Enduring questions, in the committee’s view, are ones that can serve as motivators of aspiration, scientific endeavor, debate, and critical thinking in the realm of human spaceflight. The questions endure because any answers that are available today are at best provisional and will change as more exploration takes place. Enduring questions should provide a foundation for analyzing choices that is immune to external forces and policy shifts. Enduring questions 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. The two aspirational rationales, in contrast to the pragmatic rationales, do indeed lead us to ask such questions, which require further efforts in human spaceflight if they are to be answered and which address issues of the future of humankind. They suggest an international rather than a national effort; indeed, given the breadth of the international interest and capability in spaceflight, progress in answering these questions will not depend on the U.S. spaceflight program alone.

The committee asserts that the enduring questions motivating human spaceflight are

  • How far from Earth can humans go?
  • What can humans discover and achieve when we get there?

The questions are deceptively simple, but the committee was convinced that, in the context of any national or international effort in human spaceflight, asking whether a program—or even a pathway step—helps to advance us toward the ability to answer these questions can provide a useful compass in making choices.

The possibility of human spaceflight has inspired a wide variety of questions throughout time, even before it was first accomplished in 1961 with the launch of Yuri Gagarin. Indeed, the task of this committee could be construed as answering the ultimate question: Why explore? At its most fundamental level, human spaceflight is a continuation of human exploration—an extension of the human drive to investigate uncharted territory. In the early 17th century, Johannes Kepler wrote Somnium, a work of science-based fiction that detailed human flight to the Moon on the basis of Copernican astronomy. Somnium explored how humans could conduct lunar astronomy and how the motions of Earth might be studied from the viewpoint of the Moon. The achievement of human flight to the Moon was still another 3 centuries away, but Kepler declared that it was scientifically possible to go there and to ask what we might discover and do when we get there.

The two enduring questions—How far from Earth can humans go? What can humans discover and achieve when we get there?—lead to more specific questions that are more closely linked to historically stated rationales for the U.S. human spaceflight program, such as these:

  • Does humanity have a long-term sustainable future beyond Earth?
  • What are the limits of human adaptability to environments other than Earth?
  • Can humans exploit off-Earth resources for humankind?
  • What can human exploration of celestial bodies—such as the martian system, the Moon, and asteroids—uniquely teach us about the origin of the solar system and the existence of life?
  • How can human spaceflight enhance national security, planetary defense, international relations, and other national and global goals?

In the world of policy-making, where day-to-day pressures and changes in leadership can cause abrupt shifts of direction, enduring questions can help to provide a compass to maintain stability over the long period needed to achieve challenging goals. Similar questions from other fields include these: What is the cure for cancer? How did the universe begin?4 Enduring questions can create a reference frame for comparing past, present, and future policy and for analysis of contemporary applications of it. By asking How far from Earth can humans go? and What can humans discover and achieve when we get there?, the United States can address the fundamental constraints on human exploration at any given time and consider what approaches can contribute to opening up the horizon for that exploration. Continued focus on whether a project or program helps us to answer these questions, or to

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4 National Research Council (NRC), Connecting Quarks to the Cosmos: Eleven Science Questions for the New Century, The National Academies Press, Washington, D.C., 2003, p. 60.



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