Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                          
January 31, 1992
(Phone:  202/453-1547)
 
Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)

RELEASE:  92-16

ULYSSES TO STUDY JUPITER'S MAGNETIC FIELD DURING FLYBY

   Sixteen months and 617 million miles after launch from the 
Kennedy Space Center, Fla., the Ulysses spacecraft has reached 
Jupiter, where Ulysses will use the planet's gravity to swing 
out of the ecliptic plane and on toward the poles of the sun, a 
region never explored by spacecraft.  Ulysses will accelerate 
as it nears Jupiter's northern hemisphere but will not be 
captured by the planet's gravity, said Donald Meyer, JPL's 
Deputy Mission Operations Manager.

   Instead, Ulysses will skim by the planet at a distance of 
6.3 Jupiter radii, 280,000 miles from the planet's center.  
Closest approach will occur at 7 a.m. EST on Saturday, Feb. 8.  
Ulysses will dive down at an 80-degree angle to the ecliptic 
plane and continue on a course to the sun's south pole to carry 
out its primary mission.

   Ulysses will arrive at 70 degrees south solar latitude in 
June 1994.  The spacecraft will spend 4 months studying the 
sun's southern polar region, then cross the solar equator in 
February 1995 and begin a 4-month pass of the sun's north polar 
region that June.  During its passage by the sun, Ulysses will 
study three general areas of solar physics:  the sun itself, 
magnetic fields and streams of particles generated by the sun 
and interplanetary space above the sun.

   In addition to sending Ulysses toward its primary mission, 
the unprecedented maneuver around Jupiter presents scientists 
with an added opportunity to investigate the region of space 
dominated by Jupiter's magnetic field, called the 
magnetosphere.  Science experiments will attempt to examine the 
interaction of the magnetosphere with the solar wind, a stream 
of high-energy particles emanating from the sun, and study 
various phenomena within this magnetic bubble.
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   "A variety of science investigations can take place during 
the spacecraft's journey through Jupiter's magnetosphere," said 
JPL's Dr. Edward J. Smith, NASA Project Scientist for the joint 
NASA-European Space Agency mission.

   Scientists know from previous passes by the Voyager and 
Pioneer spacecraft that the outer, middle and inner regions of 
the magnetosphere differ from one another and that these 
regions change over time.  "The experiments will study the 
structure and dynamics of Jupiter's magnetosphere," said Dr. 
Edgar Page, ESA Science Coordinator at JPL.

   "Ulysses' special complement of nine instruments will 
return data from regions never before explored as the 
spacecraft flies past Jupiter at a high latitude and in the 
dusk sector," said Smith, referring to the side of the planet 
that would be seeing sunset.

   The region, however, is a hostile environment of charged 
particles and radiation, Page and Smith said.  The intensity of 
radiation trapped within Jupiter's magnetosphere poses certain 
risks for the spacecraft as it flies past.  "It's risky 
business for any spacecraft traveling through Jupiter's 
environment," said Page, "but we needed the gravity assist of 
Jupiter to achieve our polar orbit around the sun."  

   The greatest threat from Jupiter's radiation will occur 
during a 4-day period centered on the spacecraft's closest 
approach.  Should Jupiter's radiation environment prove too 
harsh, Ulysses is programmed to shut off its instruments and 
enter a "safe mode" to protect itself for its primary objective 
of studying the sun.  The radio frequency carrier will remain 
on to keep the spacecraft in touch with ground controllers. 

   "The spacecraft's trajectory through Jupiter's 
magnetosphere will take it right between the orbits of two 
Jovian moons, Io and Europa," Smith said.  "Io's torus, a ring 
of sulfur and oxygen ions that spreads out like a cloud and 
circles Jupiter, will be of special interest to us during this 
time."  Only one spacecraft, Voyager 1, has passed directly 
through this region before, in March 1979.
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