Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration (2009)

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Appendix E History of Space Nuclear Power Systems Introduction Origins of Nuclear Power Systems for Spaceflight Through an investment of considerable resources— e ­ ngineering and scientific knowledge, human capital, and Beginning in the late 1940s several threads converged public funds—the United States has gained undisputed to make it possible to develop and use RPSs. In particular, leader­ship in the exploration of the outer solar system, that the Atomic Energy Commission (AEC) began to inves- part of the system beyond the orbit of Mars. This has been tigate production and use of radioisotopes in connection made possible since the 1950s by harnessing several core with nuclear weapons. This prompted scientific research to technologies that have enabled the nation’s scientific space- understand the radioactive decay and chemistry of various craft to travel for years on end, engage in extended scientific isotopes that are not found in nature. Second, scientists observations, and relay critical data back to Earth. Radio­ and engineers began to experiment with the development isotope power systems (RPSs) are one such technology. of small nuclear power generators for a variety of uses on RPSs generate heat from the natural decay of a radioac- Earth, especially in extreme locations and environments tive isotope, or radionuclide. This heat is transformed into (e.g., at the poles and under the seas), where scientific instru- electricity with some level of efficiency, depending upon the ments could be placed and left alone for months at a time. converter design. A variety of converter approaches have Third, advances in thermo­electricity and semiconductors been, and continue to be, investigated. In all flight systems made RTGs feasible. used to date, the heat flows from a radioactive heat source, In 1946 the newly established RAND Corporation through an array of thermocouples, and to a heat sink, gen- explored the viability of orbital satellites and outlined the erating electricity in the process. These systems are called technologies necessary for their success (RAND, 1946). By radioisotope thermoelectric generators (RTGs). RTGs are 1949 a full-scale analysis by RAND had sketched out the the preferred method for supplying the power needs of U.S. large-scale use of nuclear power sources for satellites in deep-space probes to the outer solar system and beyond, and Earth orbit (Gender and Kock, 1949). Beginning in 1951, they have also been used for some Earth-orbiting spacecraft at the request of the Department of Defense (DOD), the and to support missions to the Moon and Mars. All U.S. RPSs AEC sponsored research into nuclear power for spacecraft launched into space have been powered by 238Pu. They have to support the development of a reconnaissance satellite. provided power ranging from 2.7 watts on the very early sys- The AEC pursued two related avenues: a small nuclear tems to 500 watts on more recent flights (Lee, 1994). RPSs reactor and an RTG. These Systems for Nuclear Auxiliary have powered many types of spacecraft, including orbiters Power (SNAP) were numbered such that the odd numbers and landers. They allow spacecraft operations in extreme designated RTGs and the even numbers designated reactor environments that rule out the use of other power systems power systems. (e.g., solar arrays). By June 1952, an early classified study of the effort reported there were no insurmountable technical hurdles, and a year later, in May 1953, U.S. Air Force Headquarters authorized development of a nuclear power source for satel- The Seebeck effect. lites. The first bench-test RTGs emerged from the Mound Cassini had more than 800 watts of electrical power at launch using Laboratory (operated for the AEC by the Monsanto Research this approach. 46 

APPENDIX E 47 Corp.) in 1953 and quickly found application in Antarctica to fications. Instead, it was terminated in 2005, after it became power scientific research stations (Jordan and Birden, 1954; clear that it would have cost at least $4 billion to complete Morse, 1963). SNAP-1 (an RTG) was built at the Mound development of a spacecraft reactor module, and a total Laboratory under AEC supervision in 1954 (Anderson and of at least $16 billion to develop the entire spacecraft and Featherstone, 1960). This was followed by the use of nuclear complete the mission, not counting the cost of the launch power systems on spacecraft in the early 1960s. vehicle or any financial reserves to cover unexpected cost The possibilities of space nuclear power first entered the growth (JPL, 2005). public sphere in January 1959 when President Dwight D. The performance and reliability of space nuclear power Eisenhower posed with a SNAP-3 RTG in the Oval Office of reactor