CAPE CANAVERAL, Fla. - Space shuttle Discovery's flight to the international space station has been postponed until next week because of a sluggish valve and suspect bolts.
NASA called off Thursday night's launch because of last-minute concerns over bolts on the external fuel tank. A valve problem found later in the shuttle's main propulsion system delayed liftoff until at least Monday.
One of two valves that control the flow of fuel did not open fast enough during routine operations. The valves help the shuttle avoid jerky, pogo-stick-like motions during liftoff.
If the countdown had proceeded, computers would have sensed the sluggish valve and would have halted everything in the final nine seconds, shuttle manager James Halsell said.
Workers plan to enter Discovery's engine compartment Friday to replace the valve, a job that's been done just four times before in the history of the space shuttle program. The valve is located in a hard-to-reach area at the bottom of the compartment, and therefore requires three days of work.
This 100th space shuttle flight is a crucial space station construction mission. NASA, however, is in no rush.
''We think it's prudent to stand down,'' Halsell said. ''In other words, we do not want to get 'go fever.'''
Before launching Discovery, NASA also wants to understand what caused a bolt malfunction on the last space shuttle flight. The problem occurred during Atlantis' liftoff on Sept. 8, but was discovered only Wednesday.
While analyzing film returned to Earth aboard Atlantis two weeks ago, engineers noticed that one of the three bolts between Atlantis and the external fuel tank did not retract properly eight minutes into the flight. Photographs showed 2 inches of the 14-inch bolt sticking out on the tank.
NASA wants to determine, among other things, whether the bolt malfunction poses a danger. At worst, a protruding bolt could cause the separated tank to tumble and slam into the shuttle.
''I think the word you use would be 'catastrophic,''' said Halsell, himself a shuttle pilot. ''I would not want to expose astronauts to that risk.''
Halsell said NASA remains committed to launching space shuttles seven or eight times a year. Next time, though, the space agency may try to analyze the film more quickly, he noted.
The troublesome bolt from Atlantis is at the bottom of the Atlantic, along with what is left of the rest of the external fuel tank. The 153-foot, rust-colored tank is jettisoned once the shuttle reaches orbit.
There is evidence of bolt problems on previous flights, including Endeavour's launch in February, said launch manager Bill Gerstenmaier. But the bolts have never protruded like this, he said.
NASA would not speculate on how long it would take to replace or fix the bolts on Discovery's fuel tank, if that became necessary.
''We hold out all hope that through analysis ... we can make ourselves feel comfortable that we're safe to go fly,'' Halsell said. ''I also want to say our minds are wide open'' to the possibility of a lengthy delay, he added.
Discovery holds two new segments for the international space station, a girderlike truss and a docking port for future shuttle visits. NASA wants these parts installed before the first permanent crew lifts off at the end of the month.
NASA usually has more time between flights to review all the data from the previous mission, but has quickened its launch pace to build the international space station.
The minimum number of days allowed between shuttle launches is 21. This gap would have been 27 days if Discovery had soared Thursday.
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