Friday is the last launch day available before the U.S. space agency runs into a scheduling conflict with the Russian space agency.
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 fuel cell could have delayed any launch attempt by several weeks.
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 3½ years ago.
If Atlantis does not get off the ground on Friday — the launch is set for 11:40 a.m. EDT — 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.
By launching on Saturday, NASA would have to shorten its 11-day construction mission to the international space station, something Wayne Hale, space shuttle program manager, has said he would not like to do given the complexity of adding a 17½-ton segment to the orbiting space lab.
The mission would have to be curtailed since NASA made an agreement with the Russians to undock from the space station by Sept. 17 because Russia is sending a three-person Soyuz capsule to the space station on Sept. 18.
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.