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Executive Summary CHARGE TO THE COMMITTEE AND BACKGROUND Performing experiments in low Earth orbit has been the focus of much of the research funded by NASA's Physical Sciences Division (PSD) and its predecessors for over 30 years. This microgravity research can be divided into five broad areas, all of which focus primarily on phenomena that are strongly perturbed by gravity: biotechnology, combustion, fluid physics, fundamental physics, and materials science. To these disciplines, the Physical Sciences Division is considering adding research in such emerging areas as biomolecular physics and chemistry, nanotechnology, and research in support of the human exploration and development of space (HEDS). In response to a request from NASA, the Committee on Microgravity Research produced a phase I report (NRC, 2001), in which it proposed criteria for selecting additional research in these new areas and set forth a mission statement for the PSD. The present report is the phase II report. In it, the committee identifies more specific topics within the emerging areas on which the PSD can most profitably focus. The committee also assesses the past impact and current status of the PSD's research programs in combustion, fluid physics, fundamental physics, and materials science and gives recommendations for promising avenues of future research. At NASA's request the committee did not address work in the biotechnology area, as that area had been the subject of a recent review (NRC, 2000a). In assessing the impact of the work, the committee considered the following points: The contribution of important knowledge from microgravity research on a given topic to the larger field of which the research is a part; The progress made by microgravity research in answering the questions posed on each topic; and The potential for further progress in each area of microgravity research. Areas of future research in the existing disciplines are recommended, and guidance is given for setting priorities across these areas and within the emerging areas. The scientific impact of the existing 1

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2 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA disciplines, which was assessed by addressing the three indicators listed above, was a particularly important consideration when establishing priorities across the existing microgravity programs. The microgravity program has evolved considerably since its inception as the materials processing in space program of the Skylab era. With the exception of the biotechnology program (NRC, 2000a), in the early 1990s a major emphasis was placed on outreach to the science communities of which the microgravity disciplines were a part. This outreach took the form of biannual conferences in each of the disciplines prior to the release of a NASA Research Announcement (NRA) and an extensive canvasing of the community with notification of the opportunity to apply for support. The result was much greater visibility for NASA's combustion, fluid physics, fundamental physics, and materials science programs within the larger fields of which they are a part, and an increase in the number of proposals submitted. The impact of this outreach became clear as the committee assessed the quality of the investigators and research in the NASA program. The early 1990s also saw the establishment of the fluid physics and combustion programs in their current forms and then, in the past 5 years, an expansion of the fundamen- tal physics program. More recently, the PSD has begun to expand beyond the traditional microgravity- related disciplines to include research in which gravity may have no role, such as biomolecular physics and nanotechnology. The recent financial problems of the International Space Station (ISS) have brought a major uncer- tainty to the future of the microgravity program. Many of the facilities that were destined for the ISS have been delayed, and the crew time available for science has been drastically curtailed (NRC, 2003~. This financial crisis has also affected the ground-based research program. Whether this is a temporary setback or the beginning of the end of the microgravity program remains to be seen. Given the uncertainty, the committee did not consider what ISS resources would or would not be available when it formulated its findings and recommendations. IMPACT OF MICROGRAVITY PROGRAM In assessing the impact of the PSD-funded work in each of the existing microgravity disciplines (except, as mentioned, biotechnology), the committee employed a number of metrics. These included analysis of the citations received by papers, the citation rates for publications of research results, the prominence of the journals in which results were published, the changes to standard textbooks that resulted from research findings, documented influence on industry or NASA applications, and