BOULDER - NASA announced Thursday that it has ended its attempts to restore the Kepler Space Telescope to full working order, and is now considering new missions the spacecraft could carry out.

Kepler, a $300 million craft built by Boulder-based Ball Aerospace and Technologies Corp., launched four years ago to hunt for planets that have atmospheric conditions similar to Earth's. Workers at the University of Colorado's Laboratory for Atmospheric and Space Physics operate the NASA mission for Kepler.

One of Kepler's four gyroscope-like reaction wheels, which are used to precisely point the spacecraft, failed last summer, and a second failed in May. Kepler needs three functioning wheels to continue its search for Earth-size planets outside of our own solar system, a NASA release stated. Engineers have worked unsuccessfully for months to restore at least one of the wheels.

Kepler has already confirmed 135 exoplanets - planets outside our solar system, orbiting stars like our sun at a range of distances from a star where the surface temperature of a planet might be suitable for liquid water. It's identified 3,500 more candidates.

The spacecraft completed its primary mission in November, and NASA continues to analyze the four years of collected data, expecting hundreds more new discoveries.

But the Kepler team is also exploring whether the telescope can conduct a different type of science program using the two remaining good wheels and thrusters.

"At the beginning of our mission, no one knew if Earth-sized planets were abundant in the galaxy," said William Borucki, Kepler's science principal investigator at NASA's Ames Research Center in Moffett Field, California. "Now at the completion of Kepler observations, the data holds the answer to the question that inspired the mission: Are Earths in the habitable zone of stars like our sun common or rare?"

An engineering study will be conducted on the modifications required to manage science operations with Kepler given the two nonfunctioning wheels. The NASA release stated that the organization could decide by later this year the scientific priority of a two-wheel Kepler mission relative to other NASA astrophysics missions competing for operational funding.