File Galileo image: A global view of Ganymede's trailing side in which the colors are enhanced to emphasize color differences. The enhancement reveals frosty polar caps in addition to the two predominant terrains on Ganymede, bright, grooved terrain and older, dark furrowed areas. Many craters with diameters up to several dozen kilometers are visible. The violet hues at the poles may be the result of small particles of frost which would scatter more light at shorter wavelengths. Photo: NASA/JPL

NASA's Galileo space probe successfully swooped 1,452 miles above the surface of Ganymede on Thursday while the moon was in Jupiter's shadow, allowing a unique opportunity to study auroral glow during the eclipse.

"It looks like a nice, calm flyby," said Jim Erickson, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "The team was prepared for problems, but we're happy without any. And we'll be even happier once we've passed this orbit's closest approach to Jupiter."

Intense radiation near Jupiter poses a risk to the spacecraft's electronics. Galileo's closest approach to Jupiter on this orbit came today at 0326 GMT (10:26 p.m. EST Thursday).

Galileo has already received three times the cumulative radiation exposure it was designed to withstand and has continued making valuable scientific observations more than three years after its original two-year mission in orbit around Jupiter.

At 0900 GMT (4 a.m. EST), mission controllers received the signals indicating that the Ganymede flyby had taken place. The signals had been relayed from the Goldstone, Calif., and Madrid, Spain, stations of NASA's Deep Space Network, which operates large dish antennas around the world for communications with spacecraft.

Galileo's camera and other instruments were set to capture the flyby with images and other observations. If all goes as planned, the data will be transmitted to Earth over the next five months for processing and analysis.

Galileo makes its flyby of Ganymede while in eclipse by Jupiter as soon in animation. Photo: NASA TV/Spaceflight Now

As of 1700 GMT (12 p.m. EST), the spacecraft had recorded 31 percent of the scientific data that its instruments had been programmed to collect during this swing through the inner portion of the Jovian system, from Dec. 26 through Dec. 31. Besides studying Ganymede, Galileo is making more distant observations this week of Jupiter and the moons Io, Callisto and Europa. Some of the observations are planned as part of collaborative studies with NASA's Cassini spacecraft, which will pass Jupiter on Saturday, though at a much greater distance than Galileo is from the planet this week.

Gases in portions of Ganymede's thin atmosphere give off a shimmering auroral glow as they are struck by electrons from Jupiter's radiation belts. The phenomenon is similar to Earth's Northern Lights and to what happens inside a fluorescent light bulb. Sunlight washes out the glow, so Galileo scientists took advantage of the eclipse to study the glow for information about the chemical makeup of the gases and the structure of Ganymede's magnetic field, which affects the location of the glow.

Ganymede is the largest moon in the solar system, larger than the planets Mercury and Pluto. It is also the only moon known to have its own internally generated magnetic field. Earlier this month, scientists announced evidence that Ganymede may have a thick layer of melted, salty water under its ice-rich surface.

"Ganymede is certainly one of the most interesting places in the solar system, and we're looking forward to see what kind of new surprises Galileo may have to tell us about it," Erickson said.

Galileo was launched from NASA's Space Shuttle Atlantis on Oct. 18, 1989. It began orbiting Jupiter on Dec. 7, 1995.