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

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4 RPS Research and Development Assuming that there will be an ongoing supply of 238Pu • Advanced Stirling Converter (ASC) technology matu- for NASA missions, NASA will also need an ongoing ration is led by GRC and supported by JPL. An ASC s ­ upply of radioisotope power systems (RPSs) to power those developed by Sunpower, Inc., lies at the heart of the missions. ASRG. The ASRG is projected to have a higher specific power and a higher system energy conversion efficiency than prior RPSs. Program Overview • Sustaining launch-approval-engineering capabilities, as NASA’s RPS Program Office operates as an extension of well as related capabilities necessary to comply with the Planetary Science Division of the Science Mission Direc- National Environmental Policy Act (NEPA, 1970), torate of NASA Headquarters. The program is a multicenter, is led by JPL and supported by the Kennedy Space multiagency effort that supports strategic investments in Center. RPS technologies, validation of flight systems, and produc- • Small RPS development is intended to provide mission tion, certification, and delivery of flight hardware for NASA planners with more power options. The International spacecraft. The program manages technology portfolio Lunar Network has been suggested as an initial mis- investments by determining priorities for future RPS mission sion for a small RPS. The anticipated power level for needs in concert with NASA’s Planetary Science Division the International Lunar Network is about 40 W, with and the larger science community. The program funds the an initial launch date of 2013. This means that there is development of mission-generic, engineering-model system no time for technology development. In fact, it would hardware and, if warranted, prototype model hardware. be difficult for the DOE, NASA, and industry to design, This latter function is particularly critical for those missions assemble, test, and certify a new RPS and have it ready that require RPS development activities to be started long to go in time for a launch in 2013 even without technol- before NASA determines what organization will manage a ogy development. Looking beyond the International particular mission. Lunar Network, NASA is still in the process of setting The RPS program consists of six major elements: specific goals for a small RPS. NASA anticipates that power requirements will be on the order of 10-60 We, • Program Management is led by Glenn Research Center mission length will be 3 to 10 years, system mass will (GRC) and supported by the Jet Propulsion Laboratory be less than 15 kg, and the heat source will be a single (JPL) and the Department of Energy (DOE). Primary general purpose heat source (GPHS) module. This responsibilities include management of program scope, effort is to be led by the DOE. NASA has yet to decide budget, schedule, and risk; studies and long-range plan- which of its organizations will support this effort. ning; and education and public outreach. • The technology portfolio supports, at a low level, • Advanced Stirling Radioisotope Generator (ASRG) research and development for additional converter flight system development is led by the DOE and sup- ported by GRC. Lockheed Martin Space Company is This is comparable to the initial requirements of the Surveyor program the ASRG system integration contractor. The focus of of the 1960s that were to be accommodated by the Systems for Nuclear this effort is on reliability improvement, risk reduction, Auxiliary Power (SNAP)-11 project and for the same reason: to survive the 14-day-long lunar night. This requirement was abandoned on the Surveyor and flight readiness. program, and a SNAP-11 unit was never flown. 24 

RPS RESEARCH AND DEVELOPMENT 25 technologies with an eye toward future generations of by a single GPHS module would produce at least RPSs, subsequent to the ASRG. This includes advanced 38We at beginning of life. thermoelectrics research, led by JPL with support from GRC, and thermophotovoltaics (TPV) research, led by Program Balance GRC. The technology portfolio also includes funding for outside organizations through NASA Research Figure 4.1 shows the relative magnitude (in terms of Announcements. NASA’s budget) of each element of the RPS program. Until — The goal of advanced thermoelectric research is 2007, the RPS program was a technology development effort. to develop thermoelectric materials that are much At that time, the focus shifted to development of a flight- more efficient than traditional thermoelectric mate- ready ASRG, and that remains the current focus of the RPS rials. Success in this area could ultimately lead to program. The program received no additional funds to sup- the development of an advanced thermoelectric port this new tasking, so funding to develop a Brayton-cycle converter, which could then be used in an advanced converter and a milliwatt-scale thermoelectric converter was RTG. eliminated. In addition, the budget for the remaining RPS — A TPV RPS would be a relatively simple device technologies (advanced thermoelectrics and TPV) was cut. that uses an array of photovoltaic material adjacent As a result, the development of new generations of RPSs that to a GPHS to generate electricity. The basic device use these technologies has been delayed. (without the cooling fins) is not much larger than the With the development of the Multi-Mission Radioisotope GPHS itself. The converter efficiency is expected to Thermoelectric Generator (MMRTG), the manufacture of be at least 15 percent, so that a TPV RPS powered GPHS RTGs was discontinued, and it would be very difficult Figure 4-1 Tech Portfolio Small RPS Sustaining LAE ASC Technology Maturation for ASRGs ASRG Development Management Actual Budget Budget Request FIGURE 4.1  Relative magnitude of key elements of NASA’s radioisotope power system program. NOTE: Actual budget shown for fis- cal year (FY) 2008 and 2009. Budget shown for FY 2010 to 2014 not yet enacted. ASC, Advanced Stirling Converter; ASRG, Advanced S ­ tirling Radioisotope Generator; LAE, launch approval engineering. SOURCE: Modified from L.A. Dudzinski, NASA, ­“Radio­isotope Power S ­ ystems. Power Systems Program. Historical Overview and Current Content,” presentation to the Radioisotope Power Systems ­Committee, September 18, 2008, Washington, D.C. 

