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STRATEGIES FOR A PERMANENT LUNAR BASE
Michael B. Duke, Wendell W. Mendell, and Barney B. Roberts
NASA-Johnson Space Center, Houston, TX 77058
Abstract
Planned activities at a manned lunar base can be categorized as
supporting one or more of three possible objectives: scientific research,
exploration of lunar resources for use in building a space infrastructure,
or attainment of self-sufficiency in the lunar environment as a first step
in planetary habitation. Scenarios constructed around each of the three
goals have many common elements, particularly in the early phases. The
cost and the complexity of the base, as well as the structure of the Space
Transportation System, are functions of the chosen long-term strategy. A
real lunar base will manifest some combination of characteristics from
these idealized end members.
A MOON IN AMERICA'S FUTURE
The Earth is unique in the solar system, not only for harboring life,
but also for its relatively massive satellite. It is speculative that the
two attributes are somehow related, but certainly the Earth's companion
has left cultural and biological imprints on humanity. As cumulative
application of the scientific method has increased our understanding and
awareness of the physical universe, fascination with the habitability of
the Moon has blossomed. As late as the late century, newspaper stories
reported telescopic observations of the daily lives of lunar creatures.
The manned lunar landings of the last decade have dispelled such
romanticism forever but in turn have provided the technology and the
information necessary to fulfill a greater dream - the transport of
civilization beyond the confines of the Earth.
Cultural expansion is a recurring theme in human affairs. Motivations
for exploration or conquest vary from resource limitations (Mongol
invasions) to religion (Turkish probings of medieval Europe) to commerce
(global circumnavigations of the Sixteenth and Seventeenth Centuries).
American history especially is permeated by the doctrine of manifest
destiny. The concept of the frontier has come to symbolize for Americans
the exercise of individual freedom, which in collective expression leads
to social renewal. Contemporary popular writers cater to this mythos by
describing for an overpopulated and confused world the "high frontier" of
space. So far, the promise of space has been a reality for a few and only
a vicarious experience for most. However, humanity, and the United States
particularly, stands today at the threshold of a truly new world - the
Moon.
The promise of the Moon is not immediately evident from examination of
the current American space program. However, the space shuttle and the
proposed space station can be viewed as building blocks in a general
purpose space transportation infrastructure (fig. 1). To service
geosynchronous orbit, an upper stage is needed in addition to the shuttle.
If that upper stage is provided in the form of a reusable orbit-to-orbit
transfer vehicle docked at the space station, the transportation system
can be multipurpose. In particular, a rudimentary lunar transportation
system then will exist because the propulsion requirements for attaining
geosynchronous orbit and lunar orbit are essentially identical. A lunar
landing vehicle is required to place payloads on the lunar surface, but
its design can be a straightforward adaptation of the orbital transfer
vehicle (OTV). The space station and the reusable OTV constitute a natural
evolutionary path that, when achieved, will make accessible all near-Earth
space including the Moon. This "enabling technology" is a NASA target for
the mid 1990's.
When the requisite technology exists, the American political process
inevitably will include lunar surface activities as a major space
objective. In fact, some sort of declaration may well precede the actual
establishment of the space station. It is therefore prudent to consider
the nature of a permanent manned presence on the Moon and its potential
impact on the evolution of the Space Transportation System (STS).
Although the lunar base program is one in which the United States can
assert its leadership in space, it is inherently international in scope
and should involve as much participation as possible from other countries.
Opportunities for international cooperation exist in the planning stages,
in the science and technology development, and in operations at the lunar
base. A legal framework will be needed to guarantee that potentially
profit-making ventures adequately consider the concerns of the
international community.
USES OF THE MOON
A manned lunar base can be discussed in terms of three distinct
functions. The first involves the scientific investigation of the Moon and
its environment and the application of special properties of the Moon to
research problems. The second produces the capability to utilize the
materials of the Moon for beneficial purposes throughout the Earth-Moon
system. The last, and perhaps the most intriguing, is to conduct research
and development leading to a self-sufficient and self-supporting lunar
base, the first extraterrestrial human colony. Although these activities
take place on the Moon, the developed technology and the established
capability will benefit society on Earth as well as the growing
industrialization of near-Earth space.
