All the arguments that the committee heard in support of human spaceflight have been used in various forms and combinations to justify the program for many years. In the committee’s view, these rationales can be divided into two sets. Pragmatic rationales involve 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. Aspirational rationales involve the eventual survival of the human species (through off-Earth settlement) and shared human destiny and the aspiration to explore. In reviewing the rationales, the committee concluded as follows:

  • Economic matters. There is no widely accepted, robust quantitative methodology to support comparative assessments of the returns on investment in federal R&D programs in different economic sectors and fields of research. Nevertheless, it is clear that the NASA human spaceflight program, like other government R&D 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 on spaceflight versus other government R&D investment.
  • Security. Space-based assets and programs are an important element of national security, but 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; thus, it contributes 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.
  • Education and inspiration. The United States needs scientists and engineers and a public that has 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. 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 of human and 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, go sooner, and be much less expensive than human missions to the same locations, they cannot match the flexibility of humans to function in complex environments, to improvise, and to respond quickly to new discoveries. Such constraints may change some day.
  • Human survival. It is not possible to say whether human off-Earth settlements could eventually be developed that would outlive human presence on Earth and lengthen the survival of our species. That question can be answered only by pushing the human frontier in space.
  • Shared destiny and aspiration to explore. The urge to explore and to reach challenging goals is a common human characteristic. Space is today a major physical frontier for such exploration and aspiration. Some say that 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 that 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 costs and risks of pursuing human spaceflight, the aspirational rationales, when

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