26 RADIOISOTOPE POWER SYSTEMS and expensive to manufacture new GPHS RTGs (although budget is doubled, and funding for other RPS technologies is it may be possible to build two or three GPHS RTGs using expanded. The planned development of a small RPS would leftover thermocouples). The RPS program is now focused be a good first step toward the goal of establishing a suite on development of ASRGs; the current budget has no funding of RPSs with capabilities optimized for different mission set aside to retain the ability to produce MMRTGs, although scenarios. NASA has asked the DOE to determine what it would take to keep MMRTG production capabilities active for two years. FINDING. Programmatic Balance. Balance within NASA’s The central issue that threatens the future of RPS-­powered RPS program is impossible given the current (fiscal year 2009) missions is the short supply of 238Pu. Accordingly, RPS budget and the focus on development of flight-ready ASRG research and development should strive to meet NASA’s technology. However, NASA is moving the ASRG project mission requirements for RPSs while minimizing NASA’s forward, albeit at the expense of other RPS technologies. demand for 238Pu. In addition, a balanced program would develop RPS technologies and systems suitable for various RPS System Capabilities applications, and it would support development of RPS technology for near- and far-term use. Figure 4.2 compares the performance of past, present, Because the RPS program is focused on advanced and future RPSs. The technology development cycle for new development of a single RPS design for near-term applica- RPS technologies is typically 15 to 20 years long, and it is tion, the RPS program (in FY 2009) is not well balanced. driven by perceived mission needs (rather than actual mis- However, this imbalance is appropriate given that (1) the sion requirements) because, even for very large spacecraft FY 2009 budget is insufficient to sustain a well-balanced and very important missions, it is impossible to predict with program and (2) the focus on ASRGs is well aligned with certainty what mission requirements will be 15 to 20 years current programmatic priorities. The balance of the program in the future. Over such a long time span, space exploration would improve under the current out-year funding scenario priorities often change as changes occur in the leadership of (if enacted), as ASRG development is completed, the RPS the Administration and Congress. GPHS-RTG MMRTG ASRG ARTG TPV Past Present In Development Future Future Electric Output, BOM, We 285 125 ~140-150 ~280 to 420 ~38-50 Heat Input, BOM, We 4500 2000 500 3000 250 RPS System Efficiency, BOM, % 6.3 6.3 ~28-30 ~9-14 ~15-20 Total System Weight, kg 56 44.2 ~19-21 ~40 ~7 Specific Power, We/kg 5.1 2.8 ~7-8 ~7-10 ~6-7 Number of GPHS Modules 18 8 2 12 1 GPHS Module Weight, kg 25.7 12.9 3.2 19.3 1.6 238Pu Weight, kg 7.6 3.5 0.88 5.3 0.44 FIGURE 4.2  Performance of past, present, and future radioisotope power systems. NOTE: ARTG, Advanced Radioisotope Thermal G ­ enerator; ASRG, Advanced Stirling Radioisotope Generator; BOM, beginning of mission; GPHS, general purpose heat source; MMRTG, Multi-Mission Radioisotope Thermoelectric Generator; RTG. radioisotope thermal generator; TPV, thermophotovoltaic. SOURCE: Modified from S. Surampudi, NASA, “Radioisotope Power Systems Technology Programs,” presentation to the Radioisotope Power Systems Com- mittee, November 18, 2008, Washington, D.C. 

