***** FROM GALILEO PRESS KIT, AVAILABLE IN FULL FROM THIS SERVER AS *****
*****        /pub/pao/presskit/1995/Galileo_Press_Kit.txt           *****

RELEASE: 95-207

GALILEO'S MISSION AT JUPITER POISED TO BEGIN

     Reaching its ultimate destination after an already 
eventful space journey of more than six years and 2.3 
billion miles, NASA's Galileo mission will arrive at the 
giant planet Jupiter on Dec. 7, 1995.

     The jam-packed arrival day calls for the Galileo 
spacecraft and its recently separated atmospheric probe to 
carry out a detailed choreography of data-gathering and 
critical engineering events almost 20 years in the making.  
By day's end, Galileo should be the first spacecraft to 
enter orbit around one of the solar system's giant outer 
planets, and the first to send an instrumented probe 
directly into one of their atmospheres.  The Galileo orbiter 
spacecraft then will begin at least two years of close-up 
observations of Jupiter, its moons, its faint rings and its 
powerful radiation, magnetic field and dust environment. 

     Galileo's scientific instruments represent the most 
capable payload of experiments ever sent to another planet.  
The data they will return promise to revolutionize our 
understanding of the Jovian system and reveal important 
clues about the formation and evolution of the solar system.

     Jupiter is 318 times more massive and 1,400 times more 
voluminous than Earth, but only 1/4th as dense, since it is 
composed primarily of hydrogen (89 percent) and helium (10 
percent).  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.

     "In many ways, Jupiter is like a miniature solar system 
in itself," says Dr. Wesley T. Huntress, Associate 
Administrator for Space Science at NASA Headquarters, 
Washington, DC.  "Within Jupiter's constellation of diverse 
moons, its intense magnetic field, and its swarms of dust 
and charged particles, the Galileo mission should uncover 
new clues about how the Sun and the planets formed, and 
about how they continue to interact and evolve."

     The 2-1/2-ton Galileo orbiter spacecraft carries 10 
scientific instruments; the 746-pound probe carries six more 
instruments.  The spacecraft radio link to Earth and the 
probe-to-orbiter radio link serve as instruments for 
additional scientific investigations.

     The chain of key mission events for Galileo on Jupiter 
arrival day begins with a close flyby of the moon Io by the 
Galileo orbiter at a distance of just 600 miles.  The probe 
has been traveling a separate path toward atmospheric entry 
since it was released by the Galileo spacecraft on July 13.

     Io's gravity will change Galileo's direction to help it 
go into orbit around Jupiter.  Due to the high radiation in 
this interior region of the Jovian system, this is the 
closest that Galileo is planned to come to this moon, 
although the spacecraft will observe Io during many 
subsequent orbits.

     Four hours later, the orbiter will link up by radio 
with the previously released Galileo probe as it floats via 
parachute downward through the top level of Jupiter's 
atmosphere.  By that time, the conical probe will have 
slammed into the upper fringe of Jupiter's atmosphere at a 
top speed of 106,000 mph and endured deceleration forces as 
high as 230 times Earth's gravity.  Dropping downward on its 
eight-foot diameter parachute, the probe will make the first 
direct measurements of Jupiter's atmosphere and clouds, and 
it may encounter lightning or even water rain as it descends 
more than 125 miles from the top of Jupiter's clouds.

     The Galileo orbiter will record the measurements 
radioed from the probe for up to 75 minutes, before finally 
turning away to prepare for a crucial 49-minute-long burn of 
its main rocket engine that will insert the spacecraft into 
Jovian orbit.  The probe below is expected to succumb a few 
hours later to the increasingly intense heat it will find 
deep below the clouds.

     The orbiter will then begin its tour of at least 11 
orbits of the Jovian system, including 10 close encounters 
with three of the four Galilean satellites (four with 
Ganymede and three each with Callisto and Europa), and 
observing Io's erupting volcanoes.  In mid-March, Galileo 
will fire its main rocket engine for one last major burn to 
put itself into an orbit away from the most intense Jovian 
radiation environment.

     The Galileo mission had originally been designed for a 
direct flight to Jupiter of about two-and-a-half years.  
Changes in the launch system after the Space Shuttle 
Challenger accident, including replacement of the Centaur 
upper-stage rocket with the Inertial Upper Stage (IUS), 
precluded this direct trajectory.  Galileo engineers 
designed a new interplanetary flight path using several 
gravity-assist swingbys (once past Venus and twice around 
Earth) called the Venus-Earth-Earth-Gravity-Assist or VEEGA 
trajectory.  

     Galileo was launched aboard Space Shuttle Atlantis and 
an IUS on 
Oct. 18, 1989.  In addition to its Earth and Venus flybys, 
Galileo became the first spacecraft ever to fly closely by 
two asteroids, Gaspra and Ida.  During the second asteroid 
encounter, two of Galileo's 10 science instruments 
discovered a small moon -- later named Dactyl -- orbiting 
around Ida, the first time such an object has been 
confirmed.  Galileo's instruments also performed the only 
direct observations of the impact of the fragments of Comet 
Shoemaker-Levy 9 with Jupiter in July 1994, providing key 
insights into the early stages of the impact evolution.

     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 on-board computer 
and improvements to ground-based signal receiving hardware 
in the Deep Space Network will enable the mission to 
accomplish at least 70 percent of its original science goals 
using only its small, low-gain antenna, despite the failure 
of its high-gain antenna to unfurl properly in April 1991.  
The data return will  include an average of two to three 
images per day once the spacecraft begins transmitting 
imaging data to Earth in July 1996.

     The total cost of the Galileo mission, from the start 
of planning through the end of mission in December 1997, is 
$1.354 billion, including $892 million in spacecraft 
development costs.

     Galileo's arrival in the Jovian system represents the 
culmination of a project that began formally in 1977, and 
the realization of a dream of planetary scientists since the 
earliest days of the field.

     "The Pioneer and Voyager spacecraft that flew by 
Jupiter so quickly in the 1970s stunned us with their 
pictures of rings, active volcanoes on the moon Io and other 
unexpected findings," Huntress said.  "Right now, we can 
still only imagine the discoveries that will flow from 
Galileo as it travels for months in this most unusual and 
unearthly environment."

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

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