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Microgravity Research Opportunities for the 1990s: Chapter 2
Microgravity Research Opportunities
for the 1990s
PART I—OVERVIEW
2
Recommendations and Conclusions
INTRODUCTION
The microgravity environment presents unique opportunities to perform
important experimental laboratory research in a number of fundamental and
applied areas. The five scientific disciplinary areas that can benefit in varying
degrees are (1) fluid mechanics and transport phenomena, (2) combustion, (3)
biological sciences and biotechnology, (4) materials science, and (5) microgravity
physics. There are important scientific questions in these disciplines that cannot
be addressed by experiments at normal Earth gravity. This provides a major
impetus for the microgravity research program.
REPORT MENU
NOTICE
Microgravity research is also necessary for, and fundamental to, the
MEMBERSHIP
exploration of space. Fluid and thermal systems and materials processes that are
PREFACE
EXECUTIVE SUMMARY essential to mission-enabling technologies for extended space operations will
PART I generally behave differently under reduced gravity conditions. Microgravity
CHAPTER 1 research opportunities will frequently be driven by the technological needs of
CHAPTER 2 NASA's programs. It is well understood that engineering, applied science, and
PART II
fundamental science are interconnected. Some of the obvious engineering issues
CHAPTER 3
that should influence the microgravity research program are the ignition,
CHAPTER 4
propagation, and extinction of spacecraft fires; the fluid dynamics and transport
CHAPTER 5
processes associated with the handling, storage, and use in space of water,
CHAPTER 6
waste, foods, fuels, air environments, and contaminants; the handling, joining,
CHAPTER 7
and reshaping of materials in space; and the dynamics and chemistry of mining
PART III
and refining of resources in non-Earth environments.
CHAPTER 8
APPENDIX A
APPENDIX B
The committee reaffirms the findings of the previous and first report,
Toward a Microgravity Research Strategy,1 of the Committee on Microgravity
Research that there is little potential for a successful program to develop
manufacturing on a large scale in space for the purpose of returning high-quality,
economically viable products to Earth. The cost of transporting raw materials into
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Microgravity Research Opportunities for the 1990s: Chapter 2
space and products back to Earth will generally be much too high to be
economical. Limited amounts of certain products, however, might usefully be
made in space for scientific study.
A set of recommendations has been developed for the microgravity
sciences that has several salient features. Five science areas are recommended
for long-term support with a balance of support among these areas-fluid
mechanics and transport phenomena, combustion, biological sciences and
biotechnology, microgravity physics-and within materials science, the subarea of
metals and alloys. Fields that will benefit significantly less from a microgravity
environment are polymers, epitaxial layer growth on single-crystal substrates,
large single inorganic crystal growth, and ceramics and glasses. It is noted that
the common and major theme among the five recommended areas is that of fluid
mechanics and transport phenomena. The committee's recommendations
emphasize the development of research opportunities, modification of NASA
policies and procedures, and interactions between NASA and the investigators
that will provide an environment necessary for first-class laboratory science. The
importance of a strong ground-based program is also stressed. The roles of
microgravity research are discussed both for the advancement of general
scientific knowledge and for the development of enabling technologies for space
missions. The lists of scientific recommendations identify the obvious
opportunities for immediate and significant scientific impact.
A number of points were considered in arriving at the recommendations
for each discipline. The importance and compelling nature of the scientific
questions and the perceived impact on the field were given greatest weight and
established the context for further recommendations. Scientific questions and
objectives, however, were examined further in terms of their requirement for, and
probable benefit from, a microgravity environment. Clearly, the priority of a given
objective was reduced if alternative or superior approaches were available.
The type of microgravity facility required for the investigations and the
quality of the microgravity environment required were also considered. These
considerations addressed experimental duration, maximum allowable
acceleration level, permissible radiation levels, degree of human or robotic
interaction, and complexity of diagnostics. The following questions were asked:
Have the issues been explored sufficiently in conventional laboratories on Earth,
have adequate theoretical and computational analyses been performed, and are
models for the process available? Finally, the resource requirement and costs
were considered in the broad context of the current, and anticipated, microgravity
research program.
In addition to scientific considerations, means are recommended to
improve the scientific quality and increase the efficiency of microgravity research.
As a result, suggestions are included that relate to instrument design and
implementation, procedural matters, and interactions among the space agencies
and the investigator communities.
The research areas discussed in this report are subsets of much broader
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Microgravity Research Opportunities for the 1990s: Chapter 2
disciplines. Only a small fraction of the total activities in each occurs within the
microgravity program, and no attempt has been made to evaluate fully or
prioritize all of the research in a particular 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 similar subjects in the terrestrial environment. Experiments that
can be performed adequately at Earth gravity are not recommended for
spaceflight.