RPS RESEARCH AND DEVELOPMENT 27 POWER SYSTEM for the OUTER PLANETS to more than 3 years of domestic production of 238Pu at the FLAGSHIP 1 mission highest anticipated rate of 5 kg/year. Nevertheless, as already noted, ASRGs are not yet ready Studies of four possible Outer Planets Flagship (OPF) 1 for flight. NASA has yet to determine, for example, (1) what mission concepts began in 2007. The last two mission must be done to demonstrate that ASRGs are ready for use on concepts under consideration are the Titan Saturn System OPF 1 and (2) if those requirements can be accomplished in Mission (TSSM) and the Europa Jupiter System Mission time to meet the OPF 1 mission schedule. In general, project (EJSM) (JPL, 2009). The EJSM would consist of two parts: managers for long-life missions rely on proven technologies NASA’s Jupiter Europa Orbiter, which would be powered by and redundant subsystems for mission-critical functions such RPSs, and the European Space Agency’s Jupiter Ganymede as avionics and power. NASA’s Science Mission Directorate Orbiter, which would be powered by solar arrays. Saturn is generally expects new technology to advance to technology almost twice as far from the Sun as Jupiter, and the TSSM readiness level 6 or beyond before the mission’s preliminary mission would last 13 years, somewhat longer than the EJSM design review. With regard to ASRGs, NASA is responsible mission (9 years). for defining (1) the specific criteria that ASRGs must satisfy In February 2009, NASA and European Space Agency prior to flight and (2) a strategy to satisfy those criteria. The officials determined that EJSM is more feasible techni- problem is complex because accelerated life tests for the cally, and it is now planned to go first as OPF 1 (NASA, ASRG as a system are not possible, and the life-limiting 2009). NASA will ultimately decide whether OPF 1 will failure modes and overall reliability of the ASRG as a system use MMRTGs, ASRGs, or a combination of both. (Mission remain to be determined. Toward that end, a study team with studies indicate that all three options would work, assuming members from JPL, GRC, and the DOE has been assessing ASRGs are ready in time.) what they believe would need to be done to qualify ASRG The ASRG is projected to have a specific power of for the OPF 1 mission. As of February 2009, the results of 7 We/kg, compared to just 2.8 We/kg for the MMRTG and this effort were not available. 5.1 We/kg for the best previous RPS. This improvement in The committee believes it is unlikely that NASA would specific power is a significant consideration for deep-space baseline an ASRG for a major mission (such as a OPF mis- missions for which mass and launch-vehicle capability are sion) until it first operates successfully on another mission to typically significant system drivers. In addition, ASRGs are validate launch survivability and performance in space. The projected to have a system energy conversion efficiency more Discovery 12 mission is the earliest potential opportunity to than four times higher than MMRTGs at beginning of life, fly an ASRG, and that mission is not scheduled for launch and the projected power output of ASRGs decreases over until 2014. NASA plans to make a final decision on whether time by only 0.8 percent per year, which is half the rate of to use MMRTGs or ASRGs for OPF 1 no later than 2012. decrease of MMRTGs. Thus, it seems unlikely that NASA will decide to use ASRGs The electromagnetic interference produced by both sys- on OPF 1 unless (1) a flight-ready ASRG is developed in tems is expected to be within tolerance levels for all OPF 1 time for the Discovery 12 mission and (2) the current mis- instruments. Vibration measurements on the ASRG engi- sion schedule for OPF 1 is delayed enough to allow NASA neering unit are nearly an order of magnitude lower than the to postpone the selection of the OPF 1 power system until nominal vibration specification. Even so, vibration levels after Discovery 12 is launched and ASRGs demonstrate the will require close attention and detailed analysis during ability to operate in space for some period of time. spacecraft development. Regardless, the use of ASRGs on OPF 1 would not be driven by spacecraft design or opera- tional factors. The primary motivation for using ASRGs on Development of a Flight-Ready ASRG OPF 1 is to conserve 238Pu for other missions. For NASA as Demonstrating the reliability of ASRGs for a long-life a whole, this is an important consideration, given the large mission is critical—and it has yet to be achieved. RTGs number of RPSs to be used on OPF 1. Using ASRGs on and SRGs both begin to operate as soon as they are fueled, OPF 1 would save 16 to 19 kg of 238Pu. That is enough to and they operate continuously thereafter. The design life of power RPSs for several other missions, and it is equivalent both MMRTGs and ASRGs is 17 years. This is intended to cover 3 years of storage (between the time they are fueled  The Titan Saturn System Mission would include an orbiter, a lander, and and mission launch) and 14 years of mission time after a Montgolfière balloon, which would be filled with the atmospheric gases launch. present on Titan and then maintained aloft using the heat from an RPS to heat the gas inside the balloon. This balloon would use an MMRTG regardless NASA plans to freeze the system design specification of which RPS is chosen to power the orbiter, because an ASRG would not for the ASRG in April 2009. This is a critical and necessary produce enough waste heat to keep the balloon aloft. Only part of the decay in power output in RPS systems flown to date is due to the half-life of the 238Pu fuel; the rest is caused by degradation of the NASA defines technology readiness level 6 as a “system/subsystem thermoelectric converters in the RTGs. Expectations are that ASRG power model or prototype demonstration in a relevant environment (ground or output would degrade at a lower rate than RTGs. space)” (Mankins, 1995). 

