The decision to make the launch attempt a minute before midnight (0359 GMT Saturday), nearly a day later than planned, was issued after experts reviewed tests on a liquid hydrogen fill-and-drain valve that malfunctioned earlier in the week in Discovery's main propulsion system.

"It was announced at today's mission management team meeting that the teams need another 24 hours to review data from yesterday's fill-and-drain test before pressing forward with launch of space shuttle Discovery," NASA said Thursday

NASA had earlier begun the countdown to launch Discovery on Friday at 12:22 am (0422 GMT) with astronauts preparing for a 13-day mission to supply and repair the International Space Station (ISS).

The hydrogen fuel tank valve functioned normally in tests conducted by NASA engineers, who now believe that problems encountered when filling the shuttle's external fuel tank were due to false readings, NASA said.

But the space agency then put back the launch to allow a review of results and give teams extra time "to work on options in case the problem with the valve occured again," said NASA spokesman Allard Beutel.

In five rounds of tests performed Wednesday, technicians showed that the valve "did exactly what it should," said the agency's Mission Management Team director Mike Moses.

Yet despite the confidence, Moses said his team needed the day to "polish that off (and) really understand what is going on."

The launch, if it goes ahead, would be NASA's fourth scheduled attempt after liftoff was also delayed Wednesday and thunderstorms led NASA officials to scrub the first bid early Tuesday.

At the space station, a key task during the three scheduled spacewalks on Discovery's mission will be to replace an old liquid ammonia coolant tank, which will be substituted with a new, 1,760-pound (800-kilogram) replacement.

The new freezer will store samples of blood, urine and other materials that will eventually be taken back for studies on the effects of zero-gravity.

The astronauts were to retrieve experimental equipment from outside the ISS and return it to Earth for processing.

They will also be delivering a treadmill named after the wildly popular US satirist Stephen Colbert -- the second aboard the ISS.

The Combined Operational Load Bearing External Resistance Treadmill (COLBERT) was named after the bear-fearing host in April.

Exercise is important for astronauts spending long periods of time in space, because zero-gravity can result in muscle atrophy.

The Discovery will bring astronaut Nicole Stott to take the place aboard the ISS of Tim Kopra, who will ride the shuttle back to Earth.

The shuttle commander is to be veteran astronaut Rick "C.J." Sturckow. Other astronauts on the crew include pilot Kevin Ford and mission specialists Patrick Forrester, Jose Hernandez, John "Danny" Olivas, and Christer Fuglesang of Sweden.

Once the Discovery mission is complete, just six more shuttle flights remain before NASA's three shuttles are retired in September 2010.

The ISS is a project jointly run by 16 countries at a cost of 100 billion dollars -- largely financed by the United States.

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