Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 46
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration Appendix E History of Space Nuclear Power Systems INTRODUCTION Through an investment of considerable resources—engineering and scientific knowledge, human capital, and public funds—the United States has gained undisputed leadership in the exploration of the outer solar system, that part of the system beyond the orbit of Mars. This has been made possible since the 1950s by harnessing several core technologies that have enabled the nation’s scientific spacecraft to travel for years on end, engage in extended scientific observations, and relay critical data back to Earth. Radioisotope power systems (RPSs) are one such technology. RPSs generate heat from the natural decay of a radioactive isotope, or radionuclide. This heat is transformed into electricity with some level of efficiency, depending upon the converter design. A variety of converter approaches have been, and continue to be, investigated. In all flight systems used to date, the heat flows from a radioactive heat source, through an array of thermocouples, and to a heat sink, generating electricity in the process.1 These systems are called radioisotope thermoelectric generators (RTGs). RTGs are the preferred method for supplying the power needs of U.S. deep-space probes to the outer solar system and beyond, and they have also been used for some Earth-orbiting spacecraft and to support missions to the Moon and Mars. All U.S. RPSs launched into space have been powered by 238Pu. They have provided power ranging from 2.7 watts on the very early systems to 500 watts on more recent flights (Lee, 1994).2 RPSs have powered many types of spacecraft, including orbiters and landers. They allow spacecraft operations in extreme environments that rule out the use of other power systems (e.g., solar arrays). ORIGINS OF NUCLEAR POWER SYSTEMS FOR SPACEFLIGHT Beginning in the late 1940s several threads converged to make it possible to develop and use RPSs. In particular, the Atomic Energy Commission (AEC) began to investigate production and use of radioisotopes in connection with nuclear weapons. This prompted scientific research to understand the radioactive decay and chemistry of various isotopes that are not found in nature. Second, scientists and engineers began to experiment with the development of small nuclear power generators for a variety of uses on Earth, especially in extreme locations and environments (e.g., at the poles and under the seas), where scientific instruments could be placed and left alone for months at a time. Third, advances in thermoelectricity and semiconductors made RTGs feasible. In 1946 the newly established RAND Corporation explored the viability of orbital satellites and outlined the technologies necessary for their success (RAND, 1946). By 1949 a full-scale analysis by RAND had sketched out the large-scale use of nuclear power sources for satellites in Earth orbit (Gender and Kock, 1949). Beginning in 1951, at the request of the Department of Defense (DOD), the AEC sponsored research into nuclear power for spacecraft to support the development of a reconnaissance satellite. The AEC pursued two related avenues: a small nuclear reactor and an RTG. These Systems for Nuclear Auxiliary Power (SNAP) were numbered such that the odd numbers designated RTGs and the even numbers designated reactor power systems. 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 satellites. The first bench-test RTGs emerged from the Mound Laboratory (operated for the AEC by the Monsanto Research 1 The Seebeck effect. 2 Cassini had more than 800 watts of electrical power at launch using this approach.
