Experts to Try to Jar Galileo’s Antenna Free : Spacecraft: In final effort, scientists will ‘hammer’ away at the jammed collapsible dish to try to get it to open. Success would allow transmission of Jupiter photos at higher speed.

Just as a frustrated homeowner might whack a broken television in hopes of jarring it back to work, NASA scientists are preparing to “hammer” a jammed collapsible antenna on the $1.5-billion Galileo satellite as it heads toward Jupiter.

Scientists, having failed to fix the probe’s main antenna with more delicate means, are counting on brute strength to salvage what promises to be the most spectacular single part of Galileo’s long-awaited mission: photographs of the massive gas-ball planet and its squadron of four major and 12 lesser moons.

Without its 16-foot-wide “high-gain” antenna--a gold-plated molybdenum wire-mesh dish that resembles a giant inverted umbrella--Galileo would have to rely on a less-focused antenna. Even at its best, this “low-gain” antenna could send data to Earth at less than one-hundredth the speed of the high-gain device, greatly slowing transmission of photographs.

This last-ditch effort to deploy the Jupiter probe’s antenna will start Monday, when scientists will rotate the satellite to point the antenna toward the sun. They hope the warmth will expand metal fittings and release the stuck mechanism.

If that fails, as similar attempts did in the past, Project Manager William J. O’Neil at NASA’s Jet Propulsion Laboratory in Pasadena said controllers will flip the antenna’s electric motors on and off at rapid intervals, “hammering” the jammed mechanism in a final bid to pop it into place.

“We’re hoping to just overpower whatever is holding it,” O’Neil said.

Hammering will continue at intervals through the first couple of weeks of January, he said. If this process is unsuccessful, he said, NASA will probably give up on the primary antenna and concentrate on developing a program to pull down as much data as possible by other means.

For example, he said, the low-gain system could be pressed to increase its transmission rate to perhaps 1,000 bits per second from the normal 10. In contrast, a high-gain antenna can transmit data at about 134,000 bits per second.

Low-gain antennas broadcast their signals over a wide area, diluting the power in any one direction. High-gain antennas focus their energy in narrow beams, making it much clearer and more powerful wherever one is pointed. As a result, Galileo’s low-gain antenna not only would send information more slowly but would require Earth-based receivers to spend more time listening to the weaker signal from space.

Even using the low-gain antenna, Galileo could transmit large amounts of information on the nature and composition of the solar system’s largest planet and its four very different major moons--Io, Europa, Ganymede and Callisto.

However, the number of photographs would be significantly reduced because it takes so long to transmit each one. Rather than 60,000 images, as planned, Galileo may be able to send back only 3,000 to 4,000 photos with its low-gain antenna, O’Neil said.

Scientists are particularly eager to study Jupiter in detail because they think it may hold answers about how the solar system was formed. With a diameter 10 times that of Earth, Jupiter alone contains nearly three-fourths of the solar system’s planetary mass. Scientists believe that Jupiter, of all the planets, retained the largest amount of the primordial gas and dust out of which the solar system condensed.

Galileo is designed to investigate Jupiter in two ways, by parachuting a small “atmosphere probe” toward what some scientists believe is the planet’s curious liquid core and by putting itself into an elliptical orbit that weaves through Jupiter’s moons while looping the planet.

The interplanetary probe was launched from the space shuttle Atlantis on Oct. 18, 1989, and began a series of loops through the inner solar system to build up speed for the journey to Jupiter.

By the time it arrives at the giant planet on Dec. 7, 1995, Galileo will have traveled a total of 2.5 billion miles. It raced past Earth for the last time on Dec. 8, picking up speed from the planet’s gravity. As it approached the Earth-moon axis, it took a number of detailed, dramatic pictures of both celestial bodies.

Along the way, the instrument-packed satellite, which is slightly larger than a minivan, gave the world its first close-up of an asteroid--Gaspra, which it photographed and measured in unprecedented detail in October, 1991--and provided scientists with new images and data on Earth, Venus and the moon.

It also was used to demonstrate the potential for transmitting information over a very long distance--3.5 million miles--with laser light instead of radio waves. NASA is interested in knowing if laser transmission can send more data faster with smaller on-board electronics.

Galileo is scheduled to encounter another asteroid, Ida, next August.

However, these achievements have been shadowed by concerns over the balky antenna. The antenna was folded at the start of the mission to protect it from intense solar radiation as it gained speed by looping around Venus in February, 1990. Controllers expected to open the device--”like the petals of a flower,” O’Neil said--in April, 1991. The command was sent, but the satellite failed to respond.

On-board sensors showed that five of the antenna’s 18 graphite-epoxy ribs were jammed shut, preventing the remaining 13 ribs from opening more than about halfway. Continued pressure from the small electric motor designed to open the umbrella-like structure was able to pop open two of the jammed ribs, but three remained firmly stuck when the motor finally stalled.

Engineers studying data from the satellite and a full-sized mock-up at JPL determined that small titanium pins on the ribs probably had become wedged into their aluminum seats.

They suspect the problem developed because the antenna assembly endured two round-trips between Florida and California. The antenna was built in Florida and sent to California to be installed on the satellite. The entire satellite was then sent to Florida for launch into space. But the Challenger disaster in 1986 delayed its launch, and the satellite was trucked back to California for modification. Finally, it was returned to Florida for launch.

During these highway journeys, engineers now believe that the molydisulfide dry lubricant on the titanium pins wore away and the metal was scarred. This caused the device to jam when exposed to the hard vacuum of space.

They thought that cooling the metal parts and causing them to contract might free the ribs, so they turned the antenna away from the sun. Three of these “deep cold soaks” were tried in 1991; all were unsuccessful.

Later, they tried spinning the satellite to alternately warm and cool it, hoping to jiggle the pins free in that way. That also failed.

On Monday, with the satellite relatively close to the sun, they hope a new dose of warmth will expand key metal components and let them hammer the device open with rapid cycling of the electric motor. Ground tests indicate that in this way they can exert as much as twice the amount of pressure on the ribs as they could by simply turning the motor on and leaving it on.

“There is no way of knowing this (stuck pins) is the problem . . . but by far we think this is the most likely thing,” O’Neil said. “If it’s something else, the only thing we can do is hammer away at it and try to overcome whatever is hanging it up.”