To keep its gaze steady, Kepler needs at least three of its four reaction wheels spinning. One wheel failed last year. A second shut down in May.
When engineers last checked, the first wheel to fail could still move, but not freely.
"We would expect that it might very well seize up as soon as we try to spin it up again," Sobeck told Discovery News.
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The wheel that failed in May was completely stuck and likely will remain so.
Phase two of the Kepler revival effort will be to get one wheel moving again. Engineers will work first on the most recently failed wheel, though there is less of a chance that it can be restored.
"We'll learn any lesson on the wheel that's less likely to respond, and then we'll move on to the better bet," Sobeck said.
The options are pretty limited, he added. They include warming up the stuck wheels in the sunlight and then commanding them to spin clockwise and counter-clockwise to see if whatever is binding the wheel can be nudged out of the way.