PASADENA, Calif. -- Galileo will make its closest flyby of Jupiter's volcanic moon Io this week as controllers race to put it through ever riskier maneuvers before the aging spacecraft conks out.
Mission managers acknowledge the risks but say they want to make the most of what will likely be the spacecraft's final months. Galileo, circling the solar system's largest planet for more than four years, long ago exceeded expectations.
"We've done the job," said Jim Erickson, Galileo's project manager at NASA's Jet Propulsion Laboratory. "Now we're getting to see how much more we can milk it."
On Tuesday, the 2«-ton orbiter will fly within 124 miles of Io in a maneuver that will bombard the probe with immense radiation. Previous orbits near the fiery moon crippled Galileo's computer, though engineers managed to restart it each time.
Galileo is tentatively scheduled to zoom by the moon Ganymede in May and December. Discussions are under way about future plans, including a possible suicide plunge into Jupiter's atmosphere.
Nobody expects the $1.4 billion probe to last forever. It has been on an extended mission since completing its primary goals in 1997. Fuel for maneuvering is running low, navigation equipment is failing, and Galileo has encountered twice as much radiation as it was designed to withstand.
Engineers say the risks of extreme maneuvers are balanced by the potential science returns.
"The possibility that there is going to be a failure is always there," Erickson said. "We're way past warranty and we're incrementally pushing our luck, but that's a good thing to do."
In October, Galileo flew within 380 miles of Io, revealing what may be the most volcanically active body in the solar system. The spacecraft found more than 100 volcanoes, some of which spewed 2,700-degree lava and vented gases miles into space.
A month later, Galileo flew within 186 miles of the surface. Its camera captured lava spurting more than a mile high. Engineers were kept busy as the spacecraft's computer shut down hours before closest approach.
"With each flyby we get new and different observations," said Torrence Johnson, Galileo's project scientist. "This time, we expect to be able to observe the effects of the eruptions we saw in the October and November flybys."
Spying volcanoes is just one of Galileo's achievements. In January, during a flyby of the moon Europa, the spacecraft made what may be its most memorable finding: Magnetic field disturbances that strongly suggest a saltwater ocean exists beneath the icy crust.
It is the best evidence yet that an ocean similar to that of Earth exists elsewhere in the solar system -- and that raises the possibility of life.
Galileo also detected a magnetic field around the moon Callisto, which is now believed to have a molten core like Earth. Galileo also found, for the first time, that Jupiter has Earth-style thunderstorms in addition to the monstrous, swirling storms that Earth astronomers have seen through telescopes for centuries.
"There's kind of a food chain on Jupiter. The largest storms seem to sustain themselves by merging -- eating almost -- the smaller storms," said Andrew Ingersoll of the California Institute of Technology. "We never knew where the smaller storms got their energy, and now we do."
At first, nothing seemed to go right for the spacecraft, which is named after the Italian astronomer who first observed Jupiter's largest moons in 1610. Its launch, originally scheduled for 1986, was delayed for three years by the deadly explosion of the space shuttle Challenger.
Galileo finally was launched in 1989 from the space shuttle Atlantis. Instead of a direct, two-year voyage, it took a circuitous route using the gravity of Venus and Earth to slingshot to its destination. In all, it traveled 2.3 billion miles over six years, finally arriving in December 1995.
During the cruise, its 16-foot main antenna failed to fully deploy. Controllers used a less efficient antenna that returned data about 100 times more slowly than the main antenna.
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Source: NASA's Jet Propulsion Laboratory