28 RADIOISOTOPE POWER SYSTEMS step for assessing ASRG reliability and technical risk and for that (1) no ASC failures have thus far been experienced and producing a flight-qualified ASRG. (2) space-qualified, Stirling engine cryocoolers have operated The RPS program’s risk mitigation effort is using risk successfully in space for 12 years or more. Still, the ­reliability identification, characterization, and mitigation to reduce risk of ASCs and ASRGs over a 17-year design life remains to a level that is acceptable for a flight mission. As part of its unknown, in part because of design differences between ASCs ongoing reliability improvement and risk reduction efforts, and most cryocoolers with long-life experience in space. the RPS program has produced five ASC models. Two NASA intends to extensively test every pair of ASCs that more development models are planned before the construc- have been built. In some cases, ASC units have been tested tion of ASRG operational flight units. The progression of in the laboratory and then subjected to vibration testing to m ­ odels has featured improvements in many areas, including simulate a launch before being returned to testing. Even so, m ­ aterials that allow higher operating temperatures, thereby no individual ASC unit has accumulated more than 2 years increasing conversion efficiency and/or increasing reliability of testing. Until (1) the ASRG design specification is fro- for a given operating temperature. zen, (2) hardware manufactured according to that design is The primary life-limiting mechanisms for Stirling heat tested as a system, and (3) extensive testing is completed in engines, in general, are wear, fatigue, creep, permeation of conditions that simulate the operational environment, there helium out through the containment vessel, radiation effects will remain substantial uncertainty as to whether all failure (when used in a high radiation environment), and contami- modes of the flight design have been identified and how nation. The design of the ASC is intended to avoid each of useful existing component tests will be in predicting the these pitfalls. Wear is not generally considered an issue for reliability of ASRG flight hardware, as a complete system, Stirling engines used in ASCs because they use gas bearings for a particular mission and for the full design lifetime of in which the moving piston is centered by pumped gas. As a 17 years. In particular, even if the ASRG design specification result, no moving parts are in contact with each other (unless is frozen on schedule in April 2009, and even if subsequent the gas bearings fail for some reason). testing detects no problems with the design, it remains to be The ASC materials testing program is assessing material seen if extended tests will be able to accumulate enough time fatigue and creep. In particular, an analytical model using to justify making a switch from MMRTGs to ASRGs as the accelerated life testing data for the ASC heater head (which baseline RPS for OPF 1. is the component most susceptible to creep) has predicted The initial ASC testbed demonstrated 36 percent conver- a reliability of 0.999 for the design lifetime of 17 years at sion efficiency. Subsequent devices have continued to meet 817°C. Testing of ASCs in a simulated space environment (in or exceed performance expectations. The most advanced vacuum and at temperature) has shown that loss of helium model (the ASC-E2) has demonstrated 38.4 percent effi- via permeation is not a problem, and assessments of likely ciency (with a hot temperature of 850°C and a heat rejection radiation environments have not forced a change in the selec- temperature of 90°C). These high levels of efficiency will tion of any materials. allow the ASRG, as a complete system, to meet or exceed The ASC risk mitigation effort also includes long-life its goal of 28 to 30 percent conversion efficiency. The high tests of magnets, analyses of electromagnetic interference levels of demonstrated efficiency have also allowed the (EMI), and analysis and testing of organic materials used ASC and ASRG development efforts to focus on enhancing for electrical insulation and potting, structural bonding, reliability and manufacturability rather than improving effi- and the surface finish of moving parts. Ongoing, long-term ciency beyond that which has already been achieved. tests of magnets are scheduled to accumulate 2 years of test An ASRG quality assurance program plan has been for- data. Current levels of EMI seem to be generally satisfac- mally implemented. This plan includes DOE requirements tory. Options to reduce EMI have been identified and could for nuclear systems as well as relevant NASA requirements. be implemented, if required. All organics in the current The quality assurance effort encompasses all of the organiza- ASC design have been identified, evaluated, and approved. Additional