Further, because of the limited results available from experiments that
have been flown in space, the recommendations of this report cannot be highly
detailed or exclusive. There are a number of subjects that require further
exploratory investigation before detailed objectives can be defined. Some areas,
however, can be identified as more promising than others. This report also does
not address the NASA commercial program or international programs in
microgravity research. These topics will be addressed in the future by this
committee.
In summary, the recommendations in this report emphasize the
contribution of microgravity research to the advancement of science and
technology in general and of space exploration in particular. The report also
deemphasizes the concept of manufacturing products in space for their return to
Earth. Adjustments in NASA's structures, policies, and procedures for conducting
laboratory science in space are required by these recommendations. Finally,
priorities are suggested for topics within certain microgravity science disciplines
and subdisciplines.
The duration of experiments, the regime of parameters available to
experimenters, and the ability to demonstrate reproducibility of results in many
microgravity experiments create the need for extended-duration orbiting
platforms. The recommendations have value, however, whether such platforms
become available or the microgravity program continues only with existing
facilities.
The findings and recommendations of this report are consistent with the
preliminary statements of the first report of this committee, Toward a Microgravity
Research Stategy. This second report is broader in scope and includes
substantially greater detail in analysis and recommendations.
AREAS RECOMMENDED FOR
EMPHASIS IN MICROGRAVITY RESEARCH
Microgravity science is highly interdisciplinary and broad. The major
prospects and opportunities in each of the major disciplines are summarized in
this section. Limitations of opportunities are also discussed.
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Microgravity Research Opportunities for the 1990s: Chapter 2
Fluid Mechanics and Transport Phenomena
Fluid mechanics and transport phenomena are influenced significantly by
gravity. As a consequence, different behaviors may be expected for many fluid
configurations in a microgravity environment, both with and without heat and
mass transfer. Also, the reduction of gravitational body forces leads to dominance
by other forces normally obscured in terrestrial environments such as surface
tension and electrical forces. Because fluid mechanics and transport processes
are involved in most of the areas of microgravity research, they represent a
common theme for much of the subject. Basic research is required to understand
and describe the characteristics of transport phenomena under low-gravity
conditions.
Fluid mechanics and transport phenomena also play an essential role in
many space-based technologies. Space system designers will be challenged to
develop new enabling technologies and critical concepts that involve fluid
mechanics and transport phenomena in low-gravity environments. Unfortunately,
predictive models for the low-gravity performance and operation of those
technologies are often inadequate. In fact, some researchers believe that useful
predictive models do not exist. 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 neither reliable nor efficient.
Priority should be given to the study of phenomena that are unique to 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 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 mission-enabling technologies.
Multiphase flows. Many processes involve multiphase flows. Gravity
imposes a specific orientation on multiphase fluids, structures, and organizations
(e.g., gas-liquid, liquid-solid). In a reduced-gravity environment, orientation will be
much less important; flows and associated transport phenomena become
significantly different.
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, the complex interactions can be separated and analyzed.
Furthermore, transport effects masked at 1 g, such as Soret and Dufour
phenomena, can become important.
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Microgravity Research Opportunities for the 1990s: Chapter 2
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, and this can contribute in an important way to
understanding these physicochemical interactions.
The above topics are important to biotechnology, combustion, materials science,
and other areas of science.
The following topics deserve attention not only for their intrinsic scientific
importance but also to provide a knowledge base for space technologies and
applications:
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. 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.
Combustion
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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 (e.g., smoldering and flame spread) require long-duration
observations and measurements. Furthermore, reproducibility is an issue.
Recommendations concerning equipment and facilities include the following:
An extended orbiting platform capability will be required for many
combustion experiments.
Ground-based combustion experiments must be undertaken to
develop miniaturized diagnostics and experimental apparatus and techniques for
performing multiple repetitions of a specific experiment. Experimental space is at
a premium in microgravity research, and because of the many variables in
combustion and the difficulties in achieving reproducibility, assurance of data
quality is needed.
The following areas of emphasis are recommended in order of priority:
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 is required 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 must 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. 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.
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.
Even though gravity is not an important parameter for many practical devices, the
study of fundamental combustion processes is often impeded at 1 g because
attempts at scale-up introduce unwanted buoyancy.
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Microgravity Research Opportunities for the 1990s: Chapter 2
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
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. In addition, since
research on biological topics commonly requires experiments that take a long
time to complete, an extended-duration orbiting capability is necessary.
The following topics deserve priority in the order listed. The first two topics
have some demonstrated successes and therefore the likelihood of further
success. The last two topics are exploratory.
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 growth mechanisms affected by
gravity. Direct, high-resolution observation of the crystal growth process by a
variety of optical techniques and precise monitoring of ambient parameters such
as temperature and pH should be included. Ideal test systems should be
identified and carefully delineated.
