BY JUPITER, IT'S GALILEO!

By LEON JAROFF

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.

Galileo's encounter with Jupiter will culminate nearly two decades of work, planning, disappointment, elation and frustration for thousands of scientists and technicians at J.P.L. and at NASA's Ames Research Center in Mountain View, California, which oversees the probe. "It's been a long, hard fight, both technically and politically," says James Van Allen, the University of Iowa physicist best known for his 1958 discovery of the radiation belts that girdle Earth. Van Allen, who likens the story of the Galileo mission to The Perils of Pauline, ought to know. He headed the scientific study group that first recommended the Galileo mission to NASA in 1976 and has since seen the mission canceled and resuscitated twice.

NASA's original plan was to launch Galileo from the shuttle in 1982 on a direct, two-year flight to Jupiter. But disputes over the type of rocket most appropriate for the launch delayed the mission for four years. Then, after agreement was reached on the liquid-hydrogen-fueled Centaur rocket, the 1986 Challenger disaster not only shut down the shuttle program for nearly three years but also heightened awareness that the Centaur was too risky for a manned craft--in Van Allen's words, "like carrying a hydrogen bomb, except it's more likely to go off."

But opting for a safer, solid-fuel rocket had its drawbacks too; a rocket of this type small enough to fit into the shuttle's cargo bay would not yield enough power to thrust Galileo into a direct flight to Jupiter. "It took the project a while to invent a new way for Galileo to get to Jupiter," says J.P.L. director Edward Stone, "but there was a sense of 'It can be done.' "

AND IT WAS. THE SOLUTION WAS TO supplement the rocket's power with three "gravity assists," first from Venus, which Galileo skimmed around in a "crack-the-whip" maneuver that boosted its velocity and flung it back toward Earth, and then from Earth itself, which it swooped by twice, passing less than 200 miles from the ground before finally picking up sufficient speed to make it all the way to Jupiter.

Forced into a longer journey, the spacecraft made good use of its time. It shot pictures, calibrated its instruments, conducted scientific observations of Venus and Earth during its flybys and, among other achievements, confirmed the existence of a huge impact crater on the backside of the moon. Passing twice through the asteroid belt, it snapped the first closeup images of the asteroid Gaspra and discovered the first asteroidal moon, a tiny clump (later named Dactyl) orbiting the asteroid Ida. Then in July 1994 it shot pictures of the fragments of comet Shoemaker-Levy 9 plunging into Jupiter, capturing images of the far-side impacts not visible from Earth.

Along the way, however, Galileo suffered a serious setback. In 1991, when J.P.L. controllers attempted to deploy the spacecraft's main, 16-ft.-wide, umbrella-like antenna--which had been tucked away during the Venus encounter to protect it from solar radiation--three of the antenna's 18 ribs got stuck. Despite more than 13 months of ingenious and increasingly desperate measures to shake these ribs loose, the antenna, which had been capable of transmitting 134,400 digital bits per second (or a complete image in about a minute), remains unusable.

NASA's only alternative was to turn to a smaller, low-gain antenna that had been used to communicate during the Venus leg of Galileo's journey. But it was a poor substitute, able to transmit only eight to 16 bits per second. Wrestling with the problem, the Galileo team transmitted new, innovative software to the spacecraft, and made hardware and software changes in Earthbound receiving stations that, all told, will increase the data rate to 160 bits per second. That modest fix, and some clever software and hardware changes, say J.P.L. officials, should enable Galileo to achieve as much as 70% of its Jupiter scientific objectives.

Even that goal seemed in danger last October, when Galileo's tape recorder--now even more essential for storing data that cannot be quickly transmitted to Earth--suddenly got stuck in rewind. After a week of analysis, technicians concluded that the recorder may be unreliable under some conditions but prodded it into working again and seemed confident that they could nurse it safely through the mission.

As the spacecraft approached Jupiter last week, the Galileo team was growing increasingly nervous. "This is a very complex spacecraft," Van Allen noted, "and there are a tremendous amount of things that can go wrong. I'm still on the edge of my seat, and will be until the data start coming in. Then maybe I'll crack open a bottle of beer."

--Reported by David S. Jackson/Mountain View and J. Madeleine Nash/Chicago

With reporting by DAVID S. JACKSON/MOUNTAIN VIEW AND J. MADELEINE NASH/CHICAGO