NASA May Need Rescue Mission to Save Galileo

Just as the Galileo spacecraft nears the end of its 14-year-old struggle, its primary communications link to Earth has become so crippled that it may be necessary to mount the most distant rescue effort in the history of space exploration to salvage the $1-billion-plus mission to Jupiter.

Engineers at the Jet Propulsion Laboratory in Pasadena have not given up hope of figuring out how to get the 16-foot antenna to open properly, but there is growing concern that the only answer may be to send a communications satellite to Jupiter to relay Galileo’s discoveries to Earth.

There is no indication whether the National Aeronautics and Space Administration would be willing to undertake such a costly salvage operation, but if all else fails, that could be the only answer, according to engineers at the lab, which is managed by Caltech for NASA.

Galileo’s 16-foot-wide high-gain antenna failed to open properly when commanded to do so April 11, leaving the craft with no way to send back the enormous amounts of data it is to collect during a two-year exploration of Jupiter and its dynamic moons. The opening of the antenna was the last major hurdle Galileo faced on its journey to the largest planet in the solar system.

“We were almost there,” project manager William O’Neil said.

It has been a devastating setback for scientists and engineers who have worked 14 years to keep the often-threatened Galileo mission alive.

“It’s heartbreaking,” said Moustafa Chahine, chief scientist at JPL. Chahine said that a rescue mission with a communications relay satellite is technologically feasible, although for now engineers at the lab are concentrating on trying to make Galileo heal itself sometime during the months ahead.

“People are committed to this,” O’Neil said. “We’ll never give up.”

He added that no one knows what to do, and engineers plan a cautious approach because they do not want to damage the antenna in their efforts to fix it. There is no urgency because Galileo will not reach Jupiter until December, 1995.

The antenna was being opened in preparation for an encounter with asteroid Gaspra this October. O’Neil said the quality of the data from that rendezvous, the first with an asteroid, will be degraded somewhat, but a smaller antenna will provide nearly adequate communications until Galileo reaches Jupiter and begins collecting enormous amounts of information.

The Galileo problem comes at a time when NASA scientists are working on another major snafu, the Hubble Space Telescope, which was launched into orbit two years ago with a flawed optical system.

In the 14 years since work began on Galileo, the project has faced many threats. Its launch was delayed several times because of technical problems and great demands on the space shuttles. It was finally set for launch in 1986, but weeks before the scheduled launch the Challenger exploded, forcing such a lengthy delay that NASA seriously considered canceling the program. The decision was made to go ahead, but new safety concerns prohibited using the propulsion system that was to carry Galileo from the space shuttle to Jupiter.

As a result, less powerful solid fuel rockets had to be used for that part of the journey, and they could not provide the thrust needed for a direct trajectory to Jupiter. So Galileo was launched on a course that would allow it to fall toward the sun, thus picking up speed, and then use the gravity of Venus to fling itself back toward Earth.

Galileo flew past Earth last December, and will do so again in 1992, picking up additional speed with each passing. The second encounter will send the spacecraft toward Jupiter.

Although many scientists had worried for years over whether they would ever see the 2.5-ton spacecraft launched, it has performed perfectly during the first year and a half of its expedition.

There was no need to open the large antenna earlier because the smaller antenna provided the necessary communications with Earth. Also, the larger antenna had to remain closed to protect it from the intense heat of the sun during its flight past Venus, where solar radiation is more than twice as great as it is on Earth.

Engineers had no reason to worry about the antenna because six nearly identical antennas have been used on NASA’s primary communications satellites, and none had any problems.

No one is certain why this one failed, although the leading theory is that the sun’s heat somehow damaged the antenna, despite efforts to shield it. Ironically, it is the apparatus that was supposed to shield the antenna that has provided the clues that have allowed engineers to figure out the antenna’s condition.

The antenna opens a little like an inverted umbrella. A small disk, called a “sun gate,” at the top of the assembly was designed to keep Galileo pointed so the antenna would remain shaded from the sun.

