Space shuttle Atlantis is lowered into position for attachment to its external fuel tank and solid rocket boosters inside the Vehicle Assembly prior to a mission earlier this year. Photo: NASA

Rollout of shuttle Atlantis to pad 39A to ready the ship for launch next month will be delayed at least two days -- from Monday to Wednesday -- because of ongoing work to determine why an explosive booster separation bolt failed to fire during the Endeavour's launch Nov. 30.

Troubleshooting indicates the problem apparently was caused by a frayed or cracked wire in the integrated electronics assembly of Endeavour's left-side booster.

The left booster's IEA has flown nearly a dozen missions and engineers are trying to determine if the apparent wiring problem -- and details are sketchy -- was the result of fatigue, handling or some other factor.

The IEA is not easily accessible at the launch pad and if any repairs are ordered for Atlantis' boosters, the work must be done in the VAB. Atlantis' rollout was held up 48 hours to give engineers time to carry out X-ray inspections of the booster firing circuits.

Any additional inspections or repairs likely would delay rollout even longer.

During Endeavour's liftoff Nov. 30, one of two explosive cartridges used to separate a strut holding the base of the orbiter's left-side solid-fuel booster to the ship's external fuel tank failed to fire.

A redundant "NASA standard initiator," or detonator, inside the strut in question did fire, however, and the booster separated cleanly.

But a shuttle could be lost if both detonators in a given booster attach strut fail to fire and NASA managers want to determine what went wrong during Endeavour's launch before proceeding with Atlantis' ground processing.

The mission patch for STS-98 -- Atlantis' voyage to deliver the Destiny module to the international space station. Photo: NASA

Atlantis is scheduled for launch Jan. 18 on a long-awaited mission to deliver the U.S. laboratory module, Destiny, to the international space station. The 32,000-pound lab module is scheduled to be moved to the launch pad payload changeout room Jan. 3 for installation in Atlantis' cargo bay.

A two-day slip to Atlantis' rollout would not affect the shuttle's launch date. But it is not yet known what impact any additional inspections or rapairs might have.

The shuttle's two solid-fuel boosters fire for two minutes and four seconds, providing most of the orbiter's initial liftoff power. When internal chamber pressure drops to 50 pounds per square inch, the boosters are jettisoned to parachute into the Atlantic Ocean.

After recovery, disassembly and inspection, the four segments making up each booster are shipped back to rocket-builder Thiokol in Utah to be reloaded with propellent for another flight.

During inspections of Endeavour's left-side booster late last week, engineers discovered one of two detonators in one of the rocket's three aft struts -- the lower strut -- had failed to fire.

"The bolt does have redundant initiators on it and it's a bolt that can separate from either end to separate the strut," Endeavour's lead flight director, William Reeves, said Friday.

"The way I understand it - and I don't have all the details yet - one initiator didn't fire and the other one did the job. So that's enough said at this point. There will be a thorough examination of it and an analysis of it to determine why that one didn't fire. But the redundant system did perform its job."

Here's a description of the separation system from NASA's on-line Shuttle Reference Manual:

The SRBs separate from the external tank within 30 milliseconds of the ordnance firing command.

The forward attachment point consists of a ball (SRB) and socket (ET) held together by one bolt. The bolt contains one NSD pressure cartridge at each end. The forward attachment point also carries the range safety system cross-strap wiring connecting each SRB RSS and the ET RSS with each other.

The aft attachment points consist of three separate struts: upper, diagonal and lower. Each strut contains one bolt with an NSD pressure cartridge at each end. The upper strut also carries the umbilical interface between its SRB and the external tank and on to the orbiter.

There are four booster separation motors on each end of each SRB. The BSMs separate the SRBs from the external tank. The solid rocket motors in each cluster of four are ignited by firing redundant NSD pressure cartridges into redundant confined detonating fuse manifolds.

But given the critical nature of the system, NASA and contractor engineers want to find out what went wrong with the one that failed to make sure similar problems -- or worse -- don't affect downstream flights.