Scientific Research
A lunar base will create new opportunities for investigating the Moon
and its environment and for using the Moon as a platform for scientific
investigations. Analogous to the function of McMurdo Base in Antarctica,
the lunar base will provide logistical and supporting laboratory
capability to rapidly expand knowledge of lunar geology, geophysics,
environmental science, and resource potential through wide-ranging field
investigations, sampling, and placement of instrumentation. Access to
large, free vacuum volumes may enable new experimental facilities such as
macroparticle accelerators. The firm, fixed platform will enable new
astronomical interferometric measurements to be obtained (fig. 2). The
challenge of long-term, self-sufficient operations on the Moon can spur
scientific and technological advances in materials science, bioprocessing,
physics and chemistry based on lunar materials, and reprocessing systems.
These concepts are explored by other papers in this volume.
Exploitation of Lunar Resources
It has been argued that major industrialization of space cannot occur
without access to the resources of the Moon. Studies of immense projects
such as solar power satellites have demonstrated that at a sufficiently
large scale, it is reasonable to develop the resource potential of the
Moon to offset the high Earth-to-orbit transportation costs (Hearth,
1976). The lower gravitational field of the Moon and the absence of an
atmosphere that retards objects accelerated from the surface provides a
potential 20 to 30-fold advantage for launching from the Moon instead of
Earth. For example, at liftoff, about 1.5% of the space shuttle's mass is
payload. Most of the mass is propellant. From the Moon, approximately 50%
of the mass can be payload.
The commodity currently envisioned to be most in demand in Earth-Moon
space over the next three decades is liquid oxygen, which makes up 6/7 of
the mass of propellant utilized by cryogenic (hydrogen-oxygen) rockets,
such as the Centaur or postulated OTV's. Although it would appear unlikely
than an atmosphereless body is a source for oxygen, it is actually an
abundant element on the Moon (Arnold and Duke, 1978). It must be
extracted, however, from silicate and oxide minerals into its liquid form
for use as a propellant. Several processes have been suggested (Criswell,
1980) for accomplishing this, including reduction of raw soil by fluorine
(which is recovered) or reduction or iron-titanium oxide (ilmenite)
hydrogen (also recovered). Preliminary laboratory studies have verified
the concepts behind some of these processes.
Systems studies (e.g., Carrol et al., 1983) show that oxygen
production on the Moon could benefit STS in the early years of the next
century, even if the hydrogen component of the propellant needed to be
brought from Earth (fig. 3-5). Finding concentrations of water at the
lunar poles (Arnold, 1979) or extracting the dispersed solar wind-derived
hydrogen in the lunar regolith would greatly improve the economics of the
transportation system.
Other commodities also could be produced. Metals, such as iron or
titanium, can be extracted from the lunar soil or from specific rocks or
minerals with differing degrees of difficulty. For example, small
quantities of metal (primarily iron) from meteorites can be concentrated
with a magnetic device from large amounts of lunar soil, or, with much
larger energy inputs, titanium can be obtained from ilmenite. These
products could find applications in large space structures. Lunar titania
or alumina might be used to produce aerobrakes (heat shields) used in
OTV's. In the long term, at relatively high levels of development,
production of components for solar electric power generation in space
(e.g., solar power satellites) could be made feasible (Bock, 1979).
Lunar Autarky
A self-sufficient lunar base is a possible long-term objective that
creates new challenges in planning and development. In the near term,
emplacement of a controlled environment capsule on the Moon involves known
technology. The initial concept for a lunar habitat module is simply an
extension of the design experience from Apollo, Skylab, the space shuttle,
and space station (fig. 6). A different perspective is required to plan
systems that can utilize the Moon's native materials and energy sources to
produce a self-sufficient capability.
Most of the generic technologies for an advanced system are similar to
those employed in general space operations (life support, power, thermal
control, communications, logistics, and transportation, etc.), but they
must be modified to utilize lunar materials for growth and extension.
Ultimately, the desire to minimize or to eliminate the resupply link from
Earth required a host of applications, new to the space program, carried
to new levels of system reliability. Exploration of technologies such as
lunar metallurgy, ceramics, manufacturing processes, power systems, and
others, will reveal whether autarky is a realistic objective and can
prepare the way for achieving it an operational base. Perhaps this is the
most compelling rationale for a lunar system, as it promises eventual
self-sufficiency elsewhere in the solar system.
PHASED EVOLUTION OF A LUNAR BASE
We loosely define three scenarios, each based on one of the long-term
rationales described above: scientific research, production, and
self-sufficiency (Tables 1-3). Each scenario passes through several
phases, some of which are common to the other scenarios. The distinction
among the three views lies with the culminating phase of each.