tests are planned, for example, to verify that the Stirling-engine cryocoolers developed the technology that is the founda- organics will perform as expected at operating temperatures tion for ASCs. Cryocoolers are used in instruments operating in the infrared, and in a radiation environment. gamma-ray, and x-ray spectrum. Long-life cryocoolers are widely accepted ASRG development has included a great deal of compo- as a reliable spacecraft technology; more than 20 long-life Stirling cryo­ nent testing and analysis. ASC converters have cumulatively coolers have been used on spacecraft manufactured in the United States, undergone more than 200,000 hours (23 years) of testing at Europe, and Japan. One cryocooler operating in space (the Rutherford Appleton Laboratory 80K Integral Stirling cryocooler in the Along Track GRC, but that testing has been accumulated by many different Scanning Radiometer [ATSR-2] payload on the European Remote Sens- devices, manufactured to various different design specifica- ing 2 spacecraft) accumulated 12.8 years of continuous operation with no tions, and the testing has been conducted under ­various envi- d ­ egradation before the instrument was shut down. Six others have accu- ronmental conditions. Most important, the ­longest test time mulated half that lifetime with no degradation that affected mission life. that any single ASC has to date experienced is still a relatively However, all but one of these non-wearing, long-life Stirling cryocoolers use flexure-supported gas bearings rather than the pumped gas bearings used small fraction of the 17 year design life. It is encouraging by the ASRGs (Ross, 2008). 

RPS RESEARCH AND DEVELOPMENT 29 tions involved in developing the ASRG. In addition, the RPS The MMRTG will fly on the Mars Science Laboratory, but program is continuing to work on a configuration manage- this is the only mission that is firmly committed to using the ment plan and other related plans and processes. MMRTG. As this work is completed, the industry teams that A failure mode, effects, and criticality assessment of the developed and built the MMRTG are expected to disband, ASRG engineering unit identified 51 single-point failures and the industry facilities are expected to be reconfigured for (SPFs). By comparison, the design of the RTG-GPHS (the other purposes. It remains to be seen if NASA will sustain standard RPS used prior to the MMRTG) has only 17 SPFs. work on MMRTGs to keep the MMRTG industrial teams However, a numerical comparison of the number of SPFs and facilities intact and related infrastructure in place until a does not provide a good understanding of the relative reli- final decision is made on what system will power OPF 1. If ability of the two types of devices. The likelihood of the SPFs the ability to manufacture MMRTGs is not sustained at least must also be understood. For example, about 80 percent of until (1) the ASRG is demonstrated to be flight ready and the SPFs on the ASRG engineering unit are structural in (2) NASA commits to using ASRGs (or another comparable nature, and the designers believe that the likelihood of these RPS) for long-life, deep-space missions, then even with an failures has been reduced to very low levels through the use adequate supply of 238Pu, the United States could lose the of conservative structural designs. In any case, the issue is ability to manufacture any RPSs, at least for a time. not whether an ASRG will be as reliable as historic RTGs; the issue is whether mission managers can be convinced that FINDING. Multi-Mission Radioisotope Thermoelectric an ASRG is sufficiently reliable to meet engineering and Generators. It is important to the national interest to main- programmatic requirements for a given mission. tain the capability to produce Multi-Mission Radioisotope NASA has used fault tree and probabilistic analysis tech- Thermoelectric Generators, given that proven replacements niques to estimate that system-level reliability is 0.967 for an do not now exist. ASRG at full-power operation over the entire 17-year design life. System electronics (i.e., the electronics required to con- RECOMMENDATION. Multi-Mission Radioisotope trol and synchronize the ASCs and to convert the electrical Thermoelectric Generators. NASA and/or the Department output from ac to dc) have been identified as the major con- of Energy should maintain the ability to produce Multi- tributor to the estimated probability of failure. System-level M ­ ission Radioisotope Thermoelectric Generators. reliability at half-power operation (that is, the probability that an ASRG will have at least one of its two converters func- RPS Research and Development—Summary tioning and producing power at the end of the 17-year design life) has been estimated to be 0.984. Extended life tests will The next major milestones in the advancement of ASRGs provide additional data