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Summary The United States has publicly funded its human spaceflight program on a continuous basis for more than a half-century. Today the United States is the major partner in a massive orbital facility—the International Space Station (ISS)—that is a model for how U.S. leadership can engage nations through soft power and is becoming the focal point for the first tentative steps in commercial cargo and crewed orbital spaceflights. And yet, a national consensus on the long-term future of human spaceflight beyond our commitment to the ISS remains elusive. The task for this review originated in the NASA Authorization Act of 2010, which required that the National Academies perform a human spaceflight study that would review “the goals, core capabilities, and direction of human space flight.” The explicit examination of rationales, along with the identification of enduring questions, set the task apart from numerous similar studies performed over the past several decades, as did the requirement for the committee to bring broad public and stakeholder input into its considerations. The complex mix of historic achievement and uncertain future made the task faced by the Committee on Human Spaceflight extraordinarily challenging and multi-dimensional. Nevertheless, the committee has come to agree on a set of major conclusions and recommendations, which are summarized below. Enduring Questions Enduring questions are those that serve as motivators of aspiration, scientific endeavors, debate, and critical thinking in the realm of human spaceflight. The questions endure because any answers available today are at best provisional and will change as more exploration is done. Enduring questions provide motivations immune to external forces and policy shifts. They are intended not only to stand the test of time but also to continue to drive work forward in the face of technological, societal, and economic constraints. Enduring questions are clear and intrinsically connect to broadly shared human experience. Based on the analysis reported in Chapter 2, the committee asserts that the enduring questions motivating human spaceflight are these:  How far from Earth can humans go? and  What can humans discover and achieve when we get there? Rationales for Human Spaceflight and the Public Interest All of the arguments the committee heard supporting human spaceflight have been used in various forms and combinations to justify the program for many years. In the committee’s view these rationales divide into two sets: pragmatic rationales—economic benefits; contributions to national security; contributions to national stature and international relations; inspiration for students and citizens to further their science and engineering education; and contributions to science—and aspirational rationales—the eventual survival of the human species (through off-Earth settlement) and shared human destiny and the aspiration to explore. In reviewing these, as Chapter 2 lays out in detail, the committee concluded: PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-1

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 Economic benefits. While there exists no widely accepted, robust quantitative methodology to support comparative assessments of the returns to federal R&D programs in different economic sectors and areas of research, it is clear that the NASA human spaceflight program, like other government research and development programs, has stimulated economic activity and has advanced development of new products and technologies that have had or may in the future generate significant economic impacts. It is impossible, however, to develop a reliable comparison of the returns from spaceflight versus other government R&D investments.  National security. While space-based assets and programs are a significant element of national security, the direct contribution of human spaceflight in this realm has been and is likely to remain limited. An active U.S. human spaceflight program gives the United States a stronger voice in an international code of conduct for space, enhances U.S. soft power, and supports collaborations with other nations, thereby contributing to our national interests, including security.  National stature and international relations. Being a leader in human space exploration enhances international stature and national pride. Because the work is complex and expensive, it can benefit from international cooperative efforts. Such cooperation has important geopolitical benefits.  Inspiration of students and citizens. The United States needs scientists and engineers and a public with a strong understanding of science. The challenge and excitement of space missions can serve as an inspiration for students and citizens to engage with science and engineering, although it is difficult to measure this impact. The path to becoming a scientist or engineer requires much more than the initial inspiration. Many who work in space fields, however, report the importance of such inspiration, although it is difficult to separate the contributions from human versus robotic spaceflight.  Scientific discovery. The relative benefits of robotic versus human efforts in space science are constantly shifting as a result of changes in technology, cost, and risk. The current capabilities of robotic planetary explorers such as Curiosity and Cassini are such that although they can go farther sooner and at much lower cost than human missions to the same location, they cannot match the flexibility of humans to function in complex environments, to improvise, and to respond quickly to new discoveries. This constraint may change at some indeterminate time in the future.  Human survival. It is not possible to say whether off-Earth settlements could eventually be developed that would outlive human presence on Earth and lengthen the survival of our species. This is a question that can only be settled by pushing the human frontier in space.  