maintain nominal solar orientation of large space solar collectors. Even in synchronous orbit, some level of resistance exists. It is this resistance, however small, that makes this mode inoperative as proved in Theorem 1. However, this idea should not be completely dismissed. In practice, it is unlikely that a totally passive system will be designed because many other disturbances can contribute to alter the proper phase of motion (i.e., the proper relationship between the attitude and the attitude rate at any given time), which will eventually cause the satellite to tumble away from its quasi-inertial attitude. This implies that some kind of active control is required. The type of control concept that can take substantial advantage of the quasi- inertial attitude mode is referred to as the power-assisted quasi-inertial attitude control system. This system allows the satellite to drift for most of the time and makes phase corrections periodically. The concept proposed here is a fixed-terminal time minimum energy nonlinear control problem. This problem is formulated as follows.
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