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Toward a Microgravity Research Strategy (Chapter 1)
Toward a Microgravity Research Strategy
1
Overview of Microgravity Research
Microgravity research is scientific investigation conducted in a
gravitational field (or equivalent acceleration with respect to an inertial frame) that
is a small fraction of the gravitational acceleration, gE, on Earth. Strictly speaking,
the prefix "micro" would imply a field of 10-6 gE, but the word "microgravity" is
used in a more generic sense in this report to describe gravitational fields that are
typically less than 10-2 gE and might be lower than 10-6 gE. In consideration of
future missions to the Moon and to other planets, there might also be interest in
studies at other levels of reduced gravity, for example, 0.16 gE (Moon) or 0.3 gE
(Mars).
Gravity is certainly a weak force compared to the "strong" or "weak"
nuclear forces that bind atomic nuclei or subnuclear particles or to the
electromagnetic forces that bind atoms and molecules. Therefore, the role of
REPORT MENU gravity in physical phenomena is important only when stronger forces are already
NOTICE in balance or when other special circumstances arise. Consequently, gravity is
MEMBERSHIP important in the following cases:
SUMMARY
CHAPTER 1
1. As a driving force for convection in fluids. Differences in density,
CHAPTER 2
resulting from inhomogeneity in temperature and/or composition, can cause an
CHAPTER 3
otherwise quiescent fluid to convect, thus giving rise to convective heat and mass
CHAPTER 4
transport. In fluids, on Earth, convection of molecular species is typically orders of
APPENDIX A
magnitude more rapid than the slow migration caused by molecular diffusion due
APPENDIX B
to Brownian motion.
APPENDIX C
APPENDIX D
APPENDIX E
2. As a driving force for phase separation. Once thermodynamic
APPENDIX F
considerations (the equalization of chemical potentials dominated by
electromagnetic forces) have led to coexisting phases, such phases still can have
different densities, and phase separation can take place, even by sedimentation
over long periods of time, if the difference in density is slight.
3. As a force that helps to determine the free surface morphology of
fluids. Even at or near thermodynamic equilibrium, single phases or multiphase
systems will be bounded by surfaces or interfaces whose morphology is
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Toward a Microgravity Research Strategy (Chapter 1)
determined by a balance of forces, including gravitational forces. Examples arc
the shapes of bubbles, droplets, and liquid zones.
4. Near a critical point. Near such a point, the balance of forces is so
delicate that thermodynamic and transport phenomena can exhibit divergent or
otherwise anomalous behavior. Thus even the slight inhomogeneity in hydrostatic
pressure that arises whenever matter is in a gravitational field can lead to
important differences in observable features.
5. In the presence of very weak binding forces. There could be cases,
particularly in living systems, in which gravitational forces play a subtle but
important role, possibly because of the weak forces that bind macromolecules.
Examples might be the degree of perfection of a crystallized protein or the
development of a cell, an embryo, or a plant.
6. In the presence of very large masses or for very long times.
Examples arc relativistic phenomena such as the bending of starlight and
gravitational waves.
7. In structural members or over large distances. Examples include
stresses in buildings and bridges or phenomena within Earth and its atmosphere.
EXAMPLES OF MICROGRAVITY EXPERIMENTS
Most microgravity experiments and applications to date have involved
gravity as a driving force for (1) convection in fluids and (2) phase separation.
Examples of such experiments include the following:
1. For gravity as a driving force for convection in fluids
a. Crystal growth from fluids. The growth of crystals from either
the melt, vapor, or solution can be quite different in microgravity
conditions than on Earth because of reduced convection.
