Priorities in Space Science Enabled by Nuclear Power and Propulsion (2006)

This chapter presents the findings and recommendations developed by the committee in response to its two tasks—(1) identification of a series of promising mission concepts to help define space nuclear systems’ potential to enable major scientific advances and (2) discussion of some of the likely technological, programmatic, societal, and budgetary consequences of the potential applications of nuclear power and propulsion systems by the space science community.

The material presented in Chapters 4 and 6 clearly demonstrates that the availability of nuclear power and propulsion technologies has the potential to enable a rich variety of solar and space physics and solar system exploration missions. Of the various nuclear technologies considered, radioisotope power systems (RPSs) are directly enhancing or enabling for missions identified in the solar and space physics (SSP) and solar system exploration (SSE) survey reports as priorities for the coming decade^1,2 and also for missions mentioned there as likely candidates for consideration as priorities in subsequent decades.

None of the missions identified in the SSP and SSE survey reports as priorities for implementation in the coming decade explicitly require a nuclear propulsion system. Both survey reports did, however, call for the development of nuclear-electric propulsion (NEP) systems to enable important candidate missions in future decades. The SSE survey report was much more explicit in this regard and, in addition to recommending the development of NEP, specifically identified several candidate missions likely to benefit from this technology. The discussions in Chapters 4 and 6 identify a number of promising mission concepts that would appear to be enhanced or enabled by RPS technologies and/or NEP.

The prospective contributions of these nuclear technologies in the areas of astronomy and astrophysics are, as shown in Chapter 8, far less promising at the moment. Nuclear power and propulsion systems are not enhancing or enabling for any of the current high-priority goals of astronomy and astrophysics. Most of the astronomy and astrophysics missions envisioned work as well at 1 AU as anywhere and have power requirements that can most readily be met with photovoltaic systems. The one major exception is infrared imaging, for which a case can be made between deploying a larger telescope in the high-zodiacal-light background at 1 AU versus a smaller telescope in the lower background at ≥3 AU. There are some more exotic mission possibilities that might be enhanced or enabled by the use of nuclear technologies, but they do not uniquely address high-priority goals of the astronomy and astrophysics community.

NASA and its partners in other federal agencies such as the Department of Energy have taken some important first steps in what will be a long-term program to harness nuclear power and propulsion technologies for the benefit of space exploration activities. Nevertheless, while these technologies hold promise, their positive and negative aspects must be clearly understood. If nuclear propulsion systems are developed and demonstrated, they will give researchers access to previously inaccessible objects and destinations, enabling them to conduct comprehensive studies of a type, and with a flexibility, not previously contemplated in the history of space exploration (see text boxes in Chapters 4 and 6). However, spacecraft nuclear propulsion is in its infancy and will require a great deal of technological development. As shown in Chapter 2, the performance figures from NASA’s studies of the Jupiter Icy Moons Orbiter (JIMO) and its parametric studies of candidate post-JIMO missions reveal a significant performance gap (in terms of, for example, transit time and launch mass) between what appears to be currently feasible and what is desirable from a scientific perspective.

The material presented in Chapter 2 also suggests that NASA’s current nuclear propulsion activities may be too narrowly focused on a single technology—nuclear-electric propulsion—and might benefit from a broader assessment of other technological approaches. Unfortunately, the study committee’s ability to examine technical factors related to these concerns was somewhat limited because circumstances beyond its control indefinitely postponed the second phase of the study, which would have provided detailed inputs from a panel of space nuclear power experts.

By today’s standards, the spacecraft using nuclear propulsion systems, regardless of the exact technologies employed, will be very large, very heavy, very complex, and, almost certainly, very expensive. To what extent will the development and deployment of such technologies interfere with the diversity of space science missions? Such diversity gives scientific breadth and depth to these pursuits and is essential for the long-term health and vitality of all space science activities.

