A spare liquid hydrogen recirculation line awaits its installation into shuttle Discovery. Photo NASA/KSC

Discovery's astronauts will spend Christmas in space to repair the crippled Hubble Space Telescope, if workers can replace a damaged fuel line in time to launch the shuttle this year.

Shuttle managers decided Thursday to replace a crushed liquid hydrogen recirculation line inside Discovery's engine compartment, a job that has never been done before in the hangar let alone vertically at the launch pad.

If the work goes well, Discovery would be cleared for launch December 16 on a 10-day mission that would become just the fifth U.S. space mission to occur over Christmas. Other flights include Apollo 8 in 1968, Skylab 4 in 1973, John Blaha's stay on space station Mir in 1996 and David Wolf's Mir residence in 1997.

"At this point we have targeted the 16th," shuttle program chief Ron Dittemore told reporters. "It is a lot of work to do. We are still in the planning stages of how we are going to (replace) this part over weekend."

The four-inch diameter pipe actually contains two lines in one -- an inner liner and outer wrapper with a vacuum between the two. An inner line is where super-cold liquid hydrogen flows between the shuttle and external fuel tank to keep the main engines conditioned for ignition.

The line was first damaged in 1985, a year after Discovery's maiden flight. The initial dent was three-tenths of inch deep and six-inches long. That was not considered a problem and the shuttle continued to fly two dozen times with the slight damage.

Workers inspect a foot-long section of a hydrogen line crushed inside shuttle Discovery. Photo NASA Watch

But workers on Tuesday found the dent had grown to twice its original size, or six-tenths of an inch deep and one-foot long.

"We have flown this particular part in a dented configuration since 1985. Now all of the sudden we look at it and (the dent) has really doubled. Something happened recently, we believe," Dittemore said.

Likely causes for the new damage includes being struck by a platform or a worker stepping on the line. There is also the remote chance the vacuum in the line caused the outer wrapper to constrict, Dittemore said.

Concerns that the outer wrapper was dented deep enough to damage the inner caused officials to make the replacement.

Dittemore said if the inner line was damaged it would have scrubbed the launch and created a dangerous hydrogen leak.

"In the launch count, we would end up scrubbing because we could not maintain the temperature of the propellant and we could conceivably have a hydrogen leak. The same problem would occur in flight, you would have a leak of hydrogen and an extremely unsafe condition."

Dittemore says the six-foot long, 40-pound line is like an octopus with five tentacles that have to be connected to parts of the shuttle, and will be a challenge to replace in the cramped engine compartment at launch pad 39B.

"Because we have not (replaced) it before, the guys have gone out and taken the measurements on the spare part and compared it to the measurements of the (shuttle) door and the access they will have in the aft compartment. They have convinced themselves, although through sight, they believe they can get it in the door and maneuver it around and install it.

"They described it a little bit like moving the sofa through the door. You have to be careful and you have to have the right guidance 'turn a little bit left here, lift up there.'"

Gary Hamilton and James Stickley, both with United Space Alliance, check out the spare four-inch diameter liquid hydrogen recirculation line that will be used to replace a damaged line in Discovery. Photo NASA/KSC

The replacement work is expected to take at least three days beginning on Friday when the old line will be removed. A new, spare line should be installed starting on Saturday, NASA spokesman Joel Wells said.

Testing will follow and Discovery's engine compartment is scheduled to be closed for launch on Monday, clearing the way to begin the three-day launch countdown on Tuesday at 1:30 a.m. EST (0630 GMT).

A launch on December 16 would feature a 41-minute window opening at 9:18 p.m. EST (0218 GMT on 17th) for Discovery to lift off to begin an orbital chase with Hubble.

The shuttle would then complete its normal 10-day mission in which four astronauts will make four spacewalks to fix Hubble's broken pointing system and install new electronics.

Landing at Kennedy Space Center would be scheduled for December 26 at about 6:40 p.m. EST (2340 GMT). If bad weather at the landing site delayed Discovery's homecoming, the shuttle could be kept in space two extra days.

NASA wants Discovery on Earth no later than December 28 to protect against any problems that could occur because of the Y2K computer bug.

If Discovery doesn't lift off on December 16, NASA could still launch the mission on December 17 or 18. But in those later cases, the mission would have to be shortened, even canceling a spacewalk, so the shuttle can land on December 26, plus adding two extra days in the event weather keeps Discovery in space until December 28.

"The real driver on this whole situation is we be on the ground, safe the orbiter and then have time to do an orderly shut down of the ground computer systems prior to Y2K," Dittemore said.

"We have never said that we would preserve the holidays. We said we would protect against Y2K."

Dittemore says cutting one spacewalk, or EVA, from Discovery's mission would not be a problem.

"When we first started talking about this particular reservicing mission, we had planned for a three-EVA mission with an option to do another EVA to do some get-ahead tasks for the next servicing mission."

The main purpose of Discovery's mission should be accomplished on the first walk when astronauts Steve Smith and John Grunsfeld install six fresh gyroscopes to restart Hubble's pointing system.

"Three EVAs is all we need to do this mission. One EVA is all you need to (replace) the gyros and declare victory. The two other EVAs start adding, repairing and replacing other equipment that improve the operation of the telescope."