ABSTRACT
Nuclear power has played a significant role in the exploration of the solar system, in many cases enabling
missions that could not have been achieved otherwise. First flown by the United States in 1961, nuclear
power systems (NPSs) have consistently demonstrated unique capabilities over other types of space
power systems. NPSs are comprised of radioisotope power systems (RPSs) and fission power systems
(FPSs). RPSs generate electrical power by converting the heat released from the nuclear decay of
radioactive isotopes (typically plutonium-238) into electricity (Figure 56.1) via one of many conversion
processes that include thermoelectric, thermophotovoltaic, Stirling, Brayton, and Rankine converters.
The fundamental physical process involved in thermoelectric (TE) power conversion is the Seebeck effect,
which is the electromotive force that arises between two dissimilar materials (i.e., metals or semi-
conductors) when their junction is subjected to a temperature difference. The electromotive force
generated by the thermocouple can be used to drive an electric circuit, or if large enough, a spacecraft
power system. The key advantages of radioisotope thermoelectric generators (RTGs) are their long life,
robustness, compact size, and high reliability. RTGs are able to operate continuously, independent of
solar insolation, and are relatively insensitive to radiation and other environmental effects. Thermo-
electric converters are easily scalable, and possess a linear current-voltage curve, making power
generation easy to control via a shunt regulator and shunt radiator. They produce no noise, vibration, or
torque during operation. These properties have made RTGs ideally suitable for autonomous missions
in the extreme environments of outer space and on planetary surfaces. Figure 56.2 illustrates the general
range of space power system applicability.
