NASA calls off space shuttle Discovery launch because of a bad fuel valve.

The space agency was fueling the shuttle Tuesday for an early morning launch Wednesday.

Thunderstorms prevented Discovery from blasting off early Tuesday.

Discovery is bound for the international space station. It will haul up thousands of pounds of supplies, including six mice for a bone experiment and a treadmill named for Comedy Central's Stephen Colbert.

NASA has until the end of August to launch Discovery and its seven astronauts. Otherwise, the mission will slide into October because other countries are scheduled to launch their spacecraft.