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Microgravity Research Opportunities for the 1990s: Chapter 1
Microgravity Research Opportunities
for the 1990s
PART I—OVERVIEW
1
Introduction
Microgravity research is concerned with the identification and description
of the effects of reduced gravitational forces on physical, chemical, and biological
phenomena. Microgravity research probes a new parameter space where
gravitational acceleration no longer is equal to 1 g and, instead, can approach
values that are orders of magnitude lower. Gravity affects a wide variety of
scientific areas, some of which have profound implications for space exploration.
Scientific disciplines that are affected include fundamental physics, fluid
mechanics and transport phenomena, materials science, biological sciences, and
combustion. These disciplines are investigated predominantly as laboratory
science, which requires the use of controlled, model experiments. Laboratory
REPORT MENU
experiments often require the constant attention and interaction of the
NOTICE
experimenter, and their results are validated by their reproducibility. It is its
MEMBERSHIP
laboratory science character that distinguishes microgravity research from most
PREFACE
of the other areas of science commonly recognized as space studies. In this
EXECUTIVE SUMMARY
report, these fundamental characteristics of microgravity research are illustrated
PART I
in many different ways.
CHAPTER 1
CHAPTER 2
PART II As is usually the case in the physical and biological sciences, discoveries
CHAPTER 3
are made when a new area or novel parameter space is explored. Microgravity
CHAPTER 4
research, despite its relative infancy, is no exception. Increasingly, fundamental
CHAPTER 5
processes that were thought to be well understood under terrestrial (1-g)
CHAPTER 6
conditions have, in fact, proved to behave in altered and even startlingly
CHAPTER 7
unfamiliar ways when observed and measured in reduced-gravity environments.
PART III
Space experiments in areas such as combustion, fluid flow and transport, phase
CHAPTER 8
separation, fundamental physics, and biology have revealed new phenomena
APPENDIX A
and have demonstrated new and occasionally unpredicted behavior. Indeed,
APPENDIX B
many commonly experienced phenomena can be dramatically altered in a low-
gravity setting: for example, the dynamic behavior of gas bubbles in fluids, the
sedimentation of particles, the characteristics of the flame of a burning candle,
and the behavior of fluids within and outside their containers. This modified
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Microgravity Research Opportunities for the 1990s: Chapter 1
behavior will profoundly affect the technology for handling liquids and gases in
reduced-gravity environments. Nevertheless, virtually all scientific experimental
research in these areas has taken place in the terrestrial setting of 1 g.
Microgravity science is neither a homogeneous nor a distinct discipline. It
has evolved gradually over the past two decades into a range of multidisciplinary
space research activities in which the unifying features are basic and applied
studies of gravitational interactions with fluids and a variety of condensed states,
and with transport phenomena in physical, chemical, and biological systems. In
fact, the fluid mechanics and transport sciences constitute the core science
content of microgravity studies. Therefore, in considering new and important
directions for microgravity research, considerable emphasis is given in this report
to the study of transport phenomena. These phenomena are strongly gravity
dependent; they have extensive applications to space systems engineering; and
they play a pivotal role in many important modern technologies.
THE MICROGRAVITY ENVIRONMENT
The availability of orbiting laboratories, both crewed and uncrewed, has
provided access to an environment in which processes can be studied under
sustained levels of reduced gravity. The term microgravity is itself something of a
misnomer. Because the gravitational force is infinite in extent, low-gravity
conditions can be achieved only at substantial distances from any massive
object. In the current context, however, microgravity describes the acceleration
conditions attending free-fall, that is, the acceleration sensed within an inertial
reference frame that is at rest with respect to the local environment but
accelerating toward the center of the Earth. Such conditions occur for limited
periods in drop towers and in aircraft executing parabolic orbits. Spacecraft and
satellites in near-Earth orbit also operate at reduced gravity levels (down to 10-6
g) and have the additional advantage of providing much longer times for
exploiting this environment.
The principal characteristic of low-gravity environments is that the net
gravitational body forces are reduced substantially. This leads to decreases in
hydrostatic pressure, buoyancy-driven flows, and rates of sedimentation.
Reduction of the gravitational force, or weight, permits other forces, such as that
due to surface tension, to become important, if not dominant. An immediate
consequence is that under microgravity conditions, multicomponent and
multiphase fluid and other fluid-solid system behaviors (e.g., mixing, separation,
and interfacial phenomena) are significantly altered. Many other fluid transport
phenomena involving heat and mass transfer are also influenced by the reduction
of gravity. There is increasing recognition that transport phenomena are of central
importance in a wide range of physical, chemical, and biological systems that
play major roles in many terrestrial and space-based technologies. Specifically, if
NASA's long-range program in space science, exploration, and transportation is
ever to be realized, it is essential to obtain a comprehensive understanding of low-
gravity fluid phenomena and their consequences for extraterrestrial processing.
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Microgravity Research Opportunities for the 1990s: Chapter 1
PERCEPTIONS AND REALITIES OF MICROGRAVITY RESEARCH
The history of microgravity research is not extensive-the field is barely a
few decades old. Furthermore, only a limited number of microgravity experiments
and samples have been studied to date, and the accumulated microgravity
laboratory experience known to the U.S. scientific community amounts to less
than 1000 hours of in-orbit time. Several recent NASA shuttle missions have
been dedicated to laboratory science in microgravity (USMP-2 and IML-2) and
are expected to yield extensive data and sample returns. Some information about
the scientific results from spaceflight is provided in Chapters 3 through 7. At
present, however, all of the data have not been fully analyzed and reported.
