Scientists Try to Rescue Galileo Mission : Space: They will cool a faulty antenna in an effort to unstick it. If they fail, most data from the Jupiter trip could be lost.

The recalcitrant antenna that has refused to open on the Jupiter-bound Galileo spacecraft will be put through a deep freeze next week as engineers work desperately to save the $1.3-billion mission.

The 2 1/2-ton spacecraft, launched on a torturous journey to Jupiter two years ago, will be turned 165 degrees away from the sun so that the 16-foot-wide antenna will be in the shade. Engineers at the Jet Propulsion Laboratory in Pasadena hope that the drop in temperature will free the stuck ribs and allow the antenna to open.

Similar attempts have failed--using the sun to heat the antenna and shade to cool it--but this time the antenna will be cooled for 50 hours. Engineers hope that long exposure to the cold of space will cause the main shaft in the umbrella-like antenna to shrink slightly. That could free some small pins that were designed to guide the ribs of the antenna as it unfolded. The stuck pins apparently kept some of the 18 ribs from opening.

Engineers at the lab, which is managed by Caltech for the National Aeronautics and Space Administration, have plenty of time to work on the problem because Galileo will not reach Jupiter until 1995. If they cannot fix the antenna, most scientific data and all of the pictures that the spacecraft collects during a two-year tour of the Jovian system will be lost.

Except for the antenna, Galileo is working perfectly as it speeds through the solar system at 41,557 m.p.h., heading toward a historic encounter with an asteroid this October. Scientists believe that most of the data Galileo will collect as it passes the asteroid, named Gaspra, can be saved because it can be relayed to Earth by the spacecraft’s smaller antenna.

But the smaller device is not powerful enough to send back the mountains of data, including pictures, from the Jupiter encounter, and all but a tiny part of that will be lost if the larger antenna cannot be opened.

The large antenna opens a little like an umbrella, and engineers first ordered it to deploy on April 11. The antenna refused to open into its parabolic shape, leaving it like a collapsed umbrella.

The antenna must open fully because it operates “like a headlight beam,” said Galileo project manager William O’Neil.

After studying the problem for several weeks, engineers decided their best option would be to use thermal expansion and contraction, heating and cooling the antenna, to try to free the pins.

The spacecraft was turned so that the antenna pointed into the sun on May 20, but heat did not do the job. In June, the craft was turned the other way, shading and cooling the antenna for 32 hours, but that did not work either.

In addition to shading the antenna for a longer period next week, engineers will turn off any unnecessary electrical components aboard Galileo in hopes of making the craft even cooler.

After the cooling period is over, engineers will study data from the spacecraft for several days before turning on the motors that drive a nut up the threaded central shaft, thus pushing the ribs outward.

If the antenna opens, the long-delayed mission will be back on track. If it does not, engineers will try to figure out another strategy, but the prospects will be considerably dimmer.