NASA Press Release Text - June 24, 1996

GALILEO READY FOR CLOSE FLYBY OF JUPITER'S BIGGEST MOON

        Now residing in orbit around Jupiter, NASA's Galileo 
spacecraft is primed for its first close flyby of Jupiter's 
largest moon, Ganymede, at 2:29 a.m. EDT on June 27, 1996.

        Equipped with 10 scientific instruments, Galileo will 
fly just 524 miles above Ganymede's surface to provide the 
most detailed images and other information ever obtained 
about the icy satellite.  Galileo will be 70 times closer to 
Ganymede than Voyager 2 and 133 times closer than Voyager 1.  
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 Dec. 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, Pasadena, CA, tentatively 
scheduled for July 10.

        With a diameter of 3,269 miles, 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 Earth-like geologic formations including 
craters and basins, grooves and mountains.  The bulk of the 
satellite is believed to be 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 
Earth-based astronomers.  
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        Galileo will return high-resolution images showing 
features on Ganymede as small as 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 will 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 of 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 1/4th 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 2-1/2-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.  An innovative combination 
of new, specially developed software for Galileo's on-board 
computer and improvements to ground-based signal receiving 
hardware in the Deep Space Network will enable 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.
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        NASA's Jet Propulsion Laboratory built the Galileo 
orbiter spacecraft and manages the overall mission.  
Galileo's atmospheric probe, which plunged into the planet on 
Dec. 7, 1995, 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:

   http://newproducts.jpl.nasa.gov/galileo/

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