On the other hand, it is difficult to imagine that the space science goals of the period beyond 2015 will still be addressed with the power and propulsion technologies of the Mariners, Pioneers, and Voyagers. But it is equally difficult to imagine how it will be possible to transition smoothly from an era of Cassini, Mars Exploration Rovers, New Horizons, Discovery, and Explorers to one whose mix of activities is as diverse but now also includes super-flagship-class NEP missions.

The committee elaborates on this finding below and makes specific related recommendations.

NASA is investing in both RPSs and fission reactors as power sources for future missions. RPSs, which have a long history of enabling science investigations, could provide power for a broad range of missions, from landers at Mars or Venus to orbiters at the planets of the outer solar system. To do so they must be able to operate in a number of modes (e.g., on orbiters, landers, and rovers) and environments (e.g., extremes of hot and cold, the vacuum of deep space, or planetary atmospheres). RPSs might also be useful for different classes

of missions and should be seriously considered for small principal-investigator (PI)-led missions, such as Mars Scout and Discovery.

Of particular interest to the space science community in the near term is the ongoing development of two new types of RPSs—the multi-mission radioisotope thermoelectric generator (MMRTG) and the Stirling radioisotope generator (SRG). The committee notes that both the MMRTG and the SRG are less efficient in terms of their specific mass (i.e., kg/kWe) than the devices they are replacing. Further, the SRG has moving parts that may limit its lifetime as well as cause vibrations and electromagnetic interference.

To produce electrical power, RPSs utilize the nuclear decay of radioactive isotopes, typically ²³⁸Pu. The committee is concerned that interruptions in the production, supply, or packaging of ²³⁸Pu fuel could impact future missions. Currently Russia is the only source of ²³⁸Pu, at a cost of between $2.2 million and $2.5 million per kilogram (kg).³ This material is currently processed and packaged in the United States, principally at the Los Alamos National Laboratory.^a A steady and reliable source of ²³⁸Pu is required if the scientific potential of missions enhanced and enabled by RPS technologies is to be realized.

The current focus of Project Prometheus is on development of a nuclear-electric propulsion system. NASA’s parametric studies of the potential applications of this propulsion system indicate that a number of desirable missions require a transit time of more than 10 years (e.g., Neptune, 13 to 15 years; Kuiper Belt object, 17 to 19 years; interstellar probe, ~30 years). Transit times of a decade or more create problems for sustaining continuous systems operations, maintaining public support, and ensuring systems reliability and would lead to instrument obsolescence. The committee is worried that the failure to achieve transit times to priority destinations in the outer solar system of 10 years or less will render the NEP technology NASA is developing of little interest to the scientific community.

In Chapter 1, the committee expresses its concern that insufficient foundational work was undertaken by NASA to support a qualitative assessment of the opportunities enhanced or enabled by nuclear systems. This concern has been borne out. The committee did not find that adequate work had been done to demonstrate that a viable NEP system with wide scientific applicability could be developed. For example, estimates provided by NASA for an interstellar probe to reach 200 AU in ~30 years cannot be verified because specific assumptions have not been provided concerning important factors such as mass-to-thrust ratio, spacecraft mass, power-conversion efficiencies, payload mass, and planetary gravitational-assist scenarios. Alternative technologies, for example, nuclear-thermal propulsion (NTP) and bimodal systems, may provide a more cost-effective, faster means of transport to the outer solar system and beyond.

NASA missions that might be enabled by nuclear propulsion will require high delta-V, fast transit times, and/or high levels of electric power at the destination. To determine the benefits of nuclear propulsion for such missions will require “level-playing-field” trade-off studies that compare various propulsion options in terms of such metrics as cost, initial mass in low Earth orbit, launch-vehicle requirements, and transit time. Such studies should incorporate the impact on the systems of requiring risk mitigation through the use of redundant subsystems and extensive ground testing.

