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.
Scheduling observatory time to maximize both day-to-day science target integration time and the lifetime
of the observatory is a formidable challenge. Furthermore, it is not a static problem. Of course, every
schedule brings a new set of observations, but the boundaries of the problem change as well. As
spacecraft ages, its capabilities may degrade. As in-flight experience grows, capabilities may expand. As
observing programs are completed, the needs and expectations of the science community may evolve.
Changes such as these impact the rules by which a mission scheduled. In eight years on orbit, the Chandra
X-Ray Observatory Mission Planning process has adapted to meet the challenge of maximizing day-to-day
and mission lifetime science return, despite a consistently evolving set of scheduling constraints. The
success of the planning team has been achieved, not through the use of complex algorithms and
optimization routines, but through processes and home grown tools that help individuals make smart short
term and long term Mission Planning decisions. This paper walks through the processes and tools used to
plan and produce mission schedules for the Chandra X-Ray Observatory. Nominal planning and
scheduling, target of opportunity response, and recovery from on-board autonomous safing actions are all
addressed. Evolution of tools and processes, best practices, and lessons learned are highlighted along the
Planning an observation schedule for ground-based and space-based telescopes alike requires careful constraint management and implementation. Scientific constraints, which meet an observer's desire to maximize science returns, must be weighed against the physical constraints and capabilities of the telescope. Since its launch in 1999, the Chandra X-Ray Observatory (CXO) has provided excellent science in spite of evolving constraints, including the proliferation of constraint types and varying degrees of restriction. The CXO observation schedule is generated on a weekly basis, yet the mission planning process maintains the flexibility to turn around a target-of-opportunity (TOO) request within 24 hours. This flexibility is only possible when all personnel responsible for schedule generation - flight operations engineers, science operations personnel, and program office support - are actively involved in constraint management. A proper balance of software tools, guideline documentation, and adequate subjective judgment is required for proper constraint implementation. The decision-making process employed by mission planning personnel requires accurate, complete, and current constraint information.
The CCDs on the Chandra X-ray Observatory are vulnerable to radiation damage from low-energy protons scattered off the telescope's mirrors onto the focal plane. Following unexpected damage incurred early in the mission, the Chandra team developed, implemented, and maintains a radiation-protection program. This program - involving scheduled radiation safing during radiation-belt passes, intervention based upon real-time space-weather conditions and radiation-environment modeling, and on-board radiation monitoring with autonomous radiation safing - has successfully managed the radiation damage to the CCDs. Since implementing the program, the charge-transfer inefficiency (CTI)
has increased at an average annual rate of only 3.2×10-6 (2.3%) for the front-illuminated CCDs and 1.0×10-6 (6.7%) for the back-illuminated CCDs. This paper describes the current status of the Chandra radiation-management program, emphasizing enhancements implemented since the original paper.