Momentum from solar photons builds up in the JWST reaction wheels, and is periodically dumped by firing thrusters.

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 angular momentum accumulates

The force from solar flux acting over the area of the sun shield's outer layer represents a pressure. The pitch and roll angles of most science attitudes do not align the center of pressure with the observatory's center of mass, and this creates a torque on the observatory. The attitude control subystem (ACS) counteracts this torque with an opposing torque generated by increased net spin rate of the reaction wheels. Over time this angular momentum of the reaction wheels accumulates. The rate of momentum accumulation depends on the torque history, which is a function of the pointing and orbit history. 

Managing momentum

The angular momentum (spin rate) of the reaction wheels is actively managed to keep within operational limits. As part of generating the weekly observing schedule, the planning and scheduling system predicts the weekly momentum profile, and inserts commanded momentum unloads into the observing schedule as needed. 

Each momentum unload activity takes a few hours.  The observatory slews to a particular orientation to minimize the impact on the orbit, then fires thrusters to allow the net spin rate of the reaction wheels to be reduced.

Before launch, momentum unloads were projected to occur 4–8 times per month. During commissioning the actual rate was less frequent (12 unloads in 6 months); this is because during commissioning the observatory spent a lot of time pointed at the continuous viewing zone, a pointing where the momentum accrual rate is low. In science operations, the frequency of momentum unloads should be intermediate between these two extremes.

Because the observatory operates autonomously most of the time, there are two important sets of onboard checks to make sure the reaction wheel speeds stay within safe limits at all times. First, before starting each visit, the onboard operating system checks the current momentum state and expected rate change from the visit; if that state is not sufficient to complete that visit, an autonomous (unplanned) momentum dump is triggered.  Secondly, if for any reason one of the reaction wheels approaches its saturation limit, the attitude control system will autonomously terminate the science activities, unload momentum at the current pointing, and put the observatory into "safe mode." Recovery from safe mode would not occur until the next ground contact when real-time communications can be established. Momentum should be managed via by scheduling momentum dumps in the weekly observing plan, such that autonomous momentum unloads are rarely (if ever) executed.