Repairs on the vent line at launch pad 39B began midday Friday and, if everything goes well, are expected to conclude Saturday night after the patched leak site passes a full suite of inspections, said NASA spokesman Bruce Buckingham.Shuttle managers also have ordered workers to top off Atlantis' onboard supply of liquid hydrogen and liquid oxygen, reactants that are combined within fuel cells to generate electricity and water for the seven-member crew during the flight.
Completing those tasks in time for a Sunday launch attempt wasn't a sure thing, so mission managers decided to target Monday instead, Buckingham said.
Having already taken the news of Thursday's scrub in stride, the Expedition Four crew now onboard the 17-story frontier outpost sounded a little surprised Friday when Mission Control in Houston radioed up word to expect their company to be on the way as early as Monday.
"So they think they can get it fixed and ready to go by Monday, then?" said flight engineer Carl Walz.
"That's affirmative," replied astronaut Joan Higginbotham from the space station control room.
"That's great news," Walz added. "We'll be ready to watch them on Monday and we'll keep cleaning the place up for them."
Liftoff is expected between 2 and 6 p.m. EDT (1800 and 2200 GMT). As before, NASA will not announce the exact launch time until 24 hours prior to the shot. The reason: concern that terrorists could more easily plan an attack on the space center if they knew the precise time sooner.
The early weather outlook is extremely favorable, predicting a 90 percent chance of acceptable conditions at launch time. The same probability is stated for an attempt on Tuesday, while the chances worsen to 70 percent "go" on Wednesday.
NASA will be able to make five launch attempts in any combination beginning Monday and ending seven days later.
April 14 is the last day the shuttle can fly before the mission must be delayed into May because of a previously planned Soyuz taxi flight to the frontier outpost scheduled to begin April 25, Buckingham said.
With three Expedition Four crewmembers at the station now, officials say they don't want to have seven shuttle astronauts and three more Soyuz taxi crewmembers at the complex all at the same time.
As a result, if for some reason Atlantis still cannot get off the ground within the next week, the plan is to have Russia fly its Soyuz mission next and then NASA can try again to launch Atlantis after the Soyuz undocks on May 5, officials said.
Shuttle mission managers will next meet at 3 p.m. EDT Sunday to hear if the pipeline repair and inspections are complete and formally decide if the agency is ready to make another run at launch, Buckingham said.
If Monday remains "go," an announcement about the exact launch time would follow that meeting.
During the 11-day mission the crew is to install onto the station a $790 million collection of electronics and plumbing packed inside a 44-foot-long (13-meter-long) girder that will be the centerpiece -- literally -- of a giant beam that eventually will stretch more than the length of a football field.
Known as the S-Zero truss, the Boeing-built contraption contains 475,000 parts and includes the initial components of the world's first railroad in space, called the Mobile Transporter. The device will allow the station's Canadian robot arm to be moved along the length of the truss to assist in the complex's future construction.
It will take four spacewalks by two pairs of astronauts to make all of the structural and system connections between the S-Zero truss and the Destiny science laboratory.
If Atlantis lifts off on Monday, the STS-110 mission will conclude with the shuttle returning to a Florida landing on April 19.
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