WASHINGTON — A reaction wheel on NASA’s Kepler spacecraft continues to experience elevated levels of friction after a brief rest period, but project officials say that does not necessarily imply an imminent failure that could jeopardize the spacecraft’s planet-hunting mission.

Spacecraft engineers in early January noticed increased levels of friction in one of four reaction wheels on the spacecraft used for attitude control. To address the problem, NASA suspended science operations of the spacecraft for 10 days in January, hoping that this “wheel rest” period would resolve the issue. However, the high levels of friction continued in wheel No. 4 after normal spacecraft operations resumed in late January.

“The wheel rest period of January 17-January 27 appears to have had no beneficial impact on alleviating the elevated friction in reaction wheel #4,” Kepler mission manager Roger Hunter wrote in an update posted on the Kepler website March 29. “At this point, all mitigation steps to preserve wheel life have been implemented, and no additional steps are planned at this time.”

Those mitigation steps, deputy project manager Charlie Sobeck said in an April 3 interview, include using the wheels at a faster speed and operating them at a higher temperature to increase the effectiveness of the wheel’s lubricant. The rest period in January, which was intended to allow the lubricant to redistribute itself in the wheel, had “a low probability to work to begin with,” he said, and there are no plans to repeat it.

The elevated friction in wheel No. 4 is of particular concern because another reaction wheel on the spacecraft, No. 2, failed last July. Normal spacecraft operations require three working reaction wheels.

Sobeck said that while wheel No. 2 also exhibited elevated levels of friction prior to its failure, that does not mean wheel No. 4 is in immediate danger of failing. “The way it presents itself is very different, and we don’t quite know what to make of that,” he said. Wheel No. 2 became “somewhat chaotic” in the amount of friction it exhibited starting about six months before its failure, he said. Wheel No. 4, by contrast, has jumped between a baseline level of friction and an elevated level since its launch four years ago. After the wheel rest the friction increased to an even higher level, but then declined to the previous elevated level. The other two wheels have behaved normally throughout the mission.

In the event wheel No. 4 does fail, engineers are studying alternative ways to operate the spacecraft. Sobeck said one option is a “hybrid” mode using the remaining two reaction wheels and the spacecraft’s thrusters. This mode, though, would not have the same degree of pointing accuracy as using reaction wheels alone, degrading the data the spacecraft can collect. “Perhaps we could do some of the kinds of things we’re doing now with less photometric stability, but I think it would be very difficult,” he said.

NASA launched Kepler in March 2009 to detect minute drops in the brightness of stars caused when planets pass in front of, or transit, them. As of January, Kepler had detected 2,740 planet candidates, with 115 of them confirmed. In hybrid mode, though, Kepler’s ability to detect such exoplanets would be in doubt. “We wouldn’t be able to see Earth-sized planets around Sun-like stars,” Sobeck said. Kepler may still be able to detect transits by larger exoplanets, something he said is still under investigation.

Kepler could instead be used for alternative observations in hybrid mode, including searches for extragalactic objects or asteroids. “It would be a different flavor of mission,” he said. “It would not be Kepler as we know it, but it still might provide useful science.”

“We are developing a two-wheel science case through the exoplanet program,” Paul Hertz, director of NASA’s astrophysics division, said at a meeting of the National Research Council’s Committee on Astronomy and Astrophysics here March 7. He confirmed that exoplanet transits, at least of Earth-size planets, could not be observed by Kepler in two-wheel mode. Until that science case is reviewed, he said, “I can’t tell you how valuable Kepler two-wheel science is and whether that makes Kepler worth operating.”

Other than the reaction wheel, Kepler remains in good health, Sobeck said. The limiting factors on the spacecraft’s life are its supply of propellant and its communications link as the spacecraft slowly drifts away from the Earth in a heliocentric orbit.

The Ball Aerospace & Technologies Corp.-built spacecraft completed its 3.5-year prime mission in November and began an extended mission that runs through 2016. Sobeck projects that, if Kepler’s reaction wheels keep working, the spacecraft could operate until late 2018.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...