Space station engineers are troubleshooting an apparent leak in the vestibule between the U.S. Destiny laboratory and the newly installed Harmony connecting module that will serve as the attachment point for European and Japanese research labs.

Harmony was delivered to the space station by the shuttle Discovery last month and temporarily attached to the left-side port of the central Unity module. After Discovery departed, the station crew unbolted the main shuttle docking port from the front of Destiny, attached it to Harmony using the lab's robot arm and then moved the assembled Harmony/docking port "stack" to its permanent home on the front of the laboratory module.

During two spacewalks last week, station commander Peggy Whitson and Dan Tani plugged in power and data lines and connected four ammonia coolant supply and return lines. If all goes well, Europe's Columbus laboratory module will be attached to Harmony's right-side port next month and Japanese modules will be attached in February and April.

After Harmony was moved to the front of the station, a so-called "gross" leak check was carried out to make sure the interfaces had no major problems. No unusual leakage was observed in that test.

But during a "fine" leak check in the vestibule between the two modules, an apparent leak measuring 95.8 millimeters of mercury was observed over 15 hours. The allowable rate is 30 millimeters of mercury over eight hours. A NASA spokeswoman said the apparent leakage translated into about 3 pounds of lost air per day.

Engineers are not yet sure if the observed leak is real or whether it might be the result of an instrumentation glitch or some other problem. The spokeswoman said there was no observable change in the station's overall cabin air pressure before the fine leak test.

Engineers plan to carry out a similar fine leak test between Harmony and the forward shuttle docking port on Thursday before any possible additional tests of the Harmony-Destiny interface.

It was not immediately clear what, if anything, might be required to address the issue if follow-on tests confirm an actual leak. But a patch kit is available aboard the station to handle small leaks.

NASA managers plan to meet Friday for a final headquarters-level flight readiness review to assess the shuttle Atlantis' launch processing. Liftoff on a long-awaited mission to deliver the Columbus research module is targeted for Dec. 6.