Mission controllers have turned off one of four spinning gyroscopes that keep the international space station properly positioned in space because it was vibrating excessively.

Although the space station needs only two of the four devices functioning, having one down could be troublesome in December during space shuttle Discovery's mission to the orbiting outpost.

During one of the mission's three spacewalks, two other gyroscopes will be powered down so astronauts can hook up electrical cables to solar panels that were installed during last month's flight of space shuttle Atlantis.

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Planners need to decide whether to leave the troubled gyroscope off or spin it back up during the spacewalk, said NASA spokesman James Hartsfield in Houston.

"You honestly don't want to power off two and have only one operating," he said. "Technically, we are not calling this one failed yet, but whether it's reusable is the subject of analysis."

The vibrating gyroscope was turned off Tuesday.