Shared destiny and aspiration to explore. The urge to explore and to aspire to reach challenging goals is a common human characteristic. Space is today a major physical frontier for such exploration and aspiration. Some say it is human destiny to continue to explore space. While not all share this view, for those who do, it is an important reason to engage in human spaceflight. As discussed in Chapter 2, the pragmatic rationales have never seemed adequate by themselves, perhaps because the benefits they argue for are not unique to human spaceflight. Those that are—the aspirational rationales related to the human destiny to explore and the survival of the human species—are also the rationales most tied to the enduring questions. Whereas the committee concluded from its review and assessment that no single rationale alone seems to justify the value of pursuing human spaceflight, the aspirational rationales, when supplemented by the practical benefits associated with the pragmatic rationales, do, in the committee’s judgment, argue for a continuation of our nation’s human spaceflight program, provided that the pathways and decision rules recommended in this report are adopted (see below). The level of public interest in space exploration is modest relative to other public policy issues such as economic issues, education, and medical or scientific discoveries. As Chapter 3 documents, public opinion about space has been generally favorable over the past 50 years, but much of the public is inattentive to space exploration, and spending on space exploration is not a high priority for most of the public. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-2

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Horizon Goal As discussed in Chapter 4, the technical analysis completed for this study shows clearly that for the foreseeable future, the only feasible destinations for human exploration are the Moon, asteroids, Mars, and the moons of Mars. Among this small set of plausible goals for human space exploration,1 the most distant and difficult is a landing by human beings on the surface of Mars—requiring overcoming unprecedented technical risk, fiscal risk, and programmatic challenges. Thus the horizon goal for human space exploration is Mars. All long-range space programs, by all potential partners, for human space exploration converge on this goal. Policy Challenges A program of human space exploration beyond low Earth orbit (LEO) that satisfies the pathway principles defined below is not sustainable with a budget that increases only enough to keep pace with inflation. As shown in Chapter 4, the current program to develop launch vehicles and spacecraft for flight beyond LEO cannot provide the flight frequency required to maintain competence and safety, does not possess the “stepping-stone” architecture that allows the public to see the connection between the horizon goal and near-term accomplishments, and may discourage potential international partners. Because policy goals do not lead to sustainable programs unless they also reflect or change programmatic, technical, and budgetary realities, the committee notes that those who are formulating policy goals need to keep the following factors in mind:  Any defensible calculation of tangible, quantifiable benefits—spinoff technologies, attraction of talent to scientific careers, scientific knowledge, and so on—is unlikely to ever demonstrate a positive economic return on the massive investments required by human spaceflight.  The arguments that triggered the Apollo investments, national defense and prestige, seem to have especially limited public salience in today’s post-Cold War America.  Although the public is mostly positive about NASA and its spaceflight programs, increased spending on spaceflight is a low priority for most Americans. At the same time, most Americans do not follow the issue closely, and those who pay more attention are more supportive of space exploration. International Collaboration International collaboration has become an integral part of the space policy of essentially all nations participating in space around the world—most states now rarely initiate and carry out significant space projects without some foreign participation. The reasons for collaboration are multiple, but states, including the United States, cooperate principally when they benefit from it. It is evident that U.S. near-term goals for human exploration are not aligned with those of our traditional international partners. While most major spacefaring nations and agencies are looking toward the Moon and, specifically, the lunar surface, U.S. plans are focused on redirection of an asteroid into a retrograde lunar orbit, where astronauts would conduct operations with it. It is also evident that given the rapid development of China’s capabilities in space, it is in the best interests of the United States to be open to its inclusion in future international partnerships. In particular, current federal law preventing NASA from participating in bilateral activities with the Chinese serves only to hinder U.S. ability to bring China into its sphere of international partnerships and reduces substantially the potential international capability that might be pooled to reach Mars. Also, given the scale of the endeavor of a mission to Mars, 1 Although there is no strictly defined distinction between human spaceflight and human space exploration, the committee takes the latter to mean spaceflight beyond LEO, in which the goal is to have humans venture into the cosmos to discover new things. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-3

