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. 

 This story was updated at 5:20 p.m. EDT. 
 
 The planet-hunting days of NASA's prolific Kepler space telescope, which has discovered more than 2,700 potential alien worlds to date, may be over. 
 
 The second of Kepler's four reaction wheels — devices that allow the observatory to maintain its position in space — has failed, NASA officials announced Wednesday (May 15). 
 
 If one or both of those failed wheels cannot be brought back, the telescope likely cannot lock onto target stars precisely enough to detect orbiting planets, scientists have said. [Gallery: A World of Kepler Planets] 
 
 Staring at stars 
 
 The $600 million Kepler spacecraft spots exoplanets by flagging the tiny brightness dips caused when they pass in front of their host stars from the instrument's perspective. The mission's main goal is to determine how common Earth-like alien planets are throughout the Milky Way galaxy. 
 
 Kepler needs three functioning reaction wheels to stay locked onto its more than 150,000 target stars. The observatory had four wheels when it launched in March 2009 — three for immediate use, and one spare. 
 
 One wheel (known as number two) failed in July 2012, giving Kepler no margin for error. And now wheel number four has apparently given up the ghost as well, after showing signs of elevated friction for the past five months or so. 
 
 "This is something that we've been expecting for a while, unfortunately," NASA science chief John Grunsfeld told reporters today. 
 
 Grunsfeld is a former astronaut who flew on five space shuttle missions, including three that serviced or upgraded NASA's Hubble Space Telescope in Earth orbit. But in-space repair is not an option for Kepler, which circles the sun rather than Earth and is currently about 40 million miles (64 million kilometers) from our planet. 
 
 Astronomers have confirmed more than 800 planets beyond our own solar system, and the discoveries keep rolling in. How much do you know about these exotic worlds? Start the Quiz 0 of 10 questions complete 
 
 Alien Planet Quiz: Are You an Exoplanet Expert? Astronomers have confirmed more than 800 planets beyond our own solar system, and the discoveries keep rolling in. How much do you know about these exotic worlds? 0 of questions complete 
 
 A new mission? 
 
 The Kepler team is not taking the wheel failures lying down. Engineers will try to recover number two and number four, perhaps by turning the wheels to power through any deterioration in their mechanisms, team members have said. 
 
 "I wouldn't call Kepler down and out just yet," Grunsfeld said. 
 
 If this and other measures don't work, however, Kepler will probably get a new mission, likely one that emphasizes scanning the heavens over its previous "point and stare" operations. 
 
 The team is already thinking about what a new scanning mode might be able to accomplish. Researchers are also trying to figure out ways to conserve fuel, so Kepler can keep operating for as long as possible if it needs to start using its thrusters to help point at targets. 
 
 This chart depicts the frequencies of planets based on findings from NASA's Kepler space observatory. The results show that one in six stars has an Earth-sized planet in a tight orbit. Credit: F. Fressin (CfA) 
 
 Kepler's legacy 
 
 Whatever the future holds for Kepler, the mission will go down in history as an incredible success, researchers say. 
 
 While just 132 of Kepler's 2,700-odd planet candidates have been confirmed by follow-up observations to date, mission scientists estimate that more than 90 percent will end up being the real deal. 
 
 Further, the telescope's discoveries have allowed researchers to take an unprecedented, systematic look at worlds beyond our solar system — learning, for instance, that small, rocky planets are much more common throughout the Milky Way galaxy than gas giants like Saturn or Jupiter, at least in close-in orbits. 
 
 "Kepler has opened up the next set of questions in exoplanets," said Paul Hertz, astrophysics director at NASA Headquarters in Washington, D.C. 
 
 "Before we flew Kepler, we didn't know that Earth-sized planets in habitable zones were common throughout our galaxy," Hertz added. "We didn't know that virtually every star in the sky had planets around them. Now we know that." 
 
 Kepler also outlasted its prime mission life of 3.5 years; it has been working on an extended mission that takes it through at least fiscal year 2016. 
 
 While the observatory may not spot any more exoplanets from here on out, that doesn't mean the flood of Kepler discoveries will slow down anytime soon. 
 
 "We've really only sort of looked at half the dataset so far. We just haven't had the time and the processing hours to go through it all," Kepler deputy project manager Charlie Sobeck, of NASA's Ames Research Center in Moffett Field, Calif., told SPACE.com late last month. 
 
 Once Kepler stops finding planets, he added, "the scientific output of the mission would continue for at least another year or two before you would see a dropoff." 
 
 Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on SPACE.com.
– It seems like NASA is announcing the discovery of a new Earth-like planet every few weeks—like so, and so, and so—but those days might be over for a long while. Not that there aren't more discoveries to make, it's just that the spacecraft responsible for them has a faulty "reaction wheel" that might mean the end of the mission, reports Space.com. Kepler needs three such wheels functioning properly to zero in on targets, but one is kaput. NASA sent up the craft with a spare when it launched the $600 million mission in 2009, but that had to be deployed last summer. Now it's out of spares. Engineers aren't giving up hope of getting one of the two troublesome wheels working again, but if that proves impossible, Kepler will have to shift to a less ambitious sky-gazing mission. (It's 40 million miles away, so a manual fix is out of the question.) Kepler has been busy in its four-plus years, identifying 115 planets and another 2,740 likely candidates, reports the New York Times. "Before we flew Kepler, we didn't know that Earth-sized planets in habitable zones were common throughout our galaxy," says a NASA scientist. "We didn't know that virtually every star in the sky had planets around them. Now we know that."