By Marcia Dunn

The Associated Press

CAPE CANAVERAL, Fla. - NASA called off Thursday's launch of space shuttle Discovery because of last-minute concerns over bolts on the external fuel tank.

Within a few hours, another problem cropped up, this one involving a valve in Discovery's main propulsion system. The valve, which controls the flow of fuel, appeared to be sluggish. To replace it would take three days.

The space agency aimed for a Friday night liftoff but stressed that both problems must be solved before Discovery can fly.

This 100th space shuttle flight is a crucial space station construction mission.

"We think it's prudent to stand down for a day," said shuttle manager James Halsell. "In other words, we do not want to get go fever."'

The bolt problem occurred Sept. 8 during space shuttle Atlantis' launch 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 the shuttle and the external fuel tank did not retract properly eight minutes into the flight. Photographs showed 2 1/4 inches of the 14-inch bolt sticking out on the tank.

Engineers reviewed the film again Thursday but could not figure out what happened. So the Discovery countdown was halted, just before the start of fueling. The seven astronauts had not yet boarded the shuttle.

Before launching Discovery, NASA wants to understand the bolt malfunction and determine whether it 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."

The troublesome bolt 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 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.

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.

"If there's a success to be shown here, it's that the system does work and that we're able to respond even this close in to the next launch," he said.

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.

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