Chandra autonomously transitioned to Normal Sun Mode (NSM) during an observation late on Saturday, February 2, 2024. The transition was nominal, science instruments were safed properly, and all telemetry was as expected. The immediate cause of the anomaly was a trip of the Bright Star / Commanded Attitude Monitor. The root cause was identified as a transient high Aspect Camera Assembly (ACA) background event which caused the ACA to lose track of guide stars over approximately a 2 minute period. This anomaly has been seen earlier in the mission, the last time in 2019 – most likely due to flaking MLI that reflects sun into the aspect camera and raises the background signature to a point that guide stars are not found by software. Chandra resumed science observation on February 7, 2024 following passage through orbital low-perigee and commanded S/C recovery as planned.
