ABSTRACT
Deep space and outer planetary missions cannot effectively use photovoltaic power generation due to insufficient solar flux. For those missions, an on-board nuclear energy source or a radioactive isotope is often used to generate electrical power. The radioisotope heat is directed at a thermoelectric junction, which generates electrical potential just as in a thermocouple. This concept was briefly presented in Section 3.6, and is covered in detail in this chapter. The radioisotope thermoelectric generator (RTG) has been fully devel-
oped and used for decades for power levels in several hundred watts. Such a power source has an advantage of supplying power all the time, thus eliminating the need for a battery in a base load system having no peak power requirement. An obvious disadvantage is the heavy radiation shielding required around electronic components. Also, the nuclear fuels that are safe and easy to handle with little shielding, such as curium-244 and plutonium, are expensive. Inexpensive and easily available fuel, such as strontium-90, is unsafe. High-energy particles emitted from the radioactive isotope material are
the primary sources of energy, which heats the absorbing material. The mass of the isotope decay exponentially at a rate characterized by half-life, T1=2. The thermal radiation decreases proportionally with the remaining mass. Therefore, the thermal power P(t) radiated at any time decays exponentially from its initial value Po, as given by
PðtÞ ¼ Po exp ð0:7t=T1 2 Þ ð20:1Þ
Table 20.1 compares the isotope fuels presently used in RTGs with their half-life and specific power achievable. The advantages of an RTG are:
It provides power for a long period of time, independent of the spacecraft orientation and distance from the sun.
