The function of a remote-sensing mission is to acquire remotely-sensed data from a payload, which might be an instrument or a group of instruments, aimed at a target, which may be an area on the surface of the Earth, a moon, planet, asteroid or other body, or may be a star field or other field of view containing objects about which information is desired. The major function of the mission operations system engineers is to keep all these platform subsystems playing together in harmony, to allow the payload to perform its remote sensing function. Currently there are only three types of electrical power subsystems in common usage among remote-sensing spacecraft: solar, chemical and nuclear. Solar power is only practical for missions designed to operate close enough to the Sun to provide the needed power. Detectors of high energy protons emitted by the Sun can provide much information about our star.