the fraction of principal investigators selected as fellows of various societies, elected as members of the National Academies of Engineering or Science, or chosen for other recognition such as awards in their field. Below is a partial listing of the research topics that have had an impact on their respective field: The fluid physics research program has produced a large body of significant research in areas ranging from flows due to surface tension gradients to the dynamics of complex liquids with important applications to industrial processes such as oil recovery and to NASA flight technologies. The unique access to space provided by NASA has led to the development of ground-based and flight research programs that have enabled growth and advancement of research in such fields as thermocapillary flow, and it has attracted leading investigators to the program, including members of both the National Academy of Sciences and the National Academy of Engineering, as well as numerous fellows of professional societies. The combustion research program has made important contributions to the fundamental under- standing of such combustion behavior as the chemical kinetics of flames and flame length variation,

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EXECUTIVE SUMMARY resulting in the correction of both basic theory and college textbooks. The results of studies on smoldering, flame spread, radiative transfer, and soot production not only have led to changes in spacecraft fire safety procedures, but also have advanced knowledge about some of the most important practical problems in combustion on Earth. Some of these results are already being incorporated into industry applications such as aircraft combustor design. The NASA combustion program currently supports some of the most distinguished combustion scientists in the world, including members of the National Academy of Engineering and numerous fellows of professional societies. The fundamental physics research program has made important contributions to both basic theory and the practice of research in such areas as critical point physics and optical frequency measurement, and the work of its investigators is published frequently in the leading scientific journals. Access to the space environment enabled a definitive test of the widely applicable renormalization group theory, while ground-based research sponsored by the program led to an orders-of-magnitude reduction in the labor, physical infrastructure, and time needed for scientists around the world to perform optical fre- quency measurements. The program has attracted high-caliber talent, including six Nobel laureates and over two dozen investigators who are either members of the National Academy of Sciences or fellows of professional societies. Research in NASA's materials program has led to major theoretical insights into solidification and the crystal growth process and has resulted in both the verification and refutation of classical theories predicting materials solidification behavior and microstructural development. Much of this work also has direct relevance to important commercial processes such as casting and semiconductor production, and research results have been utilized by such diverse industries as metal-cutting tool production (to improve a production process responsible for hundreds of millions of dollars in annual costs) and jet engine manufacturing. Investigators have received numerous prestigious awards for their work in this program, and a high percentage of them are professional society fellows and members of the National Academy of Engineering and National Academy of Sciences. ~ ~ , ~ ~ HIGH-PRIORITY MICROGRAVITY RESEARCH Listed below are the areas of research judged by the committee to have a high priority within each microgravity discipline. It should be kent in mind that there are numerous additional areas of promising . . . ~ . A. . . . - 1- ' , . . . . . . .. . - . .. 1- - - O research In each of the fields that were not given the highest priority at this time and thus were not explicitly recommended. Some of these areas might achieve a higher priority in the future. In addition, the committee expects that in future years the communities will generate new research topics that will be as promising as those recommended here. ~7 ' 1 ~7 1 Fluid Physics Fluid physics should continue to play a dual role in NASA's physical sciences research program. For scientists in general, the program provides access to a unique laboratory that permits the isolation and study of the effects of nongravitational forces on fluid behavior. For NASA, the program provides the basis for acquiring knowledge necessary for the development of the next generation of mission- enabling technologies essential to NASA's human exploration and development of space. The recom- mended areas of research are these: 1 For which the Nobel Prize in physics had previously been awarded.

