NASA's durable Galileo spacecraft will skim close to the south pole of Jupiter's moon Io this week in search of new insight about that moon's volcanic surface and molten interior.

The flyby at 0123 GMT on Tuesday (9:23 p.m. EDT Monday), is taking the Jupiter-orbiting spacecraft back inside the hazardous environment of Jupiter's intense radiation belts. Io is the innermost of the giant planet's four major moons.

An engine burn to fine-tune the trajectory on Saturday was planned to send Galileo about 181 kilometers (112 miles) above Io's surface. This would be Galileo's closest approach to Io so far. The spacecraft has made five previous swings near Io since it reached Jupiter's neighborhood in 1995.

"Io is always changing, so we're eager to learn what Galileo might show us this time," said Dr. Eilene Theilig, project manager for Galileo at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Maybe it will surprise us as much as it did in the last flyby." When Galileo flew near Io's north pole in early August, scientists were watching for a gassy plume that a volcano named Tvashtar had been spraying seven months earlier. Instead, Galileo found an even taller eruption from a site where there hadn't even been a volcano before, she said.

The polar routes were chosen to position the spacecraft for magnetic field measurements that can provide hints about the moon's interior. The flight path will also provide good opportunities for studying several of Io's interesting volcanic features, including a recently discovered hot spot in the far south and Loki, the most powerful volcano in the solar system. While the spacecraft travels through the inner portion of the Jupiter system in the days surrounding the Io flyby, scientists will also be using Galileo's instruments to examine changes and details in clouds on Jupiter itself and to measure the radiation belts created by Jupiter's magnetic field.

Thursday, the Galileo team sent the spacecraft its detailed instructions for the encounter. The sequence of commands was transmitted from JPL's Deep Space Network facility near Madrid, Spain, one of three network sites worldwide with dish antennas 70 meters (230 feet) across that are used for communicating with Galileo.

Earlier, engineers sent new software to the spacecraft's camera, designed to lessen chances for a repeat of radiation-induced malfunctioning that has affected the camera on several occasions since the middle of last year.

Electronic components in the camera and elsewhere in the spacecraft have been degraded by repeated exposure to energetic-particle radiation near Jupiter. Galileo has endured more than three times the cumulative dose of radiation it was designed to tolerate. It has performed in orbit nearly three times as long as its original two-year mission.

Galileo is running low on the propellant it uses both for tweaking its trajectory and for adjusting its orientation to point its antenna. After one last flyby of Io in January 2002, the spacecraft will be on a trajectory that will take it through Jupiter's inner radiation belt and near the small inner moon Amalthea in November 2002, then out for one more long loop ending with a plunge into the crushing pressure of Jupiter's atmosphere in September 2003.

JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.