NASA says its planet-hunting Kepler space telescope has been shut down to a precautionary "safe mode" due to problems with its orientation mechanism.

The action was taken Thursday when an unexpected rise in the amount of torque needed to turn one of the telescope's reaction wheels -- spinning devices used to hold observatory's position in space -- was detected, the space agency reported.

The Kepler telescope, launched in 2009, possesses four of the reaction wheels, three to control the spacecraft around three axes and one spare.

One failed last July, and the spare was put into operation. Kepler requires three functioning reaction wheels to position itself properly, meaning another failure could potentially end the $600 million mission, SPACE.com reported.

NASA said the rest period could return the suspect wheel to normal operation.

"Resting the wheels provides an opportunity to redistribute internal lubricant, potentially returning the friction to normal levels," Kepler scientists said.

The telescope's planet-hunting efforts have produced prolific results, spotting more than 2,700 potential planets around distant stars.