For the third time this week, NASA fueled space shuttle Discovery for a Friday night 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 percent chance of acceptable weather for the planned 11:59 p.m. launch. "Our weather forecast is getting more optimistic as the evening progresses," said NASA spokesman Mike Curie.

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.

The 8-inch fill-and-drain valve is a critical part of the main propulsion system.

Discovery is packed with about 17,000 pounds of space station supplies. The most high-profile payload is a $5 million treadmill named after Comedy Central's Stephen Colbert.

Colbert tried to get a space station room named after himself and even won the online vote earlier this year, but NASA went with Tranquility instead in honor of the 40th anniversary of man's first moon landing.

Colbert said the treadmill — for "all those chubby astronauts" — is a consolation prize.

NASA must launch Discovery by Sunday, otherwise the 13-day mission will fall to October because of an upcoming space station traffic jam. A Japanese cargo ship and a Russian spacecraft are scheduled for September launches.

Flags flew at half-staff throughout Kennedy Space Center on Friday in memory of Sen. Edward Kennedy.