KENNEDY SPACE CENTER, Fla.--The shuttle Endeavour's external tank was reloaded with a half-million gallons of rocket fuel Wednesday for NASA's sixth attempt to launch a high-priority space station construction mission.
The hydrogen vent line that derailed two launch tries in June worked normally and forecasters were hopeful afternoon showers would clear the area in time for launch at 6:03:10 p.m. EDT, roughly the moment Earth's rotation carries the launch pad into the plane of the space station's orbit.
Rain showers rolled over the Kennedy Space Center shortly after 1:30 p.m. and lightning advisories were issued for the launch complex 39 area. The official forecast called for a 60 percent chance of acceptable weather by launch time.
NASA's Mission Management Team, meanwhile, is assessing the health of fuel cell No. 3, one of Endeavour's three electrical powerplants. The shuttle's fuel cells combine onboard hydrogen and oxygen to generate electricity, producing fresh water as a by-product.
Test data indicates a potential problem with fuel cell No. 3 that could prevent the device from operating normally at low power levels after the shuttle is docked to the International Space Station and drawing power from the lab's solar arrays.
If that happens -- and engineers may not know until after docking -- the fuel cell might have to operate at higher power levels to prevent producing too much water. Operating at higher power levels, however, would consume more oxygen and hydrogen than expected and force the crew to shorten the mission.
Overnight testing indicates fuel cell No. 3 was operating within normal limits, but its performance after docking is difficult to predict.
There are no other technical issues of any significance at launch complex 39A. The only question mark is the weather.
Working by remote control, engineers in Firing Room 4 at the Kennedy Space Center began pumping a half-million gallons of liquid oxygen and hydrogen rocket fuel into the shuttle's external tank at 8:38 a.m. The process was completed by around 11:38 a.m. when the countdown entered a planned two-and-a-half-hour hold.
Endeavour's crew -- commander Mark Polansky, pilot Douglas Hurley, Canadian flight engineer Julie Payette, David Wolf, Christopher Cassidy, Thomas Marshburn, and space station flight engineer Timothy Kopra -- began strapping in for launch around 2:45 p.m.
Endeavour has now been delayed five times since an initial launch attempt June 11. The first two attempts were derailed by a leaking hydrogen vent line, but the most recent three delays -- July 11, 12, and 13 -- were caused by stormy afternoon weather.
Two shuttle missions -- STS-61C in January 1986 and STS-73 in October 1995 -- share the record for launch delays, each suffering through six launch slips before finally taking off on their seventh try.
If Endeavour is grounded for a record-tying sixth time Wednesday, Polansky and company will only have one more chance on Thursday before the flight slips to July 26 to make way for launch and docking of a Russian Progress supply ship. The forecast for Thursday is 50-50.