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Microgravity Research Opportunities for the 1990s: Chapter 7
Microgravity Research Opportunities
for the 1990s
PART II—SCIENTIFIC ISSUES
7
Microgravity Physics
INTRODUCTION
The intersection between the special research possibilities offered by
microgravity conditions and the general interest of the physics community is
relatively small. This is understandable since gravitational forces play only the
most minor role in the traditional areas of physics such as high-energy, nuclear,
and condensed matter physics. Nevertheless, there are a number of problems of
interest in which access to space and the microgravity environment may allow
important research advances.
REPORT MENU
The subject of microgravity physics consists of many diverse topics united
NOTICE
by their common requirement for free-fall conditions. A similar situation holds for
MEMBERSHIP
low-temperature physics where the subject is united by the technical
PREFACE
EXECUTIVE SUMMARY requirements for low temperatures. The topics of microgravity physics may be
PART I cast into three categories for the purposes of discussion.
CHAPTER 1
CHAPTER 2
New instruments. The very low stress conditions possible in the
PART II
microgravity environment allow the construction of instruments of unique
CHAPTER 3
sensitivity. Examples are the superconducting gyroscopes of the Gravity Probe-B
CHAPTER 4
experiment and the mass balance of Everitt and Worden's equivalence principle
CHAPTER 5
test.1 Access to space allows the construction of instruments on a length scale
CHAPTER 6
CHAPTER 7 not limited by the size of the Earth, such as a long-baseline (106 km) gravitational
PART III
wave antenna. The freedom from the disturbance of microseisms that set the
CHAPTER 8
noise floor for Earth-bound gravitational wave detectors is an important benefit of
APPENDIX A
the microgravity environment.
APPENDIX B
Unique samples. Under free-fall conditions, gravitationally induced
compression effects are essentially absent and it is possible to create
experimental samples with unprecedented uniformity. This possibility is
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particularly important in the area of critical phenomena. Relief from gravitational
and vibrational stress will also be important for the preparation and study of
delicate structures such as low-density granular materials near the percolation
limit.
New dynamics in fluids. Fluid systems will provide a rich area for the
study of new dynamic phenomena that can be expected in the microgravity
environment. The absence of buoyancy forces arising from nonuniform density
distributions under free-fall conditions allows weaker forces to come into play and
produce new dynamic behavior. Understanding the behavior of fluids in space is
important not only from the point of view of fundamental science but also for the
solution of practical engineering questions involved in spacecraft design. Fluids
under microgravity conditions constitute an extensive subject that is discussed
separately in this report.
THE CURRENT MICROGRAVITY PHYSICS PROGRAM
Two extensive reviews of the field of fundamental science in space2,3
discuss in considerable depth experiments currently being developed for flight in
the near future and longer-term prospects for the field. It is useful to discuss a
number of representative experiments to gain an appreciation of the motivations,
promise, and difficulties involved in microgravity physics.
Fundamental Physics Measurements
Access to space and the microgravity environment will allow tests of
several of the most fundamental aspects of physics. The most elemental of these
is an experiment being developed at Stanford University4 to test the equivalence
principle. This principle, which asserts the identity of inertial and gravitational
mass or the impossibility of distinguishing within a system between acceleration
and a gravitational field, lies at the heart of Einstein's theory of gravitation. The
equivalence principle is so well imbedded in physics today that one tends to
forget that tests of this principle have a long and honored history dating back to
the experiments with falling bodies conducted by Galileo. In the 1960s, the
equivalence principal was confirmed5 to 3 parts in 1011. The new experiment6
promises an eventual improvement on those results by some six or seven orders
of magnitude. In the original version of the experiment, the acceleration due to
Earth's motion in its orbit was compared to the gravitational attraction of the Sun,
whereas the Stanford experiment will compare the gravitational attraction of Earth
on an orbiting test mass to the acceleration associated with that orbit. The
gravitational force on an object in low Earth orbit is three orders of magnitude
greater than the attraction due to the Sun; hence, an immediate large gain in
sensitivity is realized by going to space. A second advantage of the microgravity
environment is the escape from microseismic disturbances that would limit an
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Earth-bound experiment. As currently developed, this experiment takes
advantage of modern cryogenic technology with superconducting bearings for the
confinement of the test mass and SQUID (Superconducting Quantum
Interference Device) position detectors. A shuttle flight for this experiment would
allow an important test of the operation of this apparatus and should allow an
improvement in the test of the equivalence principle resolution to the level of 1
part in 1015. Given a successful shuttle flight, there would be a strong motivation
to fly this experiment on an independent satellite where the microgravity
environment could be substantially improved and the full promise of this
experiment might be realized.
