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Kepler, a prolific hunter for other Earths, is suddenly in trouble (+video)

Kepler's quest for an Earth-like planet orbiting a sun-like star has been put on hold, NASA said, after the spacecraft sensed it was facing in the wrong direction and put itself in 'safe mode.'

How Kepler telescope works
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The planet-hunting Kepler spacecraft’s dramatic, some would say romantic, quest for an Earth-like exoplanet orbiting a star in its habitable zone has suddenly been put on hold, NASA officials said Tuesday, while engineers try to figure out what caused the craft to lose its ability to point itself at its distant targets.

The spacecraft is essentially hibernating, NASA said at a hastily called briefing late Wednesday afternoon, having put itself into "safe mode" on Sunday after sensing it was pointing in the wrong direction.

Controllers discovered the glitch Tuesday during their twice-a-week touch-base with the craft, currently some 40 million miles away in an Earth-like orbit around the sun.

When the craft is in safe mode, thrusters control its orientation, rather than the fast-spinning “reaction wheels” that normally provide stability. The craft uses these wheels to maintain the exquisite pointing precision needed to relentlessly stare at stars long enough to detect the telltale dimming imposed by a planet as its orbit carries it in front of the star.

As controllers tried to restart the craft's reaction wheels Tuesday, one of the wheels woke up, then balked. This left the craft with two functioning wheels. It needs three to resume observing the patch of sky that for four years it has scanned for Earth-like planets in Earth-like orbits around sun-like stars.

While the malfunction is serious, NASA officials were not ready Wednesday to declare the mission over.

"The loss of the reaction wheel is not good news," said Charles Sobeck, Kepler's deputy project manager at NASA's Ames Research Center in Mountain View, Calif.

The goal now is to place the craft in an operating mode that reduces the use of its thrusters in order to preserve fuel and "take the time to figure out what to do next," he said.

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