Proceedings of the Fourth Princeton/AIAA Conference on Space Manufacturing
May 14-17, 1979
Published by American Institute of Aeronautics and Astronautics
- I. International Considerations
- II. Materials Processing
- III. Public Policy
- IV. Economic Considerations
- V. Mass Drivers
- VI. Novel Technical Concepts
- VII. Controlled Ecological Life Support Systems
- VIII. Fabrication and Products
- IX. Environmental Effects
- X. Lunar Materials Trajectories
- XI. Space Habitats
- XII. Anthropology and Psychology
- XIII. Asteroidal Resources
I. INTERNATIONAL CONSIDERATIONS
Four Dimensional Strategy. Michael A. G. Michaud, U.S. Department of State.
Abstract: Recent U.S. Administrations have declined to commit themselves to long-term goals for the U.S. civil space program, or even to major civil space systems beyond the Space Shuttle. While the civil program has leveled off in budgetary terms, national defense space programs continue to grow. If this trend continues, U.S. activity in space will be shaped increasingly by national defense interests (the Soviet space program already is heavily influenced by defense considerations). However, there is no sign in the unclassified literature that national defense space programs are guided by a three-dimensional strategy, based on a commitment to the outward projection of presence, influence, and values (Soviet programs may have a stronger ideological base). Advocates of space humanization may have no realistic choice but to work with and try to influence national defense space programs as a way to breach the conceptual and cost barriers to the large-scale use of space. But it remains to be seen whether the U.S., the USSR, or the human species as a whole has a conception of itself in three dimensions over time.
Space Industrialization: An International Perspective. Rashmi Mayur, Indian Urban Development Institute.
Abstract: Space industrializaton can provide benefits in the following areas of priorities in less-industrialized countries: disaster warning, agriculture, education, communication, resource survey, medicine, new materials, and energy.
The Potential Global Market in 2025 for Satellite Solar Power Stations. Alain Dupas and Maurice Claverie, French Centre National de la Recherche Scientifique.
Abstract: Starting from hypothesis of moderate growth for energy demand through 2000/2025, we have computed the market of Large Electrical Power Plants(LEPP) in the range 24-40 TWh/yr suited for base-load electrical needs. For this purpose, we have developped a numerical model predicting the future demands for centralized and decentralized electrical energy according to geographical position. The input of this model are: the geographical distribution of population at the present time, the energy demand growth in the different world regions, the part of energy consumption used for electricity generation in each world region. The partition of electrical demand between centralized and decentralized needs is made according to the density of electrical consumption. We have used, as energy demand growth hypothesis, results from the reports of the Conservation Commission of the World Energy Conference (1977) and the work of the Case Western Reserve University. Our model leads to a world market for LEPP in 2020/2025 of 752/942 plants, which could be provided alternatively by Conventionnal Thermal Plants, Breeder Nuclear Reactors, Fusion Reactors or SSPS (Satellite Solar Power Station). Assuming a 50% market penetration factor for SSPS between 2000 and 2025, the number of SSPS to be built during this period amounts to 199/275.
Current Space Habitat Legal Developments. Edward R. Finch Jr., Finch & Schaefler.
Abstract: This paper discusses current space habitat international legal developments. It highlights trends within international and national forums. While no specific regime is advocated for space industrialization and habitats, the present legal complexities are reviewed. [FIRST PAGE from AIAA website]
II: MATERIALS PROCESSING
Carbon Dioxide Electrolysis Using a Ceramic Electrolyte. Thomas E. Erstfeld and Oscar Mullins Jr., Lockheed Electronics Company, and Richard J. Williams, NASA Johnson Space Center.
