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Microgravity Research Opportunities for the 1990s: Executive Summary Microgravity Research Opportunities for the 1990s Executive Summary INTRODUCTION Microgravity research is concerned with the effects of reduced gravitational forces on physical, chemical, and biological phenomena. The scientific disciplines affected by gravity include fundamental physics, fluid mechanics and transport phenomena, materials science, biological sciences and biotechnology, and combustion. It is especially noteworthy that these disciplines are laboratory sciences that inherently use controlled, model experiments. Many experiments require constant attention and frequent intervention by the experimenter, which distinguishes microgravity research from the observational space sciences. Microgravity research also spans both fundamental and applied sciences. REPORT MENU Reduced-gravity experimental environments are provided by NASA NOTICE through drop towers, aircraft in parabolic trajectories, sounding rockets, and Earth- MEMBERSHIP orbiting laboratories. Some of these environments have crews and allow PREFACE extended periods of time for experimentation and for demonstrating the EXECUTIVE SUMMARY reproducibility of results. Some of the experimental platforms allow a reduction of PART I the gravity level to 10-6 times that of Earth gravity. CHAPTER 1 CHAPTER 2 PART II In a reduced-gravity environment, the decreases in rates of CHAPTER 3 sedimentation, hydrostatic pressure, and buoyancy-driven flows cause other CHAPTER 4 physical effects to become more important and more readily observable and CHAPTER 5 measurable. The acceleration due to gravity can then be treated as an important CHAPTER 6 and interesting experimental parameter. The exploration of this parameter, CHAPTER 7 through experiments at normal Earth gravity and at reduced gravity, may provide PART III a better understanding of certain physical processes, as well as lead to the CHAPTER 8 identification of new phenomena. APPENDIX A APPENDIX B The NASA microgravity research program has been widely misperceived as simply a materials processing program with the goal of providing better products for use on Earth. However, the prospects of commercial manufacture in space are limited in the foreseeable future. The justification for the microgravity research program must continue to be the promise of advances in areas of file:///C|/SSB_old_web/mgoppes.htm (1 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary fundamental and applied science. It should be recognized, on the other hand, that manned exploration of space will require increased understanding of transport phenomena, materials processing, and performance in microgravity. There have been a number of previous National Research Council (NRC) reports in this field, of which the following are particularly relevant: Materials Processing in Space1 (the "STAMPS" report, 1978) attempted to provide guidance for the future course of NASA's program. It assessed the scientific and technological underpinnings of the materials processing in space program and provided a clear understanding of the potential for exploitation of the space environment for processing materials. Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015. Fundamental Physics and Chemistry2 (1988) attempted to identify opportunities for future research efforts in relativistic gravitation and microgravity science (but not including applied research or industrial/manufacturing processes). More recently, the report Toward a Microgravity Research Strategy3 (1992) began to lay a foundation for a more mature research program, and the current report is a continuation of that effort. The research areas discussed in this report are subsets of much broader research disciplines. Only a small fraction of the total activities in each discipline occurs within the microgravity program, and no attempt has been made here to evaluate fully or to prioritize all of the research in a discipline. Only the microgravity component of each discipline is addressed in detail. Furthermore, the cost-benefit of a microgravity program has not been compared to the cost- benefit of experiments on different subjects in the terrestrial environment. Experiments that can be performed adequately under terrestrial conditions, however, are not given a priority for spaceflight. It should be understood at the outset that in evaluating the costs of doing research on such expensive vehicles as a space station, vehicle expenses are specifically not included. Microgravity research has never been the sole motivation for the space shuttle or other major space missions. These research opportunities have usually been secondary to exploratory, technological, engineering, political, educational, inspirational, and other motives for spaceflight, and until the advent of spacelab, microgravity research has been added on spaceflights launched for other purposes. Thus, in evaluating priorities, the research opportunities are taken into account but not the costs. To date, only a limited number of microgravity experiments have been conducted in space with completed analyses and reports of results. The total experience of U.S., Canadian, Japanese, and Western European scientists is less than 1000 hours for experiments in orbit. Because of the limited results available, the strategic recommendations in this report cannot be highly detailed file:///C|/SSB_old_web/mgoppes.htm (2 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary or exclusive. A number of subjects require further exploratory investigation before detailed objectives can be defined. For the same reasons, it is currently not generally possible to set specific priorities across discipline lines. Assuming experiments are scientifically sound, prioritization across disciplines is largely unnecessary because a wide range of microgravity experiments can be accommodated on shuttle flights without retarding progress in any one discipline. It may even be a mistake to attempt such prioritization since, in many cases, fundamental flight experiments are needed before prioritization can be attempted. Some areas, however, can be identified as more promising than others. This report does not address the NASA commercial program or international programs in microgravity research. These topics will be addressed in future studies of this committee. Finally, although the value and need for human intervention capabilities and long-duration flights are noted repeatedly in this report, nothing herein should necessarily be interpreted as advocating or opposing any specific NASA spacecraft or space station design initiative. SCOPE OF THE RECOMMENDATIONS The conclusions and recommendations presented in this report fall into five categories: (1) overall goals for the microgravity research program; (2) general priorities among the