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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 CHAPTER 2 1. As a driving force for convection in fluids. Differences in density, CHAPTER 3 resulting from inhomogeneity in temperature and/or composition, can cause an CHAPTER 4 otherwise quiescent fluid to convect, thus giving rise to convective heat and mass APPENDIX A transport. In fluids, on Earth, convection of molecular species is typically orders of APPENDIX B magnitude more rapid than the slow migration caused by molecular diffusion due APPENDIX C to Brownian motion. 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 file:///C|/SSB_old_web/cmgr92ch1.htm (1 of 6) [6/18/2004 11:08:13 AM]
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 file:///C|/SSB_old_web/cmgr92ch1.htm (2 of 6) [6/18/2004 11:08:13 AM]
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: file:///C|/SSB_old_web/cmgr92ch1.htm (3 of 6) [6/18/2004 11:08:13 AM]
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. file:///C|/SSB_old_web/cmgr92ch1.htm (4 of 6) [6/18/2004 11:08:13 AM]
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 2015âFundamental Physics and Chemistry. National Academy Press, Washington, D.C. Last update 7/13/00 at 10:34 am Site managed by Anne Simmons, Space Studies Board file:///C|/SSB_old_web/cmgr92ch1.htm (5 of 6) [6/18/2004 11:08:13 AM]