To determine suitable space reactor system designs and operational profiles, trade-off studies should also consider the impact of system reliability, especially for those systems designed to operate continuously without maintenance or repair for extended periods. For example, the current NEP concepts being considered for missions to the outer solar system require that the systems function without human intervention for durations between 10 and 20 years. However, no high-power-density reactor has ever been operated on Earth, without maintenance shutdowns, for any time period longer than one order of magnitude or more below such a duration. To operate reliably in space, nuclear systems must be tested extensively on Earth in environments and with operational sequences that approximate actual expected conditions as closely as possible. Such testing will require establishing operational margins—i.e., systems must be tested at full power for longer, if possible, than full mission duration. If NASA deems a full-duration-testing requirement to be infeasible for NEP systems, a full risk analysis of the system must be completed, including the option of providing for redundancy of the reactor on the spacecraft. Reliability requirements could also substantially increase system-mass requirements, depending on the extent to which redundant subsystems will be needed to meet reliability goals. Because of the weight of the massive heat rejection system they require, NEP systems also carry a considerable mass penalty.

Missions recommended for future study are described in Chapters 4 and 6. These, together with additional promising robotic missions (e.g., those resulting from NASA’s so-called Vision Missions competition) and human missions to the Moon or Mars, should be considered. Studies of trade-offs between chemical systems, solar-electric propulsion systems, solar sails, NEP and NTP systems, and bimodal systems should be completed for missions over a wide range of delta-V. These studies of trade-offs should take into account considerations of mission complexity, reliability, and risk. Only those missions for which the nuclear power significantly enhances the science return or that are otherwise effectively impossible merit the application of NEP or NTP power and/or propulsion. The most cost-effective application relative to the science return must be the deciding factor.

In Chapter 1 the committee expresses concern about two issues relating to the decadal surveys. The first issue was the potential invalidation of decadal survey science priorities that were based on technical assumptions made before the initiation of Project Prometheus. This concern has abated. Nothing that the committee heard during its study or discussed during its deliberations caused it to question the continuing validity of the science priorities articulated in the three space science decadal surveys. As partial validation of this conclusion, the committee points to the results of NASA’s recent strategic roadmapping exercise. Many of the roadmapping teams were fully briefed on the technical capabilities opened up by Project Prometheus, yet their reports are virtually devoid of missions enabled by nuclear technologies.

The second issue was related to the potential impact of super-flagship NEP missions on the programmatic balance inherent in the decadal surveys, which recommend a range of mission types and classes. This concern has not abated. The most recent decadal surveys placed high importance on overall program balance. All three surveys reiterated the need for a mix of more frequent, PI-led small- and medium-class missions combined with less frequent, more costly, larger missions. This overall program balance is considered practical, given the size of NASA budgets, and is necessary for nurturing a healthy scientific and engineering community. The potentially very large cost of NEP-class missions could threaten this programmatic balance, a possibility that is of extreme concern to this committee.

The balance between small, focused missions and more challenging missions is a pillar of the space science enterprise and must be maintained. In particular, the space science community relies directly on mission lines such as Explorer, the Solar-Terrestrial Probe, Discovery, and New Frontiers. In fact, such a mix of mission costs and sizes lends itself well to making progress and discovery on a broad front. The current projections of scientific benefit versus cost for nuclear-electric propulsion indicate that its value to the scientific community would be limited. For example, although exploring Europa remains an extremely high priority, doing so with a mission like JIMO may not be justified by the science return expected, as compared with a smaller, less expensive mission that does not use nuclear-reactor propulsion. NEP-class missions should not, in the committee’s judgment, be perceived as a replacement for any of these mission lines, and funds should not be diverted from these smaller and less expensive lines to support much more expensive nuclear-electric missions. Further, the committee sees a need for a flagship class of missions, such as the Europa Geophysical Explorer and the Solar Probe, which are intermediate in cost relative to the New Frontiers and the super-flagship NEP-class missions. By making a wide range of measurements of a given body, system, or region of space, such large-scale missions should allow for serendipitous discoveries that would not be possible with less expensive, narrowly focused missions.

