CAPE CANAVERAL, Fla. - Hoping stormy skies clear in time, NASA fueled space shuttle Endeavour for a sixth launch attempt Wednesday, the eve of the 40th anniversary of the liftoff for the first moon landing.

Endeavour, in fact, was poised at the very spot where Apollo 11's Neil Armstrong, Buzz Aldrin and Michael Collins rocketed into history on July 16, 1969, aboard a twice-as-high Saturn V rocket. The shuttle and its seven astronauts are bound for the international space station with a Japanese building block.

Thunderstorms - packing lightning, heavy rain and high wind - whipped across the launching site Monday afternoon. Conditions eased just as the astronauts departed for the launch pad. Forecasters said there was a good chance the weather would improve by the 6:03 p.m. launch time.

The astronauts had big smiles and waved as they made their way to their rocketship for the third time in four days.

Besides the weather, NASA was dealing with a possible technical concern.

Mission managers were gathering in midafternoon to discuss the condition of one of the shuttle's fuel cells. It's possible that the fuel cell - one of three identical electrical powerplants - might not be able to operate at low power during the 16-day flight, which could cut short the mission.