LEON JAROFF

After the first hour they spent atop Arizona's Kitt Peak scanning the post- midnight skies, the observers knew that their ascent to an altitude of more than 6,000 ft. had not been in vain. They had counted 33 shooting stars, the advance guard of the annual Leonid meteor shower. But none of the University of Arizona students could anticipate the spectacle that was still to come. In the small hours of that Nov. 17 morning in 1966, the fiery meteors began streaking overhead in ever increasing numbers until, as one viewer reported in Sky & Telescope magazine, "the sky began raining shooting stars."

By 5 a.m., the shower had become an awesome storm, visible over large parts of the U.S. Southwest, brightening the sky like the grand finale of a fireworks display and causing many startled spectators instinctively to shield their face. Interspersed with occasional fireballs, the meteors reached an incredible peak rate of at least 40 per sec. before the bombardment began to wane. Some shooting stars continued to fall until their trails were obscured by the glare of the rising sun.

While more modest meteor showers, usually consisting of no more than a scattering of shooting stars, take place as often as 15 to 20 times each year over various parts of the globe, dramatic displays like the 1966 Leonids occur rarely, only a few times each century. But the next great meteor storm of the 20th century could occur this week -- if astronomers' hunches are right. Conditions seem ideal, they say, for the annual Perseid meteor shower to develop into a vivid display that should be visible in many parts of the northern hemisphere on the night of Aug. 11-12.

That is when the earth will be passing close to the orbit of the comet Swift-Tuttle, which reappeared last year for the first time since 1862, swooped around the sun on Dec. 12 and headed back toward the outer solar system. Like all other comets, Swift-Tuttle sheds debris consisting largely of conglomerations of ice and dust, most of it boiled from the comet when it is in the vicinity of the sun. This material remains in orbit and gradually disperses along the comet's entire path, in effect forming a giant debris- laden tube in space. Each August when the earth passes through that tube it encounters bits of debris, or meteoroids, which hurtle into the atmosphere at 130,000 m.p.h. and, then called meteors or shooting stars, are incinerated in streaks of light.

These meteors are known as Perseids because they appear to emanate from the constellation Perseus, just as the Leonids, cast-off material from another comet, appear to radiate from a point in Leo. While most of the cometary debris consists of small particles, each tiny piece traveling at such high speed packs a mighty wallop capable of inflicting severe damage on anything it encounters. Consequently, satellites orbiting above the protective atmosphere during a heavy meteor shower are vulnerable. With this danger in mind, NASA prudently postponed last week's scheduled launch of the shuttle Discovery, which otherwise would have been in orbit during the height of the meteor bombardment. Explained a NASA spokesman: "It's too uncertain to proceed." Astronomers, too, felt concern; they could do nothing to protect the orbiting Hubble Space Telescope from the hurtling Perseids.

Predictions of an unusually spectacular Perseid display are based on several convincing omens. Astronomer Brian Marsden of the Harvard-Smithsonian Center for Astrophysics has deduced the minimum distance between the orbit of the comet and the orbit of the earth on every Swift-Tuttle flyby between 69 B.C. and A.D. 3302, "and this time," he says, "it is closer than at any other time we've calculated: only 90,000 miles."

Another reason to look for celestial pyrotechnics is the proximity of Swift- Tuttle itself; it sailed closest to the earth only nine months ago on its 130-year round trip through the solar system. The material that boiled off Swift-Tuttle on the current loop around the sun is still clinging closely to the comet and hence is already far beyond the earth's orbit. But Marsden believes that the earth this week "will probably be passing through debris that was ejected in 1862, is still relatively close to the comet and hasn't had a chance to disperse very much. So the meteors should be more ; concentrated; there should be more of them."

For similar reasons, he suggests, the Perseids were impressive in the years around Swift-Tuttle's last flyby in 1862, even though the earth's and the comet's orbits came no closer than half a million miles at that time. Indeed, two University of London astronomers suggested last week that the 1994 Perseids might just outperform this year's shower.

Despite the favorable conditions in the heavens, a terrestrial factor, cloud cover, could ruin the show for many people (only air travelers will have seats above the clouds). And even if the skies are clear, Marsden and other astronomers are hedging their bets. Predicting meteor showers, they say, is on a par with weather forecasting. "The debris is spaced irregularly along the orbit," Marsden explains, "and that's where some uncertainty comes in." Another factor is the influence of Jupiter's gravity, which along with pressure from solar radiation unpredictably alters the orbit of the cometary debris.

Should the forecast be on the nose this year, Europe will get the best view; there, the meteors will seem to spew forth from a spot high in the sky beginning a few hours after midnight on Aug. 12 (about 9 p.m. Eastern daylight time, Aug. 11) and lasting from half an hour to an hour. The shower -- or storm -- will be preceded and followed by scattered shooting stars.

Across the Atlantic, the best viewing spots should be in the easternmost regions of the U.S. and Canada, where Perseids are expected to peak soon after dark. Stargazers should seek an unobstructed view of the northeastern horizon; the meteors' radiant point will appear low in the sky, to the left and below the easily recognizable irregular W formed by the five bright stars of the constellation Cassiopeia. Unfortunately for those in the Midwest and West, early-evening skies will still be too bright for viewing, although some rear- guard Perseids should be visible after dark. And if the Perseids show up early, as the University of London astronomers speculate, before dusk on the East Coast, no one in North America will see the storm.

Despite the Continent's apparently superior vantage point, Marsden says, "I wouldn't make plans to take a special trip to Europe to see the meteors." While the 1992 Perseid showers, visible over Europe, occurred precisely at the predicted time, he notes, the 1991 Perseids, seen in Japan, were inexplicably almost three hours late. Should the meteors be that tardy this year, they would find Europe in daylight but could make a spectacle of themselves in the night skies across the entire U.S. and Canada.