CAPE CANAVERAL, Fla. - The clock is once more ticking down to a Sept. 8 launch for NASA's space shuttle Atlantis after engineers all but cleared a fuel cell issue aboard the spacecraft, mission managers said late Thursday.

Atlantis is now set to launch six astronauts - commanded by veteran shuttle flyer Brent Jett - towards the International Space Station (ISS) at 11:40:32 a.m. EDT (1540:32 GMT) and deliver a new set of solar arrays and two massive trusses to the orbital laboratory.

Only one final analysis stands between Atlantis and a Friday launch attempt here at the Kennedy Space Center (KSC), though shuttle officials expect a favorable result.

"We have had to stand down two days to make sure we are safe to fly, we have gone the extra mile," NASA shuttle program manager Wayne Hale told reporters, adding that the risk of launching Atlantis is less than replacing the fuel cell in question. "I don't feel like we're not racing to the end of the [launch] window."

The announcement, which was not entirely unanimous among shuttle managers, comes one day after an electrical short in Fuel Cell 1 - one of the three units that produce the electricity for all of Atlantis' systems - forced mission managers to scrub a Wednesday launch attempt so engineers could track the problem.

The short stifled one of three alternating current (AC) power phases for the fuel cell's vital coolant pump motor, which prevent the generator from overheating in flight. While the other two phases continued to function perfectly, engineers wanted to make sure they wouldn't fail during Atlantis' 11-day flight and force the shuttle's STS-115 astronauts to come home early without completing all their objectives - known as a minimum duration mission.

Steve Poulos, NASA's orbiter projects manager, said the problem likely lies in thin wire for the Phase A power loop in Fuel Cell 1's vital coolant motor. The phase will be switched off to avoid future shorts during Atlantis's launch, but engineers will not know for sure what prompted the failure until they dissect Fuel Cell 1.

"Until we take the fuel cell out, we won't know exactly. It's a detective story," Hale said. "There is always the element that we could be wrong."

Poulos said the Fuel Cell 1 cooling pump last flew aboard NASA's Columbia orbiter during its STS-93 mission, which experienced an electrical short during launch due to wiring issues. However, the fuel cell was found to be unaffected from that flight. Replacing the 255-pound fuel cell aboard Atlantis, especially at the launch pad, is no small order and runs a higher risk of causing more damage to the orbiter than launching the spacecraft, he added.

"We're good to go fly, we're ready to support this mission," Poulos said. "I don't have a concern that we're going to lose this fuel cell going uphill or during the mission."

Atlantis' STS-115 mission is set to install the $371.8 million Port 3/Port 4 (P3/P4) trusses and wing-like solar panels to the port side of the ISS during a series of complicated robotics tasks and three spacewalks. The spaceflight is NASA's first dedicated ISS construction mission since late 2002 and the third shuttle launch since the 2003 Columbia tragedy.

The spaceflight has been in a pinch of sorts.

Launch lighting rules are in place to allow cameras to scan Atlantis' external tank for any signs of falling foam insulation - a debris hazard similar to that which doomed Columbia and its crew in 2003 - during launch. The resulting footage would be used to guide future external tank modifications.

While NASA's full window to launch Atlantis extended through Sept. 13, the space agency has also promised Russia's Federal Space Agency that it would stand down after Sept. 8 to allow the liftoff of a Soyuz spacecraft carrying a new crew to the ISS.

The shuttle was expected to undock from the ISS on Sept. 17, with the Soyuz to launch towards the station one day later.

NASA ISS program manager Mike Sufferdini said that Atlantis can now launch on Sept. 9 and undock on Sept. 18 within hours of the Soyuz liftoff. This will maintain the much-needed buffer time for the space station's three-astronauts to jettison an unmanned Russian Progress cargo ship on Sept. 19 and be ready for the arrival of the Soyuz on Sept. 20.

"We have kind of a choreography to do," Sufferdini said. "We have all these vehicles flying around, coming and going at the space station."

Should Atlantis remain grounded Friday, Sufferdini will meet with his Russian counterparts to review the Sept. 9 launch plan.