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4 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Multiphase flow and heat-transfer technology. This is a critical technology area for space exploration and a sustained human presence in space (NRC, 2000b) and is relevant to numerous terres- trial technologies. Self-assembly and crystallization. Such research is expected to advance fundamental knowledge of phase transitions and lead to innovation in terrestrial technologies for example, the fabrication of novel materials such as photonic crystals. Complex fluid Theologies. The behavior of complex fluids, such as the particle dynamics and segregation flows of dry granular materials or magnetorheological fluids, is important to technologies needed for NASA's HEDS efforts as well as to numerous industrial applications. Interfacial processes. Surface-tension-related phenomena are important for a number of mis- sion-related technologies, and the microgravity environment offers experimentalists expanded length scales on which to observe interracial phenomena compared to Earth. Wetting and spreading dynamics. Experimental and theoretical research in these areas is neces- sary for improved understanding of thin-film dynamics in a variety of applications from coating flows to boiling heat transfer. Capillary-driven flows and equilibria. Capillary-driven flows and transport regimes associated with evaporation and condensation are important for both terrestrial and space-based applications. Coalescence and aggregation. Research on the effects of gravity (and its absence) on coales- cence and aggregation is necessary for HEDS since these processes are important to power and life support systems. Cellular biotechnology. Improved understanding of transport processes in bioreactors is impor- tant for HEDS medical applications and could lead to significant advances in the biological sciences and the biotechnology industry by improving the ability to control tissue and cell growth. Physiologicalilows. Fluids research in connection with biomedical applications (both terrestrial and space-related) will be necessary, for example, to better define paths to effective countermeasures for bone loss in microgravity and to explore the behavior of red blood cells in suspension. Combustion The microgravity combustion research program has been driven by two objectives: (1) a need to understand those physical phenomena thought to be relevant for spacecraft fire safety and (2) a desire to deepen knowledge of fundamental combustion processes on Earth. Both of these objectives are ad- dressed by the following high-priority research: Development of computer simulations of Acre dynamics on spacecraft. Earth-based fire protection techniques have evolved through thousands of years of fire-fighting experience. Since there is no such experience base for space fires, physics-based computer simulations are the only alternative. Such simulations have also proved to be of great value in assessing fire safety and control strategies for fires on Earth. Research on ignition, flame spread, and screening techniques for engineering materials in a microgravity environment. The goal of the research is the development of a science-based method for determining the fire performance of materials that are candidates for use in space. The results would also be directly usable in the space fire computer simulation codes referred to above. The two programs taken together would provide a major advance in the understanding of fires in space and in the ability to mitigate their consequences. Safety of oxygen systems. One of the critical systems on the ISS and other space, lunar, and

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EXECUTIVE SUMMARY s planetary habitats is the oxygen generation and handling system. Thus an understanding of the dynam- ics and extinguishment of fires involving oxygen is necessary. Smoldering combustion. Smoldering and transition to flaming combustion are significantly different in microgravity than on Earth and thus require additional studies. Soot and radiation. Basic processes that lead to the formation and emission of small carbon particles in high-temperature combustors remain to be understood, and radiation heat transfer has many critical implications for fire safety. Turbulent combustion. Turbulence in general and turbulence in the presence of combustion are exceedingly difficult phenomena to model and understand. Nevertheless, most industrial combustion devices and natural fires involve turbulent combustion, and thus the potential impact of this work is large. Chemical kinetics. The chemical kinetics and reaction mechanisms of practical fuels and fuel blends of interest to industry remain unknown. Nanomaterial synthesis in flames. Flames provide an inexpensive means of producing nano- particles for mass use. The work to date has generally been empirical, and opportunities exist for understanding the chemical composition and thermal structure of the flow that is conducive to synthesis of the desired forms of materials. Fundamental Physics In fundamental physics, the committee gave high priority to the successful execution of the specific experiments that have already been selected for flight on the ISS. These experiments will test important fundamental principles in physics, and in most cases an experiment's success would end any further need for space experimentation in that area. These already-selected experiments, along with new areas that have been given high priority, are as follows: Currently Selected ISS Experiments Low-temperature experiments. The results of the four planned experiments, along with