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contributions by international partners would have to be of unprecedented magnitude to defray a significant portion of the cost. This assessment follows from the detailed discussion in Chapter 4 of what is required for human missions to Mars. Recommendations for a Pathways Approach NASA and its international and commercial partners have developed an infrastructure in LEO that is approaching maturity—that is, assembly of the ISS is essentially complete. The nation must now decide whether to embark on human space exploration beyond LEO in a sustained and sustainable fashion. Having considered past and current space policy, explored the international setting, articulated the enduring questions and rationales, and identified public and stakeholder opinions, the committee draws on all this information to ask a fundamental question: What type of human spaceflight program would be responsive to these factors? This committee argues that it is a program in which humans operate beyond LEO on a regular basis—in other words, a sustainable human exploration program beyond LEO. A sustainable program of human deep space exploration requires an ultimate “horizon” goal that provides a long-term focus that is less likely to be disrupted by major technological failures and accidents along the way and by the vagaries of the political process and the economic scene. There is a consensus in national space policy, international coordination groups, and the public imagination for Mars as a major goal for human space exploration. NASA can sustain a human space exploration program that pursues the horizon goal of a surface landing on Mars with meaningful milestones and simultaneously reasserts U.S. leadership in space while allowing ample opportunity for substantial international collaboration—but only when that program has elements that are built in a logical sequence, and when it can fund a frequency of flights sufficiently high to ensure the maintenance of proficiency among ground personnel, mission controllers, and flight crews. In the pursuit of this goal, NASA needs to engage in the type of mission planning and related technology development that addresses mission requirements and integration and develops high-priority capabilities such as entry, descent, and landing for Mars; radiation safety; and advanced in-space propulsion and power. Progress in human exploration beyond LEO will be measured on timescales of decades, with costs measured in hundreds of billions of dollars and significant risk to human life. In addition, the committee has concluded that the best way to ensure a stable, sustainable human spaceflight program that pursues the rationales and enduring questions the committee has identified is to develop a program through the rigorous application of a set of Pathway Principles: Therefore, as its highest-priority recommendation, the committee recommends as follows: NASA should adopt the following Pathway Principles: I. Commit to design, maintain, and pursue the execution of an exploration pathway beyond low Earth orbit toward a clear horizon goal that addresses the “enduring questions” for human spaceflight. II. Engage international space agencies early in design and development of the pathway on the basis of their ability and willingness to contribute. III. Define steps on the pathway that foster sustainability and maintain progress on achieving the pathway’s long-term goal of reaching the horizon destination. IV. Seek continuously to engage new partners that can solve technical and/or programmatic impediments to pathway progress. V. Create a risk mitigation plan to sustain the selected pathway when unforeseen technical or budgetary problems arise. Such a plan should also include points at which decisions are made to move to a less ambitious pathway (“off-ramp,” as defined below) or stand down the program. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-4

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VI. Establish exploration pathway characteristics that maximize the overall scientific, cultural, economic, political, and inspirational benefits without sacrificing progress toward the long-term goal, these characteristics being: a. The horizon and intermediate destinations have profound scientific, cultural, economic, inspirational, or geopolitical benefits that justify public investment; b. The sequence of missions and destinations permits stakeholders, including taxpayers, to see progress and develop confidence in NASA being able to execute the pathway; c. The pathway is characterized by logical feed-forward of technical capabilities; d. The pathway minimizes the use of dead-end mission elements that do not contribute to later destinations on the pathway; e. The pathway is affordable without incurring unacceptable development risk; and f. The pathway supports, in the context of available budget, an operational tempo that ensures retention of critical technical capability, proficiency of operators, and effective utilization of infrastructure. These Pathway Principles will need to be supported by a set of operational Decision Rules as NASA, the administration, and Congress face inevitable programmatic challenges along a selected pathway. The Decision Rules this committee has developed provide operational guidance that can be applied as major technical, cost, and schedule issues arise as NASA progresses along a pathway. Because many significant decisions will have to be made before any program of record is approved and initiated, the Decision Rules have been designed to provide the framework for a sustainable program through the lifetime of the selected pathway. They are designed to allow a program to stay within the constraints accepted and developed when applying the Pathway Principles. The committee recommends: Whereas the overall pathway scope and cost are defined by applying the Pathway Principles, once on a pathway, if and when technical, cost, or schedule problems arise, they should be addressed by NASA, the administration, and Congress by applying the following Decision Rules: A. If the appropriated funding level and projected 5-year budget projection do not permit execution of a pathway within the established schedule, then do not start down that pathway.2 B. If a budget profile does not permit the chosen pathway, even if NASA is well down it, then take an “off-ramp.” C. If the U.S. human spaceflight program receives an unexpected increase in budget for human spaceflight, NASA, the administration, and Congress should not redefine the pathway such that continued budget increases would be required for the pathway’s sustainable execution, but rather the increase in funds should be applied to retire rapidly significant technology risks or increase operational tempo in pursuit of the pathway’s predefined technical and exploration goals. D. Given that limitations on funding will require difficult choices in the development of major new technologies and capabilities, give priority to those that solve significant existing technological shortcomings, reduce overall program cost, allow for an acceleration of the schedule, and/or reduce developmental or operational risk. 2 The committee recognizes that budget projections are unreliable, but they are also indispensable. One way to make the use of such projections more robust would be for NASA to conduct sensitivity analysis and evaluate plans against a range of possible 5-year budget projections that may vary by 10 percent or more. This might be done as part of the risk mitigation plan. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-5

