During the last week Chandra's observing schedule was interrupted by a spacecraft Safe Mode event that occurred at 12:41pm EST on Feb 13. Entry into safemode was nominal; examination of the error logs and telemetry indicated that the most probable cause for the anomaly is a sun position monitor trip, traceable to anomalous fine sun sensor (FSS) data. The safe mode entry time corresponds to crossing below 135 deg solar pitch, when the sun position monitor would be accepting FSS data.

Recovery procedures were initiated at the next communications pass; by approximately 11:45pm EST on Feb 13 Chandra was returned to OBC control, in Normal Sun Mode, with thermal control established.

On Feb 14 realtime procedures were employed to move the spacecraft in yaw to an attitude that minimizes momentum accumulation during the upcoming perigee passage (on Feb 15 at 3am EST).

Realtime procedures were executed on Feb 15 to implement post-safe mode cleanup steps, including patching the ephemeris-time-offset K-constant in the flight software and updating the gyro bias. An additional realtime procedure was implemented to perform a ground-commanded momentum unload. A search for stars sufficiently bright for an attitude solution was undertaken, beginning at the current normal sun position and stepping in yaw at 0.5 deg intervals, performing an ACA full field search at each position. A suitable field was identified and an aspect solution derived. At approximately 2:34am EST on Feb 16, while attempting to re-acquire stars, Chandra entered safe mode. Entry into safe mode was nominal, with the spacecraft in a safe and power positive state. A trip of the attitude/rate error monitor appears likely as cause of the safe mode. The spacecraft was in CPE attitude hold, and FSS processing was restored before the end of the comm pass.

On Feb 16 realtime procedures were executed to return Chandra to OBC control in Normal Sun Mode. Gyro biases were updated, and a CPE Sun Vector Parameter Patch was applied to prevent an extended attitude hold condition on the next Safe Mode transition. Redundant PCAD equipment was turned off; this results in reduced warming of the ACA by turning off one of the Inertial Reference Units.

Following the perigee pass at 6pm EST on Feb 17, the team will focus on returning Chandra to Normal Pointing Mode and moving to a high solar pitch angle attitude to cool critical components in advance of return to normal science operations.