Precursor Exploration. Because the scientific data base is incomplete,
particularly in the polar regions, the first step in Phase I is global
mapping of the Moon, both with relatively high resolution imagery and with
remote-sensing measurements to determine the chemical variability. This
task can be accomplished with an unmanned satellite, a Lunar Geochemical
Orbiter or LGO (Minear et al., 1977), which is a proposed mission in
NASA's planetary program and could be flown in the 1990-1992 time frame.
The LGO is in the Planetary Observer mission class, a low-cost approach to
planetary exploration recommended by the report of the Solar System
Exploration Committee (1983). Secondly, Phase I should include research on
technologies necessary to exploit lunar surfaces. Technology development
in resource problems on Earth is typically a long lead time process. At
the conclusion of Phase I, the initial site for a base will have been
defined and planned activities understood in some detail. Concurrently
with this preliminary phase in the lunar program, development of a space
station and on OTV capable of supporting a lunar base would be carried out
in NASA's STS program.
Research Output. At Phase II, an initial surface facility would
establish limited research capability for science, materials processing,
or lunar surface operations. Depending on the long-term objectives of the
lunar base program, the detailed studies and the experimental plans start
to diverge at this phase for the different scenarios. A focus on lunar
science and astronomy would result in local geological exploration, the
establishment of a small astronomical laboratory, and emplacement of
automated instruments. If production were to be the focus, a pilot plant
for lunar oxygen extractions could be set up instead, and study of the
fabrication of aerobrakes from lunar material could be initiated. If the
program goal pointed to achieving self-sufficiency, the emphasis at this
stage could be agricultural experiments utilizing lunar soil as substrate
and recycling water, oxygen, and carbon dioxide.
To accomplish Phase II in any of the scenarios, the STS must have the
capability of landing and taking off from the Moon, transporting manned
capsules (about 10,000 kg) to and from the lunar surface, and delivering
payloads of about 20,000 kg to the lunar surface. This involves delivering
approximately 40,000 kg into lunar orbit using OTV's. The requirement for
storage of the return vehicle on the Moon for extended periods (14 days to
3 months) may require new high-performance, storable propellent systems at
this phase of development.
Permanent Occupancy. At Phase III, permanent occupancy is the
objective. The surface infrastructure would include greater access to
power, better mobility in and away from the base, and more diversified
research capability. Still, depending on the long-term objectives, the
nature of the base can vary. A science base might emphasize long-range
traverses for planetological studies or extension of observational
capability with larger telescopes. A production base will incorporate
highly automated systems to produce and transfer liquid oxygen for use in
the transportation system. Advanced research for a self-sufficient base
would be making first extensions of the base utilizing indigenous
materials. The production and the self-sufficiency scenarios require a
small cousin to the Earth-orbit space station in lunar space (lunar orbit
or an Earth-Moon libration point) to provide for transfer, refueling and
maintenance of the lunar lander and the OTV's.
Advanced Base. The advanced base, Phase IV, is even more specialized.
Depending on the long-term plan, it produces more types or a greater range
of scientific investigations, adds products to the growing lunar
industrial base, or enters a phase of significant expansion of
capabilities using lunar materials as the majority of the feedstock. This
is the terminal phase for the science and production scenarios. Future
growth may occur by enlarging the number of experiments or products
produced on the Moon, but a self-sustaining capability is not included.
The production base might even develop toward a highly automated state
where permanent occupancy was unnecessary. For the production and
independence scenarios, the base should begin paying its own operational
costs. In the self-sufficiency scenario, research and development of pilot
plants aimed at a broad range of indigenous ;lunar technologies would be
pursued. The final phase of the self-sufficiency scenario is a truly
autarkic settlement, a lunar colony, in which the link to Earth can be
discretionary.
EVOLUTION OF THE PROGRAM
Figure 7 ties the possible development of a lunar base to the growth
of lunar resource support of the transportation system. Initially, the
base is totally dependent on terrestrial supply where 7 kg in low-Earth
orbit is required to place 1 kg on the lunar surface. With the
introduction of lunar oxygen first into near-Moon operations and then into
the return leg of the transportation system, the slope of the curve
changes from 7:1 to 3.5:1. As the lunar manufacturing capability increases
to the point where aerobrakes can be manufactured, the slope decreases to
somewhat slightly greater than 1:1. Further growth of lunar capability
allows expansion of base mass to be more or less independent of the
quantity of imported terrestrial mass. At the point of self-sufficiency,
only trace minerals and crew changeout are chargeable weight to lunar
operations; the slope of the curve in fig. 7 is essentially flat.
