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An Initial Review of Microgravity
Research in Support of
Human Exploration and Development of Space
2
The Current Microgravity Research Program
and HEDS Goals
HEDS GOAL 1: INCREASE KNOWLEDGE OF NATURE
USING THE SPACE ENVIRONMENT
As described above, the first of the HEDS goals adopted by NASA
mandates the scientific study of nature and its processes in microgravity for the
purpose of increasing human knowledge. Some of these studies use microgravity
as an experimental variable, and others use microgravity to enable the study of
phenomena obscured by gravity. (While microgravity is technically defined as 10-
6 of Earth's gravity, the actual spaceflight environment in which experiments are
performed ranges from 10-3 to 10-6 g). Microgravity can also permit an
experimental protocol or measurement that cannot be performed on Earth. The
current MRD science program is already closely aligned to the objectives of
HEDS Goal 1 and has been so almost from its inception. The science programs
of each of the five current MRD disciplines, described in greater detail in a
previous report of this committee,1 are briefly presented below.
Fluid Physics
The greater part of the current MRD program deals with heat and mass
transport processes in reduced gravity or microgravity that are associated with
density, temperature, and concentration gradients in gaseous, liquid, and
particulate matter, especially when changes of phase take place. Studies of
nucleation and boiling in reduced gravity are under way, as are studies of the
dynamic behaviors of droplets, bubbles, and foams and of suspensions of
particulate materials as they are transported in fluid media. A variety of interfacial
problems, for example in multilayer convection and jet impingement, arise in
many of the current studies.
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Electrohydrodynamic forces and effects, including electrophoresis, appear
in a number of these microgravity studies, in addition to capillary, thermocapillary,
and diffusional effects. Theoretical studies have been initiated in which
investigators have begun to examine the influence of reduced gravity (e.g., on the
Moon or Mars) rather than microgravity, and some studies consider the
consequences of "g-jitter" on gravity-sensitive flows.
The present program shows a strong emphasis on experimental research.
Experiments in flow physics requiring access to a long-duration, low-gravity
environment have been carried out successfully in the Space Shuttle program.
For experiments that can be conducted in shorter periods, continuing use has
been made of aircraft flight tests and of a number of drop-tower facilities for flow
physics research. While this experimental research is strongly fundamental in
character, the current MRD fluid physics program does not greatly emphasize
theory or analysis, nor does it emphasize computational fluid-dynamics
simulation (which is developed from theory) as much as do fluid mechanics
programs in other fields of aerospace research where the boundary conditions
are better understood. This is understandable for the present. However, as
NASA's interest grows in the application of fundamental scientific insights to
specific design conception, then theory, analysis, and computational simulation
will assume greater importance in microgravity work.
In general terms, issues involving the physics of fluids in low gravity
underlie a great many of the scientific and engineering technology problems of
space travel, and these are more fully discussed in subsequent sections of this
chapter and in Chapter 3. Therefore, elements of the broad current fluid physics
program of MRD will doubtless find expression within interdisciplinary studies
undertaken by NASA to support the design of general and specific systems
needed for future HEDS missions.
Materials Science
The essential quest of materials science is to understand the relationships
among processing, structure, and properties. Within this context, the MRD
program in materials science seeks to understand the influences of gravity on
those relationships. Hence, a large fraction of the science funded by MRD is
focused on understanding the fundamentals of nucleation and growth of solids
from liquids. Emphasis is also placed on elucidating the details of the genesis
and evolution of microstructure, as well as on the formation of crystal defects and
solute segregation. The ultimate goal of this research is understanding how to
improve materials properties. Additionally, because the microgravity environment
enables measurements of the thermophysical properties of liquids in stable (and
even metastable) states that might not be possible to perform in terrestrial
gravity, this area has also attracted researchers. Examples of such properties are
viscosity, heat capacity, and chemical diffusivity.
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The materials research currently funded by MRD addresses most major
classes of materials. At present, a strong emphasis exists on metals and their
alloys as well as on electronic and photonic materials; a moderate emphasis is
placed on ceramic materials, and less on polymeric materials. The reduced
emphasis on viscous polymeric materials is related to the smaller effects of
gravity on their structure. Indeed, conventional fluid mechanics show that
buoyancy-induced convection occurring in materials processes at terrestrial
gravity levels has a much greater influence on process outcomes in the case of
liquid metals and semiconductor melts than it has for highly viscous melts
associated with high-molecular-weight polymers and network (silica-based)
glasses. Within the scope of these investigations, a number of important
processes are being examined, such as directional solidification, chemical vapor
deposition, containerless processing, polymerization, and co-polymerization.
Combustion synthesis and welding and joining are also receiving some attention.
As work progresses, additional areas of research of importance to the HEDS
enterprise are expected to be identified.