Abstract: This paper discusses the results of an experimental study of the electrical aspects of carbon dioxide electrolysis using a ceramic electrolyte. The electrolyte compositions used in this study are 8% Y2O3 stabilized Zr02, 7.5% CaO stabilized Zr02, and 5% Y2O3 stabilized ThO2. Results indicate that the 8% Y2O3 stabilized ZrO2 is the best material to use for electrolysis, in terms of current as a function of voltage and temperature, and in terms of efficiency of oxide ion flow through it. The poorest results were obtained with the 5% Y2O3 stabilized ThO2 composition. Some attempts were made to predict the size of an electrolysis system which might be employed to reclaim oxygen and carbon from effluents of space manufacturing. By assuming that an industry would have to electrolyze 258,000 tonnes of CO2 per year, a total cell area of 110,000 square meters of 1 mm thickness and electrical capacity of 441 MW would be required. [FIRST PAGE from AIAA website]
Excavation Costs for Lunar Materials. W. David Carrier III, Bromwell Engineering.
Abstract: A lunar strip mining system is presented which is capable of excavating and transporting 3 million metric tons of ore per year to a central processing plant on the moon’s surface. The mining system would grow from a single front-end loader in the first year, to a fleet of ten haulers in the 30th year. The cumulative mass of equipment transported from the earth to the moon by the 30th year would range from 160 to 780 tons, depending on the assumptions and conditions. The net energy required per year would grow from an initial 8 MW-hr to a range of 160 to 930 MW-hr by the 30th year, again depending on the assumptions. Lunar personnel requirements would consist of a single individual, whose primary function would be to perform maintenance. All of the mining equipment would either operate automatically or by remote control from earth. The projected cost for the lunar mining system is approximately $12 to $37 per ton of ore over the life of the mine. Consequently, the cost of the mining system is an important part of the overall economics of exploiting lunar resources. [FIRST PAGE from AIAA website]
Near-Earth Extraterrestrial Resources. David L. Kuck, Geological and Mining Consultant.
Abstract: The moon and the Apollo-Amor asteroids are potential sources of near-earth extraterrestrial resources. Exploitation of these resources should be for space construction and for sale on Earth. The moon is the nearest and best understood source of iron, titanium, magnesium, calcium, aluminum, silicon, oxygen and chromium. Apollo and Amor asteroids are a potential source of iron, nickel, cobalt, magnesium, calcium, silicon, gallium, germanium, precious metals and oxygen. Space construction will consume silicon, titanium, calcium, magnesium, aluminum, iron, oxygen, nickel, cobalt and chromium. A commodity for sale on Earth must have a high unit value and or a high energy equivalent to pay for its recovery and transportation. Metals that can possibly be recovered and transported to Earth at a profit are gallium, germanium, and the precious metals from the Apollo-Amor asteroids. These metals not only have a high monetary value, they represent stored energy, for It requires energy for their recovery here on Earth. A 100 meter diameter hexahedrite iron asteroid contains 1235 tons of gallium, germanium and precious metals worth $2,911 billion. The same size nickel-rich ataxite contains 1380.7 tons of the same metals for a value of $3,008 billion. These metals will be the incentive for exploration and exploitation of the Apollo and Amor asteroids. The nickel-cobalt fraction of the nickel-rich ataxite is worth an additional $6,022 billion, in addition to an energy equivalent of 35.6 billion Kw-hr.
Electrostatic Separation of Lunar Soil. I. I. Inculet, University of Western Ontario.
Abstract: The paper discusses the reasons why the lunar soil and environment present the ideal combination for an electrostatic beneficiation or separation of any existing mineral values which are physically separable. It reviews the size of the lunar soil particles, the expected advantages of triboelectrification in vacuum and at low temperature, and the benefits of the lower gravity acceleration on the moon. [FIRST PAGE from AIAA website]
Overview of Methods for Extraterrestrial Materials Processing. R. D. Waldron and David R. Criswell, Lunar and Planetary Institute, and Thomas E. Erstfeld, Lockheed Electronics Company.
