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 spacecraft has successfully flown past the largest moon in our solar system -- Ganymede, which orbits around Jupiter.

Galileo dipped to 809 kilometers (503 miles) above the surface early Saturday, May 20. This was the spacecraft's first flyby of Ganymede since May 7, 1997.

"It's great that things went so smoothly," said Galileo Project Manager Jim Erickson of NASA's Jet Propulsion Laboratory. "The team was ready for any problems, but they got to relax on this one. We're really looking forward to the new pictures and learning more about this largest of all moons."

At 7 a.m. EDT (1100 GMT), mission controllers at JPL received the signal indicating that the flyby took place. The spacecraft'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 several months for processing and analysis.

To fly by Ganymede, Galileo had to approach Jupiter's powerful radiation belts. Not surprisingly, the radiation, which can affect spacecraft instruments, components and systems, did cause two main resets of Galileo's main computer. Onboard software correctly diagnosed this as a false indication, and went ahead with the Ganymede encounter unaffected.

"It appears that this workhorse spacecraft has done it again," Erickson said. Galileo has already survived three times the radiation it was designed to withstand.

Ganymede is even larger than Mercury and Pluto. Its surface is a mixture of clean, white ice and dirty, dark ice, with varied geological formations, including craters, basins, grooves and rough mountain areas.

An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

On Monday Galileo's instruments focused on Jupiter's aurora and its atmosphere, including the Great Red Spot.

The first three observations of the week are spectral scans of Jupiter's bright limb, performed by the Near-Infrared Mapping Spectrometer (NIMS). Like observations performed on Sunday, May 21, these scans will be used to provide scientists with data on the bulge created at Jupiter's equator as Jupiter rotates once every 10 hours. NIMS follows these observations with a series of 10 spectral scans of Jupiter's north polar region. The series consists of 10-minute samples separated by 60 minutes, and is designed to capture auroral activity.

Interspersed with the aurora observations are three global observations performed by NIMS and three Great Red Spot observations performed by the Solid-State Imaging camera (SSI). Taken together, the global observations performed by NIMS will map a substantial proportion of the complete range of longitudes of the planet. The Great Red Spot observations performed by SSI will provide scientists with high spatial and time resolution images of this storm, which is over 400 years old. Similar observations were taken at the beginning of Galileo's orbital tour in June 1996, so scientists will be able to observe long-term changes in the characteristics of the Great Red Spot. The Great Red Spot is so large that the Earth fits two times across it!

Galileo was launched from the Space Shuttle Atlantis on October 18, 1989. After a long journey to Jupiter, Galileo began orbiting the huge planet and its moons on December 7, 1995, and successfully completed its two-year primary mission on December 16, 1997. That was followed by a two-year extended mission which concluded in December 1999, and Galileo is now continuing its studies under yet another extension, called the Galileo Millennium Mission. JPL, a division of the California Institute of Technology in Pasadena, Calif., manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.