Combustion
MRD currently supports a rigorous flight- and ground-based effort in
combustion science in line with the fundamental science objectives of HEDS
Goal 1 through work exploring effects of gravity on flammability limits,
smoldering, flame spread, and material flammability, all of which are substantially
affected by a reduction in gravity. Related fundamental work includes
investigations of the dynamics of flame balls, structures of diffusion flames, and
characteristics of droplet and particle combustion (which, at reduced gravity, give
rise to a closer approach to spherical flames, new flame instabilities, and
modified soot formation processes), and focuses on the importance of radiative
transfer in combustion processes. Along with associated theoretical studies, this
program is leading to improved understanding of combustion phenomena at
altered gravity levels, thereby contributing scientific knowledge needed for the
HEDS enterprise.
Biotechnology
The biotechnology discipline within MRD currently supports three areas of
research: protein crystal growth, mammalian cell culture, and bioseparations.
Each is a key technology for the production of biology-based products and
involves processes that are affected by gravity.
Protein crystal growth provides the crystals that are required to determine
the unique three-dimensional structures these macromolecules adopt to perform
their biological functions. The relationship between structure and function in
proteins targeted for drug intervention, for example, has been found to be of
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critical importance to the rapid design of useful therapeutic agents. (The most
recent example of this structure-based drug design using protein crystallographic
data is the family of new HIV protease inhibitors that are the linchpins of more
effective combination therapies against AIDS.2) The process of protein crystal
growth is sensitive to gravity because of density-driven convection at growing
crystal surfaces and because of sedimentation of crystals from liquid growth
media. A first phase of spaceflight experimentation has proved that growth in
microgravity can, in some cases, produce crystals exhibiting improved X-ray
diffraction performance and more precise structure determination. A second
phase of experimentation has just begun that focuses on determining the
physicochemical mechanisms of protein crystal growth so that the knowledge can
be used to extend the beneficial effects of microgravity to the widest possible
array of proteins.
Most of the experimentation in mammalian cell culturing supported by
MRD has been aimed at the study of the basic functions of three-dimensional
cellular aggregates that form in bioreactor devices on Earth. The rotating-wall,
perfused-vessel bioreactor was designed to mimic the low-shear-stress
environment of microgravity. This cell culturing technology provides a great
advance over the use of monolayer or stirred cultures and often permits culturing
of differentiated cells and tissues that cannot otherwise be achieved.3 Culturing of
mammalian cells is important to provide cells and tissues for potential production
of biological products such as insulin, cartilage,4 and cellular proteins. In addition
to presenting research opportunities, these tissues would also be available for
transplantation and genetic therapies.5,6,7 It is anticipated that the further
reductions in shear forces that are possible in space will allow larger and more
complex tissue masses to be grown.
Biological products are isolated from culture media by a number of
techniques. Gel electrophoresis, a widely used method for purification of
biological products for both industrial and research purposes, involves separation
by size and charge in a water-based gel. Resolution is normally limited by the
gravitationally mediated phenomena of density-driven thermal convection and
sedimentation. The results of electrophoretic experiments carried out in
microgravity demonstrated that buoyancy-driven phenomena are diminished, but
new electrohydrodynamic effects have been uncovered that limit the benefits
gained by the effects of microgravity on the system.8 Biological separations will
also be important for nutrient production and waste recycling in space, which may
provide the basis of future critical mission technologies.
Low-temperature Microgravity Physics
The MRD program in low-temperature microgravity physics has
sponsored several flight-approved projects, covering both condensed matter
physics (Confined Helium Experiment, Critical Fluid Light Scattering, and Critical
Dynamics in Microgravity) and general relativity (Satellite Test of the Equivalence
Principle (STEP)). These projects use extended-duration microgravity to probe
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certain extreme physical limits, such as asymptotic approaches (within
microkelvins) to certain critical temperatures of classical (xenon liquid-gas) and
quantum (helium's lambda point) systems, where the physics of thermodynamic
fluctuations near these singularities is revealed much more clearly than on Earth.
The Quick STEP mission, which falls within this subdiscipline, is a geodesy
science satellite that will establish new limits on the gravitational-to-inertial-mass
ratio. The detection of any departure of this mass ratio from unity would present
the physics community with a significant challenge to current relativity and
gravitational theories; the absence of any systematic departure from unity would,
by contrast, establish new limits on the accuracy of current physics theories.
These projects in microgravity physics represent unique scientific opportunities
for NASA to advance our deepest understanding of how matter and energy
interact with gravity.
The Importance of Fundamental Research
Taken together, the individual discipline-specific science programs within
MRD represent an integrated approach to microgravity research that has already
contributed important knowledge of processes occurring in space and on
extraterrestrial bodies and will continue to do so in the future. Moreover, the MRD
program presents a relatively comprehensive response to the scientific
opportunities and challenges provided by the microgravity environment. Basic
microgravity research in the core disciplines should continue to be supported as
the fundamental science component of the MRD program. The fundamental
insights provided by the core disciplines can also form the basis for the evolution
of technologies required by the other HEDS goals.