Abstract: A brief survey of processing systems suitable for conversion of lunar soil fractions to refined industrial feedstocks are given. Description of a “baseline” process using hydrochemical or metallurgical separation of compounds of major and minor elements using HF acid leaching as the initial step is presented. Rough engineering parameters including power and heat rejection requirements, potential loss of earth supplied reagents during recycling, and mass: output ratios of equipment, reagent inventory, and associated power and radiator facilities are described. Minimal practical scales for such systems and manpower requirements are discussed. [FIRST PAGE from AIAA website]
Electrochemical Preparation of Useful Materials from Ordinary Silicate Rocks [Electrochemistry of Lunar Rocks]. David J. Lindstrom and Larry A. Haskin, Washington University.
Abstract: Electrolysis of silicate melts has been shown to be an effective means of producing metals from common silicate materials. No fluxing agents need be added to the melts. From solution in melts of diopside (CaMgSi2O6) composition, the elements Si, Ti, Ni, and Fe have been reduced to their metallic states (in the form of alloys with the Pt cathode). Reduction of Na and Al has not yet been observed. Platinum is a satisfactory anode material, but other cathode materials are needed. Electrolysis of compositional analogs of lunar rocks initially produces iron metal at the cathode and oxygen gas at the anode. Utilizing mainly heat and electricity which are readily available from sunlight, direct electrolysis is capable of producing useful metals from common feedstocks without the need for expendable chemicals. This simple process and the products obtained from it deserve further study for use in materials processing in space. [FIRST PAGE from AIAA website]
Glass and Ceramics from Lunar Materials. John D. Mackenzie and Rex C. Claridge, University of California at Los Angeles.
Abstract: The possibilities of producing a variety of glass and ceramic products directly from lunar soil are discussed. These include glass fibers, glass wool, glass containers, foamed glass panels, fine grained glass-ceramics for tiles, pipes and electrical insulators. Anorthite can be melted to form windows and mirrors. Silica, alumina, magnesia can be fired singly or in combination to give bricks and refractories. The uniqueness of production in space is discussed and specific examples given. [FIRST PAGE from AIAA website]
Gas Entrained Solids — A Heat Transfer Fluid for Use in Space. H. Keith Henson, Analog Precision, Inc., and K. Eric Drexler, Massachusetts Institute of Technology.
Abstract: The physical parameters leading to the high cost of low temperature radiators in space are discussed. A method is proposed to reduce these costs by the use of extraterrestrial materials in novel ways. It is proposed that the fluid filling the radiators be composed of a low pressure gas and a quantity of finely divided solids. Design parameters for radiators using gas entrained solids are derived. A few of the implications for space industrialization are discussed. [FIRST PAGE from AIAA website]
III: PUBLIC POLICY
Space Manufacturing and Public Policy Formulation. J. M. Logsdon, George Washington University.
Abstract: It appears that government support of research, development, and demonstration efforts will be essential if a capability for space manufacturing using nonterrestrial materials is to be achieved. Such a capability is one aspect of the broader process of space industrialization, which in turn makes sense only if it can be carried out as a commercial enterprise. This paper analyzes the conditions under which the federal government is likely to provide support for research and development leading to commercial uses, and the other policy actions and institutional innovations which appear linked to a successful national effort to obtain economic and social benefits from the space environment. The paper also identifies the particular problems for public policy posed by bold proposals such as basing a space manufacturing capability on the use of nonterrestrial materials. [FIRST PAGE from AIAA website]
Education for the Era of Space Industrialization. Kerry Mark Joels, National Air and Space Museum.
Abstract: Education has become a component of virtually every activity in a society with a high proportion of service careers and leisure time. Education is also a process whereby the public may be meaningfully informed about the opportunities and significance of space industrialization, and it is a market place where some of the manufactured products of space industrialization can be sold. It is challenging to contemplate how space industrialization and education will interact with each other to produce the curricula of the future.
Improving the Justification for Space Industrialization. John G. Barmby, U.S. General Accounting Office.
