In a teleconference call streamed on the Internet, Associate NASA Administrator John Grunsfeld said one of three reaction wheels that work in conjunction to keep the telescope oriented and aimed at distant stars to look for evidence of exoplanets has gone bad.
Since it launched in 2009, Kepler has discovered 132 confirmed planets beyond our solar system and thousands of candidate sightings.
Kepler requires three working reaction wheels to orient itself; it carries a spare, but a previous wheel failure had left it with just three, one of which has now also failed, NASA said.
Ground controllers said there were clear indications there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing.
Efforts to keep Kepler in operation would be ongoing, Grunsfeld said, including steps to attempt to recover wheel functionality and to investigate the possibility of a hybrid orientation mode using wheels and the telescope's thrusters.
"We're not ready to call the mission over," he said.