During a scheduled contact on Thursday, Nov. 15, 2012, engineers discovered that the Kepler spacecraft, although at science attitude, was not in finepoint, a mode where the pointing accuracy is sufficient to take precision science data. While implementing procedures to return the spacecraft to this precision-pointing mode, on-board fault protection detected a larger pointing error and placed the spacecraft in safe mode.
A safe mode is a self-protective measure that the spacecraft takes when an unexpected event occurs. During a safe mode, the spacecraft points the solar panels directly at the sun and begins to slowly rotate about a sun-aligned axis. As a safety precaution, the spacecraft turns off non-essential systems, the photometer, and both star trackers.
Engineers have evaluated data from the spacecraft subsystems and found no evidence of system malfunction. In particular, engineers have concluded that there is no indication of anomalous reaction wheel behavior.
The safe mode is likely due to the local space environment. Data suggest that particulates moving through the star trackers fields of view corrupted the spacecraft's attitude solution– its orientation relative to the guide stars. This resulted in the safe mode. Such particulate events have been observed throughout the mission, but are generally managed without resorting to a safe mode.
Two star trackers located at the base of the spacecraft, are small cameras used to measure and track the position of guide stars. The guide stars allow Kepler to determine its orientation in space.
The project team exercised the standard recovery process over the weekend, using multiple contacts with the NASA Deep Space Network, and returned to science data collection Monday, Nov. 19, 2012. The loss of science data is estimated to be six days.
To learn more about Kepler's design, see Spacecraft and Instrument.