Technical problems could keep NASA's Kepler space telescope from its goal of finding Earth-sized planets in habitable zones around other stars, astronomers say.

Launched in 2009, Kepler monitors thousands of stars for dips in brightness, an indication a planet could be passing in front of them. The space telescope needs another four years to complete its exoplanet survey but a critical hardware failure on Kepler this summer has astronomers worried the mission could end at any time, Spaceflight Now reported Tuesday.

One of the spacecraft's four reaction wheels -- spinning masses that control Kepler's orientation in space and keep the telescope locked on to target stars -- stopped July 14 due to increasing friction.

"We have to guide very accurately, and we had four reaction wheels to do this guidance," William Borucki, mission principal investigator at NASA's Ames Research Center in Moffett Field, Calif., said. "One of those was a spare, and we now have lost one of those four wheels ... The guiding is still great, but they've all had over a billion revolutions. If we lose another one, this mission terminates. We cannot track very well with two. We cannot track well enough to find planets."

Engineers will try to ensure Kepler's three active reaction wheels stay warm and operating by alternating their rotation between clockwise and counter-clockwise directions, Borucki said.

"We're trying to understand how to protect those last three wheels," he said. "People have studied these reaction wheels over the years and never came up with a good answer."

The Kepler mission was intended to last three-and-a-half years, but NASA hopes to keep the telescope operational through 2016, the report said.