The decision to make the launch attempt nearly 24 hours 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.

"Liftoff now is targeted for 11:59 pm (Friday, 0359 GMT Saturday).

NASA earlier Thursday had 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 NASA then put back the launch almost 24 hours after a review of the results to give teams extra time "to work on options in case the problem with the valve occured again," said Allard Beutel, a NASA spokesman.

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 also to retrieve experimental equipment from outside the ISS and return it to Earth for processing.

They will also be delivering a treadmill named after popular US comedy talk show host Stephen Colbert -- the second aboard the ISS. 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.

earlier related report
Discovery valve working, countdown to resume
Initial testing on a malfunctioning valve that grounded the space shuttle Discovery is encouraging and countdown will resume Thursday ahead of the next launch attempt, NASA said.

"There is no issue in the initial testing," Kennedy Space Center spokesman Allard Beutel told AFP late Wednesday.

Engineers, he said, commanded the liquid hydrogen fill-and-drain valve in Discovery's main propulsion system to open and close five times, "and it worked."

The tests seemed to indicate that the hydrogen fuel tank valve -- which scuttled a second launch attempt for Discovery that had been planned for early Wednesday -- was not broken. They suggested that the problems encountered when filling the shuttle's external fuel tank owed to false readings.

Engineers had tested the function of the valve, which facilitates the flow of liquid hydrogen from the external fuel tank to the three main engines, and had not been able to close it. The valve must be closed for the launch and opened in order to drain the tank if a launch is delayed.

Teams of engineers were also performing a pressure test of the propulsion system, and recording the leak rate of small amounts of liquid hydrogen, NASA said.

The technical team will present the results of its tests and analysis to the mission management team at a meeting Thursday.

NASA has targeted Discovery's third launch attempt for no earlier than Friday at 12:22 am (0422 GMT). But it cautioned that the decision depended "on the results of the testing and a review of the data by the mission management team."

In another sign that the US space agency was optimistic about going ahead with the planned launch, countdown was set to begin again Thursday at 8:57 am (1257 GMT).

And after thunderstorms led NASA officials to scrub the first lift-off attempt early Tuesday, the weather conditions also looked promising.

The latest forecasts gave a 70-percent chance of favorable conditions at the shuttle's rescheduled take-off time from Cape Canaveral, Florida.

But if the valve needed to be replaced, there would be no further launch attempts in August.

The shuttle has until Sunday to launch. After that, any further tries would have to be postponed until October 17 due to scheduling conflicts with rocket launches and tests, as well as upcoming launches to the International Space Station (ISS) from Japan and Russia.

earlier related report
Discovery grounded over hydrogen valve glitch
NASA was planning to launch the space shuttle Discovery early Friday after thunderstorms and a hydrogen fuel tank valve postponed two previous attempts.

The shuttle and its crew of seven astronauts were preparing for a 13-day mission to supply the orbiting International Space Station (ISS).

Ahead of the next planned launch attempt, engineers will evaluate a liquid hydrogen valve that became problematic when technicians began fueling Discovery on Tuesday, mission management team chairman Mike Moses told reporters.

It was the second scuttled launch in a 24-hour period. A first launch attempt early Tuesday was scrubbed due to stormy weather.

Moses said detailed test data about the valve would be examined before the shuttle's fuel tank is filled with propellant ahead of the planned launch shortly after midnight.

If engineers cannot determine the exact nature of the problem or if the vlave must be replaced, he warned, "then we will probably not be in a position to launch 48 hours from now."

Weather and hurricane risks could also prevent a Friday launch, he noted.

Discovery can blast off until about Sunday, after which any attempts would have to be postponed until mid-October due to scheduling conflicts.

US space agency officials said the problem valve cycles liquid oxygen from the external fuel tank to the shuttle's main propulsion system, and technicians will attempt to fix it while Discovery remains at the launch pad, positioning it for a third launch attempt Friday at 12:22 am (0422 GMT).

The delays were a reminder of turbulence that surrounded the previous mission, when space shuttle Endeavour's launch was postponed five times by weather woes and technical glitches.

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