For Galileo, One Last Voyage of Discovery ; Scientists Hope to Retrieve More Data from a Craft That Has Already Left an Indelible Mark on Planetary Research

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For nearly seven years, a robotic explorer named Galileo has stunned scientists with unprecedented views of the solar system's largest planet, Jupiter, and its moons.

This week, like a marathon runner crossing the finish line, the orbiter whipped past its final target. Now the spacecraft begins its victory lap - a final circuit of Jupiter that ends next September, when the probe will plunge into the crushing depths of Jupiter's atmosphere.

Launched in 1989, Galileo has yielded new insights about Earth's moon, uncovered a tiny moon orbiting an asteroid, revealed the Jovian moon Europa as the likeliest candidate in the solar system for extant life beyond Earth, and increased understanding about giant planets discovered around nearby stars.

Data gleaned during the past seven years on the interaction of the Jupiter's high rotation rate and its magnetic field is expected to open a window on the mechanisms that drive pulsars - rapidly spinning, dense remnants of giant stars that emit pulsed radio signals.

Galileo's indelible mark on planetary research also appears throughout a blueprint for solar-system exploration unrolled in July by the National Research Council. Several of the council's proposed planetary missions for the next decade - including a Europa orbiter and a lunar sample-return mission - trace their roots to Galileo's discoveries.

"It's been an incredible ride," says Ronald Greeley, a planetary geologist at Arizona State University, in Tempe, Ariz., and a member of Galileo's imaging team.

On Tuesday, the craft whipped past Amalthea, a russet potato of a moon roughly 83 miles long, orbiting 181,300 miles from Jupiter. The pass brought the craft to within 99 miles of the moon's surface, and an hour later, to within 44,500 miles of Jupiter's cloud tops.

"We got some good observations on this encounter," says project manager Eilene Theilig, referring to measurements that should help scientists determine the moon's composition, data on the radiation fields, and information about a gossamer ring of dust around the planet.

Yet the orbiter's path ensured that it would be bathed in the planet's intense radiation fields. Exposure to radiation levels 100 times as great as the dose deemed lethal to humans prompted the craft to enter its "safe mode." Nonessential activities have ceased until new instructions arrive from ground controllers at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. …