Mission managers decided late Thursday to order an inspection of a main propulsion system valve that controls the flow of liquid hydrogen and liquid oxygen into the shuttle's three powerful main engines.

The valve apparently failed to open during routine tests after Thursday's launch scrub.

"The likely situation is that the valve will have to be replaced," said Kennedy Space Center spokesman Joel Wells.

The valve inspections and any replacement work - coupled with the resulting need to service the shuttle's fuel cell system - will prompt a launch delay until at least Monday, Wells said.

The fuel cells provide electricity to run all spaceship systems, and onboard liquid hydrogen and liquid oxygen storage tanks must be topped off because of the four-day launch delay.

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The work ahead

Plans now call for technicians to crawl into the shuttle's rear engine compartment Friday to begin valve inspections. In the meantime, NASA engineers will continue studying the potential problem with the suspect bolt, which plays a key role in the shuttle's external tank separation system.

During a standard post-flight film review, engineers discovered that an identical bolt failed to retract properly after the September 8 launch of Atlantis on an International Space Station outfitting mission.

The 14-inch (35.5-centimeter) bolt is one of several that attach the shuttle orbiter to its expendable external tank, which holds the 500,000 gallons (1.9 million liters) of propellant needed to power the ship's main engines during an 8.5-minute climb into space.

Equipped with an explosive nut, the bolt is designed to retract back into the external tank as the bullet-shaped fuel reservoir is jettisoned from the shuttle about nine minutes into flight.

The way the system is designed, small explosive charges break the nut apart and then the bolt draws back into place within the tank. The fuel reservoir then makes a destructive plunge back into the atmosphere.

On Atlantis' flight, the nut broke apart as expected, but the bolt failed to fully retract into the tank. Consequently, about 2 and one-quarter inches (5.72 centimeters) of the bolt was left sticking out of the tank when it was jettisoned from the shuttle.

Engineers discovered the protruding bolt during a review of still photos captured by a 35mm camera mounted in Atlantis' rear engine compartment. The camera is routinely used to film the separation of the external tank in flight.

NASA managers, meanwhile, are concerned that a protruding bolt could come into contact with a shuttle orbiter and cause the massive tank to pitch up and collide with the ship - a potentially dangerous situation for an astronaut crew.

"We don't know that that would in fact happen," said veteran astronaut Jim Halsell, now a senior shuttle program manager. "But that is not a possibility that we would want to expose a vehicle - or one of our astronaut crews - to."

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Safety assured

Three investigation teams now are studying the situation.

The first is studying film from previous shuttle missions to determine if similar situations have escaped notice in the past, and a second team is conducting an engineering design review.

Shuttle Upgrades

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Perhaps most importantly, though, a third team is trying to determine whether a protruding bolt could in fact cause a dangerous collision in orbit.

"That's really the crux of what we're doing here," said Bill Gerstenmaier, another senior shuttle program manager. "Once we saw this, we weren't sure what the consequences might be, so to be thorough, we said we'd better go look at this."

Mission managers will meet Friday afternoon to review results of the bolt study.

A decision on whether to proceed with a launch attempt Monday, however, probably won't be made until a management meeting early that day.

NASA officials are hoping the engineering analysis shows that it's safe to launch Discovery despite the bolt problem on Atlantis' recent flight.

In a worst case scenario, Discovery would have to be rolled back to KSC's Vehicle Assembly Building for up-close inspections of the bolt, a move that would prompt a lengthier delay in the shuttle's International Space Station construction mission.