Engineers have concluded they will be unable to restore NASA's Kepler telescope to full form after two reaction wheel failures - effectively ending the mission's search for Earth-sized planets around other stars - but officials are seeking new ideas to use the craft's 3.1-foot telescope to search for alien worlds, asteroids, comets and supernovas, mission managers announced Thursday.

Artist's concept of the Kepler observatory. Credit: NASA
 
"We are reporting that we do not believe we can recover three-wheel operations or Kepler's original science mission," Hertz said Thursday in a conference call with reporters.

Kepler no longer has the ability to aim its telescope with the steadiness required to detect the subtle signatures of small, rocky planets like Earth. But Kepler's other systems, including its telescope and 95-megapixel camera, are still healthy and could contribute to science in other fields, according to Paul Hertz, director of the astrophysics division at NASA Headquarters.

The $600 million Kepler mission has been plagued by trouble with two of its four reaction wheels, spinning gyroscope-like devices which keep the spacecraft's telescope precisely pointed toward stars for science observations.

One of the wheels shut down after showing signs of high friction in 2012. A second wheel succumbed to high friction in May.

Kepler needs three reaction wheels to maintain the pointing necessary to see tiny blips in the brightness of stars. Slight dimming of the stars could indicate the presence of a planet passing between the stars and the Kepler telescope.

"I really feel immense satisfaction with what the mission has accomplished," said Bill Borucki, Kepler's principal investigator at NASA's Ames Research Center in California.

Built by Ball Aerospace and Technologies Corp., Kepler has collected data leading to the discovery of 135 confirmed planets, plus more than 3,500 planet candidates requiring follow-up observations.

Since the reaction wheel failure in May, engineers started working on restarting the two failed wheels. In July, controllers succeeded in getting reaction wheels no. 2 and no. 4 to spin in both directions, but telemetry showed elevated friction levels continued to plague both wheels, which are designed to spin between 1,000 and 4,000 rpm.

The reaction wheels were manufactured by Goodrich Corp., which was acquired last year by United Technologies Corp.

Charlie Sobeck, Kepler's deputy project manager at Ames, said ground controllers operated the spacecraft with three reaction wheels long enough to turn toward Earth and downlink the last bit of science data collected before the wheel failure in May. But friction levels continued to rise, and Kepler ultimately put itself into safe mode.

"The results of that show what we expected to see, which is the wheels are sufficiently damaged that they cannot sustain spacecraft pointing control for any extended period of time," Sobeck said.

Launched in 2009, Kepler is stationed in an orbit around the sun similar to Earth's. It uses a 95-megapixel camera to stare at a field of 156,000 stars in the constellations Cygnus and Lyra, looking for tiny fluctuations in the stars' brightness.

Relative sizes of all of the habitable-zone planets discovered to date alongside Earth. Left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f and Earth (except for Earth, these are artists' renditions). Credit: NASA Ames/JPL-Caltech
 
By observing a fraction of the sky for exoplanets, scientists can extrapolate their findings across the entire galaxy.

Kepler's planned mission of three-and-a-half years ended in November 2012, but NASA approved an extension of observations through 2016 to allow more time to validate the existence of the smaller, Earth-sized worlds in the mission's listing of candidate planets.

"The Kepler mission has been spectacularly successful," Borucki said. "At the beginning of the mission, no one knew whether Earth-sized planets were abundant or rare in our galaxy. Now, at the completion of Kepler observations, we know that our galaxy is filled to the brim with planets."

Kepler was built to help researchers determine how common hospitable Earth-sized planets are in the Milky Way galaxy.

Astronomers believe they have found hundreds of giant Jupiter-sized planets, some circling hellishly close to their parent stars.

Those are the easy ones.

Scientists are more interested in spotting planets in the habitable zone, a "Goldilocks" band where temperatures are just right to sustain liquid water and life.

Smaller planets in orbits farther out from the hosts take more time to confirm, with scientists needing several "transits" to confirm the dip in starlight is actually from a planet. For planets like Earth, such a discovery would take several years of observations to verify.

"Only a few of the Earth-sized planets found so far are at the right distance from their star so that liquid water could exist on the surface," Borucki said. "These planets differ from the Earth orbiting our sun in that the stars are generally smaller and cooler than the sun, and many of the planets are larger than the Earth."

Borucki said the Kepler science team still needs to analyze two years of data from the mission, and he is optimistic evidence for an Earth-sized planet around a sun-like star lies in Kepler's untapped catalog.

"We really expect the most exciting discoveries are going to come in the next few years as we search through all this data," Borucki said.

Despite Kepler's reaction wheel failures, NASA officials are interested in keeping the telescope going if scientists can make the case for continued observations.

Hertz said NASA is pursuing studies to see how Kepler could turn toward astronomical targets with two reaction wheels and conventional rocket thrusters, and to assess the types of scientific work Kepler could accomplish with diminished pointing ability.

According to Borucki, scientists have suggested using Kepler to search for asteroids, comets and study supernovas, the explosive deaths of some varieties of stars. Kepler could also stay in the field of exoplanet research by using a different technique known as microlensing to detect large Jupiter-sized worlds orbiting far away from their parent stars.

NASA will determine whether it is worth the money to continue operating Kepler for any of the proposed missions using the telescope's remaining capabilities, and the agency may put the proposals through a peer review to weigh its costs and benefits against those of other missions fighting for finite funding, Hertz said.

"We're going to consider these ideas carefully, we're going to find out which ones are practical, and we're going to try to cost each of these missions," Borucki said. "At the present time, we really do not know whether any of these concepts are practical."