The development of nuclear propulsion systems is seen as having many possible advantages for future science missions. However, the associated cost may dramatically impact near-term mission capabilities. If NASA’s science program is required to cover the development of systems, a substantial decline in expertise and capability will result, and the recovery from this decline will be slow and difficult.

Another of the committee’s initial concerns expressed in Chapter 1 was the degree to which issues have been raised relating to safety and the interagency launch-approval process. Although these issues are still a concern, the committee is less worried about them than it was at the outset. Nuclear power is used throughout the United States to provide electrical power to a significant portion of the population. Public opposition to the perceived risk of nuclear power plants and the associated production of long-lived radioactive waste, compounded by escalating costs, has led over the past three decades to a de facto cessation of the design and construction of new nuclear plants on U.S. soil. Plans for the construction of large, ground-based nuclear facilities (e.g., the Yucca Mountain high-level nuclear waste repository or the Ward Valley low-level waste facility) have also met with strong

opposition by relatively large segments of the population in the vicinity of such sites. There are many indications that a significant fraction, if not the majority, of the U.S. public is resistant to the development of new nuclear systems.

The experience with the use of nuclear power in space systems, on the other hand, has been somewhat different. Opposition to the launch of spacecraft that carry a nonnegligible inventory of radioactive material, such as the RPS-powered Galileo, Cassini, and New Horizons, has been visible and vocal, but not necessarily widespread nor ultimately successful in preventing the launch of these systems. The larger public’s interest in these, as well as Mars missions—e.g., the Mars Exploration Rovers—that carried smaller amounts of radioactive material onboard (in the form of radioisotope heater units) has been largely positive despite the missions’ nuclear power underpinning. Thus, it could be argued that the negative public perception of nuclear power is selective and may be changing, especially when a visible demonstration of successful return can be claimed. Nevertheless, the potential for public opposition to nuclear power development exists and must be considered by NASA in planning the development of nuclear space power and propulsion systems.

Historically, public acceptance has been enhanced by openness during the technology design phase and full revelation of the technology development path, including a clear explanation of the benefits and challenges of the proposed developments. Denial of risk and neglect of impacts that are potentially visible for a given development have, on the other hand, been the curse, and often the demise, of otherwise potentially beneficial nuclear technology developments. Nuclear space reactor technology has seen very limited development in the United States or, for that matter, worldwide and undoubtedly poses a number of questions regarding reliability and safety that need to be fully understood and addressed by the technical and scientific community and made comprehensible to the public at large.

Missions powered or propelled by nuclear devices have to satisfy both environmental laws and launch-approval requirements (i.e., National Environmental Policy Act [NEPA] regulations and Presidential Directive NSC-25). Safety analyses and risk assessments to satisfy both types of requirements have been developed and applied successfully for past missions carrying RPSs. However, there is no past experience with applying these processes to obtain approval for the design and launch of missions carrying more complex and powerful devices such as a nuclear reactor. Also, although no specific national or international laws, regulations, or policies have been promulgated regarding safety and environmental issues beyond the Earth environment, the questions of safe disposal of reactor fuel and protection of planetary environments may eventually be raised and perhaps used, legally or politically, to oppose missions with nuclear reactors onboard.

In summary, it would be prudent to evaluate whether the current interagency (i.e., NASA/DOE) processes for NEPA and launch approval are sufficient to address potential public or international concerns about the safety and environmental impact of nuclear reactors for space missions, or whether some additional requirements are likely to have to be addressed in the future by different means.

