The delay is forcing NASA to shorten the mission and eliminate some planned spacewalks.
If Discovery launches Sunday, at 7:43 p.m., the mission would last 11 days and include three spacewalks instead of four.
If the launch comes Monday or Tuesday, which is the last possible day, the flight would last 10 days and feature only one or two spacewalks.Some work would be passed off to the permanent station crew to do later.
"We're really trying hard to make the 15th if we can. The 16th might just be the day that's out there. The date will be what it is and we'll adjust to it," shuttle launch integration manager Mike Moses said.
After Tuesday, the shuttle launch must wait until after an already-planned Russian Soyuz mission to deliver a fresh crew to the space station.
A shuttle orbiter can't be docked at the ISS while a second Soyuz is there for a variety of reasons, including the structural strain on the space station and the inability of life support systems to support 13 astronauts and cosmonauts for an extended period of time.
That would mean waiting until April 7. NASA says it can still complete its station construction objectives during a shortened shuttle mission. The space station program prefers the earlier, shorter flight because a delay to April could postpone plans to double the station’s resident crew to six in May.
NASA also is under a deadline to fly its remaining nine or 10 space shuttle missions by the end of 2010.
"It's getting really tight no doubt about it," said Moses, a former space shuttle flight director.
Asked if they were disappointed, managers were matter of fact about ensuring safety of their spaceship and crew.
"Our business requires perfection and our vehicle is not perfect today. So, we're going to stand down, we’re going to fix the vehicle and fly when it is perfect," shuttle launch director Mike Leinbach said.
As grounds crews fueled the shuttle for launch under near-perfect weather Wednesday, gaseous hydrogen leaked from a valve located on pad equipment outside the space shuttle.
The launch team could not stop the leak by opening and closing the valve several times. Leinbach said the amount of gas leaking was "large" and he decided to stop the countdown, drain the tank and go work on repair options.
The leak is dangerous because the gas could catch fire and lead to a catastrophic explosion at the launch pad.
Indeed, the pad area will remain too dangerous for workers to even get close enough to inspect the suspect components until Thursday afternoon when all of the fuel has evaporated from the 15-story tank.
NASA plans intense inspections, tests and repairs starting then. Discovery’s crew is set to install the final set of power-generating solar arrays at the space station.
Also impacted: An Atlas V rocket launch scheduled for Saturday night. If the shuttle moves to Monday, the Atlas V and a military communications satellite could still lift off at 9:25 p.m. Saturday.
IMAGE NOTES: The two images above show the suspect ground support equipment and where it connects to the shuttle external tank. These pictures were captured by FLORIDA TODAY photographer Michael R. Brown via NASA TV.
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