Douglas Isbell
Headquarters, Washington, DC                  Oct. 8, 1999
(Phone: 202/358-1753)

Jane Platt
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE:  99-118

GALILEO SPACECRAFT HAS HOT DATE WITH VOLCANIC MOON

     NASA's Galileo spacecraft is gearing up for a daring Oct. 11 
rendezvous with Jupiter's moon Io (pronounced EYE-oh), the most 
volcanic body in our solar system. 

     Galileo will swoop down to within 380 miles (612 kilometers) 
above Io's fiery surface at 1:06 a.m. EDT, snapping the closest-
ever pictures of this intriguing celestial body.

     "Io is a natural laboratory for volcanoes," said Dr. Duane 
Bindschadler, Galileo manager of science operations and planning.  
"By studying Io close up, we'll learn more about how and when 
volcanoes erupt and why they act the way they do.  This may even 
help us predict the behavior of volcanoes on Earth."

     During the flyby, Galileo's science instruments will study 
the chemistry, heat distribution, gravity and magnetic properties 
of Io.  For scientists, this thrilling encounter promises to yield 
a bonanza of pictures and information, but for Galileo engineers, 
the flyby presents a serious challenge with uncertain results.  
Io's orbit lies in a region of intense radiation from Jupiter's 
radiation belts, which could affect the performance of spacecraft 
systems or even knock out various spacecraft instruments.  A mere 
fraction of the dose that Galileo will receive would be fatal to a 
human.  

     "We expect that the spacecraft will survive the flyby, 
although the radiation may cause its computers to reset or may 
even cause irreversible damage to critical electronic components," 
said Wayne Sible, Galileo deputy project manager.  "There is a 
possibility, if enough damage is done to the electronics, it won't 
survive the flyby.  Because of this possibility, we planned the Io 
encounters for the end of the two-year extended mission. After 
orbiting Jupiter for nearly four years, the spacecraft has more 
than fulfilled its mission objectives, so it seems reasonable to 
take a calculated risk for a much closer look at such a 
scientifically rich target."

     Galileo was originally assigned to spend two years studying 
Jupiter, its moons and its magnetic environment.  When that 
original mission ended in December 1997, it was followed by a two-
year extended mission, scheduled to end in January 2000.  While 
spending the past four years near Jupiter, Galileo has been 
exposed to radiation on an ongoing basis, which has caused some of 
its instruments to act up.  

     To prepare for any possible harm caused by radiation during 
the Io flyby, engineers have designed sophisticated software to 
help the spacecraft weed out a true crisis from a minor glitch 
caused by radiation and respond appropriately.

     Galileo, the first spacecraft to orbit Jupiter, has 
revolutionized our knowledge of Jupiter and its moons and has 
provided thousands of colorful images.  Data from Galileo support 
the premise of a liquid ocean beneath the icy crust of Jupiter's 
moon Europa, an intriguing prospect since water is a vital 
ingredient for life.  Thanks to information sent by Galileo, 
scientists know much more about the weather on Jupiter and the 
composition of its moons.  En route to Jupiter, the spacecraft 
took the first-ever close-up pictures of asteroids, when it 
photographed Gaspra and Ida, and it returned historic images of 
the destruction of comet Shoemaker-Levy 9 as its pieces slammed 
into Jupiter.

     If all goes well with the upcoming Io flyby, the spacecraft 
will make an even more daring approach of Io on Nov. 26 at an 
altitude of only 186 miles (300 kilometers).

     New Io images taken by the spacecraft are available at the 
following website:  

            http://www.jpl.nasa.gov/pictures/io

     Additional information and pictures taken by the Galileo 
spacecraft are available at the redesigned Galileo website at this 
new Internet address: 

               http://galileo.jpl.nasa.gov

     Galileo was launched from the Space Shuttle Atlantis on Oct. 
18, 1989.  It entered orbit around Jupiter on Dec. 7, 1995.  The 
Jet Propulsion Laboratory (JPL) manages the Galileo mission for 
NASA's Office of Space Science, Washington, DC.  JPL is operated 
for NASA by the California Institute of Technology, Pasadena, CA.

                             -end-