"Io is the most volcanically active body in the solar system," says David Senske, a science coordinator for Galileo at NASA's Jet Propulsion Laboratory. "Its just popping off left and right."

Mission controllers hope to maneuver the spacecraft near a volcano as it erupts -- a difficult task since volcanic activity on Io changes rapidly. The spacecraft will pass Io at an altitude of 610 kilometers (378 miles) on October 10, and about 300 kilometers (186 miles) on November 26. Io's volcanoes can spew sulfur dioxide and other materials several hundred miles above the surface.

The Io campaign marks the latest -- and possibly last -- phase of Galileo's mission. The spacecraft has been orbiting Jupiter and its moons for nearly four years, and since December 1997 has been conducting an "extended mission" focused largely on the icy moon Europa. The extended mission lasts through the end of 1999, and it is not yet clear if funds will be available to keep Galileo in operation next year.

If the mission continues, Galileo may conduct joint observations with the Cassini spacecraft when the latter flies past Jupiter in December 2000 en route to Saturn. Both spacecraft would collect data on Jupiter's magnetosphere, a vast magnetic bubble surrounding the planet; their readings could then be compared. "This would be the first time that two spacecraft are in basically the same place at the same time," says Senske.

Besides financial constraints, the spacecraft faces considerable physical hazards. The Io flybys will place Galileo in proximity to Jupiter's powerful radiation environment, which can cause spacecraft faults. On August 12, as it flew past the moon Callisto en route to Io, the probe received an unexpectedly high dose of radiation, which disrupted its observations.