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OCR for page 1
An Initial Review of Microgravity
Research in Support of
Human Exploration and Development of Space
Executive Summary
The current organizational structure of NASA includes five strategic
enterprises, one of which is the Human Exploration and Development of Space
(HEDS). Goals set by the HEDS enterprise include (1) increasing knowledge of
nature's processes by use of the space environment; (2) exploring and settling
the solar system; (3) achieving routine space travel; and (4) enriching life on
Earth through people living and working in space. The means by which NASA
proposes to accomplish these ambitious goals include a combination of scientific
research, engineering technology development, and use of the Space Shuttle
and the International Space Station (ISS) as microgravity test platforms. The first
objective stipulated within NASA's HEDS Goal 1 is that scientific research should
be conducted to understand the roles played by gravity and the space
environment in affecting the behavior of biological, chemical, and physical
systems. The second objective within HEDS Goal 1 specifies the innovative use
of major HEDS facilities, such as the Space Shuttle and the ISS, to achieve
breakthroughs in science and technology.
This preliminary report of the Committee on Microgravity Research
examines those areas of microgravity research that not only support the
objectives of Goal 1, but also have the potential to contribute to the eventual
development of the new technologies required to accomplish the remaining
HEDS goals. An initial appraisal is made of types of exploration technologies that,
for development, would require an improved understanding of fluid and material
behavior in a reduced-gravity environment.
The current microgravity research program at NASA's Microgravity
Research Division (MRD) includes five major disciplines: (1) fluid physics, (2)
materials science, (3) combustion, (4) biotechnology, and (5) fundamental
physics. In general terms, fluid physics research encompasses the phenomena of
heat and mass transport in low gravity and underlies many of the scientific and
technological problems associated with long-duration crewed missions exploring
the Moon and inner planets. A strong emphasis remains within the MRD program
on experimental microgravity fluids studies—as opposed to reliance on
computational fluid dynamics (CFD)—because the boundary conditions
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encountered in many reduced-gravity fluid physics studies are less well
understood than in conventional subfields of aerospace research. For example,
relatively weak forces, such as thermocapillary tractions and van der Waals
interactions, which may be ignored in most terrestrial flow problems, can become
dominant in microgravity. Materials science research in the MRD program tends
to be focused on basic subjects such as nucleation and growth of solids from
melts and on the evolution of microstructures—especially those involving one or
more fluid phases. These include the formation of crystal defects and solute
segregation in single-phase processing, such as semiconductor crystal growth,
as well as research aimed at achieving a better understanding of polyphase
microstructures, such as occur in eutectics and monotectics. Microgravity
materials research extends to practically important processes such as reaction
synthesis and sintering, welding and solidification, and in situ resource utilization
(ISRU) for producing structural materials from extraterrestrial bodies. Such
materials processes seem particularly relevant to technologies contemplated for
future HEDS missions. Microgravity combustion research within the
MRD—especially studies on fire safety research at the fractional gravity levels
found on extraterrestrial bodies or studies under microgravity as encountered in
spacecraft environments during deep-space transit—is critically needed to ensure
safety on future HEDS missions, where crew egress might not be an option. Such
research includes studies on flammability limits, smoldering, flame spread, and
flame stability—all of which contribute both to scientific knowledge and to the
engineering know-how needed for successfully pursuing the HEDS goals.
Research in microgravity biotechnology is considered essential for understanding
and designing reliable life-support systems, for producing nutrients and food for
crews during long-duration HEDS missions, and for safely and reliably recycling
waste aboard spacecraft for water and oxygen recovery. Current MRD studies
include activities on cell culture and bioseparations, which will contribute critically
to understanding biological options for nutrient production in spacecraft as well as
waste recycling. Low-temperature and atomic physics research using
microgravity generally probes certain extreme physical limits in both classical and
quantum systems. Research on laser cooling of atoms in microgravity can
contribute directly to the development of improved navigational systems for
achieving safe, efficient deep-space travel by providing practical atomic clocks
with greatly increased accuracy.
