NASA's Kepler space telescope is in trouble.
The telescope, launched in 2009 in search of Earth-like planets, has lost the use of one of the four wheels that control its orientation in space. Kepler, for the second time this month, has gone into safe mode, NASA reported Wednesday afternoon.
With NASA no longer able to manipulate the telescope's positioning, ground engineers also are having a hard time communicating with it since the communications link comes and goes as the spacecraft spins uncontrollably.
This is the second wheel failure Kepler has suffered.
"This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing," NASA reported. "With the failure of a second reaction wheel, it's unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry. However, no decision has been made to end data collection."
The telescope is stable and safe at this point. Engineers, though, are working to minimise the amount of fuel that the spacecraft is using while they try to control its orientation with its thrusters.
Kepler has been considered a success, wrapping up its primary three-and-a-half-year mission and entering a second phase of research last November. NASA scientists had been hoping that Kepler would continue working for another four years.
Since it began its work on May 12, 2009, the telescope has searched more than 100,000 stars for signs of Earth-like planets in the habitable zone, an area that may have water. The telescope has so far confirmed more than 100 such planets.
The telescope is onboard a spacecraft that is carrying several computers. Kepler is designed to measure the brightness of stars every half hour, allowing scientists to detect any dimming that would be caused by orbiting planets passing in front of them.
Scientists receive enough data from Kepler to determine not only the size of a planet but whether it has a solid surface and its potential to hold water, something considered crucial to the formation of life.
Last month, the space agency announced that Kepler had discovered two planets that are perfectly sized and positioned to potentially hold life.
Scientists are not saying hey actually have discovered life on the newfound planets, which are about 1,200 light years away. However, they did say they're one step closer to finding a world similar to Earth that orbits a star like our sun.
NASA announced on Wednesday that even if Kepler's mission is over, it has gathered enough information to keep scientists busy analysing it for years.