PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Franklin O'Donnell
FOR IMMEDIATE RELEASESeptember 4, 1996
GALILEO TO MAKE SECOND PASS BY GANYMEDE
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