TITLE: GCN CIRCULAR NUMBER: 31547 SUBJECT: Swift Reaction Wheel Anomaly Update (4 Feb 2022) DATE: 22/02/04 17:29:59 GMT FROM: Brad Cenko at NASA/GSFC Since Swift entered safe hold on 18 January, the Swift team has been working hard to 1) diagnose the cause of the anomaly, 2) determine if / how to attempt to recover the failed reaction wheel, and 3) prepare for operations in a 5-wheel configuration. Here we provide a brief update on the status of these efforts. As indicated with preliminary analysis, the anomaly was triggered by a failure of reaction wheel #5 (of 6). The failure mode is believed to be mechanical in nature. All other wheels are performing as expected while in safe hold. In consultation with attitude control engineers, we determined that we will not attempt to recover wheel #5 for the time being. After several more weeks of resting, we will reconsider whether to attempt recovery. In the mean time, we’ve been preparing to configure the spacecraft for 5-wheel operations. We are finalizing testing of these configuration settings, with the current intention of uploading them to the spacecraft next week. Analysis of 5-wheel operations indicates that, due to unused margin in the attitude control settings, the vast majority (> 99%) of spacecraft slews will be minimally affected by the change to 5-wheel operations. The primary impact will be for the initial slews following on-board GRB triggers, where the mean response time is estimated to increase by ~ 20 s. Once the new configuration is uploaded to the spacecraft and verified in orbit, we will begin a phased return to science operations. We will update on the time scale for this once we have verified a successful configuration change. If you have further questions, please feel free to reach out to the Swift PI, Brad Cenko, at brad.cenko@nasa.gov