The antenna -- a 16-foot mesh paraboloid stretched over 18 umbrella-like ribs -- had been furled and hiding behind a small sunshade for the almost 18 months since launch, in which the spacecraft came closer to the sun than Earth and briefly closer even than Venus. Communications, including Venus and Earth-moon science data return, had been using the low-gain antennas.

Within minutes, Galileo's flight team, watching spacecraft telemetry 37 million miles away on Earth, could see that something was wrong: The motors had stalled, something had stuck, the antenna had opened only part way.

Within weeks, a tiger team had thoroughly analyzed the telemetry, begun ground testing and analysis, and presented its first report to Project Manager Bill O'Neil and a special anomaly review board chaired by Bud Schurmeier, retired associate director.

They attributed the problem to the sticking of a few antenna ribs due to friction between their standoff pins and their sockets. The first remedial action was taken -- turning the spacecraft to warm and expand the central tower, in hopes of freeing the stuck pins.

More than 100 people, within and outside JPL, contributed to testing, simulation, analysis, consultation and review.

The tiger team, led by Division 35 Manager Ron Ploszaj, includes Gary G. Coyle, antenna task manager, and a core group including Ron Reeve, Gun-Shing Chen, Walter Tsuha, Jim Staats, Paul Rapacz, Ken Smith, Mike Johnson, and many others -- experts in mechanical, thermal, electrical and systems interfaces.

"We had people from all across the board," said Matt Landano, deputy mission director.

The state of the antenna has been as well characterized as long-distance telemetry and laboratory tests will allow, according to antenna Task Manager Gary Coyle.

The team is confident that nothing is broken and that three adjacent antenna ribs are stuck to the central tower by friction in their paired standoff pins, which are likely misaligned enough that one pin binds the upper side of its socket, its mate the lower.

This led, in turn, to the hypothesis that repeated thermal cycling, warm-cool-warm-cool, can "walk" the paired pins out, freeing the ribs.

The flight team has maneuvered the spacecraft to warm the antenna in the dun four times beginning in May 1991, and to cool it six times between July and this April.

The next warm-cool cycle is underway: the team sent a command sequence June 25 that started a warming turn on June 29, followed by a cooling turn ending July 10.

Since the spacecraft was receding from the sun throughout 1991 (to a maximum of 211 million miles on Jan. 11, 1992), and is still more than 186 million miles from the heat source, warming has had a limited effect -- about 30 degrees Celsius (50 degrees Fahrenheit) each time. Cooling turns have tended to change the temperature by as much as 140 C (250 F), shrinking the antenna tower about one millimeter (1/25 inch).

While all this was going on, the flight team also conducted Galileo's flight mission, flying a two-year elliptical orbit between the first and second Earth gravity-assists.

Along the way, the spacecraft flew by and observed the asteroid Gaspra in October 1991, and transmitted the first-ever asteroid photo over the low-gain antenna soon after: another, higher resolution Gaspra image was received last month and released June 11 (Universe, June 19).

The team is also maintaining the health of the spacecraft and guiding it toward the December 1992 gravity-assist that will send it to its Jupiter encounter in 1995.

On April 29 the antenna-deploy motors were turned on for about two seconds (the first time since the anomaly) as a diagnostic experiment; the engineers confirmed their ideas about the state of the antenna.

In addition to thermal cycling, the tiger team has developed other ideas for loosening the stuck ribs: retracting the second low-gain antenna (on a pivoting boom), pulsing the antenna motors, and increasing the spacecraft spin rate to maximum 10 rpm (normally about 3 rpm).

Finally, in case the antenna remains stuck, technical teams have been studying a mission design to use the low-gain antenna for Jupiter science return, with atmospheric-probe and remote-sensing information stored on the tape recorder and slowly played back, an option the Project team described at the June 11 press conference.

Using advanced data-processing techniques in the spacecraft computers and advanced hardware and techniques on the ground, the engineers believe they can capture a significant fraction of the total planned Jupiter science data. However, they haven't ruled out freeing and using the high-gain antenna.

"Either way," said Project Manager Bill O'Neil, "Galileo will accomplish the majority of its objectives and will be the grandest planetary mission in this last decade of the century."