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E. If there are human spaceflight program elements, infrastructure, and organizations that no longer contribute to progress along the pathway, the human spaceflight program should divest itself of them as soon as possible. Recommendations for Implementing a Sustainable Program This committee was not charged to recommend and has not recommended any one particular pathway or set of destination targets. The recommended pathways approach combines a strategic framework with practical guidance designed to stabilize human space exploration and to encourage political and programmatic coherence over time. If the United States is to have a human space exploration program, then it must be worthy of the considerable cost to the nation and great risk of life. The committee has found no single practical rationale that is uniquely compelling to justify such investment and risk. Rather, human space exploration must be done for inspirational and aspirational reasons that appeal to a broad range of U.S. citizens and policy makers and that identify and align the United States with technical achievement and sophistication while demonstrating its capability to lead and/or work within an international coalition for peaceful purposes. Given the expense of any human spaceflight program and the significant risk to the crews involved, in the committee’s view the only pathways that fit these criteria are those that ultimately place humans on other worlds. While this report’s recommendation for adoption of a pathways approach is made without prejudice as to which particular pathway might be followed, it was, nevertheless, clear to the committee from this report’s independent analysis of several pathways that a return to extended surface operations on the Moon would make significant contributions to a strategy ultimately aimed at landing people on Mars and that it is also likely to provide a broad array of opportunities for international and commercial cooperation. No matter which pathway is ultimately selected, the successful implementation of any plan developed in concert with pathways and decision rules will rest upon several other conditions. Together with the highest-priority recommendation of the pathways approach and decision rules, the committee offers the following prioritized recommendations as being those most critical to the development and implementation of a sustainable human space exploration program. Accordingly, the committee recommends: NASA should: 1. Commit to design, maintain, and pursue the extension of human presence beyond low Earth orbit (LEO). This step should include: a. Committing NASA’s human spaceflight asset base, both physical and human, to this effort and b. Redirecting human spaceflight resources as needed to include improving program management efficiency (including establishing and managing to appropriate levels of risk), eliminating obsolete facilities, and consolidating remaining infrastructure where possible. 2. Maintain long-term focus on Mars as the “horizon goal” for human space exploration, addressing the enduring questions for human spaceflight: How far from Earth can humans go? and What can humans do and achieve when we get there? 3. Establish and implement the pathway approach so as to maximize the overall scientific, cultural, economic, political, and inspirational benefits of individual milestones and to conduct meaningful work at each step along the pathway, without sacrificing progress toward long-term goals. 4. Vigorously pursue opportunities for international and commercial collaboration in order to leverage financial resources and capabilities of other nations and commercial PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-6

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entities. International collaboration would be open to the inclusion of China and potentially other emerging space powers, as well as traditional international partners. Specifically, future collaborations on major new endeavors should seek to incorporate: a. A level of overall cost sharing appropriate to the true partnerships that will be necessary to pursue pathways beyond LEO. b. Shared decision making with partners. This should include a detailed analysis, in concert with international partners, of the implications for human exploration of continuing the International Space Station beyond 2024. 5. Engage in planning that includes mission requirements and a systems architecture targeting funded high-priority technology development, most critically: a. Entry, descent, and landing for Mars, b. Advanced in-space power and propulsion, and c. Radiation safety. In this report the committee has provided guidance on how a pathways approach might be successfully pursued and the likely costs of these pathways if things go well. However, the committee also concludes that if the resulting plan is not appropriately financed, it will not succeed. Nor can it succeed without a sustained commitment on the part of those who govern the nation—a commitment that does not change direction with succeeding electoral cycles. Those branches of government—executive and legislative—responsible for NASA’s funding and guidance are therefore critical enablers of the nation’s investment and achievements in human spaceflight, commissioning and financing plans and then assuring that the leadership, personnel, governance, and resources are in place at NASA and in other federally funded laboratories and facilities in order to advance it. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION S-7