A COMET on a collision course with Jupiter is behind the Sun now, out of sight but not out of the minds of astronomers, who are busy plotting its trajectory, estimating the potential power of its impacting force and planning ways of observing the reverberating effects with an array of telescopes on Earth and in space.

The astronomers are eagerly anticipating their first opportunity next July to observe explosive impacts on a planet that could be comparable in strength to the collision of a large asteroid or comet with Earth 65 million years ago, the catastrophe implicated in the mass extinctions of dinosaurs and many other species of life.

If their predictions are on the mark, a rain of cometary debris plowing into Jupiter could blow holes the size of Texas in the thick atmosphere, send out titanic shock waves, kick up showers of dust and radiate flashes of light out among the Jovian moons. The astronomers' one regret is that all the impacts are predicted to occur on the far side of Jupiter, and so may be directly observable only by the Galileo spacecraft, then nearing a position to peek round Jupiter, and the Voyager 2, which is now reaching the farthest edges of the solar system. Other spacecraft and ground-based telescopes, though, are expected to get good views of the turbulent aftereffects, as the swiftly rotating Jupiter (once every 9 hours 50 minutes) turns its battered side toward Earth.

In even the most cautious predictions, Dr. Zdenek Sekanina, an astrophysicist at the Jet Propulsion Laboratory in Pasadena, Calif., said, it is almost certain that "impacts of the largest fragments would generate a spectacular show for an observer located at the right place at the right time."

Once before, more than a year ago, the comet came so close to Jupiter that the huge planet's gravity tore it into a multitude of pieces. The fragmented comet was first sighted last March and given the name Shoemaker-Levy 9, after its discoverers. Its pieces are scattered across 100,000 miles of space, knots of dust glistening in sunlight and surrounding solid chunks of icy material. It reminds astronomers of a string of pearls.

In the most detailed picture of the comet so far, taken by the orbiting Hubble Space Telescope this summer and made public yesterday, scientists have identified at least 20 pearls in that string. Each nucleus, though largely obscured by dust, appears to be no bigger than three miles wide. In one bright dusty knot, the Hubble picture reveals four fragments close together in celestial terms, two of the pieces being separated by only 700 miles.

An analysis of the comet's behavior and trajectory has led Dr. Jay Melosh and Dr. James Scotti of the University of Arizona to predict that the fragments should reach Jupiter one by one over a six-day period centered around July 21. As the comet gets closer, its many parts are expected to be more widely scattered, extending along a million-mile line.

The prospects for Shoemaker-Levy's encounter with Jupiter were described in interviews last week, in an analysis by Dr. Sekanina in the current issue of the journal Science and in reports yesterday in Boulder, Colo., at the annual meeting of the Division for Planetary Sciences of the American Astronomical Society.

Since the sizes of the cometary nuclei appear to be about one-third that of previous estimates, Dr. Harold Weaver, an astrophysicist at the Space Telescope Science Institute in Baltimore, concluded, their plunges into the dense hydrogen atmosphere of Jupiter should unleash only one-tenth to one-100th as much energy as had been estimated. Still it could be quite a show of pyrotechnics.

Analyzing the Hubble data, Dr. Weaver and his colleagues figured that the total energy of the multiple collisions could be equivalent to 100 megatons of TNT, or 10,000 times the total destructive power of the world's nuclear weapons at the peak of the cold war. As the objects streak through Jupiter's multicolored cloud tops at 40 miles a second, they should become blazing fireballs, gigantic versions of meteors capable of blowing tremendous holes in the dense clouds.

Jupiter, the largest planet in the solar system, with a diameter 11 times that of Earth, does not have a surface like Earth's. It is enveloped in clouds of hydrogen and some helium driven by winds into swirling bands of many hues, a kaleidoscope of reds, browns, yellows and white. At depths of a few thousand miles, the pressures become so great that the hydrogen changes from a gas to a liquid, and then, toward the core, into a metallic state.

Scientists are not sure what would happen as the cometary fragments penetrated Jupiter's atmosphere. Some, like Dr. Sekanina, think the fragments will explode when they reach depths where the atmospheric pressure is as great as Earth's at sea level. In 1908, a 160-foot-wide cometary nucleus or stony meteor apparently disintegrated over Siberia at an altitude of five miles. The explosion leveled hundreds of thousands of acres of forest.

Other scientists speculate that the fragments might penetrate much deeper into Jupiter, if they should create a shock front that acts as an insulating shield against some of the frictional heat. In any case, the objects could flash brightly as they disintegrate in the Jovian clouds, possibly providing striking backlighting to at least two of Jupiter's four largest moons. Uncertainty on What to Expect