The current research sponsored by MRD is subject to rigorous peer
review and generally is of high quality. A distinguished, broadly based scientific
community is involved with the execution of these investigations, and significant
new results are emerging from the program.9 This effort, specifically directed at
HEDS Goal 1, should be maintained at least at its current level.
NEW CHALLENGES: HEDS GOALS 2, 3, AND 4
Although MRD has established itself as an effective basic science
program and thus meets the objectives of Goal 1 as described above, it can also
play a significant role in NASA's attempt to meet the remaining HEDS goals.
Contributing to NASA's HEDS Goal 2, "to explore and settle the solar system,"
would require the recognition that the output from the MRD science program
should also be used to support the HEDS mission technologies. In other words,
the results of microgravity research should be used not only for terrestrial
applications, but also to improve the feasibility of the eventual exploration and
settlement of near-Earth space. The scientific challenges presented by this new
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approach to the use of microgravity research findings also encompass to a large
extent those posed by the subsequent HEDS goals, specifically Goal 3, "to
achieve routine space travel," and Goal 4, "to enrich life on Earth through people
living and working in space." These new goals imply long-term exposure to, and
function in, a variety of environments with gravity levels ranging from microgravity
to the gravity of Earth. One should note that with the possible exception of lunar
base missions, the space missions envisioned by HEDS would still spend a
substantial portion of their time under microgravity conditions, affecting not only
humans but also the machines, systems, and devices needed for crewed and
robotic exploration. As is described further in the next chapter, many of these
systems are directly or indirectly affected by the gravity level. A fundamental
understanding of the low-gravity behavior of fluids and materials is likely to be
critical to the successful development and performance of such systems—both to
avoid the expensive alternative of trial-and-error development and to create the
knowledge base for the generation of novel designs capable of increasing
efficiency and decreasing cost. No list of microgravity phenomena prepared today
would be sufficient in scope and depth to describe all the challenges to be
encountered in spaceflight missions in the future. However, with the goals now
stipulated by HEDS, at least some of the new challenges can be considered to
merit near-term microgravity research support, and strategies and programs can
be developed to ensure that over time all important challenges will be discovered
and addressed.
In order to understand the technology needs of HEDS, and the part to be
played by microgravity research in addressing those needs, NASA will need to
specify target missions for study. Two possible target missions that are often
cited are a return to the Moon for an extended period of human habitation and a
crewed mission to Mars.10 Such target missions help create a focus on the
specifics of the appropriate technical challenges that should drive additional
scientific research and technological development. Moreover, target missions, in
their accomplishment, also provide unique "laboratories" for performing additional
research that could help make settlement and travel to the inner planets at least
possible, if not precisely routine, in the future. Definitions of target missions
should not be used, however, to constrain the scope of either basic or applied
research to conform to near-term purposes, nor should the range of technology
interests be limited. Indeed, the technologies in which significant resources are
invested should be those that are capable of evolution and extension to meet the
long-range HEDS goals of interplanetary travel. It is especially important that
MRD research in support of these goals not be limited to specific targets.
1. Space Studies Board, National Research Council. 1995. Microgravity
Research Opportunities for the 1990s. National Academy Press, Washington,
D.C.
2. Steele, F.R. 1996. Optimism invades HIV conference. Nature Med.
2:257-258.
3. Duray, P.H., Hatfill, S.J., and Pellis, N.R. 1997. Tissue culture in
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microgravity. Science and Medicine 4:46-55.
4. Freed, L.E., and Vunjak-Novakovic, G. 1997. Microgravity tissue
engineering. In Vitro Cell Dev. Biol. Anim. 33:381-385.
5. Akins, R.E., Schroedl, N.A., Gonda, S.R., and Hartzell, C.R. 1997.
Neonatal rat heart cells cultured in simulated microgravity. In Vitro Cell Dev. Biol.
Anim. 33:337-343.
6. Baker, T.L., and Goodwin, T.J. 1997. Three-dimensional culture of
bovine chondrocytes in rotating-wall vessels. In Vitro Cell Dev. Biol. Anim. 33:358-
365.
7. Molnar, G., Schroedl, N.A., Gonda, S.R., and Hartzell, C.R. 1997.
Skeletal muscle satellite cells cultured in simulated microgravity. In Vitro Cell
Dev. Biol. Anim. 33:386-391.
8. Hymer, W.C., Barlow, G.H., Blaisdell, S.J., Cleveland, C., Farrington,
M.A., Feldmeier, M., Grindeland, R., Hatfield, J.M., Lanham, J.W., and Lewis,
M.L. 1987. Continuous flow electrophoretic separation of proteins and cells from
mammalian tissues. Cell Biophys. 10:61-85.
9. National Aeronautics and Space Administration (NASA). 1996. NASA's
Microgravity Science and Applications: Program Tasks and Bibliography for FY
1995. NASA-TM-4735, NASA, Washington, D.C.
10. Cohen, A. 1989. Report of the 90-Day Study on Human Exploration of
the Moon and Mars. NASA-TM-102999, NASA, Washington, D.C.
