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Kepler telescope's pointing system under scrutiny
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: January 24, 2013


NASA's planet-finding Kepler space telescope has suspended operations this week after a telemetry signature showed rising levels of friction in one of the spinning reaction wheels responsible for pointing the observatory toward its astronomical targets.


Artist's concept of the Kepler spacecraft. Credit: NASA
 
But officials say the problem is different than the issue that caused one of Kepler's four reaction wheels to fail in July 2012.

"All these wheels have different personalities and different behaviors," said Charlie Sobeck, Kepler's deputy project manager at NASA's Ames Research Center in Moffett Field, Calif.

Normally, the reaction wheels spin between 1,000 and 4,000 rpm in both directions, Sobeck said. The momentum generated by the spinning wheels controls the attitude of the spacecraft, which must precisely point toward stars to detect signatures of extrasolar planets.

The reaction wheels are required for Kepler's mission.

Engineers noticed higher levels of friction in reaction wheel no. 4 in early January, and managers elected to begin a ten-day "wheel rest" period Jan. 17, according to an update posted on NASA's website.

The rest period, which is scheduled to end this weekend, is a type of safe mode in which science operations are suspended, the momentum wheels stop spinning, and the spacecraft uses conventional rocket thrusters to control its orientation.

Sobeck said the rest scheme should help redistribute lubricant in the reaction wheels and allow the wheel bearings to cool down.

Ground controllers have observed intermittent friction on wheel no. 4 for much of Kepler's mission. Wheel no. 2 showed no problems until early 2012, and it failed several months later.

Officials are concerned about the health of Kepler's reaction wheels. Three of the devices are still operational after one wheel failed last year, and scientists say all three remaining wheels must operate until at least 2016 for Kepler to achieve its prime objective of finding Earth-like planets around sun-like stars.

"Now that we're down to three wheels, we're being a little more cautious than we would have been earlier in the mission," Sobeck said.

Engineers have already implemented procedures to extend the lives of the reaction wheels, including running the wheels at warmer temperatures and alternating their spin directions.

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.

Now flying more than 45 million miles from Earth, Kepler monitors the stars for dips in brightness, an indication a planet could be passing in front of it.

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.

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.

Before verifying a candidate is a real planet, astronomers need to observe multiple transits of the object. For planets orbiting far away from their parent stars, it can take several years to collect evidence to confirm their existence.

NASA last year approved an extended mission for Kepler through 2016, but its future depends on the health of the spacecraft's reaction wheels.

Three wheels are needed to keep Kepler's 3.1-foot telescope steadily pointed toward its target stars in the mission's quest to find potentially habitable planets. If another reaction wheel fails, Kepler's objectives could be narrowed to finding larger planets, a task which doesn't require the same sensitivity as detecting Earth-sized worlds.

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