A second experiment developed by the Stanford group is the very
ambitious attempt to observe the "frame-dragging" or magnetogravitational
effects predicted by Einstein's general theory of relativity. This experiment, which
has been under development for more than 20 years, is known as Gravity Probe-
B (GPB). It consists of four gyroscopes for sensing the frame-dragging effects
that will appear as extremely small torques on the gyros. The spin axes of the
gyros will then be compared with a fixed star. This experiment makes heavy use
of advanced cryogenic technology with the utilization of superconducting gyros
and superconducting readouts. The time scale envisioned for this experiment is
on the order of a year. Although GPB can be given a shakedown test in a shuttle
flight, an independent satellite will be required to satisfy the microgravity
requirements of the experiment, which lie in the 10-10-g range.
Critical Phenomena
The study of critical phenomena has become prominent in condensed
matter physics over the last 20 years. This period has seen remarkable advances
on both the experimental and the theoretical fronts, in particular with the high-
resolution experiments at the l-point transition of liquid 4He and with the
development and successful application of renormalization group techniques to
the calculation of the behavior of many systems in the neighborhood of their
critical transitions.
The subject of critical phenomena presents a very promising area for
microgravity research. The essential advantage of microgravity for studies of
critical phenomena lies in the possibility of achieving relief from the gravity-
induced compression effects always present in the Earth-bound laboratory.
These compression effects are particularly severe in liquid-vapor critical point
studies because the compressibility of the system diverges as the critical point is
approached. Near the transition, the gradient in the hydrostatic pressure leads to
a large density gradient in the sample. Thus, only a diminishing portion of the
sample will satisfy the condition for critical density as the transition is
approached. The situation with the superfluid transition in liquid 4He is somewhat
more favorable since the order parameter does not couple strongly to the density.
Nevertheless, on Earth, gravity-induced hydrostatic pressure leads to a
microkelvin shift in transition temperature for each centimeter of sample height.
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Gravity effects can be minimized, to some degree, by a reduction in the height of
the sample, but this strategy soon runs into sensitivity problems related to sample
size, and on a more fundamental level, problems associated with surface and
finite-size effects set limits to sample thickness.
The -point in liquid helium has been a paradigm for critical phenomena
studies. Liquid helium offers a number of properties that make it an almost ideal
subject for such studies.7,8 It is an exceptionally pure system. In the superfluid
state, helium possesses a very rapid relaxation time, and compared to the liquid-
vapor transition the superfluid transition is relatively insensitive to gravitational
effects. The use of samples less than 1 mm thick has allowed useful data to be
obtained at reduced temperatures, t = 1 - T/T , less than 10-7. Modern
advances in low-temperature thermometry would allow at least a two-order-of-
magnitude improvement in experimental resolution.
Motivated by the central position of the superfluid -point transition in the
critical phenomena field, Lipa and Chui at Stanford University have developed a
microgravity version of the -point heat capacity experiment.9 In the
development of this experiment, they have advanced the state of the art in high-
resolution thermometry to the point that they can resolve temperature to 1 part in
1011. Microgravity conditions are required to fully exploit this superb resolution. In
space, the heat capacity measurements can be extended two orders of
magnitude to the 10-9 level in reduced temperature beyond the best results
achieved in ground-based measurements.
