Telescope Faces Transmission Problems : Space: A crippled antenna could permanently slow the sending of data to earth. NASA is discussing a shuttle repair mission.

WASHINGTON — Declaring that the Hubble Space Telescope “is not in trouble,” NASA officials nevertheless conceded Monday that its partially crippled antenna may permanently slow data transmission and force scientists to reschedule their observation times.

Space officials also revealed that a repair mission by a shuttle crew is under preliminary discussion, although that is a remote possibility, at least for now.

In the meantime, computer scientists on the ground are racing to develop a software program that would allow the disabled antenna, now shut down, to be operated at 75% capacity.

“I feel there will be no impact on the amount of science you can do,” said Jean Olivier, telescope manager at NASA’s Goddard Space Flight Center in suburban Greenbelt, Md.

“I think only rarely will we find that you could not accomplish a basic mission requirement because of this,” he predicted.

Astronomers with projects on the much-delayed telescope were disappointed but hopeful that the glitch will delay data transmission by perhaps only a few weeks.

“After all, we’ve waited 10 billion years,” said John Bahcall, an astrophysicist at Princeton’s Institute for Advanced Study.

NASA officials said they still hope they will be able to begin the six- to eight-month process of powering up and calibrating the array of delicate instruments aboard the telescope by mid-May.

They also still expect by this weekend to obtain the first if fuzzy images of the cosmos gathered by the $1.5-billion instrument.

The persistent problem involves the telescope’s “high-gain” antenna, a bowl-shaped dish mounted on a 15-foot mast.

It and a second, “low-gain,” antenna extend from each side of the 43-foot-long telescope. They are controlled by motors that move them up and down, and from left to right.

The ability to swivel 180 degrees from side to side and from front to back allows each antenna to lock on a nearby special NASA communications satellite and relay images and data back to earth--at a volume of roughly the equivalent of a 30-volume encyclopedia in 42 minutes.

On Thursday, however, when Goddard’s controllers tried to activate the high-gain antenna, sensors suggested that it was stuck, causing the entire spacecraft to shut down, as it is designed to do in case of problems.

Using photographs and then makeshift models to test the scenario, NASA engineers late Sunday night confirmed that the antenna’s counterweight was bumping into an electrical cable, which led to the shutdown. The cable is out of position because it now appears to be about an inch too long, according to Olivier and Michael Harrington, director of the telescope’s orbital checkout.

The resulting obstruction, they said, apparently only partially affects lateral movement. Instead of moving a full 90 degrees in either direction, the antenna moves roughly 78 degrees in one, as yet unknown, direction.

“It’s kind of like you had a sore wrist and you couldn’t move it too far to one side but you could move it the other way,” Olivier said.

“We are moving now to adapt the spacecraft to operate within the confines of this situation as we understand it,” he added.

One bit of good news, according to Bahcall, is that the low-gain antenna appears to be performing better than expected, and thus may take up some of the slack.

He and NASA officials pointed out that the problem does not affect the data-gathering; rather, it merely means a reduced capacity to transmit information back to earth.

As much as 90% or more of the observations planned during the early years of the telescope do not require direct transmission back to earth, they said. Instead, they are recorded for subsequent transmission.

“You may have to use a tape recorder in a case where normally you would be able to hook up directly to a satellite,” Olivier said. “This is a very minor limitation.”

But Roger A. Bell, a University of Maryland astronomer with programs aboard the telescope, expressed concern that the need to store far more data than anticipated may tax the telescope’s capacity. “It’s a bit of a worry,” he said.

It was in answer to a reporter’s question that Olivier disclosed that there already is talk within NASA of a shuttle mission to correct the cable.

But such an undertaking would require “a significant amount of planning,” he said.