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. This effort should
follow and build on the previous effort to identify mechanisms, processes,
structure, and assemblies. 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. A comprehensive database should be
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compiled of cell types, organelles, and other assemblies, and their responses to
microgravity environments. This systematic approach has substantial potential
impact on our understanding of the underlying processes in the microgravity
environment. It should therefore have an impact on the knowledge base for
space exploration.
Materials Science and Processing
Metals and Alloys
Carefully designed and scientifically well-conceived experiments on
metals and alloys are needed to produce high-quality data and materials uniquely
derived from microgravity research. Such experiments are not useful unless they
produce results that cannot be obtained from terrestrial studies. 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 area that might benefit
from focused microgravity research include the following items listed in order of
general priority:
1. Nucleation kinetics 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, from the
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. These comprise wide classes of technologically important alloys
and composites.
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. Such studies, to be successful, require the most
demanding control of temperature, thermal gradients, growth speed, alloy
composition, and other such parametric variables. Microgravity conditions can be
useful in these instances for the pursuit of sophisticated tests of theory and the
quantification of metallurgical pattern dynamics.
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5. Some thermophysical properties can be measured advantageously in
microgravity. Accurate data on these 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 their design, including
control at the molecular level.
Although the viscous character of most high polymer melts greatly
desensitizes their response to gravitational acceleration, initial experiments in
some areas of vapor- and solution-phase processing of organic and polymer films
in microgravity show 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 properties in liquids from which bulk crystals are grown, and priority
should be given to these studies. Such studies probably require a steady, very-
low-gravity environment (less than 10-6) such as that obtainable in a free-flyer
and will provide useful data without requiring growth of bulk crystals. Precise
transport data will be 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 in 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 surface, 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 are studies of the fluid
from which crystals are grown and 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.
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Precise transport data will become particularly useful since fluid
dynamical computational capabilities are improving rapidly for terrestrial melt and
solution growth, as well as other industrial processes. These kinds of data will be
required for such computations. 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 precisely flow patterns and resulting
growth rates in model systems. The calculations are complex, and it is not certain
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 conditions.
A low priority is recommended for chemical vapor deposition studies
under microgravity conditions.
For molecular beam epitaxy methods, great gains in purity can be made
on Earth provided sufficient attention is given to reducing contamination and
increasing pumping speed.
Epitaxial layers will be too costly to manufacture in space. At a
reasonable cost, much improvement in the vacuum environment can be achieved
terrestrially. Studies in orbiting vehicles are not in order until the limits of ground-
based alternatives have clearly been reached. Ceramics and Glasses
Most ceramic synthesis and processing is achieved at high temperatures
either by solid-state processes exclusively or by those processes in which there
are only small amounts of viscous liquid phases. Glasses are formed from high-
temperature melts (the viscosity of SiO2 at 1700ºC is about 107 poise) 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 for 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:
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1. There is interesting potential for containerless melting in microgravity.
Processing of ceramics at high temperatures requires refractory containers that
remain unreactive with the specimen. Availability of a general capability for
containerless, high-temperature processing (to at least 1600ºC) would allow
contamination-free synthesis of glasses and ceramics, such as those being
considered for optoelectronic applications. Containerless melting also might allow
study of 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.
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 currently
conducted 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.
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 fluids 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.
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;
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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
experiment, 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 such as
solar wind and radiation 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 10-7-g level 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 the 10-6-g level.
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
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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 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
systems (e.g., biological systems).
There are scientific reasons to augment microgravity research in
certain areas with a variable-gravity capability of extended duration. In some
problem areas, the magnitude of the gravity vector is an important experimental
variable in 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. 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; moreover, 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
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data essential for both experimental design and modeling. These data should be
obtained by ground-based research where possible or from microgravity
measurements if necessary.
The qualitative effects of the acceleration environment on certain types
of experiments should be studied. These effects are not generally understood
and characterized.
ADMINISTRATIVE RECOMMENDATIONS
The following recommendations do not directly address scientific issues
but instead summarize administrative issues that profoundly affect the quality and
quantity of the science content of the microgravity program.
Meaningful interaction should be maintained between the principal
investigators and NASA staff during experiment development and integration,
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 must 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.
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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 the activity and promotes a
misleading impression of the potential benefits and scope of the program.
The microgravity sciences and applications 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.
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 would 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. Anything that NASA can do to
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shorten this time frame would be beneficial. This topic is explored in more detail
in Chapter 8, which also contains some additional scientific and administrative
recommendations specific to the flight program.
REFERENCE
1. Space Studies Board, National Research Council. 1992. Toward a
Microgravity Research Strategy. National Academy Press, Washington, D.C.
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