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An Initial Review of Microgravity Research in Support of Human Exploration and Development of Space Executive Summary The current organizational structure of NASA includes five strategic enterprises, one of which is the Human Exploration and Development of Space (HEDS). Goals set by the HEDS enterprise include (1) increasing knowledge of nature's processes by use of the space environment; (2) exploring and settling the solar system; (3) achieving routine space travel; and (4) enriching life on Earth through people living and working in space. The means by which NASA proposes to accomplish these ambitious goals include a combination of scientific research, engineering technology development, and use of the Space Shuttle and the International Space Station (ISS) as microgravity test platforms. The first objective stipulated within NASA's HEDS Goal 1 is that scientific research should be conducted to understand the roles played by gravity and the space environment in affecting the behavior of biological, chemical, and physical systems. The second objective within HEDS Goal 1 specifies the innovative use of major HEDS facilities, such as the Space Shuttle and the ISS, to achieve breakthroughs in science and technology. This preliminary report of the Committee on Microgravity Research examines those areas of microgravity research that not only support the objectives of Goal 1, but also have the potential to contribute to the eventual development of the new technologies required to accomplish the remaining HEDS goals. An initial appraisal is made of types of exploration technologies that, for development, would require an improved understanding of fluid and material behavior in a reduced-gravity environment. The current microgravity research program at NASA's Microgravity Research Division (MRD) includes five major disciplines: (1) fluid physics, (2) materials science, (3) combustion, (4) biotechnology, and (5) fundamental physics. In general terms, fluid physics research encompasses the phenomena of heat and mass transport in low gravity and underlies many of the scientific and technological problems associated with long-duration crewed missions exploring the Moon and inner planets. A strong emphasis remains within the MRD program on experimental microgravity fluids studies—as opposed to reliance on computational fluid dynamics (CFD)—because the boundary conditions
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encountered in many reduced-gravity fluid physics studies are less well understood than in conventional subfields of aerospace research. For example, relatively weak forces, such as thermocapillary tractions and van der Waals interactions, which may be ignored in most terrestrial flow problems, can become dominant in microgravity. Materials science research in the MRD program tends to be focused on basic subjects such as nucleation and growth of solids from melts and on the evolution of microstructures—especially those involving one or more fluid phases. These include the formation of crystal defects and solute segregation in single-phase processing, such as semiconductor crystal growth, as well as research aimed at achieving a better understanding of polyphase microstructures, such as occur in eutectics and monotectics. Microgravity materials research extends to practically important processes such as reaction synthesis and sintering, welding and solidification, and in situ resource utilization (ISRU) for producing structural materials from extraterrestrial bodies. Such materials processes seem particularly relevant to technologies contemplated for future HEDS missions. Microgravity combustion research within the MRD—especially studies on fire safety research at the fractional gravity levels found on extraterrestrial bodies or studies under microgravity as encountered in spacecraft environments during deep-space transit—is critically needed to ensure safety on future HEDS missions, where crew egress might not be an option. Such research includes studies on flammability limits, smoldering, flame spread, and flame stability—all of which contribute both to scientific knowledge and to the engineering know-how needed for successfully pursuing the HEDS goals. Research in microgravity biotechnology is considered essential for understanding and designing reliable life-support systems, for producing nutrients and food for crews during long-duration HEDS missions, and for safely and reliably recycling waste aboard spacecraft for water and oxygen recovery. Current MRD studies include activities on cell culture and bioseparations, which will contribute critically to understanding biological options for nutrient production in spacecraft as well as waste recycling. Low-temperature and atomic physics research using microgravity generally probes certain extreme physical limits in both classical and quantum systems. Research on laser cooling of atoms in microgravity can contribute directly to the development of improved navigational systems for achieving safe, efficient deep-space travel by providing practical atomic clocks with greatly increased accuracy. Although this initial report identifies the general areas of research discussed above as having the potential to make long-term contributions to HEDS technology development, the committee has attempted to prioritize neither the research nor the affected technologies, in part because NASA is currently still in the early stages of identifying its technology needs. As these needs become more clearly defined, it should be possible to identify research that can be profitably emphasized, although the need for flexibility in HEDS mission planning suggests that a strict prioritization of research is likely to remain counterproductive. Nevertheless, it is possible at this early stage to provide a number of initial recommendations, primarily programmatic, derived in the course of this review of microgravity research in support of Human Exploration and Development of Space. MRD should, on a continuing basis, assist NASA in identifying critical
