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The space shuttle Discovery - Source: Reuters -
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NASA delayed liftoff of the space shuttle Discovery because of an apparent problem with a valve in its fuel tank and said the next possible launch opportunity was Friday afternoon.
It was the second consecutive delay, after stormy weather postponed a launch attempt early on Wednesday.
Discovery and its seven-member crew were preparing for a 13-day supply mission to the International Space Station.
The valve problem surfaced as technicians began fuelling up the shuttle for launch from the Kennedy Space Center in Florida.
"When we went to close the valve (after the hydrogen tank was full) we didn't get an indication that it was closed," Mike Moses, who oversees the shuttle program at the launch site, told reporters.
Engineers believe the problem to be with instrumentation that monitors the valve, and not the valve itself, which would position NASA for a possible third launch attempt at 4:22pm on Friday.
Anything more serious and the launch likely would be postponed until October.
NASA has until Sunday for launch attempts before it would have to stand down due to a schedule conflict at Cape Canaveral Air Force Station, which shares safety, tracking and support services with Kennedy Space Center.
In addition, the International Space Station is preparing for its first cargo ship from Japan and cannot receive the shuttle and the new Japanese vehicle simultaneously.
Discovery is scheduled to deliver lab equipment, supplies and spare parts to the station.
Including Discovery's flight, NASA plans seven more shuttle missions to the space station. The 16-nation, $US100 billion project is nearing completion after more than a decade of construction 350 km above Earth.
The shuttles are being retired late next year or early 2011. NASA is working on a new type of spaceship called Orion that can travel beyond the station's orbit to the moon and other destinations.
After the shuttle program ends, the station will depend on Russian, European and Japanese vehicles, though none has nearly the lift capacity of the shuttle, which can haul 23,000kgs.
Discovery carries new science racks, a freezer, a sleep station, a second treadmill, food, clothing, supplies and spare parts for the station, including an 800kg ammonia coolant tank that will be installed during two of the spacewalks.
Among the experiments planned for the station is a bone-loss study that uses genetically altered mice as subjects.
The mice are flying to the station aboard Discovery and will be left aboard until the next shuttle mission in November.