systems using current technology remains unproven, the White House. Ultimately, the Transit 4A and 4B naviga- especially for missions with long lifetimes. In addition, the tion satellites were provided with SNAP-3B power sources committee is not aware of any substantive effort currently from the AEC. They were the first satellites to operate in under way anywhere in the world to develop space nuclear space with RPSs. Both satellites were also equipped with power reactor systems. The history of space nuclear power solar panels that supplied 35 W of power (Dassoulas and reactors suggests that space nuclear reactors, if successfully McNutt, 2007). These and subsequent missions proved the developed, could meet the needs of some missions and feasibility of using RPSs for space missions. could enable other missions that are not now under consid- eration because of power limitations. However, history also shows that the development of high-power, long-life space Space Nuclear Reactor Systems nuclear power reactors would be very time-consuming and Space nuclear power reactors are another potential option expensive. for missions where solar power is not practical. However, the United States has launched only one space nuclear power Vehicle Accidents and Malfunctions reactor (SNAP-10A), and that took place in 1965. That early system was designed to produce 40 kW of thermal power and Three U.S. spacecraft with RPSs on board have inadver- 500 W of electricity for an operating life of just 1 year, and tently returned to Earth. In all cases, the RPSs performed the failure of a voltage regulator caused the system to shut as designed; the cause of the mission failure lay with other, down after 43 days (Wilson et al., 1965). nonnuclear systems. Beginning in 1983, NASA, the DOD, and the Department The Transit 5BN-3 spacecraft with one SNAP-9A RPS of Energy invested approximately $500 million in the SP-100 on board broke up and burned up on reentry after a launch- space nuclear power reactor. This system was intended to vehicle upper stage failure. The design philosophy at that generated 2 MW of thermal power and 100 kW of ­electricity, time was to require that the 238Pu oxide fuel totally burn up but because of high costs, schedule delays, and changing during reentry into Earth’s atmosphere, which it did. national space mission priorities, the SP-100 program was As a result of that accident, the RPS design philosophy suspended in the early 1990s and later canceled. The Soviet was changed to require full containment of the fuel (i.e., no Union launched dozens of short-lived space nuclear power fuel burn up) during an inadvertent reentry from or to Earth reactors during the 1970s and 1980s, and several unfueled orbit. This design philosophy is still in effect. Soviet systems were purchased by the United States in the The Nimbus B-1 weather satellite, the first NASA satellite early 1990s. These systems were extensively ground tested to use an RPS, was intentionally destroyed during launch by a joint team of U.S., British, French, and Russian engi- due to the erratic ascent of the launch vehicle. The launch neers using electrical heaters in place of the nuclear cores. vehicle, upper stage, and payload were totally destroyed by Although the test program was successful, the United States the explosion initiated by the destruct action, and the debris did not use the Soviet equipment or technology in a flight fell into the Santa Barbara Channel off Vandenberg Air Force program (NRC, 2006). Base. The two SNAP-19B2 RPSs were recovered intact (i.e., Project Prometheus was the most recent U.S. attempt to no 238Pu oxide fuel release occurred), and the fuel was used develop space nuclear power reactors. This project began on a later mission. in 2002, and it’s initial focus was on the Jupiter Icy Moons The last accident involving a U.S. RPS was the Apollo 13 Orbiter mission. The project selected a nuclear electric mission, which has been well documented. The SNAP-27 propulsion reactor concept that was scalable from 20 kWe heat source assembly was stowed in the Lunar Excursion to 300 kWe. A nuclear electric propulsion system for a Module, which returned to Earth after the mission was deep-space mission would need to be validated for reliable aborted. It reentered over the South Pacific Ocean. Air and operation for a mission lifetime of 10 to 20 years, with no water sampling detected no 238Pu oxide fuel, indicating that maintenance or repair. However, as with the SP-100 pro- the SNAP-27 heat source assembly survived reentry intact gram, ­ Project Prometheus did not proceed to the point of (as designed) and came to rest at the bottom of the Tonga demonstrating the ability of system designs or available Trench under more than 7,000 feet of water, where it still technology to meet required performance or lifetime speci- remains. 

