To overcome the problems of power generation at large distances from the Sun, the use of RTGs provides a solution. These have been used on many spacecraft (for example, Pioneer, Voyager, Ulysses, Galileo, and Cassini), which have ventured to distant parts of the solar system, at and beyond Jupiter's orbit. The idea is that they take their own energy source with them, in the form of a radioactive material. The radioactivity heats the material, and this heat is converted to electricity through the thermoelectric effect. This was discovered, it is said accidentally, by Thomas Seebeck in 1821, and you may have done experiments in school science lessons to demonstrate the effect, using a thermocouple. Figure 9.7 illustrates a simple thermocouple: two wires made of dissimilar metals A and B are formed into a circuit, with some kind of meter to measure electrical current. If junction 1, where the two metals are joined, is heated, and junction 2 is cooled, an electrical current is generated in the circuit. The heat differential produces electricity. The RTG is a more sophisticated device, but this simple thought experiment demonstrates the underlying principle of its operation. Essentially, the temperature difference between the hot core of the radioactive material and the cold of space is exploited to produce electrical power for the spacecraft. The typical size, mass, and power output of a RTG are given in Table 9.1, and Figure 9.8 shows what they look like.

There are disadvantages to using RTGs, some of which we have already mentioned. Given that a typical RTG contains a radioactive material such as

Figure 9.7: A simple thermocouple circuit composed of wires of different metals A and B, and a meter to measure electrical current. When junction 1 is heated and junction 2 cooled, an electrical current will flow in the circuit.

Generator {RTG

Figure 9.8: The RTG on the ESA/National Aeronautics and Space Administration (NASA) spacecraft Ulysses, shown during spacecraft assembly (left) and in flight (right). (Image credits: left—NASA/Jet Propulsion Laboratory (JPL)—Caltech; right— ESA.)