OCR for page 47
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration Corp.) in 1953 and quickly found application in Antarctica to power scientific research stations (Jordan and Birden, 1954; Morse, 1963). SNAP-1 (an RTG) was built at the Mound Laboratory under AEC supervision in 1954 (Anderson and Featherstone, 1960). This was followed by the use of nuclear power systems on spacecraft in the early 1960s. The possibilities of space nuclear power first entered the public sphere in January 1959 when President Dwight D. Eisenhower posed with a SNAP-3 RTG in the Oval Office of the White House. Ultimately, the Transit 4A and 4B navigation satellites were provided with SNAP-3B power sources from the AEC. They were the first satellites to operate in space with RPSs. Both satellites were also equipped with solar panels that supplied 35 W of power (Dassoulas and McNutt, 2007). These and subsequent missions proved the feasibility of using RPSs for space missions. SPACE NUCLEAR REACTOR SYSTEMS Space nuclear power reactors are another potential option for missions where solar power is not practical. However, the United States has launched only one space nuclear power reactor (SNAP-10A), and that took place in 1965. That early system was designed to produce 40 kW of thermal power and 500 W of electricity for an operating life of just 1 year, and the failure of a voltage regulator caused the system to shut down after 43 days (Wilson et al., 1965). Beginning in 1983, NASA, the DOD, and the Department of Energy invested approximately $500 million in the SP-100 space nuclear power reactor. This system was intended to generated 2 MW of thermal power and 100 kW of electricity, but because of high costs, schedule delays, and changing national space mission priorities, the SP-100 program was suspended in the early 1990s and later canceled. The Soviet Union launched dozens of short-lived space nuclear power reactors during the 1970s and 1980s, and several unfueled Soviet systems were purchased by the United States in the early 1990s. These systems were extensively ground tested by a joint team of U.S., British, French, and Russian engineers using electrical heaters in place of the nuclear cores. Although the test program was successful, the United States did not use the Soviet equipment or technology in a flight program (NRC, 2006). Project Prometheus was the most recent U.S. attempt to develop space nuclear power reactors. This project began in 2002, and it’s initial focus was on the Jupiter Icy Moons Orbiter mission. The project selected a nuclear electric propulsion reactor concept that was scalable from 20 kWe to 300 kWe. A nuclear electric propulsion system for a deep-space mission would need to be validated for reliable operation for a mission lifetime of 10 to 20 years, with no maintenance or repair. However, as with the SP-100 program, Project Prometheus did not proceed to the point of demonstrating the ability of system designs or available technology to meet required performance or lifetime specifications. Instead, it was terminated in 2005, after it became clear that it would have cost at least $4 billion to complete development of a spacecraft reactor module, and a total of at least $16 billion to develop the entire spacecraft and complete the mission, not counting the cost of the launch vehicle or any financial reserves to cover unexpected cost growth (JPL, 2005). 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. However, history also shows that the development of high-power, long-life space nuclear power reactors would be very time-consuming and expensive. VEHICLE ACCIDENTS AND MALFUNCTIONS Three U.S. spacecraft with RPSs on board have inadvertently returned to Earth. In all cases, the RPSs performed as designed; the cause of the mission failure lay with other, nonnuclear systems. The Transit 5BN-3 spacecraft with one SNAP-9A RPS on board broke up and burned up on reentry after a launch-vehicle upper stage failure. The design philosophy at that time was to require that the 238Pu oxide fuel totally burn up during reentry into Earth’s atmosphere, which it did. As a result of that accident, the RPS design philosophy was changed to require full containment of the fuel (i.e., no fuel burn up) during an inadvertent reentry from or to Earth orbit. This design philosophy is still in effect. The Nimbus B-1 weather satellite, the first NASA satellite to use an RPS, was intentionally destroyed during launch due to the erratic ascent of the launch vehicle. The launch vehicle, upper stage, and payload were totally destroyed by the explosion initiated by the destruct action, and the debris fell into the Santa Barbara Channel off Vandenberg Air Force Base. The two SNAP-19B2 RPSs were recovered intact (i.e., no 238Pu oxide fuel release occurred), and the fuel was used on a later mission. The last accident involving a U.S. RPS was the Apollo 13 mission, which has been well documented. The SNAP-27 heat source assembly was stowed in the Lunar Excursion Module, which returned to Earth after the mission was aborted. It reentered over the South Pacific Ocean. Air and water sampling detected no 238Pu oxide fuel, indicating that the SNAP-27 heat source assembly survived reentry intact (as designed) and came to rest at the bottom of the Tonga Trench under more than 7,000 feet of water, where it still remains.