major scientific disciplines affected by gravity; (3) identification of the more promising experimental challenges and opportunities within each discipline; (4) general scientific recommendations that apply to all microgravity-related disciplines; and (5) recommendations concerning administrative policies and procedures that are essential to the conduct of excellent laboratory science. The overall goal of the research program should be to advance science and technology in each of the component disciplines. Microgravity research should be aimed at making significant impacts in each discipline emphasized. The purpose of this report is to recommend means to accomplish that goal. The essential features of the recommendations are to emphasize microgravity research for its general scientific and technological value, as well as its role in advancing technology for the exploration of space; to deemphasize the research value of manufacturing in space with the intent of returning products to Earth; to modify NASA's infrastructure, policy, and procedures so as to facilitate laboratory science in space; to establish priorities for microgravity experimentation in scientific disciplines and subdisciplines in accordance with the relative opportunities for scientific and technological impact; and to recognize fluid mechanics and transport phenomena as a central theme throughout microgravity research. The recommendations detailed in this report are based on certain findings that resulted from this study. The findings are as follows: file:///C|/SSB_old_web/mgoppes.htm (3 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary Science can be advanced by the study of certain mechanisms that are masked or dominated by gravitational effects at Earth gravity conditions. Examples include surface tension gradient-driven flows, capillary effects, multiphase flows, diffusive transport processes, and colloidal phenomena. Under terrestrial gravity conditions, these phenomena can often be dominated by effects such as buoyancy and sedimentation. There is a scientific need to understand better the role of gravity in many physical, chemical, and biological systems. To understand the relative importance of certain gravitational effects compared to nongravitational effects under terrestrial conditions, it is helpful to study a phenomenon at more than one gravitational level. Thus, gravitational force can become a controlled experimental parameter. For example, the role of buoyancy versus the role of surface tension gradients in driving a fluid flow could be examined at different gravitational levels. A significant portion of microgravity research programs should be driven by the technological needs of the overall space program. Examples of important engineering issues include the ignition, propagation, and extinction of spacecraft fires; the fluid dynamics and transport associated with the handling, storage, and use in space of water, waste, foods, fuels, air environments, and contaminants; the handling, joining, and reshaping of materials in space; and the dynamics and chemistry of mining or refining resources in extraterrestrial environments. Microgravity research primarily involves laboratory science with controlled, model experiments that inherently require attention and intervention by the experimenter. Such experiments also require the opportunity to demonstrate reproducibility. The potential for manufacturing in space in order to return economically competitive products to Earth is very small. Research on materials processing in microgravity, however, could prove to be important for materials science and materials processing technology on Earth. Fluid mechanics and transport phenomena represent both a distinct discipline and a scientific theme that impacts nearly all microgravity research experiments. The ground-based research program is critically important for the preparation and definition of the flight program. The need for an extended-duration orbiting platform has been identified as critical in many microgravity research experiments because of the time required for experimentation, the wide parametric ranges, and the need to demonstrate the reproducibility of results. In developing this report, it has been assumed that a space station will be available within a decade. The file:///C|/SSB_old_web/mgoppes.htm (4 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary recommendations have value and should be implemented, however, even if a space station does not materialize and the microgravity research program continues only with the current facilities. Although this report does not set out a complete strategy for microgravity research, it presents some of the important elements of such a strategy, including: A summary of the current state of knowledge of microgravity science; A discussion of some of the fundamental questions to be answered; A presentation of the goals in this field; The science objectives within each discipline; An evaluation of the potential for microgravity research to provide advances within each discipline; The experimental requirements for achieving the science objectives of each discipline; A description of the other resources required for a successful microgravity science program; and A limited prioritization of research topics within each discipline. The two aspects of a strategy for microgravity research that are not presented in this report are (1) a prioritization of microgravity research objectives across disciplines and (2) a cost-benefit analysis of anticipated microgravity results. Experience has shown how difficult it is to set research priorities, even within a single homogeneous science discipline. Reaching agreement on priorities for microgravity research relative to all other science research was judged so unlikely that it has not been attempted in this report. The other practical reason for not setting stricter priorities stems from the nature of shuttle flights. Frequently, payloads are assigned to a flight because their requirements for space, power, and so on, fit what is available, and scientific priority is less important. No cost-benefit analysis was attempted because the assumption is that orbiting platforms will be available for microgravity research. Decisions on the availability of platforms such as a shuttle or space station are essentially programmatic issues in which microgravity research is only one of many considerations. RECOMMENDATIONS FOR THE SCIENCE PROGRAM file:///C|/SSB_old_web/mgoppes.htm (5 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary The disciplines of microgravity research discussed in this report include fluids and transport, combustion, biological sciences and biotechnology, materials science, and microgravity physics. Materials science is further categorized by constituent subfields: metals and alloys, organic materials and polymers, inorganic crystals, epitaxial