Two of four reaction wheels seized up, critical for precise photometry
Excerpt from the Kepler Mission Manager Update. (h/t to Dr. Leif Svalgaard)
At our semi-weekly contact on Tuesday, May 14, 2013, we found the Kepler spacecraft once again in safe mode. As was the case earlier this month, this was a Thruster-Controlled Safe Mode. The root cause is not yet known, however the proximate cause appears to be an attitude error….
…We attempted to return to reaction wheel control as the spacecraft rotated into communication…but reaction wheel 4 remained at full torque while the spin rate dropped to zero. This is a clear indication that there has been an internal failure within the reaction wheel, likely a structural failure of the wheel bearing. The spacecraft was then transitioned back to Thruster-Controlled Safe Mode.
An Anomaly Review Board concurred that the data appear to unambiguously indicate a wheel 4 failure, and that the team’s priority is to complete preparations to enter Point Rest State. Point Rest State is a loosely-pointed, thruster-controlled state that minimizes fuels usage while providing a continuous X-band communication downlink. The software to execute that state was loaded to the spacecraft last week, and last night the team completed the upload of the parameters the software will use.
The spacecraft is stable and safe, if still burning fuel…In its current mode, our fuel will last for several months. Point Rest State would extend that period to years.
…We will take the next several days and weeks to assess our options and develop new command products. These options are likely to include steps to attempt to recover wheel functionality and to investigate the utility of a hybrid mode, using both wheels and thrusters.
With the failure of a second reaction wheel, it’s unlikely that the spacecraft will be able to return to the high pointing accuracy that enables its high-precision photometry. However, no decision has been made to end data collection.
Kepler had successfully completed its primary three-and-a-half year mission and entered an extended mission phase in November 2012.
Even if data collection were to end, the mission has substantial quantities of data on the ground yet to be fully analyzed, and the string of scientific discoveries is expected to continue for years to come….
Read the full Mission Manager Update.
In a New York Times article, astronaut John Grunsfeld, now in charge of NASA Science Mission Directorate under which Kepler operates, is quoted: “For Mr. Grunsfeld, who played mechanic to the Hubble telescope during several lengthy spacewalks, the Kepler malfunction looked particularly frustrating. ‘Unfortunately, it’s not in a place where I can go and fix it,’ he said.”
What does reaction wheel 4 look like?
(see photo at right)