Abstract: Space industrialization concerns the practical applications of space such as Energy Services, Information Services, Products, and People in Space. Many of the spacecraft aspects may be technically feasible but there are some problem areas. NASA-sponsored studies appear to be overly optimistic regarding economic and political feasibility while the social, costing, and management problems seem to be underestimated. It is concluded that some space industrialization programs have potential; however, it is premature to embark upon the ambitious schemes suggested by certain advocates. Many of the problems which could accrue from a major space industrialization effort are expected to be the same as those faced by the Shuttle and the present applications satellites. Hence, the Shuttle and its payloads could be used to resolve the conflicts sooner and generate confidence. It is advisable for NASA to establish a substantial planning and analysis group composed of individuals with economic and marketing skills who understand competitive commercial operations. If the private sector does not choose to fund the equipment and operations for many aspects of space industrialization, consideration might be given to making NASA an operational agency as well as an R&D agency. [FIRST PAGE from AIAA website]
Space Manufacturing and Technocratic Centralism. J. D. Salmon, University of West Florida.
Abstract: Space industry has the potential to become a centralized, technocratic, elite-controlled system with very important impact upon the concept and practice of political democracy. Opposition to SI is likely to focus heavily upon this prospect, and that opposition could delay or even prevent accomplishment of SI. Recognition of this fact should lead to implementation of measures to broaden public knowledge of and access to the SI program, and to seek methods by which actual decentralization can be made part of the program.
An Analysis of the Socio-Political Status of Efforts Toward the Development of Space Manufacturing Facilities. Robert D. McWilliams, University of Mississippi.
Abstract: This paper examines the current socio-political status of on-going efforts toward the development of space manufacturing facilities in the United States. The primary focus is on three situational matters. These include: (1) the nature of current popular American opinion toward federal spending on space exploration, (2) the question of whether or not there may currently be a pro-space utilization social movement emerging in America, and (3) an analysis of the current congressional mood toward the space program. Subsequent to discussion of these matters, a suggestion is offered to proponents of space industrialization as to what might be the best course of promotional action toward achieving their goals. This mainly involves a brief analysis of the beneficial role space industrialization could play in the relationship between human technology and the planetary environment. Lastly, it is concluded that, while the successful development of space manufacturing is a socio-political possibility it is by no means a certainty, and will have to be aggressively pursued. [FIRST PAGE from AIAA website]
The Role of Public Interest Groups in Space Policy. Charles M. Chafer, Foundation for Public Affairs and Institute for the Social Science Study of Space.
Abstract: Space manufacturing facilities (SMFs), if developed beyond the experimental and demonstration stages, will fundamentally affect extant terrestrial economic, political, and social institutions. SMFs will be integrated into these institutions by means of the political process. In the United States, public interest groups have become important members of the “issue networks” which propose, and help enact and implement public policy. Several national public interest groups have already addressed, and in most cases opposed, various aspects of space industrialization. “Conflict management” is proposed as a means of mediating conflicts which arise between public interest groups and those institutions and organizations advocating space industrialization. Public interest groups with state/local or international focus have yet to analyze space industrialization, but these groups may emerge as the most important elements of the public interest sector over the next decade. If space industrialization conflicts involving these groups are to be minimized, it may be crucial to demonstrate that SMFs can meet the requirements of “appropriate technology.”
IV: ECONOMIC CONSIDERATIONS
Financing Alternatives for Space Industrialization. J. Peter Vajk, Richard D. Stutzke, and Mark S. Klan, Science Applications, Inc., and Robert Salkeld and G. Harry Stine, Consultants.
Abstract: Large scale space industrialization projects challenge conventional methods of financing commercial undertakings. As part of the Department of Energy’s Satellite Power System Concept Evaluation Program, we examined the problems of financing both the lengthy and expensive R&D phase and the capital-intensive commercial implementation phase of an SPS program. Ten alternative schemes, ranging from purely public to purely private, were developed. One of these, a purely private enterprise approach, is already underway. Some of the alternatives presented here may be readily adapted to other space industrialization projects and to large-scale terrestrial projects as well. [FIRST PAGE from AIAA website]
Economic Considerations in Space Industrialization. Robert U. Ayres, Carnegie-Mellon University, Leslie W. Ayres, Variflex Corporation, and David R. Criswell, Lunar and Planetary Institute.