layer growth, and ceramics and glasses. The expected degree of scientific success in each of these areas naturally varies: General Priorities Fluids and transport, combustion, metals and alloys, microgravity physics, and certain areas of biotechnology offer the greatest likelihood for substantial advances. In these areas, focused experiments can be performed that will expand our knowledge of fundamental phenomena and processes. The areas of polymers, ceramics or glasses, inorganic crystals, and epitaxial layer growth, on the other hand, do not at present offer great promise of new advances. Limited support is still warranted, however, for certain fundamental studies. Certain areas of the biological sciences and biotechnology should be viewed from a different perspective. Much of the work in these areas is phenomenological. Such research should be pursued to discover the scope and potential for utilizing the microgravity environment. Areas Recommended for Emphasis Major prospects and opportunities in each of five disciplines are summarized in the following sections. Fluid Mechanics and Transport Phenomena Fluid mechanics and transport phenomena play a dual role in the microgravity research program. They stand as distinct disciplinary areas but also appear as themes running through other disciplines. The presentation in this report reflects the role of fluids and transport in the support of other microgravity disciplines. Most physicochemical transport phenomena are influenced significantly by gravity. As a consequence, unusual behavior is expected in low-gravity environments for many fluid configurations. Also, the reduction of gravitational forces leads to dominance by other forces normally obscured in terrestrial file:///C|/SSB_old_web/mgoppes.htm (6 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary environments, such as surface tension and electrohydrodynamic effects. Basic research is required to understand and describe the unusual characteristics of transport phenomena under low-gravity conditions. Transport phenomena also play essential roles in many processes that are important to mission-enabling technologies. Predictive models for low-gravity performance and operation of those technologies are frequently inadequate. New models are needed. Strictly empirical approaches are not preferred for low- gravity applications because they are costly and time consuming, and can result in products or systems that are unreliable or inefficient. Priority should be given to the study of phenomena that are prominent in the low-gravity environment and to those that are critical to space mission- enabling technologies and commercial developments. Among the basic topics that may be studied uniquely with special advantages in the low-gravity environment are the following: Surface tension gradient-driven flows and capillary effects. These are frequently obscured in a terrestrial environment but may become significant or dominant in reduced gravity. These topics warrant investigation because they are not well understood and because surface tension-driven flows are ubiquitous for many spaceflight-enabling technologies. Multiphase flows. Many processes involve multiphase flows. Gravity imposes a specific orientation on multiphase fluids and structures (e.g., gas- liquid, liquid-solid). In a reduced-gravity environment, multiphase flows and associated transport phenomena become significantly different because of the altered orientation of the various phases. Diffusive transport processes. At 1 g, multicomponent fluids experience various modes of thermosolutal convection. In addition, there are other effects due to different diffusivities for heat and mass. With reduced- buoyancy convection, these complex interactions can be separated and analyzed. Furthermore, transport effects masked at 1 g, such as Soret and Dufour phenomena, can become important. Colloidal phenomena. At 1 g, the nature of surface and short-range forces, and their consequences in colloidal systems, are often difficult to study because of the complications associated with competing gravitational effects. Microgravity provides an opportunity for study of colloidal systems in which short- range forces are dominant, which can contribute in an important way to the understanding of these physicochemical interactions. The above topics appear prominently in many areas of science, including combustion, biotechnology, materials science, and physics. Therefore, fluid mechanics and transport phenomena should be viewed as a common theme throughout many of the microgravity disciplines. file:///C|/SSB_old_web/mgoppes.htm (7 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary The following topics deserve attention for both their intrinsic scientific importance and their applications to space technologies: Convective processes at low Reynolds number. Investigation of low Reynolds number flows with density variations, at reduced gravity levels, will reveal the altered nature of transport phenomena in a new range of parametric conditions. Transport processes with a phase transition. The modified processes of condensation, evaporation, and boiling in a low-gravity environment with dominant interfacial forces require study. Complex materials. Porous, granular, and colloidal media, and foams are complex materials whose structures and functions can differ in microgravity. Research is necessary to understand the behavior of such materials in a low- gravity environment and may also lead to a better understanding of their behavior at 1 g. Materials processing. Buoyancy, sedimentation, and interfacial phenomena influence such processing methods as fluidized-bed hydrogenation, electrowinning, and vapor-phase pyrolysis and therefore should be investigated. Physical processes in life- and operating-support systems (enabling technologies). Some of the effects indicated above apply to processes such as power generation and storage, water purification, oxygen production, and fuel and fluid storage and management. Numerous parameters govern the topics listed above. Judicious choice of the parameter ranges and configurations will be required to ensure that the information obtained is directly applicable to the reduced-gravity environment. Clearly, fluids and transport phenomena appear as critical technical issues in many spaceflight-enabling technologies. Combustion Combustion involves fluid mechanics, mass and heat transport, and chemical reaction-all directly or indirectly subject to numerous gravitational effects. The number of parameters to be investigated is large. Several phenomena require long-duration observations and measurements (e.g., smoldering and flame spread). The following research areas are recommended for emphasis according to rank-order: file:///C|/SSB_old_web/mgoppes.htm (8 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary 1. The highest-priority area, and one of intense practical interest, is that of fires in spacecraft and potential extraterrestrial bases. Microgravity and reduced- gravity research is required because of the potential for disaster posed by fires. Much is still unknown about fires in altered gravity conditions. A variable- gravity capability would be useful for a full understanding of gravitational effects and for implementation of fire safety measures in spacecraft. 