CAPE CANAVERAL, Fla. - NASA is now targeting Friday for the launch of its shuttle Atlantis, but the mission can only lift off if engineers settle a fuel cell system glitch that prevented a planned Sept. 6 space shot, mission managers said late Wednesday.

Wayne Hale, NASA's shuttle program manager, said Atlantis is set to launch no earlier than 11:40:32 a.m. EDT (1540:32 GMT) on Sept. 8 - the last available day in its current flight window - as engineers attempt to solve a power supply issue afflicting the cooling system of one of three vital fuel cells.

"We have had no failures, I should say, of this type before," Hale told reporters here at NASA's Kennedy Space Center (KSC), where Atlantis stands poised for flight at Pad 39B. "This is an interesting thing."

NASA's three space shuttles each use a trio of fuel cells to generate electricity during spaceflight, and have experienced some failures or issues - both before and during a shuttle flight - in the past. They run on liquid hydrogen and liquid oxygen and power not only the orbiter, but the cooling systems that prevent the fuel cells themselves from overheating.

While the afflicted unit - Fuel Cell 1 - is operating fine on two power supply lines known as "phases," a third phase appears not to be reaching its Freon coolant pump. It is possible, shuttle managers said, that additional analysis will find that those two phases are enough to support Atlantis' STS-115 construction mission to the International Space Station (ISS).

Engineers noticed the glitch early Wednesday and opted to scrub Atlantis' planned mid-day launch aimed at sending six astronauts, a massive pair of trusses and two wing-like solar arrays to the ISS. The shuttle's STS-115 mission is NASA's third since the 2003 Columbia accident and would mark the first dedicated ISS construction flight since late 2002. Aside from the fuel cell issue, Atlantis seems to be in good health.

"The rest of the ship is in good shape," Hale said.

A packed schedule

NASA could launch Atlantis as is without violating any shuttle flight rules - which require three operating fuel cells to fly - but that would leave Fuel Cell 1 one phase failure away from a forced shutdown to prevent overheating, said Ed Mango, NASA's deputy orbiter project office manager.

If the fuel cell failed during Atlantis' 11-day mission, the spaceflight would have to be cut short - a rare occurrence in NASA's 25-year shuttle history - and its crew forced to return to Earth without accomplishing all of its goals. No less than three spacewalks, two inspections of Atlantis' heat shield, the installation of two portside trusses and the deployment of two solar panels make up the busy schedule for Atlantis' STS-115 astronauts.

"We want to fly a good mission," Hale said. "We know that we could have a safe mission if the fuel cell were not available to us on orbit. But we want to have a successful mission, and that's something that we want to have all three of our fuel cells working properly for the full mission duration to accomplish."

Mango said engineers currently have a mountain of data to pore through, which would not be complete in time for a Thursday launch attempt. Even a Friday space shot is not rock solid, he added.

"If we get through all that data and we look up and it's Friday and we're able to go launch, then we'll recommend to go launch," Mango said. "If we are not ready for Friday, then we're going to make that recommendation to the [Mission Management Team]. We're going to let the data drive us."

A chief concern is whether the phase issue in Atlantis' Fuel Cell 1 coolant pump is an isolated glitch or a symptom of some larger problem with all three of the orbiter's electricity generators. Currently, NASA engineers don't believe that's the case since the signature has only appeared in Fuel Cell 1, but they want to be sure before deciding on a flight rationale or asking for an additional delay.

An October launch?

Hale said that shuttle managers are even considering a potential Sept. 9 launch, though the option is not preferred since it would leave no room for an extra docked day at the ISS should the solar array deployment go awry or engineers require additional inspections of Atlantis' heat shield.

Verifying the integrity of shuttle heat shields in orbit has been a priority for NASA since the loss of the Columbia orbiter after its own heat shield was critically damaged by external tank debris during launch. The orbiter broke apart during reentry on Feb. 1, 2003.

"We think it's prudent to keep a contingency day in our hip pocket," Hale said.

A Mission Management Team meeting is scheduled for 1:00 p.m. EDT (1700 GMT) on Thursday to review NASA's progress in tracking the fuel cell system issue, with a press briefing to follow some time after that, space agency officials said.

One restriction governing Atlantis' launch is a NASA rule to lift off in daylight to allow cameras to record the performance of the orbiter's external tank. If NASA keeps that rule and misses Atlantis' current launch window, the shuttle would have to wait until the next daylight opportunity - a few days surrounding Oct. 26. If the restriction - which Hale said he chose over the recommendations of shuttle managers - is removed, Atlantis could launch toward the ISS at the end of September or early October.

Flight conflicts

Also complicating the Atlantis' liftoff schedule is the planned Sept. 18 liftoff of a Russian Soyuz spacecraft to ferry two ISS Expedition 14 astronauts and space tourist Anousheh Ansari to the orbital laboratory. Space station flight rules require some buffer time between the shuttle's departure and the Soyuz's arrival to give the ISS crew some rest time, NASA has said. The Soyuz is expected to dock at the ISS on Sept. 20, kicking off more than a week of crew change activities before Ansari and two of the station's current Expedition 13 astronauts return to Earth on Sept. 29.

Today's launch scrub is the latest delay for Atlantis' STS-115 mission. The spaceflight was pushed from an Aug. 27 liftoff following a launch pad lightning strike and subsequent systems checks, only to be forced to first leave - then return - to Pad 39B as mission managers prepared the orbiter to weather a tropical depression.

NASA astronaut Robert Satcher told SPACE.com that while shuttle safety is paramount, a sense of disappointment is inevitable whenever a launch is scrubbed.

"Sure, everybody's human and that's human nature," Satcher said. "But at the same time, I think everybody is professional and we understand that these things happen."

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