Nevertheless, there have been some notable successes. For example,
the Lambda Point Experiment (LPE) was flown on the shuttle in October 1992
(USMP-1). Analysis of the data indicates that an improvement of nearly two
orders of magnitude over previous data obtained on Earth has been achieved.
This was done in an unbiased way, and heat capacity data approaching within a
few nanokelvins of the Lambda point were obtained. This experiment gives a
clear demonstration that highly sophisticated experiments involving the most
sensitive and advanced instrumentation can be performed reliably in the
microgravity environment.
For many years, microgravity research was perceived as focused
primarily on materials processing. Indeed, the microgravity line item in NASA's
budget has even been titled "Materials Processing in Space." Although NASA's
microgravity research program certainly includes research conducted on
materials science and materials processing, the reality is that this program is
much broader in its scope. Specifically, current microgravity research, viewed as
a laboratory science, is also broadly directed toward fundamental studies in
physics, chemistry, and biology. Access to prolonged periods in space, as well as
to other short-duration, ground-based microgravity facilities, is beginning to
provide researchers with the opportunity to apply the methods of the physical and
biological sciences to a new regime of low-gravity experiments. Thus, the
acceleration of gravity is becoming a meaningful, independent, experimental
parameter. This approach is analogous to that employed for more than a century
in low-temperature physics, in which diverse phenomena are studied in a
cryogenic regime where temperature is the primary independent variable that
may be set arbitrarily close to zero.
In summary, microgravity research involves strong elements of traditional
laboratory science, in contrast with the observational science that constitutes
most of the NASA space science program. Experiments often involve controlled
model systems and exacting parameter settings. Numerous experimental
conditions must be explored, and repeat experiments must be conducted for
each investigation in order to understand the phenomena of interest and
establish, according to scientific standards, the validity and reproducibility of the
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Microgravity Research Opportunities for the 1990s: Chapter 1
data. By contrast, however, the U.S. microgravity research program has
produced in its limited experience of in-orbit experimental time only a few score
samples and sets of data that meet high scientific standards. This situation is not
surprising in view of the fact that humans have been able to make observations in
the absence of gravity for less than a quarter of a century and that serious
laboratory experiments have been feasible in space for only about 15 years. The
decade of the 1990s represents the first phase of experimentation in microgravity
and the potential advent of a true laboratory science in space.
A number of previous National Research Council (NRC) reports have
evaluated the field of microgravity research at different stages of its development.
The 1978 report Materials Processing in Space (the "STAMPS" report)1 provided
guidance during the program's early years, whereas Space Science in the
Twenty-First Century: Imperatives for the Decades 1995 to 20152 attempted to
identify future research opportunities. More recently, the 1992 report Toward a
Microgravity Research Strategy3 began to lay a foundation for a more mature
research program, and the current report is a continuation of that effort. More
detailed descriptions of these studies can be found in the preface.
Although this report does not provide a complete strategy for microgravity
research, it does present some of the important elements of a strategy, including
the following:
A summary of the current state of knowledge of microgravity science;
A discussion of some of the fundamental questions to be answered;
A presentation of the goals of the field of microgravity science;
The science objectives within each discipline;
An evaluation of the potential for microgravity research to provide
advances within each discipline;
The experimental requirements for achieving the science objectives of
each discipline;
A description of the other resources required for a successful
microgravity science program; and
A limited prioritization of research topics within each discipline. The
two aspects of a strategy for microgravity research that are not presented in this
report are (1) a prioritization of microgravity research objectives across
disciplines and (2) a cost-benefit analysis of anticipated microgravity results.
Experience has shown how difficult it is to set research priorities, even within a
single homogeneous science discipline. Reaching agreement on priorities for
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Microgravity Research Opportunities for the 1990s: Chapter 1
microgravity research relative to all other science research was judged so
unlikely that it was not attempted in this report. The other practical reason that
stricter priorities were not set for microgravity research stems from the nature of
shuttle flights. Frequently, payloads are assigned to a flight because their
requirements for space, power, and so on, fit what is available, and scientific
priority per se is less important. No cost-benefit analysis was attempted because
the assumption is that orbiting platforms, launched for other purposes, will be
available for microgravity research. Decisions on the availability of platforms such
as the shuttle or space station are essentially programmatic issues in which
microgravity research is only one of many considerations. Furthermore, the cost-
benefit of a microgravity program has not been compared to the cost-benefit of
experiments on different subjects in the terrestrial environment. Experiments that
can be performed adequately under terrestrial gravity conditions, however, are
not given a priority for spaceflight in this report. Finally, although the value and
need for human intervention capabilities and long-duration flights are noted
repeatedly in this report, nothing herein should necessarily be interpreted as
advocating or opposing any specific NASA spacecraft or space station design
initiative.
The remainder of Part I of this report discusses current research
opportunities and presents the major conclusions and recommendations of the
report. Specific conclusions and recommendations for each of the five disciplines
of microgravity research are discussed first and are followed by general
recommendations and conclusions that prioritize research issues and provide
some general guidance for administrative policy and procedures. Part II presents
a status chapter for each of the five scientific disciplines. Chapter 3 discusses the
status of fluid and transport science and also serves to introduce the other
science sections because fluid flow is a theme common to most of the other
areas. Some discussion concerning facilities and research administration is
presented in Part III, which covers programmatic issues.
REFERENCES
1. Space Science Board, National Research Council. 1978. Materials
Processing in Space. National Academy of Sciences, Washington, D.C.
2. Space Science Board, National Research Council. 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.
3. Space Studies Board, National Research Council. 1992. Toward a
Microgravity Research Strategy. National Academy Press, Washington, D.C.
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