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. Controllers are analyzing data on the shutdown to determine the best way to reawaken the craft and download the information Galileo has stored from the flyby.
Failure to recover the data would be disappointing.
"You find me not only sitting and staring at my computer" analyzing Galileo data, "but biting my nails," says Margaret Kivelson, a space physicist at the University of California at Los Angeles, whose magnetic-field measurements are set to continue during Galileo's final orbit.
Yet if the orbiter never gathered another bit of information, it would still be deemed an overwhelming success, researchers say. The mission already has been extended three times beyond its nominal December 1997 end - a testament to the craft's design and operation, despite technical glitches along the way.
When Galileo arrived, it dropped a probe into Jupiter's atmosphere. Atmospheric data from the probe and subsequent information from the orbiter "have told us a lot about the formation of Jupiter," says Torrence Johnson, project scientist for the mission at JPL.
Images of Jupiter's moon Europa revealed a geologically young surface of ice replete with long cracks and ridges similar to those generated when ice packs shift on the ocean's surface in Earth's polar regions. But images yielded little evidence that a currently active ocean might lie beneath the ice, Dr. Greeley says.
The smoking gun came two years ago, when Dr. Kivelson's team completed their analysis of magnetic measurements taken at Europa. These suggested that an ocean of salty water, or perhaps salt-water slush, is circulating beneath the ice. Similar measurements revealed evidence of oceans beneath the icy sheaths covering the moons Callisto and Ganymede. Because water is one of the pillars supporting the emergence of organic life, and water on Europa appears to be relatively close to the surface, the moon has become a prime candidate in the search for life beyond Earth.
Galileo's final orbit also brings its share of poignancy. One of NASA's longest-running missions, Galileo has become "a family thing," Dr. Johnson says. "It's easy to anthropomorphize these things, but in reality Galileo is really us out there. We've sent an extension of our senses to these places," says Johnson. "Now let's go out and do the next one."