Galileo Set to Rendezvous With Jupiter and Moons

After nearly 20 years of planning, construction, delays, and a dramatically altered flight plan, it's payback time for the nearly 50 researchers involved in the Galileo mission to Jupiter.

Shortly before 3 p.m. PST on Thursday, 746 pounds of human ingenuity will slam into Jupiter's atmosphere. For 75 minutes, this probe will relay information about the atmosphere to its mother ship high above. Then, as it plunges ever deeper into the planet, climbing temperatures and pressure will vaporize the probe until it becomes part of the planet itself.

Once the probe finishes its work, the Galileo orbiter will begin a two-year, 11-orbit study of Jupiter, the solar system's largest planet, and its moons. It will get close-up looks at Ganymede, Europa, and Callisto, and longer-range looks at Io and several other moons.

Galileo represents ''the most complex spacecraft we've ever sent to the outer planets,'' says Jay Bergstralh, Galileo program scientist at the National Aeronautics and Space Administration's headquarters in Washington, D.C. It also is the second to the last of its generation of flagship planetary missions. The final spacecraft in this grouping is the Cassini mission to Saturn, scheduled for launch next October.

As the critical day approaches, the scientists involved are mindful of the $1.4 billion mission's stakes. Given the technical hitches that already have occurred on Galileo, and mindful of the loss of the Mars Observer just as it was to enter orbit around the Red Planet in August 1993, they remain wary. ''The big thing that one fears is failure,'' concedes Alvin Seiff, the principal investigator on the probe's atmospheric structure instrument. ''We've had good success but we're not home yet. We stand to learn so much that it sometimes seems worthwhile taking a risk.''

The Galileo project's origin dates back to 1976, when a committee led by James Van Allen proposed the probe-orbiter combination. Scientists' curiosity had been piqued by the Pioneer 10 and 11 missions that returned the first close-up images of Jupiter. Meanwhile, the Voyager project had begun, which would send two spacecraft past Jupiter on their way to the ends of the solar system. Observations from the two would raise even more questions about the planet and its moons that only a long-term dedicated mission could help answer. Once approved, work began on the project in 1977. Galileo was scheduled for launch from the space shuttle in January 1982.

The project's complexity soon became apparent. ''There were well over 100 major design changes,'' Dr. Bergstralh says, centered around the question of ''what we were going to use to launch.''

The mission plan originally called for a direct flight to Jupiter, which would have taken about 2-1/2 years. Designers settled on a liquid-fuel upper stage that would ignite after the craft was released from the shuttle's cargo bay. Launch was reset for March 1986. But in January that year, the shuttle Challenger exploded 70 seconds into its launch, killing the seven astronauts on board. The fleet was grounded indefinitely, and the liquid-fuel upper stage on Galileo was determined to be too dangerous to carry aboard a shuttle. Galileo planners were forced to use a solid-fuel upper stage that was too weak for a direct flight. They realized, however, that what the upper stage couldn't supply, gravity could. By boosting the spacecraft's energy with the help of a Venus flyby and two Earth flybys, Jupiter was within reach - albeit in six years and 2.3 billion miles.

During that extra time en route, Galileo reaped scientific dividends. In a fitting parallel with its namesake, the Italian astronomer who discovered moons orbiting Jupiter and so altered humanity's notions about its place in the universe, the spacecraft discovered a tiny asteroid, Dactyl, orbiting a larger asteroid, Ida. That has been Galileo's ''most spectacular discovery'' to date, Bergstralh says. ''The idea of satellite asteroids has been kicking around for about 20 years, but it's been controversial.'' Galileo settled the question.

In addition, the craft with its unique array of instruments has yielded a new understanding of the cloud structure on Venus. And it has given astronomers a more detailed analysis of the composition and distribution of minerals on Earth's moon. This will help lay groundwork for a new robotic mission to the moon, Lunar Prospector.

The trip hasn't been trouble free, however. In April 1990, controllers signaled the spacecraft to unfurl its high-gain antenna, which was to be used for long-distance communication to Earth. The antenna jammed, rendering it useless. In addition, in October, the craft's data recorder jammed. Although the problem was fixed, the amount of available tape has been reduced. In all, mission planners expect that Galileo can complete at least 70 percent of its scientific objectives.

As for the atmospheric probe, researchers are striving for a 100 percent return. The probe, with its seven instruments, is designed to answer a number of questions, says Richard Young, probe project scientist the NASA Ames Research Center. ''One of the main reasons we want to go to Jupiter is to understand its composition. We already know that Jupiter has close to the same composition as the sun - mostly made up of hydrogen and helium. But it's the abundance of key trace elements, such as carbon, sulfur, nitrogen, and oxygen, and rare gases such as xenon, argon, krypton, and their isotopic ratios that give us important clues as to how the planets formed and what the early composition was of the solar nebula, the gas and dust cloud from which all planets formed.''

Another item of interest is the ratio of helium to hydrogen in Jupiter. ''We know that on Jupiter the helium-to-hydrogen ratio is less than it is on the sun,'' Dr. Young explains. ''We know that on Saturn the helium-to-hydrogen ratio is even less than it is on Jupiter. So by understanding the helium-to-hydrogen ratio we have some idea as to what sort of evolutionary processes are taking place in the giant planets.''

Moreover, researchers want a better understanding of the atmosphere's structure - how temperature and pressure vary with depth.

Another point of interest: Unlike Mercury, Venus, Earth, and Mars, which radiate back into space about as much energy as they receive from the sun, Jupiter, Saturn, and Neptune radiate twice as much energy as they receive. ''That means they have an internal heat source,'' Young says. ''We want to understand how that heat is deposited through the atmosphere. Also we want to determine how the sunlight is deposited in the atmosphere.''

Important clues to that internal heat source lie in the banded wind structure on the planet. At the equator, the winds blow at upwards of 250 miles per hour in the direction of the plant's rotation. Roughly 10 degrees north and south of the equator, the winds reverse direction, while maintaining a speed of 150 to 200 m.p.h. The wind bands reverse direction several more times at they approach the poles. ''One of the key science objectives of the probe is to understand that wind structure,'' Young says. The orbiter will track the probe to see how deep into the atmosphere the winds extend.

Beyond information on the planet itself, Jupiter could yield clues about the evolution of the galaxy. ''Jupiter has given us a great gift,'' says Tobias Owen, a professor at the University of Hawaii's Institute of Astronomy. Because the planet never became massive enough to ignite and become a star, ''there has been no nuclear chemistry there in 4.5 billion years. Whatever was there was trapped and has not been modified.''

When the 2-1/2-ton Galileo orbiter has finished monitoring the probe, it embarks on its study of the planet's environment and moons, ''very interesting worlds in themselves,'' says Bergstralh.

One opportunity likely to be missed is a close swing by the moon Io, the solar system's most geologically active body. It might still be possible should Galileo be functioning well at the end of its tour.