The United States and the former Soviet Union responded to the need to develop long lasting, safe, and economical power sources in a variety of ways. The contrast between the former Soviet Union space program and that of the U. S. is evident in the number of nuclear sources launched into orbit. The principles of radioactive decay are quite simple. The power generated by a RTG at any time t depends on the number of initial radioisotopes, the decay energy, and the half-life. The radioisotope of choice is 238 Pu.
Thermoelectricity is a reliable way to convert heat energy directly into electricity.
The Galileo RTG has operated perfectly. The release of energy from fission relies on the same conversion of mass into energy as stated by the Einstein equation. The SP-100 nuclear reactor, designed in the U. S., would produce 100 kWe of continuous power for at least 7 years. The SP-100 can be used for Base power on the Moon or Mars. A comparison of the different kinds of fission reactors in orbit or on the drawing board reveals their similarities and differences.