OCR for page 48
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration SPACE NUCLEAR POWER AND OUTER-PLANET MISSIONS A major shift in the use of RPSs came with NASA’s decision to pursue outer-planet exploration. This initiative was driven by the discovery of “grand tour” trajectories that could enable relatively short missions to the planets of the outer solar system by using multiple planetary gravity assists.3 This planetary configuration is rare, occurring only about every 176 years, but it was due to occur in the late 1970s and led to one of the most significant space exploration efforts undertaken by the United States (Dethloff and Schorn, 2003). The nearly identical Pioneer 10 and 11 spacecraft were launched in 1972 and 1973, respectively, to make the first trips through the asteroid belt to Jupiter and beyond. Both relied on RPSs to provide power far from the Sun. Pioneer 10 flew past Jupiter in late 1973. It transmitted data about the planet and continued on its way out of the solar system. Pioneer 11 provided scientists with an even closer view of Jupiter, whose gravity was used to send Pioneer 11 to Saturn before it, too, departed the solar system. Pioneer 11 ended its mission in 1995, when the last transmission from the spacecraft was received. NASA continued to receive signals from Pioneer 10 until 2003, when the spacecraft was 7.6 billion miles from Earth. The success of the Pioneer missions would not have been possible without the four SNAP-19 RTGs that each spacecraft carried as their sole source of power. Each Pioneer spacecraft also had a dozen radioisotope heater units (RHUs), each generating 1 W of thermal energy, to heat selected components (Wolverton, 2004). A third spacecraft, the flight spare Pioneer H, is displayed in the National Air and Space Museum. After the success of the Pioneer missions, two Voyager spacecraft were built to conduct intensive flyby studies of Jupiter and Saturn, in effect repeating on a more elaborate scale the flights of the two Pioneers. These spacecraft were scaled back versions of the proposed Grand-Tour spacecraft, which was rejected at the time for budgetary reasons. Voyager 1 and 2 were launched in 1977, each with three Multi-Hundred Watt (MHW) RTGs. With the successful flyby of Saturn’s moon Titan by Voyager 1 in November 1980, Voyager 2 was targeted for one of the grand-tour trajectories.4 Voyager 2 subsequently had close flybys of Saturn (August 1981), Uranus (January 1986), and Neptune (August 1989), providing the bulk of all human knowledge about the latter two “ice giant” planets (Dethloff and Schorn, 2003). Voyager 1, which is traveling faster than Voyager 2, is now farther from Earth than any other human-made object. Now traveling out of the solar system, both Voyager 1 and Voyager 2 have passed the “termination shock” of the solar wind and continue to send back the first information ever received from the outer boundary of our solar neighborhood. The Voyagers are expected to return scientific data until the RPSs can no longer supply enough electrical energy to power critical systems. With the adoption of power sharing among the still-operating instruments, the final transmission is expected to occur in about 2020. Whether Voyager 1 will reach the heliopause, the “boundary” between the shocked solar wind and interstellar plasma, by then is unknown. NASA has continued to use RPSs on missions to the outer planets and on selected long-term missions closer to the Sun when necessary to enable the mission. In 1989, NASA deployed the Galileo spacecraft from a space shuttle and sent it on a 6-year, gravity-assisted journey to Jupiter, where it became the first spacecraft to orbit the giant planet (Launius and Johnston, 2009). The flight team for Galileo ceased operations in 2003, and the spacecraft was deorbited by command into Jupiter’s atmosphere to guard against any potential future contamination of Jupiter’s moon Europa by an uncontrolled spacecraft impact. Galileo carried two newly developed general purpose heat source (GPHS) RTGs. These units produced 300 W of electricity at beginning of life and had a total mass of 55.9 kg, giving these devices the highest specific power of any RPS the United States had ever flown. The Ulysses spacecraft was also launched from a space shuttle in 1990 with one GPHS RTG to undertake a sustained exploration of the Sun. To enable a trajectory nearly over the Sun’s poles, the spacecraft was sent to Jupiter to use a gravity assist to rotate the heliocentric orbital plane of the spacecraft by almost 90°. Ulysses made the first and only observations of fields and particles in interplanetary space out of the ecliptic plane. It recently fell silent because of problems with its telecommunications system. Cassini became the first mission to orbit Saturn. It is an international program involving the United States, the Italian Space Agency, and the European Space Agency. Conceived in 1982, Cassini was launched in October 1997 with three modified GPHS RTGs and multiple RHUs. Cassini arrived at Saturn and began orbiting the planet in July 2004. It also sent a probe (Huygens) to the surface of Saturn’s moon Titan early in 2005. Huygens is the first outer-planet mission built by the European Space Agency. Now in extended mission, Cassini continues to make fundamental discoveries in the Saturn system (Launius and Johnston, 2009). New Horizons is the most recent mission to employ RPS generators. It will be the first spacecraft to visit Pluto and the Kuiper Belt. Launched in January 2006, New Horizons conducted a Jupiter flyby 13 months later to increase speed. New Horizons will make its closest approach to Pluto on July 14, 2015. The half-ton spacecraft contains scientific 3 A gravity assist is used to speed up or slow down the speed of a spacecraft by a close flyby of a planet that exchanges momentum between the spacecraft and the planet. Prograde approaches to planets in the outer solar system increase spacecraft speed, enabling them to reach planets farther from the Sun faster than they could otherwise. 4 As the backup for Voyager 1, Voyager 2 would have been targeted to Titan if Voyager 1 had failed.