2. In fire research, several subfields of combustion need to be investigated under microgravity conditions. Ignition, flammability limits, smoldering, flame spread, and extinguishment are all deserving of detailed study. Variables should include fuel type and phase, fuel/oxidizer ratio, ambient oxidizer concentration, forced and free convection, ignition source, and extinguishment methodology. 3. Turbulent combustion processes are highly important on Earth but cannot be probed at 1 g at the small size scales that typically occur. Laboratory scale-up at 1 g introduces unwanted buoyancy. Reduced gravity would allow a scale-up in overall size without the introduction of major buoyancy effects, thus permitting access to the smallest scales of turbulence that are important to the problem. Here, the Reynolds number based on a forced flow velocity would remain fixed in the scaling process. Even though gravity is not an important parameter for many practical devices, the laboratory study of combustion processes is often impeded at 1 g. 4. Research on laminar premixed and diffusion flames and spray-flow interactions should be conducted, again because of the importance of these processes on Earth. The same problem occurs here as with turbulent flames. That is, scale-up to allow study at 1 g introduces unwanted buoyancy effects. In addition, other general recommendations are noteworthy. Reproducibility must be verified. Many combustion phenomena require a survey of a wide range of parameters. Also, some combustion phenomena are long-duration events. Consequently, an extended orbiting platform capability will be required for many combustion experiments for complete and serious study. Relevant to all areas above, ground-based experiments should be undertaken to develop miniaturized diagnostics and experimental apparatus and techniques for performing multiple repetitions of a specific experiment. Biological Sciences and Biotechnology The biological sciences and biotechnology are experimental disciplines that are highly dependent on empirical approaches to the solution of problems and on the continued discovery and development of useful research systems. Unlike many other branches of microgravity science such as fluid dynamics or materials science, there may be no firm foundation of theory, only a limited accumulation of experience. Thus, a reasonably high tolerance for scientific risk (which is clearly distinct from safety risk) should be allowed in investigations in file:///C|/SSB_old_web/mgoppes.htm (9 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary the biological sciences. The science community should be prepared, however, to support research in this area on the basis of its long-term potential and importance, especially its relevance to human spaceflight. Since research on biological topics commonly requires experiments that take a long time to complete, an extended-duration orbiting capability is necessary. Priorities should be given to scientific issues in biological sciences and biotechnology in the following order: 1. In studies to improve methods of crystallization of macromolecules for use in diffraction studies, additional experiments should focus on defining quantitatively those macromolecular crystal properties and growth mechanisms affected by gravity. There is demonstrated success in this research area and it is recommended for the highest priority. 2. Further experimentation is needed in both terrestrial and microgravity environments to develop new methods, materials, and techniques to exploit the potential of microgravity, where it exists, for improvements in biochemical separations. These separations are important both in terrestrial applications and in materials processing to support human spaceflight. 3. A dedicated effort should be made to evaluate the potential advantages of the microgravity environment for the study of cellular interactions, cell fusion, and multicellular assembly and to identify candidate cell systems that show maximum response to being cultured in such an environment. Efforts should also be made to identify and characterize subcellular mechanisms that sense and mediate responses to the magnitude and direction of gravitational forces. 4. A systematic effort should be made to identify those cellular and biomolecular processes, structures, assemblies, and mechanisms that may be affected by gravity, and to design and carry out experiments to explore the effects of microgravity on appropriate systems. Materials Science and Processing Metals and Alloys. The microgravity environment provided by an orbiting spacecraft offers new opportunities in the control of melting and solidification processes. Reduction of convective velocities permits, in some cases, more precise control of the temperature and composition of the melt. Body force effects such as sedimentation, hydrostatic pressure, and deformation are similarly reduced. Weak noncontacting forces derived from acoustic, electromagnetic, or electrostatic fields can be used to position specimens while they are being processed, thus avoiding contamination of reactive melts with their containers. Since many experiments in this field require long time scales for completion, the capability of an extended-duration orbiting platform is needed to obtain full benefits from microgravity research. Topics in the metals and alloys file:///C|/SSB_old_web/mgoppes.htm (10 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary area that would benefit from a focused effort include the following prioritized items: 1. Nucleation control and the achievement of metastable phase states, such as metallic glasses and nanostructures, are areas that could benefit from achieving deep supercooling in the microgravity environment, by elimination of container surfaces, and from the reduction of melt flows due to buoyancy-driven convection. 2. Microgravity experiments on Ostwald ripening and phase coarsening kinetics would add quantitative, fundamental information about the key metallurgical issues of interfacial dynamics during thermal and solutal transport, and the question of microstructure evolution in general. 3. Observations of aligned microstructures processed reproducibly under quiescent microgravity conditions should help to provide well-defined thermal processing limits for polyphase directional solidification of eutectics and monotectics. 