Galileo Set to Rendezvous with Jupiter and Moons Mission Will Mark the First Time a Space Probe Has Entered the Giant Planet's Mysterious Atmosphere

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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. …