the results of experiments that have already flown, are expected to provide a full picture of the equilibrium behavior of systems near critical points, including the role of boundaries and the dynamical response to perturbations. Relativity and precision clock experiments. The results of these experiments are expected to substantially improve the precision and stability of atomic clocks. Other NASA clock application experiments. By flying other types of clocks simultaneously with the atomic clock experiments, such fundamental ideas as the Einstein weak equivalence principle can be tested. New Areas Antimatter search and measurements. A positive identification of heavy antimatter would be highly significant for astrophysics and cosmology. Elemental composition survey. Measurement of the cosmic-ray elemental composition up to and beyond the "knee" in the cosmic-ray spectrum should provide the best clues to the origin of cosmic rays. Materials Science Materials science has played a central role in many of the discoveries that have shaped our world, from integrated circuits to low-loss optical fibers and high-performance composite materials. These

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6 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA research areas, which also contain many subdiscplines, will continue this tradition of science-driven discoveries of great importance to both the nation and NASA: Nucleation process within, and properties of; undercoated liquids. The nucleation process plays a prominent role in setting materials properties. Currently the conditions governing the nucleation of stable and metastable phases are not well understood. Dynamics of microstructural development during solidification. The ability to directly link processing conditions to the resulting materials properties is still not at hand because the mechanisms governing the development of microstructure during solidification are not well understood. Morphological evolution of multiphase systems. The properties of a material are linked to the size, shape, and spatial distribution of the component phases. Understanding the morphological evolu- tion of these systems will allow prediction of the manner in which the properties of a material evolve. Computational materials science. It is now possible to design a material using simulations to obtain a desired set of properties. This capability will create a new paradigm for designing industrially relevant materials because the materials will be created with a minimum of costly, time-consuming experiments. This approach can have a significant impact on NASA as it ensures that the desired materials properties of interest to NASA will be attained, and in a greatly reduced time and at lower cost. Thermophysical data of the liquid state in microgravity. Accurate thermophysical data for the liquid state is required for computational modeling of materials processing. Nanomaterials and biomimetic materials. There are many promising avenues for materials research at the nanoscale and at the interface between the biological and materials sciences. These new directions are discussed in Chapter 7, "Emerging Areas," and are listed below. HIGH-PRIORITY RESEARCH IN THE EMERGING AREAS Emerging technologies, particularly at the confluence of the biological, physical, and engineering sciences at the nanoscale, offer NASA an ideal opportunity to address its own technology needs by leveraging knowledge gained from the worldwide investments in these fields. NASA should stay in a position to capitalize rapidly on anticipated advances in nanotechnolo~v This includes huildin~ and O. O maintaining scent In-house expertise and ensuring that the ~~o reaches out to new communities since many disciplines are involved, including physics, chemistry, biology, materials science, medical science, and engineering. Important technologies for fabricating new materials and devices will origi- nate from novel approaches to molecular assembly, combined with nano- and microfabrication tools and the exploitation of design principles inspired by nature. The following topics were identified by the committee as the most promising areas of future research relevant to NASA's needs and PSD capabili- ties: Methods for long-term stabilization of proteins in vitro. Long-term preservation of protein function is essential to the utilization of proteins in space in sensors, for diagnostics, and in bioreactors on extended flight missions. Cellular responses to gravity-mediated tissue stresses. Developing a mechanistic understanding of how applied loads and stresses affect cellular processes and the underlying molecular processes will lead to a better understanding of the impact of low-gravity conditions on human health. Technologies to produce nanoengineered hybrid materials with multiple functions. Investments in nanoengineered materials consisting of diverse molecular species or phases, or hybrid materials,