The great distances, exposure to intense cosmic radiation, and physiological response to zero gravity all

support the concept of using an advanced propulsion system to shorten the duration of human missions to Mars. Nuclear power and propulsion systems could also provide the large amounts of electricity necessary for life-support systems on human missions and for support of surface science and exploration activities. Surface and space power systems are likely to be different, however, and it is not clear that the NEP system currently under study by Project Prometheus is adequate for either application. NTP systems may provide better thrust for manned Mars missions. Nuclear reactor power and propulsion systems for human exploration missions must be qualified to a much higher level of reliability than power and propulsion systems for robotic missions, and will also need to be validated for reliable operation at full power and for the lifetime of the mission.

Applications of nuclear power for human exploration, especially NEP for piloted missions, may involve problems associated with reactor-generated radiation. Current systems, as illustrated by the JIMO/Prometheus Baseline (PB) 1 design, have neutron and gamma fluxes at the science platform that are far in excess of those considered safe for humans. The mass of shielding required to reduce exposure to safe levels may negate the propulsion advantages of NEP. Radiation is also a concern, although one more easily mitigated, for surface-based systems powering human-occupied stations. In the absence of containment systems on the scale of Earth-based generating systems, such power systems will have to be located at a remote site, possibly surrounded by a protective regolith berm, with power transmission cables to deliver the electric power to the crew station.

A heavy-lift launch capability would potentially enable new classes of space science missions. However, the added costs must be included when comparing JIMO-class missions using nuclear propulsion technologies to other mission options. In addition, development of a heavy-lift launch vehicle or the use of multiple launches followed by assembly in orbit would also have an impact on schedule, again negatively affecting already high cost estimates.

Considerations relating to these technical and programmatic issues include the following:

• Fraction of launch mass for science. On typical planetary missions flown over the last 30 years, the fraction of science payload mass to total mass has varied between 0.09 and 0.17. For example, the Cassini spacecraft, currently in orbit around Saturn, has a science payload to mass fraction of >0.1: that is, somewhat more than 600 kg is allocated to the science payload (body-mounted science instruments totaling approximately 300 kg,

and an atmospheric entry probe of mass greater than 300 kg) on a spacecraft with an overall launch mass of less than 6,000 kg. By contrast, the JIMO spacecraft, for example, would have a launch mass in excess of 25,000 kg, with only 1,500 kg of that allocated to the science payload, yielding a science payload to mass fraction of <0.06. The much larger masses necessitated by large NEP systems should offer the opportunity for much larger science payloads.

• The Deep Space Network and the Planetary Data System. A selling point of NEP-class missions is that their abundant electrical power would enable a new generation of high-power, high-capability instruments that could collect unprecedented amounts of data. In Chapter 1 the committee expresses initial concerns about the ability of both the space science infrastructure—exemplified by the Deep Space Network and the Planetary Data System—and the data-analysis programs to accommodate such large data volumes. These concerns remain valid. In fact, using current capabilities, it is unlikely that the complete dataset collected by such missions could be transmitted back to Earth. Accommodating this high volume of data will require some combination of the following: advanced onboard processing to reduce the amount of data that must be sent back to Earth; high-bandwidth communications facilitated, in part, by greater transmitting power; and improvements to the Deep Space Network.

  Advanced onboard processing is increasingly important. In fact, astrophysics missions are now being proposed that have substantial (~10 kW) power demands (see Table 7.1) driven by the need for data correlation and compression. Onboard processing does, however, have a significant drawback: the danger that potentially important serendipitous discoveries will be precluded because the telemetry stream is inadvertently biased toward results that are expected a priori. Therefore, increasing Earth-based receiving apertures, particularly if implemented using arrays of small antennas, may be the more flexible approach in that these receiving apertures could, for example, support several missions simultaneously. The Planetary Data System and other data repositories will have to be expanded so that the large volumes of data returned by NEP-class missions can be adequately archived and distributed to the scientific community. Appropriate data-analysis programs will have to be established and/or augmented to meet the needs of these future missions and ensure that the data returned are fully analyzed.

