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Introduction One of the remarkable features of a spacecraft is that it pro- vides the opportunity for carrying out scientific experiments in a microgravity environment. The principal message that the task group wishes to convey in this report ~ that there are, indeed strong reasons for exploiting this opportunity. In the following introductory paragraphs, the task group will describe in fairly general terms the rationale for research in mi- crogravity science, and outline what it feels are some important constraints that must be observed to implement a successful pro- gra~n. The central portion of this report ~ devoted to illustrative descriptions of a few scientific projects. The report concludes with a summary of the task group's recommendations. The task group stresses at the outset that a NASA program in microgravity science mutest be fundamentally different from other space-based projects in its underlying motivation and structure. Unlike, for example, a space telescope or a gravity probe, which provide unique windows to the world beyond the surface of our planet, microgravity science provides just one especially new way of looking at a vast range of phenomena. Thus, general areas of opportunity are emphasized here rather than specific projects. The task group will describe a number of such projects in some detail. 59
60 The task group's primary purpose in doing this is to illustrate why a long-term program Is interesting and useful. There are two related reasons for interest in microgravity from the most fundamental scientific point of view. The more mundane is that gravity often induces effects that obscure essential features of the phenomena being studied. The denser product of a phase transition or a chern~cal reaction settles to the bottom of the sam- ple before all the intrinsically unportant processes have gone to completion. Or the homogeneity of the sample is destroyed be- cause its own weight compresses it more at the bottom than at the top, thus impairing measurements. Such effects occur cormnonly in experiments in hydrodynamics, in novel materials, in biological systems, and elsewhere. We are seeing enormous progress in such areas largely because new experimental and computational tech- niques allow us to obtain and interpret information in the kind of detail undreamed of just a few years ago. The examples described later in this report illustrate a number of cases where elimination of gravitational effects leads to a major simplification. The second, more dramatic reason for interest In m~crogravity science is the real possibility of Recovering completely new phe- nomena. Obviously, we cannot point with any certainty to areas where such discoveries might be made, but some fascinating hints do exist. In the examples section of this report the task group will describe the theoretical prediction of a new state of matter called a "fractal aggregate"an extremely tenuous form of a space-fi~ling solid whose effective dimensionality, In a mathematical sense, is less than three. Such a material would be intrinsically unstable in a gravitational field, but an understanding of its properties in a freely suspended state might be profoundly meaningful. In addi- tion, there are a number of other situations discussed below where removal of gravitational effects could conceivably reveal entirely new and unexpected phenomena. In presenting specific examples of how a m~crogravity environ- ment may be used to carry out significant scientific research, the task group will focus on several broad categories of phenomena distinquished by the manner in which gravitational forces affect the states or processes involved. First is a set of situations in which careful measurements of a system's equilibrium properties are desired, but where the system's state of equilibrium in a gravi- tational field IS different from what it would be without that field. The classic example of such a situation is liquid helium near its
61 larnb~a point or, more generally, a very wide variety of systems that undergo dramatic changes in their thermodynamic properties in the neighborhood of second-order phase transitions. These so- called "critical phenomena" have been of immense interest during recent years because their theoretical interpretation has a bearing on scientific topics ranging from applied metallurgy to elementary particle physics and cosmology. The theory of critical phenomena has become detailed and sophisticated. To test its limits of valid- ity, we now need extremely accurate measurements. A practical I~rnit to the accuracy of earth-based experiments is imposed by the gravitationally induced nonuniformity of the equilibrium state of the system; the pressure Is greater at the bottom than at the top. As a result, only a very small part of the system can be held precisely at its critical point. In other words, gravity limits the effective site of the experimental sample by deforming it, and thus limits the precision of measurements. Planning for a lamb~a-point experunent In space is now well under way and Is described below. Also discussed below ~ a similar situation in which gravitational deformations of granular media inhibit accurate observation of failure mechanisms In such materials. The second category of examples described here includes situa- tions In which gravity causes stationary states or processes not just to deform but actually to become unstable. The gravitationally induced collapse of a fractal aggregate is an especially interesting example of such a situation. Another example in this category is a phase-separating mixture of fluids. Here the fundamentally interesting diffusive interactions between emerging droplets are obscured by buoyancy-induced flow. In this example we are deal- ing with a class of phenomena involving nonequilibrium states of matter, a largely underdeveloped field of scientific enquiry. The in- terest here is in processes rather than simply states of equilibrium, and the potential relevance of the microgravity environment stems from the fact that gravity makes it difficult or impossible to disen- tangle one process from another. In the case of the fluid mixture, the evolution of a precipitation pattern uncler conditions of diffu- sion control is of great fundamental interest; but simple diffusion ordinarily is disrupted by sedimentation the lighter precipitates float to the top and the heavier ones sink- before quantitatively satisfactory measurements can be completed. A final category of examples includes strongly nonequili~ rium processes very much at the frontier of modern research in
62 which the simplification achieved by elirn~nating extraneous grav- itational effects might make it easier to discover the fundamental principles that are involved. The specific examples presented here are surface-tension-driven flows in hydrodynamics, pattern for- mation in crystal growth, combustion of particulate clouds, and electrically driven flows, all of which figure in current plans for research on the Space Shuttle. However, the task group believes that the potential for m~crogravity research in this general area is much broader than has yet been appreciated; in particular, many of the basic issues in this area are directly relevant to bio- logical processes. The difference between this category and the previous one is that this category treats systems that are being driven persistently away from equilibrium and that exhibit com- plex dynamical behavior. Basic scientific questions include how to predict whether this behavior will be regular or chaotic, what spatial patterns will be formed, and whether those spatial patterns themselves wid be stable. A common feature of many phenomena where such questions are relevant is that underlying symmetries may