regarding reliability, but there is not are to freeze the design of the ASRG, to conduct system test- enough time or money to build enough ASRGs and then test ing that verifies that all credible life-limiting mechanisms them for long enough to determine rigorously what level of have been identified and assessed, and to demonstrate that reliability they will have over a 17-year lifetime. However, ASRGs are ready for flight. However, neither the DOE nor this has been the case for earlier RPSs—and for other critical NASA have formal guidance or requirements concerning spacecraft hardware as well. There has never been a numeric what constitutes flight readiness for RPSs. In general, RPSs reliability requirement specification for an RTG, and NASA (and other systems) on spacecraft for deep-space missions does not intend to establish one for the ASRG. are flight ready when the project manager for that mission says they are flight ready. Given this situation, ongoing efforts to advance ASRG technology and demonstrate that RPS Facilities it is flight ready are being guided by experience with past NASA appears currently to be well positioned with regard programs and researchers’ best guess about the needs and to key RPS research and development facilities. These facili- expectations of project managers for future missions. While ties are located at GRC and JPL. The facilities at greatest this approach has enabled progress, the establishment of immediate risk are those associated with advanced RPS formal guidance and processes for flight certification of research (e.g., advanced thermoelectric and TPV research RPSs in general and ASRGs in particular would facilitate facilities). NASA has not yet lost any critical RPS facilities, the acceptance of ASRGs as a viable option for deep-space and the projected budget seems adequate to sustain neces- missions and reduce the impact that the limited supply of sary research and development facilities. However, there are 238Pu will have on NASA’s ability to complete important concerns related to other facilities that are necessary for the space missions. production of flight systems. FINDING. Flight Readiness. NASA does not have a broadly accepted set of requirements and processes for This section deals with facilities associated with development and fabri- demonstrating that new technology is flight ready and for cation of RPS technologies and RPS converters. DOE 238Pu production and RPS assembly and testing facilities are addressed in Chapter 2. committing to its use. 

30 RADIOISOTOPE POWER SYSTEMS RECOMMENDATION. Flight Readiness. The RPS pro- HIGH-PRIORITY RECOMMENDATION. ASRG gram and mission planners should jointly develop a set D ­ evelopment. NASA and the Department of Energy should of flight-readiness requirements for RPSs in general and complete the development of the Advanced Stirling Radio- Advanced Stirling Radioisotope Generators in particular, as isotope Generator (ASRG) with all deliberate speed, with the well as a plan and a timetable for meeting the requirements. goal of demonstrating that ASRGs are a viable option for the Outer Planets Flagship 1 mission. As part of this effort, RECOMMENDATION. Technology Plan. NASA should NASA and the Department of Energy should put final design develop and implement a comprehensive RPS technology ASRGs on life test as soon as possible (to demonstrate reli- plan that meets NASA’s mission requirements for RPSs ability on the ground) and pursue an early opportunity for while minimizing NASA’s demand for 238Pu. This plan operating an ASRG in space (e.g., on Discovery 12). should include, for example: References • A prioritized set of program goals. • A prioritized list of technologies. JPL (Jet Propulsion Laboratory). 2009. Outer Planets Flagship Mission. Available at http://opfm.jpl.nasa.gov/. • A list of critical facilities and skills. Mankins, J.C. Advanced Concepts Office, Office of Space Access and • A plan for documenting and archiving the knowledge Technology, NASA, Technology Readiness Levels, A White Paper, base. April 6, 1995. • A plan for maturing technology in key areas, such as NASA (National Aeronautics and Space Administration). 2009. NASA and reliability, power, power degradation, electrical inter- ESA Prioritize Outer Planet Missions. Available at http://www.nasa. gov/topics/solarsystem/features/20090218.html. faces between the RPS and the spacecraft, thermal NEPA (National Environmental Policy Act). 1970. National Environmental interfaces, and verification and validation. Policy Act of 1969, as amended, 42 USC Sections 4321-4347. Available • A plan for assessing and mitigating technical and sched- at http://ceq.hss.doe.gov/Nepa/regs/nepa/nepaeqia.htm. ule risk.,., Ross, R.G., Jr. 2008. Cryogenic Cooling in Space: 50-years of Lessons Learned. Second meeting of the Radioisotope Power Systems Commit- tee, Pasadena, Calif., October 28.