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It will be NASA's third attempt to send Discovery to the international space station with a load of supplies. Fueling should get under way Friday afternoon.
Forecasters say there is a 60 percent chance of acceptable weather. Thunderstorms are the concern for the launch, which is scheduled for 11:59 p.m.
Discovery and its seven-member crew tried to blast off early Tuesday, but storms kept them on the pad. Then fuel valve trouble struck and scuttled Wednesday's try.
Engineers believe the hydrogen fuel valve inside Discovery is fine and that an indicator switch is faulty.
Managers have agreed to a workaround plan, if the indicator acts up again.
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