4. Studies of the formation of solidification cells and dendrites under well- defined microgravity conditions can add to our expanding knowledge of complex metallurgical pattern formation and, more generally, of the fundamental physics of nonlinear dynamics. Microgravity conditions can be useful in these instances for the pursuit of sophisticated tests of theory and the quantification of metallurgical pattern dynamics. 5. Some thermophysical properties can be measured advantageously in microgravity. Accurate data on such properties, frequently essential for the modeling of metallurgical processes and materials responses, are often not available from standard terrestrial measurements. Polymers. Polymers potentially represent the broadest classes of engineered materials, permitting great innovation and precision in design, including control at the molecular level. Although the viscous character of most high polymer melts greatly desensitizes their response to gravitational acceleration, nonetheless some polymer solutions have relatively low viscosities. Moreover, some of these organic systems provide materials with interesting applications in the fields of nonlinear optics and metrology. A few initial experiments on the vapor- and solution-phase processing of organic and polymer films in microgravity have shown improved texture and smoothness over terrestrial counterparts, suggesting that this area of research merits further study. Growth of Inorganic Single Crystals. The microgravity environment will be particularly useful for the study of transport phenomena in the liquids from which bulk crystals are grown, and priority should be given to these studies rather than to the growth of large crystals. Such studies probably require a steady, very-low- file:///C|/SSB_old_web/mgoppes.htm (11 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary gravity environment such as that obtainable in a free-flyer and will provide useful data without requiring growth of bulk crystals. Precise transport data will become particularly useful for fluid dynamics computations, which are rapidly improving for terrestrial melt and solution growth. Any experiments on bulk crystal growth must be judged by their potential to contribute to the scientific understanding of the fundamental processes of crystal growth. The recommendations for this area of research are as follows: The design and execution of microgravity experiments that lead to a better fundamental understanding of crystal growth have proved elusive, and the committee recommends against the growth of large inorganic crystals under low gravity. The best approach to understanding the details of such growth will likely derive from fluid dynamical modeling and the modeling of processes at the fluid- solid interface, along with terrestrial studies of crystal growth. This analytical approach may provide the rationale for the growth of benchmark-quality inorganic crystals in microgravity. Assuming that some of the versatility of terrestrial experimentation can be achieved in the microgravity environment, there are opportunities for microgravity research that will have an impact on terrestrial bulk crystal growth. Priority should be given to transport studies, including studies of solute and self- diffusion, heat diffusion, and Soret diffusion. All of these involve the fluid from which crystals are grown, and they are amenable to routine study with repeatedly used apparatus. However, they may require a more stable environment than that provided by the space laboratory. Indeed, since such studies will undoubtedly be sensitive to the acceleration environment, they may also be useful for the study of this environment as a variable in the low-gravity range. Precise transport data will become particularly useful since fluid dynamical computational capabilities (for which these data are required) are improving rapidly for terrestrial melt and solution growth, as well as other industrial processes. Furthermore, transport measurements in industrially important fluids may be an important microgravity application outside the realm of large inorganic crystal growth. Growth of Epitaxial Layers on Single-Crystal Substrates. During the terrestrial growth of epitaxial layers by vapor deposition, an effort is made to minimize the effects of flow, buoyancy, and boundary layer uniformity by rotating or spinning substrates during the growth process. In addition, attempts are under way to calculate precise flow patterns and resulting growth rates in model systems. The calculations are complex, and it is uncertain whether they will be able to predict occurrences in real systems that are useful for industrial production. Since the manufacture of heterostructures by vapor deposition methods will be accomplished terrestrially, it is not clear whether any relevant data can be obtained solely under microgravity. A low priority is recommended for chemical vapor deposition studies under microgravity conditions. file:///C|/SSB_old_web/mgoppes.htm (12 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary For molecular beam epitaxy (MBE) methods, great gains in purity can, if necessary, be made on Earth, if sufficient attention is given to reducing contamination and increasing pumping speed. The committee concludes that epitaxial layers will be too costly to manufacture in space and that, at a reasonable cost, much improvement in the vacuum environment can be achieved terrestrially. Molecular beam epitaxy studies in orbiting vehicles are not appropriate until the limits of ground-based alternatives have been clearly reached. Ceramics and Glasses. As discussed above, a low-gravity environment will reduce buoyancy-driven convective flows in liquids. Most ceramic synthesis and processing is done at high temperature either by solid-state processes exclusively or by processes in which there are only small amounts of viscous liquid phases. Glasses are formed from high-temperature melts, where the suppression of convective mixing is generally undesirable because convection promotes compositional homogeneity. A second case in which liquids are important to ceramics is in the synthesis of ceramic powders or films from aqueous solutions or colloidal suspensions. Frequently, the requirement is to achieve a high density of nuclei and consequent fine particle sizes of the precipitated powders. Thus, rapid mixing is used, and there is no reason to suppress convection. For these reasons, low-gravity studies are of limited advantage in this discipline. Nonetheless, a low-gravity environment might be of benefit to ceramics research and development in the following prioritized areas: 1. There is interesting potential for containerless melting. Processing of ceramics at high temperature requires refractory containers that remain unreactive with the specimen. Availability of a general capability for containerless, high-temperature processing (to at least 1600(C) would allow contamination-free synthesis of glasses and ceramics, such as those being considered for optoelectronic applications, as well as the study of glass nucleation and crystal growth without heterogeneous nucleation on a container wall. 2. A fundamental study of crystal nucleation and growth in glass melts in microgravity would be interesting. Crystallization is an important process, desirable in glass-ceramics and undesirable in optical glasses, that requires further characterization and understanding. 