JPL FACT SHEET
June 1994

           COMET SHOEMAKER-LEVY 9 IMPACT WITH JUPITER:
                PLANETARY SPACECRAFT AS OBSERVERS

     Three NASA spacecraft missions launched some years ago to
explore various planets and the sun will observe the events
surrounding the Shoemaker-Levy 9 impacts with Jupiter from unique
perspectives far removed from Earth.  They are Voyager 2, Galileo
and the joint NASA/European Space Agency Ulysses mission.

     Voyager and Galileo will be the only two spacecraft actually
in view of the nightside impact points for the many comet
fragments.  

     Voyager 2, launched in August 1977 and now leaving the solar
system after flying by and observing Jupiter, Saturn, Uranus and
Neptune, will be some 6 billion kilometers (3.7 billion miles)
from Jupiter in mid-July when the events begin.  It will use its
ultraviolet spectrometer and its planetary radio astronomy
instrument to detect and measure emissions stimulated by the
comet impacts. 

     The Galileo spacecraft is now on the final leg of its flight
path to Jupiter after a complex interplanetary trajectory
involving gravity-assist flybys of Venus and Earth and two
encounters with asteroids. It is scheduled to send a probe into
Jupiter's atmosphere and begin a two-year Jupiter orbital tour in
December 1995.

     Galileo will be about 240 million kilometers (150 million
miles) from Jupiter between July 16 and July 22, 1994, when the
comet fragments are colliding with the planet, and will view the
nightside impact regions directly.  At that range, its camera can
resolve Jupiter as a disc 60 pixels across, comparable to the
capability of many large Earth-based telescopes.  (A pixel, or
picture element, is one point in the digitized picture.)

     Galileo's near-infrared mapping spectrometer, the
photopolarimeter and the ultraviolet spectrometer will measure
all of the light from Jupiter, attempting to detect increases at
the time of the impacts.  Plasma-wave sensors and the dust
detector will be operating to detect radio emissions from the
impacts and to search for changes in Jupiter's dust environment.

     The Galileo imaging team plans to try several different
strategies to capture the Shoemaker-Levy phenomena.  One approach
is to collect many time-lapse exposures in mosaic patterns,
because the times of impact will be uncertain to within many
minutes.  The timing uncertainties and data storage limitations
mean that this procedure can be used to observe only a few of the
fragment collisions.  Multi-color imaging -- in which the camera
takes several exposures in a row through various colored filters
-- will be attempted for other impacts.

     The commands to execute this complex, multiple-instrument
observation program must be transmitted to the spacecraft many
days in advance, with limited opportunity for later updates.  The
spacecraft will then carry out the program when the time comes,
while the flight team monitors its actions and receives some
science data sent back at a low rate over Galileo's low-gain
antenna.

     After the Shoemaker-Levy events have concluded, the flight
team will search through data recorded on Galileo's onboard tape
recorder and will command the spacecraft to selectively transmit
observations of interest.  This selective procedure has been
proven in the two asteroid encounters.  It may be as late as
October or November 1994 before the majority of Galileo's data
from these events are transmitted to Earth from the spacecraft.

     Ulysses, the NASA/ESA mission to study the poles of the sun,
will also search for possible radio emissions caused by
Shoemaker-Levy.  During July the spacecraft will be in the midst
of its southern passage around the sun -- measuring the solar
wind as well as fields and particles from positions less than 20
degrees from the sun's south pole.

     At that time Ulysses will be about 400 million kilometers
(250 million miles) from the sun, and about twice that distance
from Jupiter.  Its combined radio and plasma wave instrument will
also search for radio emissions from Jupiter's magnetosphere that
show effects of the Shoemaker-Levy impacts. 

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      ___    _____     ___
     /_ /|  /____/ \  /_ /|     Ron Baalke     | baalke@kelvin.jpl.nasa.gov
     | | | |  __ \ /| | | |     JPL/Telos      | 
  ___| | | | |__) |/  | | |__   Galileo S-Band | If you follow the herd, you
 /___| | | |  ___/    | |/__ /| Pasadena, CA   | will eventually end up in the
 |_____|/  |_|/       |_____|/                 | slaughter house.