Although this initial report identifies the general areas of research
discussed above as having the potential to make long-term contributions to
HEDS technology development, the committee has attempted to prioritize neither
the research nor the affected technologies, in part because NASA is currently still
in the early stages of identifying its technology needs. As these needs become
more clearly defined, it should be possible to identify research that can be
profitably emphasized, although the need for flexibility in HEDS mission planning
suggests that a strict prioritization of research is likely to remain
counterproductive. Nevertheless, it is possible at this early stage to provide a
number of initial recommendations, primarily programmatic, derived in the course
of this review of microgravity research in support of Human Exploration and
Development of Space.
MRD should, on a continuing basis, assist NASA in identifying critical
OCR for page 3
technologies that would benefit future HEDS missions and then seek
opportunities in microgravity research to contribute to their efficient realization.
MRD should, however, remain both flexible and cautious in evaluating such
opportunities. Major advances in technology can result from basic research
undertaken without regard to current technological priorities, which have yet to be
even identified. In addition, the timing of such technological advances is often
unpredictable.
In supporting HEDS, MRD should continue to focus on maintaining its
broad program of microgravity research. Although not all of the technological
advances needed for HEDS missions will be the direct result of basic research,
the unfolding knowledge base and collective experience of microgravity
investigators focused within the MRD program will continue to represent unique
NASA resources with which to approach the scientific questions underlying many
of the barriers to space exploration.
MRD should be prepared to stimulate and support critical microgravity
research to help discriminate among competing HEDS technologies, specifically
providing information so that NASA can make informed choices among them.
The process of gathering and exchanging information relevant to
research selections that could support HEDS missions should be strengthened.
Specialized workshops, cross-divisional teams, advisory panels, and study
groups attended by mission technologists and microgravity scientists are among
the suggested mechanisms for achieving this recommendation. Such activities
would encourage the exchange of ideas between technologists and scientists,
provide better communication and ongoing awareness of the technology needed
for MRD, and also allow timely transfer of microgravity research findings to HEDS
technologists.
The goals of HEDS involve the development of complex technological
systems that require integration of microgravity information derived from research
in disparate fields of science. MRD may find it advantageous to initiate a limited
number of cross-disciplinary projects to develop experience in selecting and
managing research projects that operate across traditional boundaries of the
microgravity science disciplines.
Some HEDS missions will involve operating systems at fractional
gravity levels, such as the 0.16 Earth gravity encountered on the Moon or the
0.37 Earth gravity encountered on Mars. It is, however, often unclear as to
whether or not thresholds of the gravity level exist at which various physical,
chemical, and biological phenomena and processes undergo change. MRD
should consider giving more attention to research studies carried out at fractional
gravity levels where HEDS technologies might directly benefit from the scientific
advances. Knowledge generated from such studies could be used to evaluate the
need to provide artificial gravity by using continuous spacecraft rotation.
Ongoing investments by NASA in robotics and automation research
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are expected to benefit both manned and unmanned HEDS missions, which must
operate sufficiently far from Earth that highly autonomous operations and control
become necessary because of the long transit time of signals. MRD should
ensure that microgravity issues in teleoperations and robotics research are given
sufficient attention and should maintain an active and current awareness of these
issues.
The International Space Station (ISS), when available for scientific use
shortly after the beginning of the next millennium, should provide MRD with
unique long-duration microgravity opportunities for evaluation of technical
systems deemed important to future HEDS missions. MRD should take
advantage of the ISS as a microgravity platform for investigating closed-cycle,
long-term operation of various physical, chemical, and biological systems
considered to be within its research purview.
In view of the normally long time-scale needed for the evolution of
basic scientific concepts into practical applications, MRD should begin now to
study and understand the scope and long-term implications of microgravity
research areas relevant to accomplishing HEDS goals. Any adjustments to the
emphasis or scope of MRD research must then be carefully assessed with
respect to overall program balance, scientific merit, external interest, and HEDS
mission relevance.
The systematic and periodic application of NASA Research
Announcements (NRAs) and peer review has improved the quality and selection
of the science supported by MRD. These benefits to NASA and the nation are so
extensive that these mechanisms should be preserved to ensure scientific
objectives that support and enhance the HEDS enterprise. The recent inclusion
of a call for research on ISRU and two-phase flow in the 1996 NRAs for materials
science and fluid physics is commended as timely and responsive.
