Galileo's Long Goodbye

As NASA temporarily grounds its shuttle fleet following the Columbia disaster, an unmanned spacecraft that has been exploring the solar system for 14 years is nearing the end of its mission — and still revealing the secrets of a planet hundreds of millions of miles from Earth.

It took the Galileo probe five years to reach Jupiter. It photographed asteroids, and the stormy red spot, and then turned its cameras on the giant planet's moons, each a world in its own right.

The photos and measurements sent back to Earth have provided tantalizing clues suggesting three of those moons — Callisto, Europa and Ganymede — have liquid oceans, an idea that has fueled the imagination of scientists who speculate the salty water could harbor extraterrestrial life.

The probe also caught Io, the most volcanically active body in the solar system, in the act as it belched lava and plumes of dust mixed with gas.

Like many extended journeys, Galileo's will end soon with countless miles traveled, its fair share of breakdowns and album after album stuffed with snapshots.

At the end of this month, mission operations at NASA's Jet Propulsion Laboratory for the craft will end. After that, scientists will check on it once a month until September, when it is directed to crash into Jupiter.

Galileo could be allowed to simply remain in orbit, but scientists feared it might collide with Europa and contaminate that body with microbes from Earth, possibly damaging its environment.

The $1.5 billion mission met 70 percent of its science objectives and made a number of serendipitous discoveries along the way — despite a range of problems.

"There was always something to fix. It kept me busy," said engineer Greg Levanas, Galileo's full-time troubleshooter.

The biggest challenge was the failure of its umbrella-like main antenna, which did not unfurl as planned two years after launch. That forced NASA to rely on a smaller antenna, severely squeezing the amount of data Galileo could beam back. The spacecraft still relayed a wealth of data about the planet and moons previously imaged in the 1970s by Pioneer 10 and 11 and Voyager 1 and 2.

Galileo also was battered by the intense radiation found close to Jupiter.

The spacecraft — itself powered by radioactive plutonium — withstood four times the cumulative dose of harmful radiation it was designed to take. But the bombardment took its toll, causing technical hiccups and occasional losses of data. In December, it zapped the spacecraft's reel-to-reel tape recorder, though engineers were able to fix it.

From the beginning, it seemed the mission was constantly overcoming adversity.

Congress first approved the project in 1977, but it took another dozen years to get the spacecraft — named for Galileo Galilei, who discovered Jupiter's four largest moons in 1610 — to the launch pad. It was waylaid in part by the 1986 Challenger disaster that grounded NASA's space shuttle fleet.

In 1989, the more than 20-foot spacecraft finally blasted into space aboard shuttle Atlantis and began its 2.3 billion-mile trip.

Along the way, it took the first close-up images of an asteroid. It topped the feat two years later when it spied a tiny moon in orbit around another space rock, another first.

As Galileo approached Jupiter in 1994, it used its unique vantage point to photograph fragments of the comet Shoemaker-Levy 9 as they smashed into the atmosphere, producing brilliant flashes of light on the planet's far side that would have otherwise remained unseen.

Close to Jupiter, Galileo released a probe that hit the planet's atmosphere at more than 106,000 mph. The probe worked for nearly an hour, transmitting data about the planet's chemical composition, winds, clouds and natural radiation environment.

At its height, 300 people worked on the mission at JPL. Now just two dozen do. The end is slated for Sept. 21, when Galileo is expected to plunge into Jupiter's stormy atmosphere after completing its 35th loop around the planet.

NASA recently announced plans for the Jupiter Icy Moons Orbiter, a nuclear spacecraft that would further reconnoiter Callisto, Europa and Ganymede. It won't launch before 2011 but should pave the way for the further robotic exploration of the Jupiter system, NASA said.

The only other spacecraft in the neighborhood of Jupiter is Cassini, on track to arrive at Saturn next year.

Given the risks of human spaceflight, made clear by the Columbia disaster, planetary exploration will remain the province of such robot explorers for decades to come, experts said.

Other than Mars, the rest of the solar system is too distant or too inhospitable for astronauts to explore.

"It's a big, tall order," said Louis Friedman, executive director of The Planetary Society, a space exploration advocacy group. "I would say it's not on the horizon."

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