Four hours before Galileo was scheduled to pass 186 miles above Io on Thursday, radiation triggered a fault in the onboard computer's memory, shutting down all nonessential operations aboard the craft, said Jane Platt, a spokeswoman for NASA's Jet Propulsion Laboratory.
A team of engineers managed to salvage most of the mission by quickly transmitting new commands.
The same problem occurred during an Oct. 10 flyby over the volatile moon, she said. In both cases, J.P.L. engineers scrambled to create new commands to allow the spacecraft computer to work around the problem.
Late Thursday, the craft made its closest approach. After traveling 386 million miles, its signal reached Earth 35 minutes later.
"With so little time to spare, it would have been easy to think, 'There's no way can we do this'," said Galileo project manager Jim Erickson. "But our team members jumped to the challenge, in some cases leaving behind half-eaten Thanksgiving dinners."
Galileo's functions were not restored until about 8:45 p.m. PST, but that was still enough time to complete more than half the mission's objectives, Platt said.
It will take several weeks for all of the information it collected to be transmitted to Earth.
The small spacecraft continued to function normally late Thursday and NASA officials are considering extending its mission even further, Platt said.
Galileo's closest looks at Io were scheduled near the end of its extended mission because of the high risk of harm to spacecraft systems. Planners weighed the risk against the opportunity to gather high-resolution images and other data about Io's volcanos and plumes.
Several onboard systems were damaged during the October flyby of Io, which circles Jupiter in an orbit bathed in the powerful radiation of the solar system's biggest planet.
In that case, new commands reached Galileo two hours before it passed 380 miles above Io's surface. The spacecraft returned data showing that the moon is even more active than previously known, with more than 100 erupting volcanoes and vast lava flows.
Scientists say it has been 15 million years since the last comparable lava eruption occurred on Earth and more than 2 billion years since lava as hot as that on Io, 2,700 degrees Fahrenheit, flowed here.
Galileo project scientist Torrence Johnson likened observing Io to going back to Earth's early years and seeing a process that has long been dead in the rest of the solar system.
The spacecraft was walloped by radiation many times greater than anticipated on a close approach to Jupiter in August, passing within 281,000 miles of the gas giant's cloud tops. Onboard "fault protection" software handled a series of problem during that encounter.
Io is one of Jupiter's four largest moons. The current flyby also allowed observations of Callisto and Europa.
Galileo was launched from the space shuttle Atlantis on Oct. 18, 1989, and began orbiting Jupiter in December 1995. After its primary mission ended in December 1997, it continued on a two-year extended mission focusing on Europa and capped off with the Io flybys.