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EXECUTIVE SUMMARY 7 could provide NASA with new materials that can sense, respond, self-repair, and/or communicate with the user. Integrated nanodevices. Emerging technologies for engineering micro- and nanodevices able to sense, process acquired data, and take action based on sensory inputs could contribute significantly to achieving NASA's goals. Power generation and energy conversion. Nanotechnology promises to increase the efficiency of energy conversion, decrease weight, and increase the overall energy density for energy storage. Knowledge base for stabilizing cell function in vitro. Efforts to stabilize cells may represent an effective strategy for producing needed cell types to meet emergencies on demand while eliminating the need to keep an extensive inventory of cell types available in space. RESEARCH PRIORITIES AND PROGRAM DIRECTIONS In order to assess and compare research across the microgravity disciplines, the committee critically examined the potential impact of the research on the scientific field of which it is a part, on NASA's technology needs, and on industry or other terrestrial applications. The committee's evaluation of research in each of these categories is expected to assist NASA program planners by providing the insight into likely risks and potential rewards of the research necessary to create a vibrant microgravity research program that has an impact in all of these areas. Because of the brief history and rapid development of the fields of research in the emerging areas, it was not possible to evaluate research in those areas using the same criteria applied to the research in combustion science, fluid physics, fundamental physics, and materials science. While the likelihood that PSD-funded research in emerging areas will have significant impacts on NASA capabilities cannot be evaluated at this time, the magnitude of the impact of successful research is potentially very high. Therefore the committee ranked the research topics in the emerging areas only relative to each other and suggests that the PSD utilize this prioritization to help allocate funds that have been set aside for these emerging areas. Prioritizing Microgravity Sciences Research When comparing research across disciplines, the committee considered only those areas already identified above as having a high priority for one of the disciplines. To evaluate the recommended research areas, the committee separately judged the likelihood that the research would have a significant impact in (1) the scientific field of which it is part, (2) industry or terrestrial applications. and (31 NASA technology needs. ~ Within each of these categories the committee looked specifically at both the magnitude of the potential impact that the research would have on that category, and the likelihood that the research would be successful in achieving that impact. The impact and probability of success were assessed independently of each other since it was possible for areas with a potential for high impact to have a low probability of success and vice versa. The results of the committee's assessment are plotted in Figures ES.1, ES.2, and ES.3. Note that the setting of actual research priorities must depend on NASA's programmatic goals and that those goals determine both the desired end result, such as scien- tific discovery, and the level of acceptable risk. The purpose of these plots, then, is to provide NASA with tools that it can use to rationally select the best research, regardless of which combination of scientific discovery (Figure ES.1), terrestrial applications (Figure ES.2), or NASA technology needs (Figure ES.3) NASA chooses to emphasize or what trade-offs between research risk and reward it is willing to accept.

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8 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Most, Important IL o ~ O _ ~ F ~ Z ~ Important ~ ~ 43 ~ ,~ , 3c ~ 1 ~ 15 10 Low PROBABILITY OF ACHIEVING IMPACT 18 ( 14C ' / High FIGURE ES.1 Assessment of research topics in terms of their likely impact on scientific knowledge and under- standing. Most Important IL o ~ O _ ~ _ ~ z Important ~ ( ) Low PROBABILITY OF ACHIEVING IMPACT High FIGURE ES.2 Assessment of research topics in terms of their likely impact on terrestrial applications such as industry's technology needs.

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EXECUTIVE SUMMARY Critical Most Important IL o ~ O _ ~ _ ~ z Important ~ it: ~ As' J Low PROBABILITY OF High ACHIEVING IMPACT ) J FIGURE ES.3 Assessment of research topics in terms of their likely impact on NASA's technology needs. 9 FIGURES ES. 1, ES.2, and ES.3: Only subjects already considered by the committee to be of high priority in at least one discipline are included in this analysis, and therefore the magnitude scale ranges only from important to very important (or critical). A subject may not have a high impact in every category and therefore may not appear in every figure. Numbers inside the same circle should be considered to occupy approximately the same position in the figure. The numbers in the figures represent the research topics as follows: 1. Multiphase flow and heat transfer; 2. Complex fluids: (a) self-assembly and crystallization, (b) complex fluid theologies; 3. Interfacial processes: (a) wetting and spreading, (b) capillary-driven flows and equilibria, (c) coalescence and aggregation (liquid phase); 4. Biofluid dynamics: (a) cellular biotechnology, (b) physiological flows; 5. Turbulent combustion; 6. Chemical kinetics; 7. Soot and radiation; 8. Smoldering combustion; 9. Development of computer simulations of fire dynamics on spacecraft; 10. Oxygen systems fire safety; 11. Ignition, flame spread, and screening techniques for engineering materials; 12. Antimatter search/measurements; 13. Elemental composition survey; 14. Complete the current set of fundamental physics ISS experiments: (a) low-temperature experiments, (b) relativity and precision clock experiments, (c) other NASA clock application experiments; 15. Nucleation process within, and the properties of, undercoated liquids; 16. Dynamics of microstructural development during solidification; 17. Morphological evolution of multiphase systems; 18. Computational materials science; 19. Collection of thermophysical data of liquid state in microgravity.

