Illustration of Galileo at Jupiter

NASA's Galileo spacecraft will snap three-dimensional pictures of giant, icy fissures and look for further signs of a magnetic field when it dives past Jupiter's moon Ganymede on Friday, Sept. 6.

Galileo will sail just 262 kilometers (163 miles) over the frozen moon's north pole at 19:00 Universal Time (12 noon Pacific time) Friday. The flyby, Galileo's second encounter with Ganymede since its arrival at Jupiter last December, will be the spacecraft's closest swing by any of Jupiter's moons during its two-year prime mission.

During the flyby, Galileo will collect pictures of two regions on Ganymede, Uruk Sulcus and Galileo Regio, that were imaged during the spacecraft's first flyby in late June. This will allow scientists to create stereo pairs offering a three-dimensional view of Ganymede's icy terrain.

"The areas on Ganymede that we saw during the first flyby have huge contrasts of light and dark that fool the eye," said Galileo Project Scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory. "What your eye interprets as a slope may not really be one. These 3-D views will give us a better idea of what is paint on Ganymede's surface, so to speak, vs. what is real topography." In particular, Johnson said scientists are eager to understand better the patterns of fissures and cracks that riddle the moon's surface.

Scientists also hope that this week's flyby will settle a current controversy -- whether or not Ganymede boasts an internally generated magnetic field. Data collected by Galileo's space physics experiments during the first Ganymede flyby show that the moon is interacting with Jupiter's enormous magnetic field in some way, but scientists do not yet agree on whether this means that Ganymede itself has a magnetic field.

"The upcoming flyby should conclusively settle the question of whether Ganymede has an internal magnetic field," said Dr. Donald Gurnett of the University of Iowa, principal investigator for Galileo's plasma wave spectrometer. "Because the spacecraft passes over a different region of Ganymede, there is a very specific signature that we should see if one exists."

Besides the imaging and space physics efforts, Galileo will train other instruments including its near-infrared mapping spectrometer and its ultraviolet spectrometer on Ganymede during the flyby to study the moon's northern regions. Throughout the encounter period, Galileo's instruments studying magnetic fields and charged particles will collect data on the environment near Jupiter that will be sent to Earth as they are received.

During Galileo's close-approach period throughout the week, the spacecraft will also be making observations of the icy surface of Jupiter's moon Europa, and will take global pictures of the heavily cratered jovian moon Callisto. As on other flybys, Galileo will also keep watch on the volcanic moon Io to look for active eruptions. In addition, the spacecraft will take a picture of Amalthea, one of Jupiter's handful of much smaller moons measuring just 100 kilometers (60 miles) across.

Data from most of the science instruments will be stored on Galileo's onboard tape recorder and transmitted to Earth Sept. 8 through Nov. 2. On Nov. 4, Galileo will carry out its third flyby of the Jupiter orbital tour, a close approach to Callisto.

With a 5,262-kilometer (3,269-mile) diameter, Ganymede is the largest moon in the solar system -- bigger than Mercury and about three-quarters the size of Mars. It possesses a variety of familiar Earthlike geologic features including craters, basins, grooves and mountains. The bulk of the moon is about half water ice and half rock.

The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. JPL manages the Galileo Project for NASA's Office of Space Science, Washington, D.C.

Additional information on the Galileo mission and its results can be found on the World Wide Web at

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