NASA's planet-hunting Kepler spacecraft appears to be hobbled by a faulty wheel and may be near the end of its four-year mission, space agency scientists said Wednesday.

Kepler, a $600 million mission, was launched in 2009 on a search for other planets. So far, it has found 2,700 candidates, including a handful that may be habitable worlds, not too hot and not too cold.

The problem is a reaction wheel that keeps the spacecraft pointed but has stopped working, said John Grunsfeld, associate administrator for the Science Mission Directorate at NASA.

"We are not ready to call the mission over," said Grunsfeld, adding that scientists are still going to try and figure out how to get the wheel restarted.

The problem was discovered on Tuesday, when the spacecraft went into a pre-programmed safe mode that kicks in "if the observatory has trouble knowing where it should point," said Grunsfeld.

Scientists have been expecting that such a problem may arise, since the wheels have a limited lifespan and a different reaction wheel broke down in July.

NASA has not turned that wheel, reaction wheel 2, back on since, and the spacecraft needs a minimum of three wheels to function in the way it was intended.

Charles Sobeck, deputy project manager at NASA's Ames Research Center in California, said the signs indicate "internal failure within the wheel," and that it will take experts a few weeks to decide the next steps.

In the meantime, they will reduce the spacecraft's fuel consumption, essentially parking it in space, roughly 40 million miles (64 million kilometers) from Earth, while NASA decides what to do.

"The eventual performance we are going to get to, we just don't know at this time," said Sobeck.