Abstract: The industrial categories of the United States economy are surveyed to identify those which can function using dominantly lunar raw materials or lunar derived feedstocks (LDF) and solar energy. Sixty-four standard industrial categories (SIC) appear to be compatible with LDF inputs; another 166 SIC’s might be adaptable to LDF and space industry if substitution of materials and/or terrestrial supplements were introduced. Analytic tools are presented to use in deciding optimal strategies by which a generalized economy can be developed in space in an optimal manner within given constraints of capital, products derived at a given time, local production costs, cost of import from earth and other factors.
Scaling and the Start-Up Phase of Space Industrialization. David R. Criswell, Lunar and Planetary Institute.
Abstract: By terrestrial standards very little mass is needed to construct the space portion of a 10,000 megawatt (10 GW) power system. Use of lunar materials makes it reasonable to consider alternatives to silicon solar cells for conversion of sunlight to electricity and thereby avoid present major problems associated with solar cell production. Machinery needed on the moon to excavate lunar materials and deliver them to a transport system, to beneficiate lunar materials, to produce glasses and ceramics from lunar materials and to chemically process lunar materials into their major oxides and elements are minor mass fractions of the total mass of equipment needed in space to produce an SPS. In addition the processing equipment can throughput several hundred times their own mass each year with very little requirement for makeup mass from earth.
The Benefits of Solar Power Satellites. Peter E. Glaser, Arthur D. Little, Inc.
Abstract: It is becoming increasingly apparent that the concerns about the future supply of energy resources are real and that new approaches will have to be developed to replace nonrenewable with renewable energy sources. An SPS Reference System has been selected to meet a requirement of the SPS Concept Development and Evaluation Program Plan of Feburary 1978. The intent of the SPS Reference System is to document an SPS configuration for purposes of economic and environmental assessments and for comparisons with alternative technical approaches. The reference system does not represent an SPS configuration which is likely to be implemented but serves as an initial base for future technological developments. The SPS program holds the promise of ending the escalating energy problems which have plagued societies in recent years and that are certain to cloud future progress. All indications are that the SPS is one of the few available options which can confront the challenge of the inevitable transition to renewable sources of energy on the planet Earth.
Start Up Considerations for a Space Manufacturing Enterprise. Joseph H. Engle, University of Illinois at Chicago Circle, and J. Peter Vajk, Science Applications, Inc.
Abstract: We discuss costing considerations for inclusion in planning a Space Manufacturing Enterprise. Cost Categories are: RDT&E, Procurement, Lift (of materiel and personnel to low earth orbit), Depreciation, Space Personnel, Mission Control, Administration, Interest, Inflation and Taxes. A hypothetical facility is examined. Assumed RDT&E begins in 1980, with actual Space operations beginning in 1985. Habitats and work facilities are established in Low Earth Orbit, on and near the Moon, and at a manufacturing facility in Space where ore from the Lunar Surface can be chemically processed, fabricated and combined with special components from Earth to produce Solar Power Satellites (SPS’s). Each SPS, placed in geosynchronous orbit, transmits, via microwave, 10 gigawatts of received power to suitable earth stations. Four SPS’s are built during the 1985-92 development period. The facility can then provide 2.4 SPS’s per year through the efforts of 3,096 workers in Space in the steady state. Raw costs to reach steady state are estimated at $101.6 billion (1975 dollars). The 1980 Present Value Discount Cost (exclusive of taxes and inflation) is $44.6 billion. The initial investment is recovered by 1994, and the average rate of return before taxes by the year 2000 is 7.4% per year.
Optimization of Space Manufacturing Systems. David L. Akin, Massachusetts Institute of Technology.
