(CNN) -- The future of NASA's planet-hunting Kepler space observatory was in question Wednesday after a part that helps aim the spacecraft stopped working, the U.S. space agency said.
Controllers found Tuesday that Kepler had gone into a "safe mode" and one of the reaction wheels needed to orient the spacecraft would not spin, Associate NASA Administrator John Grunsfeld told reporters. NASA engineers are trying to figure out whether they can get the balky part back into service or whether they can resume control by another method, Grunsfeld said.
"We're not ready to call the mission over," he said. But at roughly 40 million miles from Earth, "Kepler is not in a place where I can go up and rescue it."
The Kepler mission has identified 132 planets beyond our solar system since its launch in 2009, leading scientists to believe that most stars in our galaxy have planets circling them. It has gone into a "safe mode" with its solar panels facing back at the sun, giving controllers intermittent communication with the craft as it spins.
The probe was built with four reaction wheels and needs three of them -- one for each axis -- to aim its telescope precisely at a distant star, deputy project manager Charlie Sobeck said. One failed in July 2012, leaving it with only two functioning wheels at this time, Sobeck said.
Kepler shut itself down after it was pointing in the wrong direction, Sobeck said. He said it was "reasonable to suspect" the failure of the reaction wheel was the cause, but scientists hadn't confirmed that.
The $600 million mission was designed to operate for three and a half to six years. Even if controllers are unable to return it to service, NASA said the mission has produced enough data to keep scientists busy for up to two years.
"Another four years would have been frosting on the cake, but we have a nice cake now," said William Borucki, the project's principal scientist.
CNN Correspondent John Zarrella contributed to this report.