Sometimes reduced convection can create a product that has a
more uniform composition and structure but is not necessarily a
better one from the standpoint of applications. Moreover, the
virtual absence of convection results in a process that is controlled
by molecular diffusion and, therefore, is usually more predictable
and amenable to modeling.
b. Fundamental phenomena in crystal growth. The fundamental
mechanisms governing phenomena that occur during crystal
growth, particularly those that pertain to crystal-fluid interface
morphology (dendrites, cells, and the structure of in situ eutectic
composites), are not well understood and often are masked by
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Toward a Microgravity Research Strategy (Chapter 1)
convection that is unavoidable on Earth. Carefully designed
experiments conducted in microgravity might guide choices
among alternative theories.
c. Convection phenomena. In microgravity, buoyancy forces in the
equations of fluid dynamics can be negligible, or at (cast
adjustable, thus allowing the study of other phenomena. Examples
are convection driven by secondary mechanisms, such as surface
tension gradients (sometimes called Marangoni convection), and
turbulence that is dominated by inertial and viscous forces.
d. Measurement of the transport properties of fluids. The
transport properties of fluids, characterized by such features as
diffusion coefficients and Soret coefficients (diffusion driven by
thermal gradients), can be measured much more accurately in
microgravity experiments, leading to data with far less scatter than
data obtained on Earth (sometimes even to transport coefficients
with signs opposite to those measured on Earth) and to the
possibility of identifying controlling mechanisms.
e. Combustion phenomena. Typically, buoyancy-driven convection
plays a major role in the transport of materials associated with
combustion processes that occur over large lengths and/or at
small velocities. The very notion of a flame invites the image of
rapidly rising incandescent gases. However, flammability and
flame propagation are quite different in microgravity than on Earth.
It should be possible to conduct experiments with nearly spherical
symmetry that would permit comparison of the results with
tractable models.
f. Fire safety aboard spacecraft. From the perspective of fire
safety aboard spacecraft, it is important to understand and control
the practical consequences associated with combustion
phenomena.
2. For gravity as a driving force for phase separation
a. Immiscible alloys and multiphase solids. Phase separation
resulting from sedimentation can be avoided in microgravity, thus
possibly giving rise to uniformly dispersed composite materials.
However, uniform dispersal will be possible only if other
phenomena (e.g., Brownian motion and Marangoni convection)
can be overcome. Furthermore, two-phase fluid flow can be
studied with attention to such characteristics as "slug flow," which
can influence heat transfer coefficients.
Among the experiments based on other roles of gravity are the following:
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Toward a Microgravity Research Strategy (Chapter 1)
3. For gravity as a force that helps to determine the free surface
morphology of fluids
a. Containerless processing. Given the virtual weightlessness of
materials in microgravity, it should be possible to process highly
reactive or ultrapure materials by avoiding contact with a
container, although some slight force is necessary to maintain
positional stability.
b. Drop dynamics. The dynamics (e.g., vibrational modes and
bifurcations) of freely floating droplets can be studied
experimentally and compared with theoretical models to enhance
our understanding of nonlinear continuum mechanics.
4. For gravity near a critical point
a. The Lambda Point Experiment, whose objectives are to
measure the heat capacity of a sufficiently large volume of
superfluid helium very near to its critical temperature and to make
accurate measurements of critical point exponents, in order to
better test current theory.
b. The Zeno Experiment, which is designed to measure the large
density fluctuations in xenon near the critical point that are
responsible for the phenomenon of critical opalescence.
Gravity's role in the presence of very weak binding forces has not led yet
to any readily identifiable experiments. However, the growth of protein crystals
could be affected by convection phenomena, as in la above, or by the more
subtle action of gravitational forces that can distort macromolecules. Similarly,
weak gravitational signals could influence the direction of the growth of plants
(e.g., as in geotropism) or the development of cells.
Comments on experiments based nn the influence of gravity in the
presence of very large masses or for very long times, others such as Gravity
Probe B, and other experiments similar to those mentioned in 4a and 4b above
are made in Space Science in the Twenty-First Century.1
Phenomena related to the role of gravity in structural members or over
long distances are beyond the scope of this report, but they must be considered
in the context of habitation and structures on the Moon or the planets. For
example, the engineering of a building or a bridge on the Moon (0.16 gE) might
entail selecting materials that are entirely different from those that would be
chosen for use on Earth.
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Toward a Microgravity Research Strategy (Chapter 1)
REFERENCE
1. Space Science Board. 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.
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