Updating Chandra high-radiation safing in response to changing observatory conditions

The science instruments on-board the Chandra X-ray Observatory may be damaged by high fluxes of energetic particles produced in Solar storms. For twelve years the on-board commanding used to safe the instruments when a high-radiation environment was identified included stopping all on-board scheduled activity. Over this time the conditions of the observatory have changed: the orbit has evolved to lower perigee and the thermal protective surfaces have degraded. The likelihood of an unplanned unload of spacecraft angular momentum, a spacecraft component exceeding thermal limits, or an eclipse passage without planned commanding occurring following a Solar storm during the upcoming Solarmaximum due to the lack of scheduled commanding led us to update the Chandra response to a high-radiation environment; commands are sent to safe the science instruments but vehicle related commanding (maneuvers, angularmomentum unloads, eclipse commands) are allowed to continue. While this was a conceptually simple change, it touched all elements of the program, including flight software, the planning and commanding systems, flight-load verification tools, and ground-based data processing. A key to successful and timely implementation was the establishment of a working group with representation from all elements of the program.