The crew of the international space station ran into unexpected problems today detaching a docking port on the left side of the multi-hatch Unity module. After considerable troubleshooting on the ground, flight controllers cleared the crew to press ahead and pressurized mating adapter No. 3 was detached around 8:20 a.m.

PMA-3 is being moved from Unity's left hatch to a downward-facing port to make way for attachment of a new module during a visit by the shuttle Discovery in late October. Today's move was expected to take about an hour.

With the station's robot arm locked onto a grapple fixture on PMA-3, the lab crew sent commands to back out the first of four motorized bolts in the common berthing mechanism early today and received fault messages indicating possible problems. The crew pressed ahead and received similar, though intermittent, messages on the next two bolts they tried.

"Flight controllers continuing to troubleshoot some fault indicators seen in attempts to undock PMA-3 from the port side of the Unity node in preparation to its mate to the nadir docking port on Unity 90 degrees away," mission control commentator John Ira Petty said around 7:45 a.m. "Grapple of PMA-3 by Canadarm 2 proceeded normally. The undock procedure seemed to be going normally as well until a little before (6:20 a.m.) there was a fault indicator that occurred on one of the bolts. That indicator appeared after the bolt had reached an indicated load of zero.

"Troubleshooting was unable to identify an instrumentation problem. A decision was made to continue with the undocking procedure and subsequent fault indicators were seen on additional bolts. These indications were intermittent. So flight controllers and engineers on the ground asked for another hold, asked the crew to stand down in their procedures to undock PMA-3 while they scratch their heads over this."

After additional discussion on the ground, flight controllers decided it was safe to proceed with the undocking procedure, concluding the fault messages did not indicate any sort of problem that might cause damage to the common berthing mechanism. The fourth bolt was backed out without incident.

But the fault indicators must be explained to resolve any lingering concern about whether the bolts will work as required to lock the Harmony module to Unity's left-side hatch in October.

"Through all these discussions, it was of paramount importance that nothing be done to compromise the docking of the Harmony node to be brought ot the international space station ... by Discovery," Petty said.

With Harmony mated to Unity's left port, the station crew will detach a shuttle docking port - PMA-2 - from the front of the Destiny lab module and attach it to Harmony. Then Harmony/PMA-2 will be detached from Unity and moved to the front end of Destiny.

Harmony will serve as the connecting point for European and Japanese research modules scheduled for launch late this year and early next. But before any of that work can begin, Harmony must be successfully mated to Unity and that key step requires a healthy common berthing mechanism.