Nasa's planet-hunting Kepler telescope is broken, potentially jeopardising the search for other worlds.
The failure could mean an end to the US$600 million ($727 million) mission's search, although the space agency wasn't ready to call it quits yesterday. The telescope has discovered scores of planets but only two so far are the best candidates for habitable planets.
"I wouldn't call Kepler down-and-out just yet," said Nasa sciences chief John Grunsfeld.
Nasa said the spacecraft lost the second of four wheels that control its orientation in space. With only two working wheels left, it can't point at stars with the same precision.
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In orbit around the Sun, 65 million km from Earth, Kepler is too far away to send astronauts on a repair mission like the way Grunsfeld and others fixed a mirror on the Hubble Space Telescope. Over the next several weeks, engineers on the ground will try to restart Kepler's faulty wheel or find a workaround.
Kepler was launched in 2009 in search of Earth-like planets. So far, it has confirmed 132 planets and spotted more than 2700 potential ones. Its mission was supposed to be over by now, but last year, Nasa agreed to keep Kepler running until 2016 at a cost of about US$20 million a year.
Just last month, Kepler scientists announced the discovery of a distant duo that seems like ideal places for some sort of life to flourish. The other planets found by Kepler haven't fit all the criteria that would make them right for life of any kind.
While ground telescopes can hunt for planets outside our solar system, Kepler is much more advanced and is the first space mission dedicated to that goal.