OCR for page 49
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration TABLE E.1 Radioisotope Power Systems for Space Exploration Name and Model Used on (Number of RTGs per User) Maximum Output Maximum Fuel Used (kg) RPS Mass (kg) Electrical (W) Heat (W) 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 Pluto and its three moons, investigate Pluto’s atmosphere, measure the solar wind, and assess interplanetary dust and energetic particles. After it passes Pluto, NASA plans to fly the spacecraft by one or two Kuiper Belt objects. Since sunlight at the Kuiper Belt is more than 1,000 times less intense than at Earth, New Horizons relies on a GPHS RTG for power (Ottman and Hersman, 2006). Table E.1 lists key parameters for U.S. RPSs that have been used in space, the missions on which they were used, and the fuel, mass, and output. All have been fueled by 238Pu. REFERENCES Anderson, G.M., and F.H. Featherstone. 1960. The SNAP Programme: U.S. AEC’s Space-Electric Power Programme. Nuclear Engineering 5:460-463. Dassoulas, J., and R.L. McNutt, Jr. 2007. RTGs on Transit. Space Technology and Applications International Forum, Albuquerque, N.M., February 11-15. Dethloff, H.C., and R.A. Schorn. 2003. Voyager’s Grand Tour: To the Outer Planets and Beyond. Washington, D.C.: Smithsonian Institution Press. Gender, S.L., and H.A. Kock. 1949. Auxiliary Power Plant for the Satellite Rocket: A Radioactive Cell-Mercury Vapor System to Supply 500 watts for Durations of up to One Year. Santa Monica, Calif.: The RAND Corporation. JPL (Jet Propulsion Laboratory). 2005. Project Prometheus Final Report. 982-R120461. Available at http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/38185/1/05-3441.pdf. Jordan, K.C., and J.H. Birden. 1954. Thermal Batteries Using Po-210. MLM-984. Miamisburg, Ohio: Mound Laboratory. Launius. R.D., and A.K. Johnston. 2009. Smithsonian Atlas of Space Exploration. New York City: HarperCollins. In press. Lee, J.H. 1994. Aerospace Nuclear Safety: An Introduction and Historical Overview. International Topical Meeting: Advanced Reactor Safety, Pittsburgh, Pa., April 17-21. Morse, J.G. 1963. Energy for Remote Areas: Generators fueled with radionuclides are supplying power in small terrestrial and space systems. Science 139:1175-1180. NRC (National Research Council). 2006. Priorities in Space Science Enabled by Nuclear Power and Propulsion. Washington, D.C.: The National Academies Press. Ottman, G.K., and C.B. Hersman. 2006. 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. RAND Corporation. 1946. Preliminary Design of an Experimental World-Circling Spaceship. SM-11827. Santa Monica, Calif.: The RAND Corporation. Wilson, R.F., H.M. Dieckamp, and D. K. Cockeram. 1965. SNAP 10A Design, Development, and Flight Test. AIAA Second Annual Meeting, San Francisco, Calif., July 26-29. AIAA Paper No. 65-467. Wolverton, M. 2004. The Depths of Space: The Pioneers Planetary Probes. Washington, D.C.: Joseph Henry Press.