One of the international space station's four control moment gyroscopes, used to keep the outpost properly oriented without jarring, fuel-consuming rocket firings, was taken off line late Monday because of concern about repeated instances of excessive vibration.

While CMG-3 has not been officially declared failed, flight planners are scrambling to assess the potential impact of a failure on the shuttle Discovery's upcoming mission in December to rewire the space station to take advantage of newly delivered solar arrays.

During the third of three planned spacewalks during Discovery's flight, CMGs 1 and 4 will have to be shut down while spacewalking astronauts unplug and replug electrical cables routing solar panel power to two of four main electrical circuits.

If CMG-3 is not available to help control the station's orientation, rocket thrusters might have to be used to augment CMG-2.

But the newly installed P4 solar array panels, which stretch 240 feet from tip to tip, are designed to rotate to track the sun as the station circles the globe. It's not yet clear whether the fragile masts that support the extended arrays can withstand jarring rocket firings while rotating. And they need to rotate to generate the necessary power.

Depending on the results of an engineering assessment, CMG-3 could be put back on line full time, brought up for specific events like the spacewalk in question or left off line but available for use as an emergency backup. Engineers also are assessing whether the arrays can simply be locked in a power-favorable position, if necessary, during the spacewalk.

The station's four control moment gyroscopes maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith, hatch during shuttle mission STS-92 in October 2000.

Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without using rocket firings that would jar sensitive microgravity experiments. Or the new P4 solar arrays, installed during the shuttle Atlantis's mission in September.

The station's orientation can be maintained with just two CMGs in a worst-case scenario.

On June 8, 2002, CMG-1 suffered a malfunction and shut down. Station astronaut Carl Walz reported hearing an unusual noise inside the Unity module. He said the noise appeared to be coming from the module's zenith area. Mission control then told Walz engineers were working an issue with a spin bearing in CMG No. 1. Walz said the noise was quite noticeable inside the module.

"We're hearing a pretty loud, audible noise, kind of a growling noise, from inside the node," Walz reported.

"It looks like we have a mechanical failure of the spin bearings on CMG-1," an astronaut in mission control replied. "It's currently spinning down right now. The growling noise is undoubtedly due to vibration."

That CMG was replaced with a spare during the first post-Columbia shuttle mission in 2005. No other backups are available, but the failed gyro currently is being refurbished. Depending on what happens with CMG-3, the refurbished gyro could be added to an upcoming mission.