Scientists announced in June 2013 that three planets orbiting star Gliese 667C could be habitable. This is an artist's impression of the view from one of those planets, looking toward the parent star in the center. The other two stars in the system are visible to the right.
(CNN) -- So, got any ideas for what to do with a used space telescope?
NASA hopes to find a new job for its planet-hunting Kepler probe after efforts to restore precision control of the orbiting telescope have failed, the space agency announced Thursday.
Kepler has been sidelined since mid-May, after the second of four devices used to aim the spacecraft's telescope gave out. Controllers have been trying to restart at least one of those two devices, known as reaction wheels, since July.
"The wheels are sufficiently damaged that they cannot sustain spacecraft pointing control for any extended period of time," Charlie Sobeck, Kepler's deputy project manager, told reporters Thursday afternoon.
That means Kepler's original science mission -- the search for Earth-like planets far beyond our solar system -- is over, said Paul Hertz, the head of NASA's astrophysics division.
But the space agency is trying to figure out whether it can find other missions that don't require that kind of pinpoint control -- and if so, "whether that science is compelling enough to justify continued investment in Kepler operations," Hertz said.
The roughly $600 million mission has so far confirmed 135 planets and identified nearly 3,500 possible planets. It findings have led scientists to believe that most stars in our galaxy have planets circling them. Two of them -- found about 1,200 light-years away -- are considered the best candidates so far for hosting life.
Controllers have remained in communication with the craft, which is more than 45 million miles from Earth. And since scientists still have more than two years of data to comb through, NASA doesn't consider the mission over, said Bill Borucki, the project's principal scientist.
"We have all this data we have not yet analyzed, and we expect many, many more discoveries," he said.
Kepler needs three of its four reaction wheels to tweak the telescope, which is aimed at a sliver of the cosmos around the Northern Hemisphere constellations Cygnus and Lyra. The No. 2 reaction wheel failed in 2012, while the No. 4 unit quit in May.
In a series of tests that began in mid-July, controllers got both reaction wheels to turn. In early August, they stopped tests on wheel No. 4 after finding high levels of friction, but continued testing the second until encountering the same problem last week, Sobeck said.
Sobeck said the probe can still be aimed using the two remaining reaction wheels, and its thrusters -- just not as finely as it could by reaction wheels alone. The thrusters still have enough fuel remaining to last several years if used carefully, he said.
The probe was launched in 2009 and has already surpassed its three-and-a-half-year minimum expected lifespan.
Borucki said NASA has asked scientists to propose alternatives within a few weeks. The agency will then decide "which of these will be practical and which of these we can do for a reasonable cost."
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This diagram shows the planets thought to orbit star Gliese 667C, where c, f, and e appear to be capable of having liquid water. The relative sizes, but not relative separations, are shown to scale.
This diagram lines up planets recently discovered by Kepler in terms of their sizes, compared to Earth. Kepler-22b was announced in December 2011; the three Super-Earths were announced April 18, 2013. All of them could potentially host life, but we do not yet know anything definitive about their compositions or atmosphere.
This illustration depicts Kepler-62e, a planet in the habitable zone of a star smaller and cooler than the sun. It is located about 1,200 light-years from Earth in the constellation Lyra.
This illustration depicts Kepler 62f, a planet in the habitable zone of a star smaller and cooler than the sun, in the same system as Kepler 62e.
This diagram compares the planets of our own inner solar system to Kepler-62, a five-planet system about 1,200 light-years from Earth. Kepler-62e and Kepler-62f are thought capable of hosting life.
The planet Kepler-69c is located about 2,700 light-years from Earth in the constellation Cygnus. This is an illustration of the planet, which is the smallest yet found to orbit in the habitable zone of a sun-like star.
This diagram compares the planets of our own inner solar system to Kepler-69, which hosts a planet Kepler-69c that appears to be capable of hosting life, in addition to planet Kepler-69b.
This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft.
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