Engineers appear more optimistic today about the possibility of launching the shuttle Endeavour Friday on a space station assembly mission. No decisions have been made on what to do about the shuttle's damaged robot arm, but NASA plans to restart Endeavour's countdown overnight Tuesday on the assumption the arm will be cleared to fly as is.

NASA managers plan a final meeting Wednesday evening to review test data and possible repair options before giving Endeavour's crew a final clear to launch. The only options currently on the table are to A) fly as is; B) repair the arm damage at the pad and C) remove the arm at the pad and fly the mission without it.

The 50-foot-long Canadian-built robot arm was damaged last week when it was accidentally hit by an access platform that was being inserted into Endeavour's cargo bay to repair a leaking crew cabin oxygen line. The leaking O2 line derailed an attempt to launch Endeavour on Nov. 11. The oxygen line was quickly replaced and over the weekend, engineers replaced an associated nitrogen line just to be on the safe side.

The arm damage is potentially serious because the fragile crane is needed to pull Endeavour's payload, a $390 million 14.5-ton solar array truss segment, from the cargo bay. The arm operator, shuttle commander James Wetherbee, then will hand the truss segment off to the Canadarm2 space crane for attachment to the space station. Canadarm2 cannot pull the truss from the cargo bay itself because it would run into one of the support cradles used by the shuttle's robot arm while trying to reach the payload's grapple fixture.

The carbon-composite structure of the shuttle's robot arm suffered a small area of delamination near its shoulder joint that may have degraded its structural integrity. To find out, engineers plan to deliberately damage a spare arm segment at a plant in Canada and then to measure its ability to handle the "loads," or forces, the real arm will experience during launch, when it pulls the truss from the shuttle's cargo bay and during re-entry.

Depending on the results of that work and other on-going data analysis, shuttle program manager Ronald Dittemore will make a decision Wednesday evening on whether to continue Endeavour's countdown, whether to order repairs at the pad or whether to have engineers simply remove the arm and its support cradles and launch the shuttle without an arm in place. Mission planners at the Johnson Space Center in Houston are assessing potential trajectories for the station's Canadarm2 arm operator, Peggy Whitson, to pull the P1 truss from the cargo bay in the event the latter option ultimately plays out.

Repair or removal of the shuttle arm would delay launch to around Dec. 4-6. The latest Endeavour can launch is around Dec. 9, the latest projected start of a so-called "beta angle cutout" when the plane of the station's orbit in relation to the sun prevents the lab's solar arrays from generating enough power for a docking mission. The beta cutout ends around Christmas day. But if Endeavour is not off the ground by the start of the cutout, the flight almost certainly will be delayed to early January.