3. Mass transport and diffusion studies of glass and ceramic melts under microgravity conditions should generate more precise data than those available from terrestrial measurements. One area of research that might be aided by reduction of convection is obtaining accurate data on diffusion in ceramic melts. 4. The suppression of free evaporation from melt surfaces could allow synthesis at higher temperatures than can be performed on Earth. file:///C|/SSB_old_web/mgoppes.htm (13 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary 5. The epitaxial growth of films from solution, including biomimetic synthesis (self-assembling monolayers) of ceramics, should be studied. Microgravity Physics Experiments in this area, with the exception of the general relativity test Gravity Probe-B (GPB), all represent extensions of work that is, or can be, conducted in Earth-based laboratories. Much of the scientific value of the proposed space experiments will depend on the strength of the connection to Earth-bound research. Given the long time scale for the ground-based development through flight of a space experiment, there is concern that the scientific goals of the experiment might be bypassed by new developments or by major shifts in the value ascribed to the work. The topics of microgravity physics fall into two general categories: (1) development of new instruments (e.g., superconducting gyroscopes for the GPB experiment and the mass balance for the equivalence principle test), and (2) preparation and study of unique samples such as uniform fluid free from gravity- induced density gradients or low-density granular materials near a percolation limit. This program can include such important studies as fundamental physics measurements (e.g., verification of the equivalence principle), critical phenomena, dynamics of crystal growth, and low-density aggregate structures. A recent successful experiment in this area is the Lambda Point Experiment (LPE) that was flown on the shuttle in October 1992 (USMP-1). Analysis of the data indicates an improvement of nearly two orders of magnitude over previous data obtained on Earth. This has been done in an unbiased way, and heat capacity data have been obtained approaching a few nanokelvin of the Lambda point. This experiment demonstrates clearly that highly sophisticated experiments involving the most sensitive and advanced instrumentation can be profitably performed in the microgravity environment. The quality of the microgravity environment must be examined critically in the context of each possible experiment. Minimum acceleration is the most obvious parameter of concern for many of the contemplated experiments; however, the time span over which a high-quality, low-gravity environment can be maintained may be of equal importance. For some experiments, accidental large accelerations (perhaps resulting from sudden movements of personnel or the firing of small thrustors) might destroy the object of study, for example, a low- density granular structure. With regard to the quality of the microgravity environment available for experiments, only the center of the orbiting spacecraft is in true free-fall and then only to the extent that orbital drag effects and other external influences are negligible. In a gravity gradient-stabilized spacecraft, there will be a steady rotation of any experiment about the center of mass of the entire spacecraft once each orbit. For a low Earth orbit, this will result in accelerations on the level of 10- file:///C|/SSB_old_web/mgoppes.htm (14 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary 7 g at a distance 1 meter from the center of mass of the entire orbiting system. On both the space shuttle and the space station, only a few experiments will be located close enough to the center of mass to ensure acceleration levels below 10-6 g. A number of scientifically meritorious projects, such as the equivalence principle experiment and GPB, will require spaceflight independent of any crewed space facilities. In the future, we may anticipate a continued requirement for low- temperature facilities in space, since low temperatures are important for the highest-resolution measurement techniques, particularly those based on SQUID (Superconducting Quantum Interference Device) technology. If a space station is to be a useful contributor to the area of fundamental science, access to liquid-helium facilities will be mandatory. GENERAL SCIENCE RECOMMENDATIONS FOR MICROGRAVITY RESEARCH Following are a number of general science recommendations that are important for microgravity research: The reproducibility of results is a crucial element of laboratory science, and flight investigators should be given the opportunity to address reproducibility in their research. Nonetheless, a balance should be established between the reflight opportunities necessary for reproducibility and the flight of experiments that address new scientific issues. The need for scientific judgment, trained observation, and human intervention and participation in certain laboratory microgravity experiments requires a greater use of payload specialists with expertise and laboratory skills directly related to the ongoing experiments. Some flight experiments would also benefit from the direct investigator interaction made possible by teleoperation capabilities, and NASA should support the development and deployment of such techniques in future microgravity experiments. As a rule, microgravity experiments should be designed and conducted to provide specific explanations for meaningful scientific questions. Scientific objectives should be clear and specific. In addition, NASA must continue to support ground-based experiments and the development of underlying theories. Theoretical understanding and ground-based experimental results should support the need for microgravity experiments and the likelihood that experimental objectives will be met. Notwithstanding this, exploratory experiments can be valuable in advancing understanding of some complex file:///C|/SSB_old_web/mgoppes.htm (15 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary systems (e.g., biological systems). There are scientific reasons to augment microgravity research in certain areas with a variable-gravity capability of extended duration that covers the microgravity to 1-g range. In a laboratory science where controlled model experimental systems are essential, the