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Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE                                                 July 8, 1991

       The Ulysses spacecraft fired two small thrusters today to set the final course for its early February 1992 rendezvous with Jupiter.

       The trajectory correction maneuver, conducted by the mission operations team at the Jet Propulsion Laboratory, was the last change to the spacecraft's flight path before Ulysses completes its 484-million-mile journey to Jupiter. The 81/2-minute maneuver began at 6:24 a.m. Pacific Daylight Time.

       NASA and European Space Agency officials said the maneuver was minor and was designed to alter the spacecraft's trajectory by about 0.29 meters per second (less than one foot per second). The trajectory correction will trim approximately four minutes off Ulysses' arrival time at Jupiter.

       Ulysses is presently traveling in the ecliptic plane -- the plane in which the planets of the solar system orbit -- at a heliocentric velocity of approximately 68,000 kilometers per hour (44,000 miles per hour). The spacecraft is slowing as it nears Jupiter, but is still closing on the planet at a little more than one million kilometers per day (832,000 miles per day).       

       The 810-pound spacecraft is expected to enter the Jovian magnetosphere about Feb. 3, 1992, and begin making measurements during a 10-day sweep past the planet. The magnetosphere is the

       region of space dominated by the magnetic field of the planet.

       Ulysses, on its approach, will fly through a previously unexplored region of Jupiter's magnetosphere at about 30 degrees north latitude on the morning side of the planet. Closest approach of 6.3 Jupiter radii (279,300 miles) from the center of the planet will occur at 4 a.m. Pacific Standard Time on Saturday, Feb. 8, 1992.

       Approximately a week later, the spacecraft will exit the magnetosphere at high southern latitudes on the evening side of the planet.

       Many of the Ulysses science experiments are expected to yield new information about Jupiter's magnetic field and particle environment. Ulysses will determine the composition of energetic particles trapped in the magnetic field, look for X-ray emissions and investigate the properties of the solar wind in the region influenced by the planet.

       After the encounter, the momentum gained from the gravitational pull of Jupiter will swing Ulysses out of the ecliptic plane and onto a trajectory leading over the southern solar pole. In June 1994, the spacecraft will reach 70 degrees south solar latitude and begin its primary mission of exploring the polar regions of the sun.

       Ulysses is a five-year mission managed jointly by NASA's Office of Space Science and Applications and the European Space Agency. Tracking and data collection are provided by NASA's Deep Space Network, which is managed by the Jet Propulsion Laboratory.