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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.