After a "wheel rest" safe mode that began on January 17, 2013, NASA's Kepler spacecraft returned to science data collection at 5 p.m. PST on January 28, 2013. During the 10-day resting safe mode, daily health and status checks with the spacecraft using NASA's Deep Space Network were normal.
The recovery from wheel rest began at 11:30 a.m. PST on January 27, 2013, and proceeded without issue. The spacecraft responded well to commands and transitioned from thruster control to reaction wheel control as planned.
Early this month during a semi-weekly contact with the spacecraft, elevated friction was detected in reaction wheel #4. As a precaution for wheel safety, and as a measure to mitigate the friction, the reaction wheels were spun down to zero-speed and the spacecraft was placed in a thruster-controlled safe mode.
Science data collection was halted during this rest period and the spacecraft solar panels were pointed at the sun to maintain positive power. This is similar to a normal safe mode configuration, but with thrusters maintaining attitude instead of reaction wheels.
Since the failure of reaction wheel #2 in July 2012, the performance of the spacecraft on three wheels has been excellent. Reaction wheel #2, we now know, worked relatively well until January 2012, when it began to exhibit elevated and somewhat chaotic friction that led to failure.
Reaction wheel #4 has been something of a free spirit since launch, with a variety of friction signatures, none of which look like reaction wheel #2, and all of which disappeared on their own after a time.
Resting the wheels can provide an opportunity for the lubricant in the bearings to redistribute and potentially return the friction to nominal levels. Over the next month, the engineering team will review the performance of reaction wheel #4 before, during, and after the safe mode to determine the efficacy of the rest operation.