NASA’s planet-hunting Kepler spacecraft has been shut down by the failure of one of the reaction wheels that keep it pointed, the space agency announced Wednesday.

“I wouldn’t call Kepler down and out yet,” said John Grunsfeld, a former astronaut and Hubble repairman who is NASA’s associate administrator for space science, at a news conference.

But he and others said that if engineers could not restore the wheel or find some other way to keep the spacecraft’s telescope precisely pointed, the failure could end one of the most romantic and successful of NASA’s missions: the search for Earth-like planets in habitable orbits around other stars. Just last month, astronomers reported that Kepler had found two planets, only slightly larger than Earth, orbiting a star 1,200 light-years from here in the Goldilocks zone, where liquid water is possible.

More potentially habitable planets, even smaller and more Earth-like, lurk in the pipeline, astronomers say, but have not yet been confirmed. “We believe there are planets there that we haven’t found yet,” said William Borucki of NASA’s Ames Research Center, the founder and leader of the Kepler effort.