In January, engineers noticed that one of the reaction wheels that keep the spacecraft pointed was experiencing too much friction. They shut down the spacecraft for a couple of weeks to give it a rest, in the hope that the wheel’s lubricant would spread out and solve the problem. But when they turned it back on, the friction was still there. Until now, the problem had not interfered with observations, but on Tuesday, the spacecraft went into a so-called safe mode, and the engineers determined that the reaction wheel had stopped.

Kepler was launched with four reaction wheels, but one failed last year after showing signs of erratic friction. Three wheels are required to keep Kepler properly and precisely aimed, and now there are only two. The lack of three working wheels probably robs it of the ability to point precisely enough to detect Earth-size planets.

Project managers hope to remedy the situation by rocking the wheel back and forth, or perhaps resurrecting the wheel that failed last year, an effort that will probably take several months, according to Kepler’s deputy project manager, Charles Sobeck of the Ames Research Center. It would take that long as well to figure out what else the telescope, which is itself in fine condition, can be used for if it can no longer hunt planets.

Mr. Borucki said that the Kepler project had been a long journey — and a phenomenal success — and that he was not ready to pronounce it over. When Kepler was conceived, he said, nobody knew if any other stars had planets; now we know that almost every star in the galaxy has a planet and that the nearest exoplanet might be only 10 light-years away. “I’m delighted and surprised with what we have done,” he said.

For Mr. Grunsfeld, who played mechanic to the Hubble telescope during several lengthy spacewalks, the Kepler malfunction looked particularly frustrating. “Unfortunately, it’s not in a place where I can go and fix it,” he said.