Overview

The power system supplies the life blood of the spacecraft. As long as the spacecraft has power, it can perform its mission. Almost all other failures can be worked around by ground operations, but a loss of power is a fatal heart attack for the spacecraft. In the early years of spaceflight, the power system was also the limiting factor in mission duration. Sputnik I consisted of a structure, a battery, a transmitter, and an antenna. The highly successful mission lasted exactly as long as the battery: 21 days. Explorer I, the first U.S. spacecraft, lasted longer but also ended abruptly with battery depletion. One-shot battery systems of the type Sputnik and Explorer I used are called primary batteries for reasons lost in space history. The short duration of time provided by primary batteries was clearly unsatisfactory for most missions. Solar panels were developed to convert the sun's energy into power, and solar panel battery power systems became the backbone of unmanned spacecraft design from 1959 to the present.

As planetary missions outbound beyond Mars were considered, it became necessary to develop power sources independent of solar energy because of the great solar distances involved. (Recall that incident solar energy decreases as the square of the distance from the sun.) The development of radioisotope thermoelectric generators (RTGs) resulted. RTGs convert the energy released by the decay of a radioisotope into electrical power.The power levels of an RTG are moderate, but the life of an RTG is very long, depending...