Video: Discovery's mission
updated 8/27/2009 12:38:26 AM ET 2009-08-27T04:38:26

A suspect fuel valve on the space shuttle Discovery functioned correctly five times during launch pad tests on Wednesday, NASA said.

The results mean there's still a chance that NASA will go ahead with a launch attempt at 12:22 a.m. ET Friday, but the decision won't be made until Thursday afternoon, just hours before the appointed time for liftoff.

The fill-and-drain valve on Discovery signaled a failure to close on Tuesday, while the external fuel tank was being filled for launch. The apparent failure forced a halt to the countdown and led to a 48-hour postponement, giving engineers time to drain the tank and diagnose the problem.

The 8-inch (20-centimeter) valve, part of the main propulsion system, is used to control the flow of liquid hydrogen from the fuel tank. It has to be closed before launch, and opened in order to drain the tank following a launch delay.

The valve worked fine during the first launch attempt early Tuesday, but thunderstorms prevented Discovery from blasting off at that time. The weather outlook was better during Tuesday afternoon's countdown, but then the valve problem cropped up.

Engineers suspected that the problem had to do with the indicator system rather than the valve itself, said Mike Moses, chairman of NASA's mission management team.

The results from Wednesday's test supported that view. In an online status report, NASA said the valve "opened and closed when commanded five different times with no issues."

"Teams are now performing a pressure test of the propulsion system, and recording the leak rate of small amounts of liquid hydrogen," according to the status report.

The mission management team is to weigh the engineering teams' findings Thursday afternoon. If they can be assured that the valve will actually work properly, managers could decide to clear the shuttle for launch even if indicators show during the fueling process that the valve has not closed.

The countdown to a Friday launch is due to pick up at 8:57 a.m. ET Thursday, even before the managers meet.

NASA says its current opportunity to launch Discovery to the international space station lasts until Sunday. After that time, the space agency would have to put Discovery's liftoff on hold to let the U.S. Air Force as well as Russian and Japanese space agencies launch other spacecraft. Discovery's mission would have to wait until mid-October.

Discovery is loaded with thousands of pounds of space station supplies, experiments and equipment, including a new treadmill named after Comedy Central’s Stephen Colbert. The shuttle is also due to take up a new crew member for the station, rookie NASA astronaut Nicole Stott, and bring station resident Tim Kopra back down to Earth.

More on international space station | shuttle Discovery

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