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Wheel fails on NASA's Kepler probe, halting its search for alien planets

An artist's conception shows NASA's Kepler space telescope observing a planet making a transit across an alien star. (Star and planet not to scale.)
An artist's conception shows NASA's Kepler space telescope observing a planet making a transit across an alien star. (Star and planet not to scale.)NASA

NASA's planet-hunting Kepler space telescope suffered a second failure in its reaction-wheel control system, forcing a suspension of its search for alien planets while the space agency determines whether the four-year mission is truly finished.

"It's certainly not good news," Charles Sobeck, deputy project manager for the $600 million mission at NASA's Ames Research Center, told reporters Wednesday.

But Sobeck and other mission managers emphasized that there was still a chance that the probe could be revived. "I wouldn't call Kepler down and out just yet," said John Grunsfeld, associate administrator for science at NASA Headquarters.

The problem has to do with the reaction wheels that are part of Kepler's fine-pointing system. The space telescope identifies worlds in far-off solar systems by watching for the telltale dips in starlight when the planet's disk passes over its parent sun. But in order to make those observations, Kepler has to hold itself in a precise position with the aid of four gyroscopic reaction wheels. One of the wheels failed last July, but Kepler could still do the job with the other three.

On Sunday, however, the spacecraft put itself into safe mode when it couldn't stay in its proper orbit around the sun, 40 million miles (64 million kilometers) from Earth. When the mission team did its regular check-up with Kepler on Tuesday, they found that a second reaction wheel wasn't working. In a mission update, NASA said the problem was probably caused by "a structural failure of the wheel bearing."

That forced an end to Kepler's planet quest. "We need three wheels in service to give us the pointing precision to make this work," the mission's principal investigator, William Borucki of NASA Ames, told NBC News.

Sobeck said the spacecraft itself could remain stable as long as it had fuel for its thrusters, but the thrusters aren't capable of providing the precise pointing that Borucki and his colleagues need. Over the next several months, members of the Kepler team will assess their technical options, and gauge what kind of science could be accomplished using those options, said Paul Hertz, astrophysics director at NASA Headquarters. 

There's still a chance that the reaction-wheel system could be restored — for example, by trying to spin the wheel backward and forward, just as you might spin the wheels of a car that's stuck in a snowdrift. Sobeck said it might even be possible to revive the wheel that was shut down last July. "When we turn it on, it just might start spinning, we don't know," he said.

But mission managers also acknowledged that Kepler's planet-hunting days may be over. Hertz pointed out that the spacecraft outlasted its 3.5-year primary mission, and was currently into an extended mission costing $20 million a year.

Even if the spacecraft's control system can't be revived, it will still take another couple of years to analyze the trillions of bits of data already collected, Borucki said. The mission has already identified 132 confirmed planets and 2,740 additional candidates yet to be confirmed. Some of those worlds are thought to lie within the habitable zones of their planetary systems.

"The prime reason for the existence of this mission is to determine whether Earths are common or rare in our galaxy," Borucki said. So far, the evidence suggests that there are billions of Earth-size planets in the Milky Way. Scientists have not yet identified an Earth-size planet in an Earthlike orbit around a sunlike star, but Borucki voiced confidence that the crucial evidence was tucked away somewhere in the readings that have already been beamed down from Kepler.

"I am really delighted, frankly, with what we've accomplished," he said.

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.