Late Tuesday afternoon, launch officials stopped the countdown midway through the fueling process. The seven astronauts had not yet boarded the shuttle for the scheduled early Wednesday morning flight to the international space station. A new launch date was not yet set right away.

NASA spokesman Allard Beutel says engineers were trying to open and shut the fill-and-drain valve in Discovery’s engine compartment when it looked to be broken. The exact condition of the valve is not yet known.

Thunderstorms prevented Discovery Space Shuttle 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.

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