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technologies that would benefit future HEDS missions and then seek opportunities in microgravity research to contribute to their efficient realization. MRD should, however, remain both flexible and cautious in evaluating such opportunities. Major advances in technology can result from basic research undertaken without regard to current technological priorities, which have yet to be even identified. In addition, the timing of such technological advances is often unpredictable. In supporting HEDS, MRD should continue to focus on maintaining its broad program of microgravity research. Although not all of the technological advances needed for HEDS missions will be the direct result of basic research, the unfolding knowledge base and collective experience of microgravity investigators focused within the MRD program will continue to represent unique NASA resources with which to approach the scientific questions underlying many of the barriers to space exploration. MRD should be prepared to stimulate and support critical microgravity research to help discriminate among competing HEDS technologies, specifically providing information so that NASA can make informed choices among them. The process of gathering and exchanging information relevant to research selections that could support HEDS missions should be strengthened. Specialized workshops, cross-divisional teams, advisory panels, and study groups attended by mission technologists and microgravity scientists are among the suggested mechanisms for achieving this recommendation. Such activities would encourage the exchange of ideas between technologists and scientists, provide better communication and ongoing awareness of the technology needed for MRD, and also allow timely transfer of microgravity research findings to HEDS technologists. The goals of HEDS involve the development of complex technological systems that require integration of microgravity information derived from research in disparate fields of science. MRD may find it advantageous to initiate a limited number of cross-disciplinary projects to develop experience in selecting and managing research projects that operate across traditional boundaries of the microgravity science disciplines. Some HEDS missions will involve operating systems at fractional gravity levels, such as the 0.16 Earth gravity encountered on the Moon or the 0.37 Earth gravity encountered on Mars. It is, however, often unclear as to whether or not thresholds of the gravity level exist at which various physical, chemical, and biological phenomena and processes undergo change. MRD should consider giving more attention to research studies carried out at fractional gravity levels where HEDS technologies might directly benefit from the scientific advances. Knowledge generated from such studies could be used to evaluate the need to provide artificial gravity by using continuous spacecraft rotation. Ongoing investments by NASA in robotics and automation research
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are expected to benefit both manned and unmanned HEDS missions, which must operate sufficiently far from Earth that highly autonomous operations and control become necessary because of the long transit time of signals. MRD should ensure that microgravity issues in teleoperations and robotics research are given sufficient attention and should maintain an active and current awareness of these issues. The International Space Station (ISS), when available for scientific use shortly after the beginning of the next millennium, should provide MRD with unique long-duration microgravity opportunities for evaluation of technical systems deemed important to future HEDS missions. MRD should take advantage of the ISS as a microgravity platform for investigating closed-cycle, long-term operation of various physical, chemical, and biological systems considered to be within its research purview. In view of the normally long time-scale needed for the evolution of basic scientific concepts into practical applications, MRD should begin now to study and understand the scope and long-term implications of microgravity research areas relevant to accomplishing HEDS goals. Any adjustments to the emphasis or scope of MRD research must then be carefully assessed with respect to overall program balance, scientific merit, external interest, and HEDS mission relevance. The systematic and periodic application of NASA Research Announcements (NRAs) and peer review has improved the quality and selection of the science supported by MRD. These benefits to NASA and the nation are so extensive that these mechanisms should be preserved to ensure scientific objectives that support and enhance the HEDS enterprise. The recent inclusion of a call for research on ISRU and two-phase flow in the 1996 NRAs for materials science and fluid physics is commended as timely and responsive.