Spacecraft Faces Intense Radiation
By JOHN ANTCZAK
Associated Press Writer

PASADENA, Calif. (AP) _ NASA's Galileo spacecraft plunged through Jupiter's dangerously intense radiation belts Thursday on course to pass just 186 miles above the surface of the fiery moon Io. The closest approach was scheduled for 8:05 p.m. A signal of the event, traveling 386 million miles, was expected to take 35 minutes to reach mission officials at NASA's Jet Propulsion Laboratory. 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 volcanoes and plumes. Several onboard systems were damaged during an October flyby of Io, which circles Jupiter in an orbit bathed in the powerful radiation of the solar system's biggest planet. On that approach, radiation triggered a fault in the onboard computer's memory and Galileo went into ``safe mode,'' shutting down all non-essential operations. JPL engineers created new commands to allow the spacecraft computer to work around the problem and Galileo's functions were restored just two hours before the flyby. Galileo passed 380 miles above Io's surface on Oct. 10 and returned data showing 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 problems 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.