NASA's Kepler telescope has resumed its quest for planets around other stars after 10 days off to rest the spacecraft's suspect reaction wheels, officials announced Tuesday.

The Kepler spacecraft undergoes preparations for launch in 2009. Credit: NASA/KSC
 
The planet-hunting telescope returned to science data collection Monday, NASA said in an update posted on its website.

Officials suspended Kepler's science mission Jan. 17 to give a break to the craft's three operational reaction wheels. A fourth reaction wheel stopped functioning in July 2012.

The wheels spin between 1,000 and 4,000 rpm in both directions, generating momentum to guide Kepler's telescope toward stars and keep it stable while it looks for signs of planets.

Kepler finds exoplanets by detecting slight dips in the light coming from stars. Kepler looks for evidence of a planet passing between the telescope and the star, and the transit registers as a change in the brightness of the host star.

Launched in March 2009, the $600 million observatory is stationed in an Earth-trailing solar orbit and aims its 3.1-foot telescope toward constellations Cygnus and Lyra, observing a 10-degree-wide field containing 4.5 million detectable stars. Kepler is focusing on approximately 156,000 stars for the purposes of its research.

The precise work requires steady pointing from Kepler's reaction wheels. Three of the wheels are needed to achieve Kepler's prime objective of finding potentially habitable Earth-sized planets around sun-like stars, according to Charlie Sobeck, Kepler's deputy project manager at NASA's Ames Research Center in Moffett Field, Calif.

Managers put Kepler into safe mode Jan. 17 when reaction wheel no. 4, one of the spacecraft's three remaining reaction wheels, showed rising friction. The reaction wheels were spun down and the observatory switched to chemical rocket thrusters to control its attitude for a 10-day "wheel rest" period.

The wheel rest period was designed to allow the wheel bearings to cool and lubricant to redistribute inside the wheel housings, hopefully resolving the friction issue.

The problematic reaction wheel has exhibited intermittent elevated friction levels since launch, Sobeck said. Reaction wheel no. 2, which failed in July, showed a different, more chaotic friction signature before it was lost.

Engineers will review the performance of wheel no. 4 over the next month to evaluate the effectiveness of the wheel rest scheme, NASA said in an update. More wheel rest periods may be needed in the future if the procedure proves effective, according to Sobeck.

The mission was supposed to accomplish its primary goals in three-and-a-half years, but noisy data coming from the telescope's 95-megapixel camera and flickering light from Kepler's target stars have hampered astronomers as they comb through data looking for planet signatures.

Because of the analysis challenges, scientists say the mission will need twice the time originally planned to gather enough data to confirm the presence of Earth-sized planets in the habitable zones of stars within Kepler's field-of-view.

Kepler's mission was last year extended through 2016 to continue its hunt for exoplanets. Engineers have instituted several measures, including the wheel rest safe mode and running the wheels at warmer temperatures, in an attempt to ensure the devices stay healthy.

Researchers using Kepler data have found 105 new planets around other stars, and the mission's data archive has evidence for more than 2,700 planet candidates.