Despite intense radiation near Io, the spacecraft completed all its planned activities during the flyby at 6:32 a.m. Pacific Standard Time on Tuesday, Feb. 22, at an altitude of 198 kilometers (124 miles).

Data gathered during the flyby include observations designed to study changes in Io's volcanoes since Galileo's previous flybys of Io in October and November of 1999. There was also a radio science experiment performed while Jupiter was partly blocking the radio path from the spacecraft to Earth. By studying distortions in radio signals in these situations, scientists learn more about Jupiter's atmosphere.

While Galileo was approaching Io, radiation did apparently trigger two computer resets, but previously-installed onboard software in essence told the spacecraft they were "false alarms," and the flyby continued unaffected. The resets occurred on Feb. 22 at approximately 1:38 a.m. PST, and again sometime between 5:30 and 6:30 p.m.

Nearly two days after the close flyby, a third reset occurred on the spacecraft at 4:45 a.m. PST on Thursday, Feb. 24. This put the spacecraft in "safing," or standby mode, which temporarily stopped all non-essential operations until further commands were received from Earth. Normally, that reset would have been handled by the same onboard software that took care of the first two resets. However, the third reset happened when the spacecraft had already completed its Io flyby and had begun playing back data from its onboard tape recorder. During playback, the software that would prevent safing is disabled. Once the flight team diagnosed the problem, normal operations were restored on the spacecraft later in the day, at 9:30 p.m. Playback of the Io data will resume on Saturday, Feb. 26.

Galileo engineers were somewhat surprised that this third computer reset happened well after the Io flyby, when the spacecraft was quite a distance away from Jupiter (29 Jupiter radii, which is 2.1 million kilometers or 1.3 million miles) and therefore not as close to the most intense radiation. This served as another reminder of the powerful effects of natural radiation in space. Galileo has already survived more than twice the radiation it was designed to withstand, and its experiences will help mission planners design future spacecraft headed for high-radiation environments.

Galileo was launched from the Space Shuttle Atlantis in 1989 and arrived in orbit around Jupiter in December 1995 for a two-year study of the huge planet, its moons, and its magnetic environment. That primary mission was completed successfully, and was followed by a two-year extension, which ended last month. Galileo is now embarking on another extension, called the Galileo Millennium Mission.