FIGURE 1.1 A schematic showing the approximate cost ($) per watt of electrical power (We) required for radioisotope power systems (RPSs), photovoltaic systems at various distances from the Sun, and fission reactors of the type that might be used in nuclear-electric propulsion (NEP) systems. Most current space science missions have relatively modest power requirements, typically in the range of 0.1 to 1 kilowatts of electricity (kWe). For spacecraft operating at or near 1 AU from the Sun, photovoltaic systems have the advantages of cost-effectiveness and reliability. At greater distances from the Sun, RPSs become the favored option for satisfying modest power requirements. Fission reactors may fill a niche for supplying the large amounts of electrical power required by NEP systems and new types of power-hungry scientific instruments or to support human exploration activities. The plot is based on data supplied by the Boeing Company and is courtesy of Michael Kaplan.

serious handicap for space missions limited to Earth orbit, short visits to the Moon, or robotic missions to Mars. But the capability of missions to the outer planets (Jupiter, Saturn, Uranus, Neptune, and Pluto) and their moons is restricted if they must rely on only chemical, solar, and/or radioisotope systems for electric power and propulsion. While RPSs are inherently low-power systems, most space science experiments have only low power demands. Thus, highly capable missions such as Cassini, which is equipped with a dozen instruments, can manage sufficiently on some 800 watts of electricity. The outer planets are so far from the Sun, however, that the output of solar-power systems at such distances is minuscule. Also, the duration of missions to the outer planets is so long that the average power available to a spacecraft over its lifetime from a chemical power system would also be minuscule, and the total energy available from a chemical system would be limited by the need to carry along the requisite fuel.

Low power and energy limits are problems because power is needed to operate science instruments, communications systems, and propulsion systems. Nuclear reactor systems, which can provide relatively high power over long periods, make it possible to design missions with more numerous and more capable science instruments, high-bandwidth communications systems, shorter transit times, and greater flexibility to change the course and speed of spacecraft enough to:

  • Conduct extended investigations (rather than brief flybys) of bodies of interest;



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