The Boulder-built Kepler spacecraft, which has dazzled scientists and sparked the public's imagination in its four-year hunt for new planets, has malfunctioned and could be facing the end of its mission.

NASA reported Wednesday that the Kepler craft, developed by Boulder's Ball Aerospace & Technologies Corp. and operated by the Laboratory for Atmospheric and Space Physics at the University of Colorado, has lost the second of four wheels that control the telescope's orientation in space.

The spacecraft has defaulted to a self-protective "safe" mode and is no longer properly operational.

"The mission as we have known it is over," said John Troeltzsch, Kepler program manager at Ball Aerospace.

If engineers can't find a fix, the failure means the $600 million Kepler mission will no longer be able to scan deep space for exoplanets -- planets outside of our solar system.

"It has been an incredible mission," Troeltzsch said. "When I think about what has happened in the last 15 years with exoplanets and how Kepler has enabled a whole new branch of astronomy, it's an amazing success."

NASA sciences chief John Grunsfeld is not signing off on Kepler's obituary.

"I wouldn't call Kepler down-and-out just yet," Grunsfeld said.

For the past four years, Kepler has focused its telescope on a patch of the Milky Way including the constellations Cygnus and Lyra, hosting more than 150,000 stars, recording slight dips in brightness -- a sign of a planet passing in front of the star.

But with the instrument failure, "We can't point where we need to point. We can't gather data," deputy project manager Charles Sobeck said.

Kepler was launched in March 2009 in search of Earth-like planets. From its position in solar orbit about 41 million miles from Earth, Kepler has confirmed 132 planets and spotted more than 2,700 potential ones, to date.

Originally, Kepler's mission was intended to be over by now. Last year, however, NASA agreed to keep Kepler aloft and active through 2016, at a cost of about $20 million a year.

Just one month ago, it was announced that Kepler had discovered two distant planets, labeled as Kepler-62e and Kepler-62f. The planet pair appeared to be in what some scientists call "the Goldilocks zone," with high potential for hosting some form of life, because they are an ideal size and in the most desirable position relative to their star.

Bill Possel, director of mission operations and data systems at LASP, said one of Kepler's four reaction wheels, instruments critical to aiming Kepler's powerful on-board telescope, had failed about nine months ago.

"Now, another appears to have failed in the last few days, and so we are in the process of trying to understand how that wheel failed," Possel said. "And there are things that we can do to get it turned on again, but at this point we are not doing the Kepler science of looking for planets."

Possel held out some hope for Kepler's future.

"I have been doing space work for the last 30 years, and it is always amazing that people come up with creative ideas. It's times like this that people's creative juices get flowing, and they come up with ways to make missions that look like they're dead come back to life.

"I'm going to stay positive and optimistic for the next weeks to months that we can get this thing going."

Possel said even if Kepler cannot collect one more piece of new data, the fruits of its mission are nowhere close to being exhausted.

"Bill Borucki (Kepler science principal investigator at the Ames Research Center in California) is still looking -- we have been collecting data for four years, and his scientists have only analyzed two years of that data, so they have a lot of data still to go through," Possel said.

Additionally, he said, "Here at LASP we have had over 100 students who have been involved in Kepler, getting it ready for launch, launching it and (managing) its operations.

"And every one of those students are telling their parents, 'I'm operating a $600 million spacecraft, and doing some science.' That is so cool."

The Associated Press contributed to this report.

Contact Camera Staff Writer Charlie Brennan at 303-473-1327 or brennanc@dailycamera.com.