The team began anomaly response procedures on July 19, 2012, by removing an unresponsive reaction wheel from the spacecrafts attitude control processes, downlinked diagnostic data and returned to normal science data collection.
Initial assessment shows that the anomaly appears to be isolated to reaction wheel #2. The RWA consists of four reaction wheels mounted on non-orthogonal axes. The wheels are actively redundant. This means that all four wheels share the load for momentum management and attitude control of the spacecraft. The spacecraft design is tolerant to the failure of a reaction wheel and can meet all mission objectives operating with three wheels.
The spacecraft remained at science attitude when the anomaly occurred, but transitioned from fine-point to coarse-point mode. In this mode, the telescope continued to monitor the target stars, but the larger pointing jitter negates the scientific use of the data. The loss of science data is estimated to be six days.
Over the course of the next few weeks, the team will analyze all available telemetry to determine cause of the reaction wheel anomaly, and ascertain the potential of returning wheel #2 to service. Meanwhile, the spacecraft has been returned to mission science observations using three reaction wheels.
Updates will be provided as additional information is available.
Provided by NASA
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