RELEASE: 96-180


Douglas Isbell
Headquarters, Washington, DC September 4, 1996
(Phone: 202/358-1753)

Franklin O'Donnell
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

NASA's Galileo spacecraft will snap three-dimensional pictures of giant, icy fissures and look for further evidence of a magnetic field when it dives past Jupiter's moon Ganymede at 3 p.m. EDT on Friday, September 6.

Galileo will sail just 163 miles over the frozen moon's north pole. 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 new 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 image 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 (JPL), Pasadena, CA. "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, versus 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 in Iowa City, principal investigator for Galileo's plasma wave spectrometer. "Because the spacecraft will pass over a different region of Ganymede than it did the first time, there is a very specific signature that we should see if one exists."

In addition to the imaging and space physics-related measurements, 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 that study magnetic fields and charged particles will collect data on the environment near Jupiter that will be sent to Earth as it is received.

During Galileo's close-approach period throughout the week, the spacecraft also will be making observations of the icy surface of Jupiter's moon Europa, and it will take global pictures of the heavily cratered Jovian moon Callisto. As on other flybys, Galileo also will 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 60 miles across.

Data from most of the science instruments will be stored on Galileo's onboard tape recorder and transmitted to Earth from September 8 through November 2. On November 4, Galileo will carry out the third flyby of its Jupiter orbital tour, a close approach to Callisto. With a 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 2-1/2-ton Galileo orbiter spacecraft was launched aboard Space Shuttle Atlantis on October 18, 1989. JPL manages the Galileo Project for NASA's Office of Space Science, Washington, DC.