48 RADIOISOTOPE POWER SYSTEMS Space Nuclear Power and Voyager 1, which is traveling faster than Voyager 2, is Outer-Planet Missions now farther from Earth than any other human-made object. Now traveling out of the solar system, both Voyager 1 and A major shift in the use of RPSs came with NASA’s Voyager 2 have passed the “termination shock” of the solar decision to pursue outer-planet exploration. This initiative wind and continue to send back the first information ever was driven by the discovery of “grand tour” trajectories received from the outer boundary of our solar neighborhood. that could enable relatively short missions to the planets of The Voyagers are expected to return scientific data until the outer solar system by using multiple planetary gravity the RPSs can no longer supply enough electrical energy to assists. This planetary configuration is rare, occurring only power critical systems. With the adoption of power sharing about every 176 years, but it was due to occur in the late among the still-operating instruments, the final transmission 1970s and led to one of the most significant space explora- is expected to occur in about 2020. Whether Voyager 1 will tion efforts undertaken by the United States (Dethloff and reach the heliopause, the “boundary” between the shocked Schorn, 2003). solar wind and interstellar plasma, by then is unknown. The nearly identical Pioneer 10 and 11 spacecraft were NASA has continued to use RPSs on missions to the launched in 1972 and 1973, respectively, to make the first outer planets and on selected long-term missions closer trips through the asteroid belt to Jupiter and beyond. Both to the Sun when necessary to enable the mission. In 1989, relied on RPSs to provide power far from the Sun. Pioneer 10 NASA deployed the Galileo spacecraft from a space shuttle flew past Jupiter in late 1973. It transmitted data about the and sent it on a 6-year, gravity-assisted journey to Jupiter, planet and continued on its way out of the solar system. where it became the first spacecraft to orbit the giant planet Pioneer 11 provided scientists with an even closer view of (Launius and Johnston, 2009). The flight team for Galileo Jupiter, whose gravity was used to send Pioneer 11 to Saturn ceased operations in 2003, and the spacecraft was deorbited before it, too, departed the solar system. Pioneer 11 ended its by command into Jupiter’s atmosphere to guard against any mission in 1995, when the last transmission from the space- potential future contamination of Jupiter’s moon Europa by craft was received. NASA continued to receive signals from an uncontrolled spacecraft impact. Pioneer 10 until 2003, when the spacecraft was 7.6 billion Galileo carried two newly developed general purpose miles from Earth. The success of the Pioneer missions would heat source (GPHS) RTGs. These units produced 300 W of not have been possible without the four SNAP-19 RTGs that electricity at beginning of life and had a total mass of 55.9 kg, each spacecraft carried as their sole source of power. Each giving these devices the highest specific power of any RPS Pioneer spacecraft also had a dozen radioisotope heater units the United States had ever flown. (RHUs), each generating 1 W of thermal energy, to heat The Ulysses spacecraft was also launched from a space selected components (Wolverton, 2004). A third spacecraft, shuttle in 1990 with one GPHS RTG to undertake a sustained the flight spare Pioneer H, is displayed in the National Air exploration of the Sun. To enable a trajectory nearly over and Space Museum. the Sun’s poles, the spacecraft was sent to Jupiter to use a After the success of the Pioneer missions, two Voyager gravity assist to rotate the heliocentric orbital plane of the spacecraft were built to conduct intensive flyby studies of spacecraft by almost 90°. Ulysses made the first and only Jupiter and Saturn, in effect repeating on a more elaborate observations of fields and particles in interplanetary space scale the flights of the two Pioneers. These spacecraft were out of the ecliptic plane. It recently fell silent because of scaled back versions of the proposed Grand-Tour space- problems with its telecommunications system. craft, which was rejected at the time for budgetary reasons. Cassini became the first mission to orbit Saturn. It is an Voyager 1 and 2 were launched in 1977, each with three international program involving the United States, the Italian Multi-Hundred Watt (MHW) RTGs. With the successful Space Agency, and the European Space Agency. Conceived flyby of Saturn’s moon Titan by Voyager 1 in November in 1982, Cassini was launched in October 1997 with three 1980, ­ Voyager 2 was targeted for one of the grand-tour modified GPHS RTGs and multiple RHUs. Cassini arrived ­ rajectories. Voyager 2 subsequently had close flybys of t at Saturn and began orbiting the planet in July 2004. It also Saturn (August 1981), Uranus (January 1986), and Neptune sent a probe (Huygens) to the surface of Saturn’s moon Titan (August 1989), providing the bulk of all human knowledge early in 2005. Huygens is the first outer-planet mission built about the latter two “ice giant” planets (Dethloff and Schorn, by the European Space Agency. Now in extended mission, 2003). Cassini