The prolific planet-hunting spacecraft that has already discovered some of the most intriguing exoplanets known has abruptly lost the capacity to carry out its mission, NASA officials announced May 15.

NASA’s Kepler spacecraft, which launched in 2009, relies on an array of flywheels, or reaction-wheel assemblies, to stabilize the pointing of its telescope toward a field of stars in the Milky Way. Kepler needs three of its four reaction wheels in working order to carry out its exoplanetary mission, and the spacecraft had already lost one wheel in July 2012. Now a second wheel appears to have failed, and unless it can be revived the spacecraft’s search for extrasolar worlds may be over.

“Basically we need three wheels in service to give us the pointing precision we need to find planets,” Kepler principal investigator Bill Borucki of the NASA Ames Research Center said in a teleconference with reporters. “Without three wheels it’s unclear whether we could do anything of that order.”

One of the remaining reaction wheels, which had been giving indications of trouble for some time, was discovered to have failed yesterday. The spacecraft is programmed to go into a protective safe mode when it loses its ability to orient its telescope. “Yesterday we turned on the antennas on the ground and we found the spacecraft was in safe mode,” said Charles Sobeck, deputy project manager for Kepler at NASA Ames, adding that mission controllers then attempted to fire up the reaction wheels. “We did that and we initially saw some movement of the wheels, but wheel four went back to zero speed.” He said the plan now was to put Kepler into a parking mode that will preserve its fuel while mission planners decide what to do next.

Kepler has already identified more than 100 confirmed exoplanets, including nearly all of the worlds discovered to date that are comparable in size to Earth. But mission scientists have yet to deliver the prize that astronomers have been waiting for: an Earth-size planet orbiting in the habitable zone of a sunlike star—in other words, the first planet that might pass for our own. Even more important, it is hoped that Kepler will reveal how common such planets are, thereby providing an indication of how many potentially habitable locales exist in the galaxy.

But Borucki said that “we’re well on our way” to determining the prevalence of roughly Earth-size planets in the habitable zone, noting that the spacecraft has already collected roughly two years of data “that has not been fully searched for planets.” Added John Grunsfeld, associate administrator for NASA’s Science Mission Directorate, “We’re not ready to call the mission over, but by any measure this has been a very successful mission.”