The Lambda Point Experiment (LPE) was flown on the shuttle in October
1992 (USMP-1). Analysis of the data indicates an improvement of nearly two
orders of magnitude over previous data obtained on Earth. Unfortunately, the
final analysis was delayed by several factors. The first was the failure of NASA to
provide the investigators with the full data set on an expeditious time scale. At
least four months passed after the touchdown of USMP-1 before a complete
record of the raw experimental data became available for analysis. A second
factor was the discovery, as is often the case during innovative research, of an
unanticipated experimental problem. The cosmic-ray background encountered in
orbit was found to produce stochastic thermal spikes in the response of the high-
resolution thermometers, resulting in a somewhat degraded performance. The
principal investigator has developed an algorithm for combing the data for the
worst of these spikes. This was done in an unbiased way, and heat capacity data
approaching within a few nanokelvin of the -point have been obtained.
The LPE has been an outstanding success. This experiment gives a clear
demonstration that highly sophisticated experiments involving the most sensitive
and advanced instrumentation can be performed profitably in the microgravity
environment.
A second critical point experiment is also in an advanced state of
preparation. This is the light scattering experiment developed for measurement of
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the correlation length at the liquid-vapor critical point of xenon.10 This experiment
should be able to test the theory of density fluctuations in the nonhydrodynamic
limit much closer to the transition than would be possible on Earth. It has been
expected that the phenomenon of critical slowing (i.e., the divergence of the time
scale for thermal equilibrium as the transition is approached) will be an important
experimental constraint. This would be especially true for an experiment
conducted on the shuttle, where there will be stringent time constraints. There are
indications, however, that there may be an actual speeding up of equilibrium near
the transition.11 If true, then the feasibility of liquid-vapor critical point
experiments will be much enhanced.
FUTURE EXPERIMENTS
UNDER MICROGRAVITY CONDITIONS
Critical phenomena, particularly studies at low temperature, have
occupied a central position in the current program of microgravity research. This
is due to the advanced level of experimental and theoretical sophistication
achieved in this area of condensed matter physics as a consequence of the
vigorous progress of the last two decades. It is expected that the future will see
continued experimental advances from the marriage of modern high-resolution
measurement techniques with the vastly improved sample uniformity possible
under microgravity conditions.
New directions are possible for the study of critical phenomena in space.
Up to this time the emphasis has been on measurement of static properties,
particularly the specific heat in the neighborhood of the critical transition. For the
future, several new directions are possible that represent important extensions
beyond the successful completion of LPE. Chief among these will be studies of
systems far from equilibrium, nonequilibrium dynamic phenomena, and studies of
finite size effects in well-characterized geometry. Again, liquid helium will be the
system of choice, given the large body of experience and its experimental
advantages. Enthusiasm for further work with liquid helium in space will be
strongly enhanced by success with LPE.
Although low-temperature critical phenomena experiments are the most
advanced and tend to drive the field, there are many possibilities for studies of
critical phenomena in other systems that can benefit from microgravity conditions.
Wetting phenomena, nucleation and growth, and the critical behavior in
microemulsions and polymer systems, as well as the behavior of fractal
aggregate systems near their percolation threshold, hold considerable potential
for microgravity research.
Beyond critical phenomena, there are interesting studies to be made on
the static and dynamical properties of low-density aggregate structures and
suspensions. Freedom from sedimentation effects will be most important in such
work. One interesting aspect of low-density aggregate structures is their fractal
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nature. On Earth, there is a minimum to the density of an aggregate structure that
can be stable since such structures tend to collapse of their own weight, because
their strength decreases more rapidly with decreasing density than does the
gravitational stress.
