For the third time this week, NASA fueled space shuttle Discovery for a launch to the international space station after resolving a fuel valve problem.
The seven astronauts waved as they headed out to the launch pad under clearing skies. Thunderstorms surrounded the area earlier in the evening, and three lightning strikes were recorded within five miles of the pad.
Forecasters said there was a 60 per cent chance of acceptable weather for the planned 11:59 p.m. (0359 GMT) launch. "Our weather forecast is getting more optimistic as the evening progresses,'' said NASA spokesman Mike Curie on Friday.
Discovery and its crew tried to blast off early Tuesday, but storms kept them on the pad. Then the fuel valve trouble struck and scuttled Wednesday's try.
Engineers suspected the hydrogen fuel valve inside Discovery was fine and that an indicator switch was faulty. Mission managers agreed to a workaround plan, if the indicator acted up again. But everything worked during Friday's fueling, and applause filled the firing room when the indicator switch properly showed the valve to be closed.