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10 ASSESSMENT OF DIRECTIONS IN MICROGRAVITY AND PHYSICAL SCIENCES RESEARCH AT NASA Priorities in the Emerging Areas All of the areas recommended below satisfy the criteria identified in the phase I report for choosing research in the emerging areas (NRC, 2001~. The development of methods for the long-term stabiliza- tion of proteins in vitro and research on cellular responses to gravity-mediated tissue stresses are of higher priority than the others, because these areas are not typically supported by other agencies. The research on exploiting nanotechnology for power generation and energy conversion is also ranked "most important" because of the great importance of power generation and energy conversion in NASA's spaceflight program and the major impact these technologies may have on this program. The remaining areas, ranked as important, are heavily supported by agencies such as the Defense Advanced Research Projects Agency, the Department of Energy, the National Science Foundation, and the Department of Defense as well as by other divisions within NASA. Thus the PSD should partner with these agencies or other divisions within NASA to pursue such research. In the past, the PSD has successfully partnered with other agencies, such as the National Cancer Institute. The recommended topics are given below. Note that these are not rank-ordered within each category. Most Important Develop methods for the long-term stabilization of proteins in vitro. Work on understanding cellular responses to gravity-mediated tissue stresses. Exploit nanotechnology for power generation and energy conversion. Important lions. Develop enabling technologies to produce nanoengineered hybrid materials with multiple func- Develop integrated nanodevices. Develop methods for stabilization of cellular function in vitro. Program Balance When considering the question of the overall balance within the PSD between microgravity re- search and research in the emerging areas, the committee looked at several factors. These included the degree of support received by topics in emerging areas from other government agencies and other divisions within NASA, the considerable potential of the microgravity research disciplines to yield important new results, the potentially high impact of successful research in emerging areas, and the ability of the PSD to provide unique resources or knowledge. These and other factors argued for a balanced PSD program of research that retains the unique potential for studying the effects of gravity on phenomena in combustion, fluid physics, materials, fundamental physics, and biotechnology topics such as tissue culturing. The committee concluded that the proportion of the physical sciences program devoted to the emerging areas should remain relatively modest, perhaps 15 percent of the program, until such time as a clear justification arises for increasing its size. This fraction of the program should allow NASA to have an impact on a limited number of highly focused topics within the broad emerging areas while leveraging the research of other agencies. It would also permit the majority of the research in the microgravity areas to continue to produce the high-impact results described in the discipline chapters.

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EXECUTIVE SUMMARY 1l Peer Review The committee has commented numerous times in past studies on the role that rigorous peer review has had in greatly improving the quality of the research funded by the Physical Sciences Division, and strongly recommended its continued use in future funding selections (NRC, 1994, 1997, 2000b). As the program moves into new areas of research it is worth emphasizing again that any research proposal submitted to the program no matter how relevant to an area considered highly desirable for inclusion in the program should be funded only if it has undergone a rigorous peer review and has received both high marks for scientific merit and a high ranking compared with competing proposals. REFERENCES National Research Council (NRC), Space Studies Board. 1994. "On Life and Microgravity Sciences and the Space Station Program," letter from SSB Chair Louis J. Lanzerotti, Committee on Space Biology and Medicine Chair Fred W. Turek, and Committee on Microgravity Research Chair William A. Sirignano to NASA Administrator Daniel S. Goldin (Febru- ary 25~. National Research Council, Washington, D.C. National Research Council, Space Studies Board. 1997. An Initial Review of Microgravity Research in Support of Human Exploration and Development of Space. National Academy Press, Washington, D.C. National Research Council, Space Studies Board. 2000a. Future Biotechnology Research on the International Space Station. National Academy Press, Washington, D.C. National Research Council, Space Studies Board. 2000b. Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies. National Academy Press, Washington, D.C. National Research Council, Space Studies Board. 2001. The Mission of Microgravity and Physical Sciences Research at NASA. National Academy Press, Washington, D.C. National Research Council. 2003. Factors Affecting the Utilization of the International Space Station for Research in the Biological and Physical Sciences. The National Academies Press, Washington, D.C., in press.