the control program will stop the gimbal movement, and the rotors will be left pointing more forward than before. The inflow of unwanted forward angular momentum has been routed through the CMGs and dumped into the rotors; the forward component of their total angular momentum vector is now greater than before. If these events are repeated, the angular momentum vectors of the individual rotors will bunch more and more closely together round the forward direction. In the limiting case, they will all end up parallel, and the CMG cluster will now be saturated in that direction; it can hold no more angular momentum. If the CMGs were initially holding no angular momentum about any other axes, they will end up saturated exactly along the forward axis. If however (for example) they were already holding a little angular momentum in the "up" (yaw left) direction, they will saturate (end up parallel) along an axis pointing forward and slightly up, and so on. Saturation is possible about any axis. In the saturated condition attitude control is impossible. Since the gyroscopic torques can now only be created at right angles to the saturation axis, roll control about that axis itself is now non-existent. There will also be major difficulties with control about other axes. For example, an unwanted left yaw can only be countered by storing some "up" angular momentum in the CMG rotors. This can only be done by tilting at least one of their axes up, which will slightly reduce the forward component of their total angular momentum. Since they can now store less "right roll" forward angular momentum, they will have to release some back into the spacecraft, which will be forced to start an unwanted roll to the right. The only remedy for this loss of control is to desaturate the CMGs by removing the excess angular momentum from the spacecraft. The simplest way of doing this is to use reaction control system (RCS) thrusters. In our example of saturation along the forward axis, the RCS will be fired to produce an anticlockwise torque about that axis. The CMG control program will then command the rotor spin axes to begin fanning out away from the forward direction, producing gyroscopic torques whose resultant is clockwise about the forward direction, opposing the RCS as long as it is firing, and so holding the spacecraft steady. This is continued until a suitable