Another consideration in the growth of lunar activities is the
economic "balance of trade" between Earth orbit and the lunar surface. The
value of lunar products may support lunar operations before a true mass
balance is achieved. It is difficult to calculate the economic value of
lunar oxygen and other products in low-Earth orbit. However these "lunar
credits" are shown qualitatively in fig. 7 at the point where a closed
ecological life support system (CELSS) and a significant manufacturing
capability are available. The slope of the "credits" line will be a
function of many things, such as the amount of oxygen required to support
non-lunar activities, the value of science and research enabled by the
lunar base. Finally, the dashed line of constant slope indicates the
continued total dependency that would exist if these technologies are not
pursued on the Moon, that is, if a self-sufficiency element is not
included in the lunar base program.
The real lunar base will evolve as some combination of the above
scenarios. Determination of the right mix requires research, development,
and debate. Even if a program is started now, several years should be
devoted to study of the detailed lunar base scenario. The time is
available because the development of the space transportation
infrastructure and the completion of the orbital science survey will take
7-10 years. Proper preparation will make it possible to decide on a
specific lunar base design in the early 1990's. That time frame is
consistent with the development of the infrastructure that will enable the
lunar base program to be carried out to its full potential. The first
manned landings could occur early in the first decade of the next century;
permanent occupancy could be achieved by the year 2007, the fiftieth
anniversary of the Space Age.
There are potential technological problems that may slow the
development of the lunar base, and at each phase there will be serious
questions as to whether to proceed and how and when to proceed. A
commitment need not be made now to the whole plan. Nevertheless, the
long-term objective is one of immense significance in human history and
should not be casually discarded. It is inevitable that humankind will
settle the Moon and other bodies in the Solar System. We live in a
generation that has already taken very significant steps along that path.
With careful planning, we can nuture the capability to move from the
planet, to provide benefits to Earth, and to satisfy humanity's spirit of
adventure.
REFERENCES
Arnold, James R. (1979) Ice in the lunar polar regions, J. Geophys.
Res., 84, 5659-5668.
Arnold, James R. and Duke, Michael B. (1978) Summer Workshop On
Near-Earth Resources, NASA CP-2031, NASA, Washington, DC, 1-7 pp.
Bock, Edward H. (1979) Lunar Resources Utilization for Space
Construction, Final Report for Contract NAS9-15560, General Dynamics
Convair Div., Advanced Space Programs, San Diego, CA.
Carroll, W. F., Steurer, W. H., Frisbee, R. H., and Jones, R. M.
(1983) Extraterrestrial materials - Their role in future space
operations, Astronaut, Aeronaut, 21, 80-85.
Criswell, David R. (1980) Extraterrestrial materials processing and
construction, Final Report Contr. NSR 09-051-001, Lunar and Planetary
Institute, Houston, TX.
Hearth, Donald P. (1976) Outlook for Space, NASA SP-386, NASA, Wash., DC,
237 pp.
Minear, J. W., Hubbard, N., Johnson, T.V., and Clarke, V. C., Jr. (1977)
Mission Summary for Lunar Polar Orbiter, JPL Document 660-41, Rev. A., Jet
Propulsion Laboratory, Pasadena, CA.,36 pp
Solar System Exploration Committee (1983) Planetary Exploration Through
Year 2000: A Core Program, U.S. Government Printing Office, Washington,
DC, 167 pp.
Figure Titles
Figure 1. The Space Transportation System of the future may service a
station in geosynchronous orbit as well as a lunar base via a station in
lunar space. The lift capacity of the Shuttle fleet may be augmented by an
unmanned heavy lift vehicle, designed to ship fuel and consumables to
space.
Figure 2. A radio telescope located on the farside of the Moon would be
shielded from background noises generated by terrestrial sources. Although
depicted here with a parabolic dish in a convenient crater, an initial
lunar instrument may well be a phased array of dipole antennas.
Figure 3. Liquid oxygen fuel (LOX), manufactured on the Moon and delivered
to low-Earth orbit may become a profitable export for a lunar base. A
critical parameter in analyses of the system is the mass payback ratio,
defined as the ratio of the excess lunar LOX in LEO to the liquid hydrogen
fuel delivered from Earth to LEO.
