less intense than at Earth. Continuing outward to Pluto, sunlight is 99.94 percent less intense. RPS-powered Voyager, Galileo, Cassini, and New Horizons spacecraft have enabled the United States to explore every planet in this dark, outer region of the solar system. Much of their success has been due in large part to having a reliable power source that provides enough power to operate complex instruments at a data rate high enough to optimize the capabilities of the scientific instruments they carry.

RPSs are also useful for missions to the surface of the Moon (especially during the long, cold lunar nights and in the permanently shadowed regions near the lunar poles); for missions to the surface of Mars (with its dust storms and extended winters); for extended missions below Venus’s cloud deck; and for other missions where solar power is not practical, for example, because the dynamic range of solar power would preclude the use of solar arrays.2

Space nuclear power reactors are another potential option for missions where solar power is not practical. However, the performance and reliability of space nuclear power reactor systems using current technology remains unproven, especially for missions with long lifetimes. In addition, the committee is not aware of any substantive effort currently under way anywhere in the world to develop space nuclear power reactor systems. The history of space nuclear power reactors suggests that space nuclear reactors, if successfully developed, could meet the needs of some missions and could enable other missions that are not now under consideration because of power limitations. For example, Project Prometheus, which was NASA’s most recent attempt to develop space nuclear power reactors, selected a nuclear electric propulsion reactor concept that was scalable from 20 kilowatts of electrical power (kWe) to 300 kWe. However, history also shows that the development of a high-power, long-life space nuclear power reactor would be very time-consuming and cost billions of dollars (see Appendix E).

Since 1961, the United States has launched 45 RPSs on 26 spacecraft dedicated to navigation, meteorology, communications, and exploration of the Moon, Sun, Mars, Jupiter, Saturn, and elsewhere in the outer solar system (see Table 2.1). This critical work could not have been accomplished without RPSs. Current RPS-powered space missions include the Cassini spacecraft, with three RPSs, which is studying Saturn and its moons; and the New Horizons spacecraft, with one RPS, which is studying Pluto and the Kuiper Belt. The Mars Science Laboratory spacecraft is scheduled for launch in 2011 with an RPS-powered rover. Over the longer term, RPSs are expected to support continued exploration of extreme environments of the Moon, Mars, and Venus, as well as the dimly lit outer reaches of the solar system and beyond. Such missions will be severely constrained or eliminated unless RPSs are ready and available (see Table 2.2).

FINDING. Importance of RPSs. RPSs have been, are now, and will continue to be essential to the U.S. space science and exploration program.

WHY 238Pu?

Plutonium-238, which does not occur in nature, is created by irradiating neptunium-237 (237Np) targets in a nuclear reactor. Although many studies over the past 50 years have assessed the advantages and disadvantages of using a wide variety of isotopes as a fuel for RPSs, every RPS launched into space by the United States has been fueled by 238Pu.3 Studies examined by the committee demonstrate that the longstanding decision by the Department of Energy (DOE) and NASA to rely on 238Pu is correct and well-justified. No other radioisotope meets or exceeds the safety and performance characteristics of 238Pu, particularly for long-duration, deep-space exploration missions (see Appendix D). Plutonium-238, which has a half-life of 88 years, is the only isotope that meets all of the general criteria for RPS fuels, as follows:

  • It generates heat for a sufficient length of time (i.e., it has a radioactive decay half-life of sufficient length).

  • The type and quantity of the emissions produced by the radioactive decay of the fuel allow it to be handled safely.

  • It has high specific power (heat per mass) and high power density (heat per volume).

  • It has a fuel form that is noncorrosive, water-insoluble, and chemically stable, and it demonstrates good engineering properties at high temperatures.

  • It can be produced in sufficient quantity at an affordable cost.

FINDING. Plutonium-238 Supply. Plutonium-238 is the only isotope suitable as an RPS fuel for long-duration missions because of its half-life, emissions, power density, specific power, fuel form, availability, and cost. An assured supply of 238Pu is required to sustain the U.S. space science and exploration program.


The Atomic Energy Act of 1954, as amended (Public Law 83-703, 1954), establishes comprehensive requirements regarding the possession, use, and production of nuclear


A specific example is a solar probe mission using Jupiter for a gravity assist in order to pass the Sun in an orbit highly inclined to the plane of the ecliptic. For a mission such as this, the spacecraft experiences such a wide range of solar intensity that current technology is unable to provide the spacecraft with a low-mass solar power system.


The Systems for Nuclear Auxiliary Power (SNAP)-3 Program used both polonium-210 and plutonium-238 as nuclear fuel for RTGs during ground tests (Dieckamp, 1967). Over the years, some papers have erroneously reported that SNAP-3 RTGs fueled with polonium-210 were operated in space. That is not the case.

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