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       Now residing in orbit around Jupiter, NASA's Galileo spacecraft is primed for its first close flyby of Jupiter's largest moon, Ganymede, at 6:29 Universal Time on June 27 (11:29 p.m. Pacific Daylight Time on June 26).

       Equipped with 10 scientific instruments, Galileo will fly just 844 kilometers (524 miles) above Ganymede's surface to provide the most detailed images and other information ever obtained about the icy satellite. Images and other data gathered by the spacecraft will be radioed back to Earth in the hours and months following the flyby.

       On June 23, Galileo's particle detectors and magnetic fields instruments began making nearly continuous measurements as the spacecraft approached Ganymede. Its optical instruments will shortly begin their periodic observations, including the first round of picture-taking (other than engineering images taken for navigation purposes) since months before the spacecraft entered orbit around Jupiter on December 7, 1995.

       If spacecraft operations near Ganymede and the subsequent transmission to Earth of initial science data occur as planned, selected images of Ganymede taken by Galileo will be released in a televised news conference at the Jet Propulsion Laboratory, tentatively scheduled for July 10.

       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 formations including craters and basins, grooves and mountains. The bulk of the satellite is about half water ice and half rock.

       Portions of its surface are relatively bright, clean ice while the other regions are covered with darker "dirty" ice. The darker areas appear to be ancient and heavily cratered, while the lighter regions display evidence of tectonic activity that may have broken up the icy crust. A thin layer of ozone has been found in Ganymede's surface by astronomers.

       Galileo will return high-resolution images showing features on Ganymede as small as 10 meters (about 33 feet) across. Instruments on board will assess Ganymede's surface chemistry and search for signs of an atmosphere around the big moon. Measurements will be made to characterize Ganymede's gravity field and to determine if it possesses a magnetic field.

       In the days just before and after the Ganymede flyby, Galileo's other studies include a search for auroral activity on Jupiter's nightside and observations of other Jovian moons: Io, Europa and Callisto. The "Io torus," a hot, doughnut-shaped ring of charged particles swirling about Jupiter at Io's distance, will be another target of study, as will Jupiter's Great Red Spot.

       Galileo's Ganymede encounter marks the start of a steady stream of data to be returned to Earth by Galileo's instruments throughout the course of its two-year tour of the Jovian system, which continues through December 1997. Beginning in July, data return will include an average of two to three images per day.

       The remainder Galileo's mission is to complete 11 orbits of Jupiter, conducting multiple close flybys of the moons Ganymede, Europa and Callisto, with numerous, more distant studies of the moon Io also scheduled throughout the tour. Studies of Jupiter itself are planned throughout the tour, and nearly continuous studies of Jupiter's enormous radiation and magnetic fields will be conducted.

       The fifth planet from the Sun is known primarily for the banded appearance of its upper atmosphere and its centuries-old Great Red Spot, a massive, hurricane-like storm as big as three Earths. Jupiter generates the biggest and most powerful planetary magnetic field, and it radiates more heat from internal sources than it receives from the Sun.

       Given its large size and its many natural satellites, Jupiter is often described as a miniature solar system. Jupiter has 318 times more mass and 1,400 times more volume than Earth, but is only one-fourth as dense, since it is composed primarily of hydrogen and helium. It is orbited by at least 16 moons (and Galileo -- its first artificial satellite).

       The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft was launched aboard Space Shuttle Atlantis on Oct. 18, 1989. It carries the most capable payload of scientific experiments ever sent to another planet.

       Communications to and from Galileo are conducted through NASA's Deep Space Network, using tracking stations in California, Spain and Australia. A combination of new, specially developed software for Galileo's onboard computer and improvements to ground-based signal receiving hardware in the Deep Space Network have enabled the spacecraft to accomplish at least 70 percent of its original mission science goals using only its small, low-gain antenna, despite the failure of its high-gain antenna to unfurl properly in April 1991.

       NASA's Jet Propulsion Laboratory, Pasadena, CA, built the Galileo orbiter spacecraft and manages the overall mission. Galileo's atmospheric probe was managed by NASA's Ames Research Center, Mountain View, CA.

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

Note to Editors: A NASA Video File to accompany this news release will be aired on NASA Television today at 2, 4, 7 and 9 p.m. PDT. NASA Television is broadcast on Spacenet 2, transponder 5, channel 9, C band, located at 69 degrees west longitude, horizontal polarization, frequency 3880.0 megahertz, audio 6.8 megahertz. A computer-generated rendering to accompany this news release is available from JPL's Public Information Office, (818) 354-5011.

6/24/96 MBM