NASA's Kepler probe, tasked with searching distant stars for Earth-like planets, may not be able to continue its mission. On a conference call, NASA confirmed that the spacecraft is currently in "safe mode" after a vital reaction wheel failed, likely because of a problem with the wheel bearing or other components. Kepler launched with four reaction wheels, but one failed last year, and three are necessary to keep the craft oriented.

Though Kepler is currently stable, scientists would need to find a way to either get it running again or route around it in order to continue the primary mission. "We would not call Kepler down and out just yet," said one of the conference participants, despite a press release saying it was "unlikely" that it could resume normal data collection. And right now, the Kepler team has barely started researching a potential solution. This will be done over the coming weeks and months.

"We would not call Kepler down and out just yet."

Kepler first launched in 2009, and its primary mission ended in 2012. It then entered an extended mission period, with funding earmarked until 2016. But if a solution is not found, that lifespan may be much shorter. These problems with the reaction wheel have been ongoing: in January, NASA suspended operations for ten days, attempting to resolve abnormally high levels of friction. The issue, though, continued after it resumed, despite attempts to better lubricate the wheel. Without three wheels, Kepler can't point itself effectively enough to make the fine observations needed to find exoplanets. NASA is still investigating whether a two-wheeled craft could be used for other tasks that require less precision.

Despite the failure, NASA says it still has a surprising amount of torque on the wheels, and it's considering the possibility of getting the first failed wheel to start turning again. If it can't, scientists say there's still a wealth of data — two years' worth, in fact — left to be processed. "I'm very optimistic that the data that we have" will allow for the discovery of more planets, said one participant during the briefing.

Since its launch, Kepler has discovered a number of planets that share characteristics with Earth, looking for telltale anomalies that could indicate a planetary orbit. In April, the mission found two "Super Earths" inside a star's habitable zone, the orbital area that could support liquid water. Most importantly, though, it's found that smaller planets are not nearly as unusual as once assumed, and that many more could be found in the future. Regardless of whether Kepler can be resurrected, more planet-hunting projects are in the works. The Transiting Exoplanet Survey Satellite (TESS), a new probe developed in collaboration with Google and MIT, is set to launch in 2017.