Atom trapping and laser cooling techniques have been advanced in
recent years to the point where atomic transitions can be studied under
conditions of free-fall. In "atomic fountain" experiments, groups of ultracold atoms
are projected upward into a microwave cavity. The dwell time for these atoms,
under free-fall conditions, is limited by the size of the cavity and Earth's
gravitational field. Current experimental configurations allow observation times as
long as 0.5 s and should allow the development of an atomic clock with the
remarkable accuracy of 1 part in 1016. This type of experiment is a clear
candidate for the microgravity environment, where the experimental observation
time could be greatly extended with a further major increase in clock accuracy.
The dynamics of crystal growth is another subject that can profit from
microgravity conditions. To date, there has been considerable interest in the
possibility of improved crystal size and quality with growth under microgravity
conditions where convection is suppressed. Another aspect to the crystal growth
problem is the influence of gravity on the facet size and ultimate equilibrium
shape of the crystal. These questions could probably be best addressed through
studies of growth of free-floating helium crystals under microgravity conditions.12-
14 Helium crystals are particularly good candidates for microgravity research
because they can be rapidly grown, melted, and annealed. Surface relaxation is
so rapid that melting-freezing waves are easily created. The roughening
transition, readily observed in helium crystals, makes their shape strongly
influenced by gravity.15 A large reduction in gravitational influences would
facilitate studies of this unique type of phase transition.
RECOMMENDATIONS AND CONCLUSIONS
The experiments mentioned above, with the exception of the GPB, all
represent extensions of work that is currently performed or can be performed in
Earth-based laboratories. Much of the scientific value of the proposed space
experiments will depend on the strength of the connection to Earth-bound
research. Given the long time scale for the development through flight of a space
experiment, there is a real danger that the scientific goals of the experiment may
be bypassed by new developments or by major shifts in the value ascribed to the
work. There is also the possibility that the principal investigator (PI) may lose
contact with the field. This is particularly likely if the PI becomes heavily involved
with the development of flight hardware for a long period of time. These
considerations argue for considerable support of ground-based research in the
same general area as the candidate flight experiments. Also, it is desirable to
keep approved projects under periodic scientific review throughout the life of the
experiment.
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The quality of the microgravity environment is a topic that must be
examined critically in the context of each possible experiment. Minimum
acceleration is the most obvious parameter of concern for many of the
contemplated experiments; however, the time span over which a high-quality, low-
gravity environment can be maintained may be of equal importance. For some
experiments, accidental large accelerations, such as might result from the
sudden movement of personnel or the firing of small thrustors, may destroy the
object of study, for example, a low-density granular structure.
When considering the quality of the microgravity environment available for
experiment, one should remember that only the center of the orbiting spacecraft
is in true free-fall and then only to the extent that orbital drag effects and other
external influences such as solar wind and radiation are negligible. In a gravity
gradient-stabilized spacecraft, there will be a steady rotation of any experiment
about the center of mass of the entire spacecraft once each orbit. For a low Earth
orbit, this will result in accelerations on the 10-7-g level at a distance 1 meter from
the center of mass of the entire orbiting system. On both the space shuttle and
the space station only a few experiments will be located close enough to the
center of mass to ensure acceleration below the 10-6 level.
It can be expected that a number of scientifically meritorious projects,
such as the equivalence principle experiment and GPB, will require spaceflight
independent of the crewed space facilities.
In the future, one may anticipate a continued requirement for low-
temperature facilities in space since low temperatures are important for the
highest-resolution measurement techniques, particularly those based on SQUID
technology.
If the space station is to be a useful contributor to the area of
fundamental science, access to liquid-helium facilities will be mandatory.
REFERENCES
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Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New
York.
2. Hazelrigg, G.A., and J.M. Reynolds, eds. 1986. Opportunities for
Academic Research in a Low-Gravity Environment. Progress in Astronautics and
Aeronautics, Vol. 108. American Institute of Aeronautics and Astronautics, New
York.
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Microgravity Research Opportunities for the 1990s: Chapter 7
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