Friday had been the last launch day available before the U.S. space agency ran into a scheduling conflict with the Russian space agency. But NASA managers now believe they can try Saturday, if needed, and they were finalizing negotiations with the Russians.
There was a 30 percent chance bad weather would interfere at the 11:40 a.m. EDT Friday launch time.
On Thursday, NASA decided not to change out an electricity-generating fuel cell whose coolant pump had given erratic readings, causing a scrub a day earlier. Replacing the 250-pound pump could have delayed any launch attempt by several weeks and might have been riskier than leaving it in place, said Steve Poulos, shuttle orbiter projects manager.
The most likely cause of the erratic reading was thinning wires on a motor which hadn't been used in seven years, Poulos said.
"Pulling out a 250-pound piece of hardware and getting it in and out ... and at the end of the day not having a problem, well there's just risk associated with that," Poulos said. "It became a very easy recommendation for me ... to say 'We're good to go fly."'
The decision to try a launch on Friday wasn't unanimous. Officials in NASA's safety office felt the fuel cell should be changed out not because of a safety issue, but they were concerned about whether the mission could be carried out successfully. The fuel cell's manufacturer, UTC Power, wanting NASA to use a pristine unit, also recommended against flying until it was swapped out, said Wayne Hale, space shuttle program manager.
But late Thursday night, the company said it was fine with the decision to fly.
"We're very comfortable with flying," said Henry DeRonck, general manager for space at UTC Power. "There is very low risk of anything getting further worse."
After this weekend, the next daylight launch opportunity is not until the end of October. NASA rules say Atlantis must lift off in daylight so that its big external fuel tank can be photographed for any signs of broken-off foam of the sort that destroyed Columbia 3 1/2 years ago.
If Atlantis does not get off the ground on Friday, NASA officials had two options they were reluctant to exercise that would permit a launch attempt before the end of October: Try on Saturday, or relax the daylight-launch rule.
NASA managers originally believed the 11-day construction mission would have to be shortened if Atlantis were launched on Saturday. NASA had made an agreement with the Russians to undock from the space station by Sept. 17 because Russia is launching a three-person Soyuz capsule to the space station on Sept. 18.
But NASA was negotiating with the Russians about possibly undocking Sept. 18 if the U.S. space agency decides to launch the space shuttle on Saturday, said Mike Suffredini, space station program manager.
Relaxing the daylight rule would open up launch chances in late September and early October.
Atlantis' astronauts will restart construction on the half-built international space station for the first time since the Columbia disaster.