During deployment of the antenna, small motors drive a nut up a threaded shaft, in turn pushing 18 ribs outward, thus opening the antenna much the same as an umbrella.

When the antenna is fully open, the ribs should come in contact with sensors that tell engineers that the antenna is deployed.

“It was when we didn’t see that that our faces drained,” O’Neil said.

Other symptoms, including rising electrical current drawn by the stalled motors, told the engineers that the antenna was stuck. It did not tell them what specifically had gone wrong, so engineers were left with a technological mystery.

That is where the sun gate came into the picture. The sun gate reflects a cone of sunlight, similar to a flashlight beam, onto sensors that tell Galileo if the antenna is drifting toward the sun and thus in danger of overheating. As they examined the data from Galileo, engineers discovered that the cone of sunlight had been distorted.

Further analysis told them that one rib of the antenna--and only one rib--had failed to open and was blocking part of the sunlight. O’Neil said the ribs were supposed to open 68 degrees from the shaft, but the data showed that one rib had only opened about 35 to 40 degrees.

“There is a chance heat caused the rib to warp” during the Venus flyby, thus keeping it from deploying properly, he said.

After the antenna failed to open, the motors were shut off and it partially folded back up again. Today, it resembles a claw rather than the open dish it was supposed to become.

In its present condition, it is useless.

A high-gain antenna is “like a headlight beam,” O’Neil said. Its shape must be perfect in order to transmit with the pinpoint precision needed to ship prodigious amounts of data from Jupiter.

Engineers can try again to open the antenna, but the antenna cannot reclose completely because the motors cannot be reversed. Ground controllers can turn them on again, and hope that this time the balky rib will snap into place, but they do not want to do that until they understand the problem fully.

“We’re concerned about breaking something,” O’Neil said.

For the next few months, a similar antenna will be put through tests at JPL, including radical changes in temperature, to see if the problem can be duplicated on the ground.

Finally, they will order Galileo’s computer to turn the motors on again. If the antenna refuses to open, the Galileo mission will be severely curtailed. One part of the mission, a probe that is to pierce through the dense atmosphere of Jupiter, can still work because the smaller antenna aboard Galileo could transmit that data to Earth.

But “those gorgeous images would be lost,” O’Neil said. Detailed pictures of Jupiter--and especially its volcanic moon Io--are among the most cherished treasures Galileo was sent to get.

A rescue mission would be possible, but there are no cost estimates available.

A communications satellite could be launched directly to Jupiter by a Titan 4 rocket, and it could be there waiting for the spacecraft because of the circuitous journey required for Galileo.

It would take three years for the satellite to get to Jupiter, even on a direct course, and if the launch is delayed so long that Galileo arrives there first, that is no problem, O’Neil said.

Galileo could be put into a “parking orbit” and travel around Jupiter until the satellite arrives.

The relay satellite is what O’Neil calls his “ace in the hole,” but even he admits this is a problem he never expected to see. The antenna is relatively a “simple mechanism,” and although every maneuver in space is filled with anticipation and anxiety, no one expected the antenna to fail.

That has left a highly sophisticated spacecraft, with the finest in electronics and computers that talk to each other, crippled by a simple mechanical failure.

“It’s on its way,” after years of frustration, O’Neil said. “It’s beautiful. All we’re lacking is the antenna.”

TELESCOPE PLANNED: A $74.6-million grant will allow Caltech to build a twin to theworld’s largest telescope. B1

GALILEO IN TROUBLE

The 16-foot antenna on the Galileo spacecraft has failed to open. If it cannot be fixed, the spacecraft will not be able to transmit most of its scientific data and photographs of Jupiter, where it should arrive in 1995, back to Earth. 1. Engineers believe one rib in the antenna may have warped, preventing it from opening. 2. On April 11, during this portion of Galileo’s flight, the malfunction occurred. 3. One option being considered is to send a communications relay satellite to Jupiter. The satellite would take data transmitted by Galileo and relay that on to NASA’s large antennas on Earth.