be broken by gravitational forces. The front-to-back symme- try of the environment of a propagating flame front or the tip of a growing crystal may be broken by buoyancy; or an interesting instability of a hydrodynamic system may actually be stabilized by gravity because of symmetry breaking. In all of these cases, gravitational effects perturb, obscure, or completely destroy the dynamical behavior of interest. The focus of this report is entirely on basic science. The task group will not try to evaluate or make recommendations con- cerning specific technological applications. In general, however, the task group believes that ultimately the greatest commercial benefits of microgravity science will be gained from interactions between basic and applied research, rather than from direct ef- forts to manufacture or process materials in space. As we gain understanding of the conditions under which gravity fundamen- tally alters structures and dynamic processes, novel applications are likely to develop. The task group views the creative interplay of basic and applied science in microgravity to be of the highest priority, with the benefits likely going in both directions. For ex- ample, semiconductors are the best understood materials in the world today, largely because of the commercial importance of the transistor. In turn, the transistor was invented on the basis of new quantum mechanical understanding of the dynarn~cs of electrons in
63 solids. The task group believes that we should be looking for such breakthroughs in technology- as well as science when exploiting the microgravity environment. Let us turn now to some issues that seem to the task group to have major implications for the implementation of a national program in microgravity science. First is the role of the computer, which has revolutionized the way In which research is performed, and which must be taken into account in deciding whether and how to carry out experiments in space. It seems highly likely that within the next decade well before a microgravity facility is apt to be in full operation aboard a space station we will be able to perform accurate numerical sunulations of many of the systems discussed in this report. Fully three-dimensional simu- lations of fairly complex hydrodynamical flows, flame fronts, or solidification patterns seem to be only slightly beyond the reach of current sup ercomputers and numerical algorithms. They will almost certainly be possible within a few years. Does this mean that experiments in space will be irrelevant? In some cases, the answer to this question may be yes. If we are quite sure of the physical ingredients of the simulation model, and have been able to test the accuracy of the computer code against ground-based experunents, then there would seem to be little sense in spending the effort and resources necessary to fly an experimen- tal "analog computers on a spacecraft. On the other hand, if there is fundamental uncertainty about the physical principles, then nu- merical simulation may actually be the key to making microgravity experiments feasible and meaningful. The ideal microgravity ex- periment will be one in which the computer has been used, not just to design the apparatus and control its operation, but also to make the observations more meaningful by providing quantitative predictions. Thus, the new level of interaction between theory and experiment made possible by the computer provides a rationale for microgravity science that would not exist otherwise. The computer can help investigators design experiments effectively to highlight critical features of the phenomena being studied; and properly designed experiments can test and refine theoretical hypotheses in a way that, ultimately, will lead to better simulations. The last issue that the task group will address here concerns what might be called the ~infrastructure" of microgravity science. As noted earlier, this field is quite different from other, more uniquely space-relate`] areas of research. The differences pertain
64 both to the nature of the research and to the way in which it ~ carried out. A thoughtfully implemented m~crogravity program may eventually have an ~rnpact on all fields of science and tech- nology that are concerned with properties of materiab. Thus, few areas of scientific activity should be completely uninterested in it. Conversely, few of these areas of activity are completely dependent on a m~crogravity facility in order to pursue their investigations. In contrast, astronomy, astrophysics, and relativity are coming to rely heavily on space-based observational facilities. All of the research problems discussed here have emerged in fields that have existed outside of space science and will continue to exist with or without a laboratory on a space station. The task group has argued that a m~crogravity facility may be quite useful in many of these areas and might even be crucial for certain special projects. In addition, it argues that, in order to take advantage of these oh portunities, we must recognize the special needs of the scientists who wall do the work. A crucial ingredient of a successful microgravity program, the task group believes, should be a weli-equipped, scientifically staffed research center that functions much as a national laboratory does. The purpose of this center would be to provide leadership in both the scientific and the technological aspects of niicrogravity re- search. Such a center should support full-time, m-house activities and should also serve scientists from universities, industry, and other national laboratories by enabling them to carry out micro- gravity research in a timely, cost-effective manner. This proposal is intended to encourage a broad range of the nation's most capable scientists to commut their efforts to m~crogravity research. Micro- gravity exper~rnents generally will be relatively small but crucial parts of broader research programs in which related problems are berg attacked using different techniques. If they are to work at all, the m~crogravity projects will have to fit comfortably into this overall scheme, both with regard to timing and with regard to the amount of effort required. Thus, the costs to investigators in both time and resources will be crucial factors in their decisions about whether to attempt experunents in space. A visible and effective national research center would address many of these practical questions. Its permanent staff could relieve principal investiga- tors of the need to master all of the special techniques required for space experiments and, with experience, develop efficient and
65 flexible procedures that wait make the m~crogravity laboratory ac- cessible to those who are best able to take advantage of it for basic scientific purposes. This staff would also be able to help investiga- tors shorten the path to success, perhaps by using supple, quickly implemented experiments as opposed to major long-term projects. In summary, the task group believes that a m~crogravity re- search facility can justifiably be made a significant component of plans for the future of space science. Although the m~crogravity program may ultimately have a major impact on technology, its principal rationale should be its importance for basic research. Successful implementation of such a program requires extensive ground-based preparation of individual projects, starting with the careful definition of the basic questions to be addressed, and con- t~nuing with detailed simulations and design studies for experi- ments. This is a serious effort for which, the task group believes, the rewards will be substantial. Finally, the task group views the active participation of out- standing young scientists as a crucial element in the long-term productivity of the m~crogravity program.