Figure 4. The mass payback ratio for lunar LOX delivered to LEO is
sensitive to the design characteristics of the OTV used as a lunar
freighter. The fractional mass of the OTV aerobrake and the oxidizer to
fuel ratio are key parameters. Manufacture of aerobrakes on the Moon would
enhance system performance.
Figure 5. A simple cost-benefit analysis assumes that a lunar oxygen
production facility has its capital costs amortized solely by "profits" on
delivery of LOX to LEO. While lunar oxygen is competitive with shuttle
delivery in all cases, introduction of a cost-efficient heavy lift vehicle
reduces the advantage under more conservative cost estimates for the lunar
operation. If costs of lunar LOX are shared with other activities, the
advantage is restored.
Figure 6. The first lunar base habitats and laboratories could be space
station modules, buried in the lunar regolith for protection from solar
flare radiation. Interface modules not only interconnect the buries
structures but also can be stacked to create exits to the surface.
Figure 7. Initially, almost 7 kg must be lifted into LEO for every kg
landed on the Moon. As lunar oxygen is introduced into the transportation
system, the ratio improves as a unit mass goes from Earth to Moon with
only little overhead in the system. In a Phase IV advanced base, the
growth of lunar surface infrastructure becomes only weakly dependent on
imports from Earth. A favorable balance of trade is ultimately
conceivable.
Table 1. Lunar Base Growth Phases: Sciences Base Scenario
A growing capability to do lunar science and to use the Moon as a research
base for other disciplines, using lunar resources to a limited extent to
support operations.
Phase I: Preparatory exploration
. Lunar orbiter explorer and mapper
. Instrument and experiment definition
. Site selection
. Automated site preparation
Phase II: Research Outpost
. Minimum base, temporarily occupied, totally resupplied from Earth
. Small telescope/Geoscience module
. Short range science sorties
. Instrument package emplacement
Phase III: Operational Base
. Permanently occupied facility
. Consumable production/Recycling pilot plant
. Longer range science sorties
. Geoscience/Biomedical laboratory
. Experimental lunar telescope
. Extended surface science experimental packages
Phase IV: Advanced Base
. Advanced consumable production
. Satellite outposts
. Advanced geoscience laboratory
. Plant research laboratory
. Advanced astronomical observatory
. Long-range surface exploration
Table 2. Lunar Base Growth Phases Production Base Scenario
A lunar base that is intended to develop one or more products for
commercial use. Manned activity may be continuous, but a high degree of
automation is expected.
Phase I: Preparatory exploration
. Lunar orbiter explorer and mapper
. Lunar pilot plant definition
. Site selection
. Automated site preparation
Phase II: Research outpost
. Minimum base, temporarily occupied, totally resupplied from Earth
. Surface mining pilot operation
. Lunar oxygen pilot plant
. Lunar materials utilization research module
Phase III: Operational base
. Permanently occupied base
. Expanded mining facility
. Consumables supplied locally
. Oxygen production plant
. Lunar materials processing pilot plant(s)
Phase IV: Advanced base
. Large scale oxygen production
. Ceramics/Metals production facility
. Locally derived consumables for industrial use
. Industrial research facility
Table 3. Lunar Growth Phases: Lunar Self-Sufficiency Research Base
Scenario
A lunar base that grows in its capacity to support itself and expand its
capabilities utilizing the indigenous resources of the Moon, with the
ultimate objective of becoming independent of Earth.
Phase I: Preparatory exploration
. Lunar orbiter explorer and mapper
. Process definition
. Site selection
. Automated site preparation
Phase II: Research Outpost
. Minimum base, temporarily occupied, totally resupplied from Earth
. Surface mining, pilot operation
. Lunar oxygen production pilot plant
. Closed systems research module
Phase III: Operational base
. Permanently occupied facility
. Expanded mining facility
. Lunar agriculture research laboratory
. Lunar materials processing pilot plant(s)
Phase IV: Advanced base
. Lunar ecology research laboratory
. Lunar power station-90% lunar materials-derived
. Agriculture production pilot plant
. Lunar manufacturing facility
. Oxygen production plant
. Lunar volatile extraction pilot plant
Phase V: Self-sufficient colony
. Full-scale production of exportable oxygen
. Volatile production for agriculture, Moon-orbit transportation
. Closed ecological life support system
. Lunar manufacturing facility: tools, containment systems, fabricated
assemblies. etc.
. Lunar power station - 100% lunar materials - derived
. Expanding population base