ability to vary an important parameter continuously should be developed whenever feasible. Therefore, if a space station centrifuge is to be developed, it is desirable to evaluate shared utilization for microgravity research. NASA should categorize experiments according to their minimum facility requirements to maximize scientific return and cost-effectiveness. Drop towers, aircraft on parabolic trajectories, sounding rockets, and orbiting platforms supply a range of acceleration levels, acceleration spectra, and experimental durations and provide opportunities for human interaction and demonstration of reproducibility of results. Some facilities are more suitable than others for precursor experiments to evaluate instruments and procedures and to demonstrate feasibility. General-purpose facilities (versus experiment-specific equipment) should not be imposed on principal investigators if it might degrade the scientific results. Costs are not necessarily lowered when equipment is designed for a wide range of experiments. The scientific benefits could be substantially reduced by the inherent compromises. For each flight experiment, the acceleration vector should be accurately measured locally, frequently (or continually), and simultaneously with other experimental measurements. Since the magnitude and direction of the net local acceleration environment can significantly affect experiments, they must be correlated with the primary experimental data. The net acceleration vector varies temporally and with position relative to the spacecraft center and can have major effects even at small magnitudes. Materials studied in microgravity experiments should be adequately characterized on Earth. For some materials, there is a lack of the thermophysical data essential for both experimental design and modeling. These data should be obtained by ground-based research, where possible, or from microgravity measurements, when necessary. The qualitative effects of the acceleration environment on certain types of experiments should be studied. These effects are not generally understood and characterized. FLIGHT OPPORTUNITIES AND CHALLENGES file:///C|/SSB_old_web/mgoppes.htm (16 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary The peculiar character of microgravity research as a laboratory science in space requires real-time interactions between scientists and their experiments. This special requirement indicates another role for humans in space, in addition to the traditional use of astronauts for exploration and the deployment and repair of satellites. Moreover, a laboratory-based researcher should be able to carry out a large number of experiments to cover required ranges of the experimental parameters and demonstrate the reproducibility of results. The researcher should be able to modify the apparatus and diagnostics, when necessary, to ensure successful experimentation. An assortment of facilities is available for research in a low-gravity environment. These range from drop towers and aircraft to orbiting platforms. The duration required for experiments in some of the subdisciplines of microgravity research may exceed the flight time available on some platforms. Additional spacelab missions, particularly those with extended-duration capability, might adequately serve those subdisciplines. For other subdisciplines such as some aspects of biotechnology and materials science, however, longer flight times would provide significant benefits in terms of the quality of scientific yields. Even for those subdisciplines with shorter-duration experiments, the limited number of spacelab flights and the high costs associated with them severely limit the experimenter with regard to demonstration of reproducibility, time for readjusting or repairing equipment, and reflight opportunities. These limitations, inherent to the spacelab system, can be obviated only by the longer flight durations available on a space station or a recoverable satellite. It is important, however, that the design of a space station or recoverable satellite provide a stable microgravity environment that minimizes unacceptable disturbances. The present microgravity research infrastructure does not readily accommodate the needs of laboratory research. Although drop towers and airplanes flying parabolic trajectories can be used for special or precursor experiments and some materials processing can be done on free-flyers, the spacelab and space station are better suited for microgravity laboratory research. Experience with spacelab for microgravity research, however, indicates that (1) it is likely to take years to develop an experiment that can yield high-quality scientific data and (2) it is an extremely expensive process. The time required from the selection of the principal investigator to launch with new hardware is 5 to 6 years for mid-deck (noninteractive) experiments and 6 to 8 years in spacelab. Reflights with minimal modifications to the equipment require 1 year for mid-deck and 2 years for spacelab experiments. Minute elements of an experiment must be documented in detail and subjected to safety tests. Experiments are often delivered up to a full year before launch for integration into the spacelab and then into the orbiter. Because the number of spacelab missions is limited, as many experiments as possible are scheduled for each flight. Thus, the number of experimental runs is limited even if all goes as planned on the mission. As a result, compromises are made that might be favorable for one experiment but not others. For those experiments that suffer in the compromise process, essential science might be sacrificed. Also, because of heavy demand, experimenters are often guaranteed only one flight. file:///C|/SSB_old_web/mgoppes.htm (17 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary Surmounting the various administrative hurdles confronting an experiment requires interaction with three different NASA centers. The conditions imposed on the experiment by various centers are not always mutually consistent. The principal investigator is often excluded from the discussions. Most of this complexity results from concern for the safety of the crew and spacecraft, and results in long times for experiment development and flight and, consequently, increased cost. Concern for safety in human spaceflight missions should always remain a primary consideration, but safety issues could be fully addressed in a streamlined administrative process that does not compromise the scientific objectives of the mission. The requirement for laboratory science is that human participation in the experimental process be guaranteed, although in some cases, uncrewed flights with remote control of experiments are effective substitutes for physical human presence. Human presence, however, will be necessary for many spaceflight experiments in the foreseeable future. The entire