• Radiation-hardened components and radiation-tolerant detectors. The development of new, more capable, radiation-hardened electronic components, particularly processors, will enhance measurement capabilities. Similarly, the development of new detector materials, new detection concepts, and a better understanding of the detectors’ response to and damage by radiation from advanced nuclear power and propulsion technologies would enhance, and in some cases enable, the scientific return of missions in these radiation environments.

• Contamination mitigation for instruments I. A spacecraft equipped with nuclear power and propulsion systems must accommodate a wide range of instrument types that are sensitive to local environmental conditions. Instruments that might benefit from power and/or propulsion from nuclear reactors will clearly be compromised or damaged by high-energy particles and photons from the reactor. In addition, the reactor and its power-conversion and radiator systems are likely to be sources of DC magnetic fields, electromagnetic interference over a wide range of frequencies, waste heat, vibrations, and outgassed effluents. The ion thrusters also contaminate the environment with plasma, magnetic fields, and electromagnetic interference, and can also erode exposed surfaces. Power and propulsion systems must be clean and stable in terms of transient magnetic and electric fields, chemical contamination, radiation and charged-particle levels, and vibration to allow characterization of, for example, planetary magnetic fields; planetary magnetic-field/solar-wind interactions; phenomena associated with local plasmas; useful ion and neutral mass spectrometer studies in upper atmospheres, near ring systems, and near small bodies; precision measurements of gravitational fields; and precise instrument pointing and stability. Sample-return missions must ensure that the samples are not altered by high neutron and gamma fluxes and fluences. If NEP systems are to enhance or enable space science objectives, their effects as sources of interference must be fully investigated and mitigating strategies developed that avoid to the maximum extent possible the use of heavy shielding.

• Contamination mitigation for instruments II. Nuclear reactors operating within Earth’s magnetosphere are well established as causing significant interference to balloon-borne and orbiting gamma-ray observatories and hampering their ability to conduct astronomical measurements. This interference is caused by primary and secondary gamma rays and, in addition, electron-bremsstrahlung and positron-annihilation radiation. These difficulties are particularly problematic for reactors located inside Earth’s magnetosphere, because of the long timescale (up

to several minutes) for trapping of particles in the geomagnetic field. The interfering signals can have a duration ranging from a few seconds to many minutes and can also have highly variable amplitudes and rates.

The amplitude of the photon events depends simply on the location of the reactor relative to the detector, whereas the particle-induced event rates are more complicated to quantify because they also depend on the orientation and strength of geomagnetic field lines near the location where they were emitted, as well as on the time between emission and detection. All these effects have been well documented in data from the Gamma-Ray Spectrometer aboard the Solar Maximum Mission, which operated from 1980 to 1989 and which suffered from significant background of this type generated by the Russian Cosmos orbiting ~100-kW nuclear reactors.^4-7 These factors suggest that unless it can be demonstrated that this type of interference will not occur, nuclear reactors on future spacecraft should only be operated well outside Earth’s magnetosphere.^b

1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.

2. National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003.

3. G.R. Schmidt, R.L. Wiley, R.L. Richardson, and R.R. Furlong, “NASA’s Program for Radioisotope Power System Research and Development,” Space Technology and Applications International Forum—STAIF-2005, M.S. El-Genk, ed., American Institute of Physics, Melville, N.Y., 2005.

4. E. Reiger et al., “Man-Made Transients Observed by the Gamma-Ray Spectrometer on the Solar Maximum Mission Satellite,” Science 244: 441, 1989.

5. G.H. Share et al., “Geomagnetic Origin for Transient Particle Events from Nuclear Reactor-Powered Satellites,” Science 244: 444, 1989.

6. E.W. Hones and P.R. Higbie, “Distribution and Detection of Positrons from an Orbiting Nuclear Reactor,” Science 244: 448, 1989.

7. O’Neill et al., “Observations of Nuclear Reactors on Satellites with a Balloon-Borne Gamma-Ray Telescope,” Science 244: 451, 1989.