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FOR IMMEDIATE RELEASE      April 19, 1991

      Intensive analysis of the problem that prevented deployment of the Galileo spacecraft's high-gain antenna is continuing at NASA's Jet Propulsion Laboratory.

      A "tiger team" of specialists from a variety of engineering disciplines -- including consultants from contractor companies -- has been assembled to study the problem and how to correct it.

      Galileo Project officials say they expect to carry out considerably more analysis and ground tests before determining a date to make another deployment effort. The deployment difficulty poses no immediate problems for the spacecraft, which otherwise is functioning properly.

      The problem arose Thursday, April 11, when the spacecraft's umbrella-like main communications antenna was commanded to deploy.

      Commands to unfurl the antenna were issued by Galileo's computers on schedule April 11. The deployment action -- very similar to opening a conventional umbrella -- was expected to be concluded in less than three minutes.

      Data from Galileo, however, indicate that the antenna unfurled partially but did not completely unfold. One side of the antenna appears to be deployed more fully than the other side, suggesting that some restriction may be affecting a portion of the antenna.

      Data that the JPL team has been studying include readings from a spacecraft sun sensor and from its spin detectors, which offer engineers information on the current state of the antenna. In addition, data from Galileo's power system provide details on how the deployment attempt proceeded and possible clues on the nature of the restriction.

      Engineers say that continued analysis of the data -- and tests of identical antenna equipment on the ground -- are important to avoid any action that could damage onboard equipment.

             The 16-foot-diameter high-gain antenna -- a modified version of the design used in NASA's Earth-orbiting Tracking & Data Relay Satellites -- has a surface made of gold-plated molybdenum wire woven into a mesh. The mesh is stretched across 18 graphite-epoxy ribs and connected with quartz cords.

      The antenna has been stowed behind a sun shield since Galileo's launch in October 1989, to avoid heat damage while the spacecraft flew closer to the sun than the orbit of Earth.

      The antenna deployment is driven by a set of redundant motors which turn a worm gear. This gear pushes a nut connected to levers which spread the antenna's ribs, much as an umbrella is opened.

      Unfurling of the antenna is necessary for Galileo to send scientific data to Earth at much higher rates over greater distances than it can with the two low-gain antennas it has used since launch.

      Project officials say Galileo will still conduct its planned flyby of the asteroid Gaspra on October 29 even if the antenna is not open. In that event, pictures and other data would be stored on the spacecraft's onboard tape recorder and relayed to the ground when Galileo approaches for its flyby of Earth in December 1992.

      JPL manages the Galileo Project for NASA's Office of Space Science and Applications.


4/19/91 FOD