infrastructure and extensive procedures that are currently extant have been developed essentially for missions in space with purposes other than laboratory science. They should now be reassessed and perhaps modified extensively to take full advantage of the unique microgravity environment. The biotechnology program has been affected by some confusion concerning its administrative oversight. Biotechnology is tied strongly to other microgravity programs because it shares fluids and transport phenomena as the common scientific theme through which gravity becomes an important parameter. The administrative issue concerns the need for cooperation and coordination between the microgravity research administration and the life sciences administration for research on cellular and subcellular processes and mechanisms. In summary, the major administrative challenges before NASA in the microgravity research domain are the following: (1) interactions among centers and between centers and headquarters should be simplified and unified; (2) principal investigators should be continually involved with the development of experiments and; (3) biotechnological research and life sciences programs should be well coordinated. ADMINISTRATIVE RECOMMENDATIONS The following recommendations do not directly address the scientific issues, but rather summarize administrative issues, including some points discussed above, that profoundly affect the quality and quantity of the scientific content of the microgravity program. Meaningful interaction should be maintained between the principal investigators and NASA staff during experiment development and integration, file:///C|/SSB_old_web/mgoppes.htm (18 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary and communication among these groups must be continued following flight. It is essential to minimize the overlap of responsibilities in the NASA infrastructure to accomplish this goal. It is essential that the overlap of responsibilities among NASA centers and between centers and headquarters be substantially reduced in order to optimize the influence of the principal investigators and the likelihood of successful experiments. Measures should be taken to improve coordination and cooperation between the life sciences administration and the microgravity research administration concerning biological and biotechnological research on cellular and subcellular processes and mechanisms. The strong scientific coupling between biotechnology and other microgravity disciplines through the fluids and transport theme should be recognized in any administrative reorganization. All data, including acceleration measurements, should be made available immediately to the principal investigator. Current delays in providing data extend many months beyond the flights. NASA should coordinate the operations of various offices so that priority is given to the processing of experimental data. It is necessary to increase substantially the number of ground-based investigations to ensure the future supply of high-quality flight experiments. Consequently, the budget for ground-based research should be increased as a fraction of the entire microgravity program. To the maximum extent feasible, funding for ground-based research in the microgravity program should be protected from temporary budgetary fluctuations. In addition to its role in developing flight experiments, the ground- based program can provide important scientific and technical data for other purposes. A ground-based study can therefore be judged successful even if a flight experiment does not result. Microgravity research is much broader than the topic of materials processing in space and should be identified as microgravity research in all official documents, including the federal budget. Microgravity research better describes the activity and its actual and potential accomplishments. Materials processing in space characterizes only a fraction of this research activity and promotes a misleading impression of the potential benefits and scope of the program. The microgravity sciences program and the commercial development program have a large area of overlap and common interest. Coordination between these programs and an equally stringent review process for each should be fostered by NASA for their mutual benefit. file:///C|/SSB_old_web/mgoppes.htm (19 of 21) [6/18/2004 11:15:14 AM]
Microgravity Research Opportunities for the 1990s: Executive Summary NASA should establish mechanisms for continuous, unrestricted submission of research proposals in order to optimize quality and take advantage of scientific advances. Unrestricted submission of research proposals would parallel approaches of other funding agencies and enable continual scientific advance. To manage the diverse disciplines in microgravity research, it is necessary to increase the breadth and experience of the scientific staff at NASA headquarters. A rotation of prominent scientists on leave from universities, national laboratories, and industry is one mechanism to be explored. Introduction of active scientists in the administration of the program would be highly beneficial. Prompt documentation of experimental results should be required and enforced. Reports of all experiments, including unsuccessful efforts, should be accessible to all interested parties. A concern is that some investigators might not report results because they are proprietary or inconclusive. The lack of an available report could lead to unnecessary duplication of efforts. NASA should organize and maintain an accessible archive of microgravity research results. This archive should contain a bibliography of all published scientific papers and reports on microgravity subjects and should preserve the original spaceflight data sets, such as photographs and electronically recorded data. An additional point of concern is that given the long time scale for the development through flight of a space experiment, there is a real danger that the scientific goals of the experiment might be bypassed by new developments or by major shifts in the value ascribed to the work. There is also the possibility that the principal investigator may lose contact with the field. Several of the above recommendations may be useful in this regard. If NASA can shorten this time frame, it would be beneficial. REFERENCES 1. Space Science Board, National Research Council. 1978. Materials Processing in Space. National Academy of Sciences, Washington, D.C. 2. Space Science Board, National Research Council. 1988. Space Science in the Twenty-First Century: Imperatives for the Decades 1995 to 2015. Fundamental Physics and Chemistry. National Academy Press, Washington, D.C. 3. Space Studies Board, National Research Council. 1992. Toward a Microgravity Research Strategy. National Academy Press, Washington, D.C. file:///C|/SSB_old_web/mgoppes.htm (20 of 21) [6/18/2004 11:15:14 AM]
