JWST Momentum Management
The JWST Observatory's momentum is managed both predictively and in real time by the attitude control system to keep the observatory under control at all times.
During science observations, solar photon pressure causes angular momentum to build up within the reaction wheels. This angular momentum must be dumped periodically by firing thrusters.
How momentum builds up
During science observations, the observatory will be pointed at a target, in an orientation at which the sun shield center of pressure is not aligned with the observatory center of mass. As solar photons hit the large sun shield, they place a torque on the observatory as a whole. The attitude control subystem (ACS) counteracts this torque by appropriately changing the spin rate on the reaction wheels, with the consequence that angular momentum accumulates in the reaction wheels. Momentum accumulation depends on the solar pitch angle, the roll orientation of the telescope, and the visit duration at a particular pointing position. The angular momentum (spin rate) of the reaction wheels must be managed to be kept within operational limits.
The planning and scheduling system predicts the momentum profile for a given section of schedule delivered to the observatory, based on an assumed starting momentum and schedule of observatory pointings. Momentum changes can be managed at some level by the way a sequence of observations is planned; this is done by observing at an orientation that builds momentum in a particular reaction wheel, followed by an observation at an orientation that removes momentum from that wheel.
However, managing momentum is only one of a number of planning constraints. At some point, one or more wheels will need to be adjusted to stay within operational bounds. The planning and scheduling system inserts planned momentum unloads into the schedule as needed, based on the modeling of expected momentum buildup, currently expected to be 1–2 times per week. Each unload activity takes a few hours, in which the observatory slews to a particular orientation to minimize the impact on the orbit and then fires thrusters as needed to allow the spin rate of the reaction wheels to be adjusted. The observatory then rejoins the preplanned observing timeline.
Because loss of pointing control from saturating one or more reaction wheels could endanger the entire observatory, an important safeguard is built into the ACS. Since JWST operations are event-driven, the actual sequence of activities can differ from what was planned. For example, if a guide star acquisition fails on one observation, that observation is dropped and the observatory moves on to the next planned observation. This will obviously make the real momentum profile different from what was planned.
The onboard operating system checks the current momentum state before starting each visit. If the momentum state is judged not to be sufficient to safely complete that visit, it will autonomously request a momentum unload be performed before the visit begins. Also, while margins are built into the planned timeline, if for any reason one of the reaction wheels approaches its saturation limit, the ACS will autonomously terminate the science activities, unload momentum at the current pointing, and put the observatory into a "safe mode." Recovery from safe mode would not occur until the next ground contact when real-time communications can be established. The operating system checks prior to each visit should prevent this safety net from ever being needed, but the safety net is there as a stop gap against a dangerous situation for the observatory.