BY JUPITER, IT'S GALILEO!

ON A STARRY NIGHT IN 1610, IN THE city of Padua, Italy, Galileo Galilei pointed his newly handmade telescope skyward, stared into the eyepiece and gasped in excitement. Through the lenses of the world's first astronomical telescope, four white spots were clearly visible floating near a brightly shining planet. Galileo had discovered Jupiter's four major moons, the first (except for Earth's own moon) ever seen around a planet.

This week, after a six-year, 2.3 billion-mile odyssey, a 2 1/2-ton, instrument-crammed spacecraft named after the Italian astronomer will hurtle past two of those moons, Europa and Io, then swing into orbit around Jupiter. There, if all goes well, it will conduct the most thorough study ever of the solar system's largest planet and its swarm of moons (Jupiter is known to have at least 16).

"In many ways, Jupiter is like a miniature solar system," says Wesley Huntress, a NASA space science administrator. "The Galileo mission should uncover new clues about how the sun and the planets formed and how they continue to interact and evolve."

As Galileo goes into orbit, a probe that it will have released 147 days earlier will plunge into the upper Jovian atmosphere at 106,000 m.p.h., its heat shield glowing. Two minutes later, after friction has slowed its descent, the probe will deploy a parachute at around 400 m.p.h. and drift downward, sniffing at gases, measuring temperatures and pressures, observing cloud structures and lightning and transmitting data back to its mother ship. Finally, about an hour into its descent, the probe will be vaporized by the steadily increasing temperatures it encounters below the dense clouds. Its fate, says a NASA official, will be to "join the atmosphere it came to observe."

For scientists at NASA's Jet Propulsion Laboratory (J.P.L.) in Pasadena, California, where the Galileo probe was largely designed and built, the moment of highest drama during the Dec. 7 Jupiter encounter will occur at 3:04 p.m. (P.S.T.). At that instant, a signal that will have been sent from the spacecraft 52 minutes earlier will arrive at J.P.L., having traveled 600 million miles at the speed of light. "A positive signal means the probe has survived the most difficult entry ever and is transmitting to Galileo," explains William O'Neil, the Galileo project manager. "That pretty much says it all."

After storing the probe's transmitted data in its tape recorder, Galileo will begin its tour of the Jovian system. In a route that will take it into 11 far-ranging orbits during the next 23 months, it will swoop as close as 160 miles above three of Jupiter's four major moons, Europa, Callisto and Ganymede, flying by each of them several times. On these passes--hundreds of times closer than those achieved by Voyagers 1 and 2 in 1979--it will shoot pictures and, with remote sensing instruments, analyze the chemical composition of the moons. In the course of its many orbits, Galileo will also investigate Jupiter's fourth major moon, the volcanically active Io, but only at a safe distance; the moon lies within an intense radiation belt that could endanger the spacecraft's electronics systems.