CAPE CANAVERAL, Fla. -- Shuttle Atlantis won't roll back to its Kennedy Space Center assembly building until Sunday because NASA is draining toxic rocket fuel out of storage tanks aboard the vehicle.

The fuel off-loading operation is being done just in case the shuttle has to be disassembled to complete repairs to the ship's hail-hammered external tank.

If that turns out to be the case, the shuttle's International Space Station assembly mission might be delayed beyond late April. And if the tank cannot be repaired at KSC, then the mission likely will be postponed until June.

The off-loading operation involves draining nitrogen tetroxide and monomethyl hydrazine from storage tanks that feed the shuttle's twin maneuvering engines as well as its 44 nose-and-tail steering jets.

The same propellants are being drained from the shuttle's three auxiliary power units, which provide the hydraulic power needed to steer the shuttle's three engines in flight and control its wing flaps, rudder-speedbrake, landing gear, brakes and nosewheel steering system during atmospheric reentry and landing.

The hydraulic power units that enable the nozzles on the shuttle's twin solid rocket boosters to be gimbaled also are being drained.

The concern is that toxic rocket fuel could leak if the shuttle is disassembled and towed back to its processing facility.

NASA is planning to remove the shuttle's payload from the ship's cargo bay early Saturday. The 17.5-ton station truss segment will be moved into holding cell at launch pad 39A. The six-hour trip from pad 39A to the assembly building is scheduled to begin at 7 a.m. Sunday.

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