continues to make fundamental discoveries in the Saturn system (Launius and Johnston, 2009). A gravity assist is used to speed up or slow down the speed of a space- New Horizons is the most recent mission to employ RPS craft by a close flyby of a planet that exchanges momentum between the generators. It will be the first spacecraft to visit Pluto and spacecraft and the planet. Prograde approaches to planets in the outer solar the Kuiper Belt. Launched in January 2006, New Horizons system increase spacecraft speed, enabling them to reach planets farther conducted a Jupiter flyby 13 months later to increase speed. from the Sun faster than they could otherwise. New Horizons will make its closest approach to Pluto on As the backup for Voyager 1, Voyager 2 would have been targeted to Titan if Voyager 1 had failed. July 14, 2015. The half-ton spacecraft contains scientific 

APPENDIX E 49 TABLE E.1  Radioisotope Power Systems for Space Exploration Maximum Output Name and Model Used on (Number of RTGs per User) Electrical (W) Heat (W) Maximum Fuel Used (kg) RPS Mass (kg) SNAP-3B Transit-4A/B (1) 2.7 52.5 ~0.2 2 SNAP-9A Transit 5BN-1/2/3 (1) 25 525 ~1 12 SNAP-19 Nimbus B1 (2) 40.3 525 ~1 14 Nimbus III (2) Pioneer 10/11 (4) Modified SNAP-19 Viking 1/2 (2) 42.7 525 ~1 15 SNAP-27 Apollo 12-17 ALSEP (1) 73 1480 3.8 20 MHW-RTG LES-8/9 (2) 470 2400 ~4.5 38 Voyager 1/2 (3) GPHS-RTG Galileo (2) 285 4500 7.6 56 Ulysses (1) Cassini (3) New Horizons (1) NOTE: ALSEP, Apollo Lunar Surface Experiments Package; GPHS, General Purpose Heat Source; LES, Lincoln Experimental Satellite; MHW, Multi-hundred Watt; SNAP, Systems for Nuclear Auxiliary Power. SOURCES: Data from G.L. Bennett, “Space Nuclear Power: Opening the Final Frontier,” AIAA 2006-4191, pp. 12-13, presentation at 4th International Energy Conversion Engineering Conference and Exhibit (IECEC), San Diego, Calif., June 26-29, 2006; G.K. Ottman and C.B. Hersman, “The Pluto-New Horizons RTG and Power System Early Mission Performance,” AIAA-2006-4029, 4th International Energy Conversion Engineering Conference, San Diego, Calif., June 26-29, 2006; R.D. Cockfield, “Preparation of RTG F8 for the Pluto New Horizons Mission,” AIAA-2006-4031, 4th International Energy Conversion Engineering Conference, San Diego, Calif., June 26-29, 2006; R.R. Furlong and E.J. Wahlquist, “U.S. Space Missions Using Radioisotope Power Systems,” Nuclear News, April 1999, p. 29. instruments to map the surface geology and composition of JPL (Jet Propulsion Laboratory). 2005. Project Prometheus Final ­Report. Pluto and its three moons, investigate Pluto’s atmosphere, 982-R120461. Available at http://trs-new.jpl.nasa.gov/dspace/­ bitstream/2014/38185/1/05-3441.pdf. measure the solar wind, and assess interplanetary dust and Jordan, K.C., and J.H. Birden. 1954. Thermal Batteries Using Po-210. energetic particles. After it passes Pluto, NASA plans to MLM-984. Miamisburg, Ohio: Mound Laboratory. fly the spacecraft by one or two Kuiper Belt objects. Since Launius. R.D., and A.K. Johnston. 2009. Smithsonian Atlas of Space sunlight at the Kuiper Belt is more than 1,000 times less E ­ xploration. New York City: HarperCollins. In press. intense than at Earth, New Horizons relies on a GPHS RTG Lee, J.H. 1994. Aerospace Nuclear Safety: An Introduction and Historical Overview. International Topical Meeting: Advanced Reactor Safety, for power (Ottman and Hersman, 2006). Pittsburgh, Pa., April 17-21. Table E.1 lists key parameters for U.S. RPSs that have Morse, J.G. 1963. Energy for Remote Areas: Generators fueled with radio­ been used in space, the missions on which they were used, nuclides are supplying power in small terrestrial and space systems. and the fuel, mass, and output. All have been fueled by Science 139:1175-1180. 238Pu. NRC (National Research Council). 2006. Priorities in Space Science Enabled by Nuclear Power and Propulsion. Washington, D.C.: The National Academies Press. References Ottman, G.K., and C.B. Hersman. 2006. The Pluto-New Horizons RTG and Power System Early Mission Performance (AIAA-2006-4029), 4th Anderson, G.M., and F.H. Featherstone. 1960. The SNAP Programme: International Energy Conversion Engineering Conference, San Diego, U.S. AEC’s Space-Electric Power Programme. Nuclear Engineering Calif., June 26-29. 5:460-463. RAND Corporation. 1946. Preliminary Design of an Experimental World- Dassoulas, J., and R.L. McNutt, Jr. 2007. RTGs on Transit. Space Tech- Circling Spaceship. SM-11827. Santa Monica, Calif.: The RAND nology and Applications International Forum, Albuquerque, N.M., Corporation. February 11-15. Wilson, R.F., H.M. Dieckamp, and D. K. Cockeram. 1965. SNAP 10A Dethloff, H.C., and R.A. Schorn. 2003. Voyager’s Grand Tour: To the Outer Design, Development, and Flight Test. AIAA Second Annual Meeting, Planets and Beyond. Washington, D.C.: Smithsonian Institution Press. San Francisco, Calif., July 26-29. AIAA Paper No. 65-467. Gender, S.L., and H.A. Kock. 1949. Auxiliary Power Plant for the Satel- Wolverton, M. 2004. The Depths of Space: The Pioneers Planetary Probes. lite Rocket: A Radioactive Cell-Mercury Vapor System to Supply 500 Washington, D.C.: Joseph Henry Press. watts for Durations of up to One Year. Santa Monica, Calif.: The RAND Corporation.