Abstract: Four separate analyses are detailed: transportation to low earth orbit, orbit-to-orbit optimization, parametric analysis of SPS logistics based on earth and lunar source locations, and an overall program option optimization implemented with linear programming. It is found that smaller vehicles are favored for earth launch, with the current space shuttle being right at optimum payload size. Fully reuseable launch vehicles represent a savings of 50% over the space shuttle; increased reliability with less maintenance could further double the savings. An optimization of orbit-to-orbit propulsion systems using lunar oxygen for propellants shows that ion propulsion is preferable by a 3:1 cost margin over a mass driver reaction engine at optimum values; however, ion engines cannot yet operate in the lower exhaust velocity range where the optimum lays, and total program costs between the two systems are ambiguous. Heavier payloads favor the use of a MDRE. A parametric model of a space manufactur-ing facility is proposed, and used to analyze recurring costs, total costs, and net present value discounted cash flows. Parameters studied include productivity, effects of discounting, materials source tradeoffs, economic viability of closed-cycle habitats, and effects of varying degrees of nonterrestrial SPS materials needed from Earth. Finally, candidate optimal scenarios are chosen, and implemented in a linear program with external constraints in order to arrive at an optimum blend of SPS production strategies in order to maximize returns.
The Economics of Strikes and Revolts during Early Space Colonization: A Preliminary Analysis. Mark M. Hopkins, The Rand Corporation and Harvard University.
Abstract: Strikes and revolts can have devastating consequences. In extreme cases the entire space colonization system, including the accumulated output of satellite solar power stations, can be destroyed. Fortunately, appropriate design and administrative policies can reduce the problem to manageable proportions.
V: MASS DRIVERS
Overview and Outline of Mass-Driver Two. Gerard K. O’Neill and William R. Snow, Princeton University.
Abstract: An overview of the Princeton-M.I.T. second mass-driver is presented. Mass-Driver Two is a 13.1 cm caliber system which uses a two coil superconducting bucket and a two-phase in quadrature drive system. Discrete drive coils are individually energized with timing supplied by position-sensing optical detectors. Intermediate energy storage is provided by sector capacitors which are recharged every half cycle by an external power source. A vacuum environment is provided for the superconducting bucket by a 4 inch i.d. glass pipe with the drive coils surrounding it. Magnetic flight is generated by eddy current repulsion from six copper guide strips lining the glass pipe. The length is 2.5 meters equally divided between acceleration and deceleration sections. Nominal acceleration is 5000 m/s2 giving a maximum bucket velocity of 112 m/s. Regenerative braking is used to decelerate the bucket. Current densities of 25 KA/cm2 are achieved in the superconducting bucket coils and are maintained by a cryogenic service station. Studies of guidance forces, acceleration forces, measurement of drive shielding losses, and possible coupling of acceleration forces into modes of transverse and rotational oscillation of the bucket will be performed. [FIRST PAGE from AIAA website]
Mass Driver Two: Cryogenic Module. Kevin Fine, Fred Williams, Peter Mongeau, and Henry H. Kolm, Massachusetts Institute of Technology.
Abstract: The cryogenic module of Mass Driver Two comprises a 3.25 inch (82.55 mm) o.d. bucket with two 44 kilo-ampere-turn coils made with .028 inch (.71 mm) dia niobium-titanium multi-filamentary cable in a copper matrix, impregnated with lead alloy for thermal inertia, as well as the service station to refrigerate, energize and eject the bucket. The station is housed in a six inch flanged pyrex cross which connects to the four inch pyrex tube of the mass driver itself, built by the Princeton group. The bucket is refrigerated by being forced against a copper braid cradle attached to the bottom of a liquid helium reservoir which protrudes into the cross from above. The bucket is energized inductively by turning off two superconducting coils which are also attached to the helium reservoir, and which have maintained the correct flux linkage through the bucket coils during their cool-down through the critical temperature. Once charging is completed, the clamping pressure is released and the bucket is injected into the mass driver by means of two normal-conductor pulse coils surrounding the horizontal branches of the cross. [FIRST PAGE from AIAA website]
Mass Driver Alignment by Bucket Steering. Jonathan Leavitt, Princeton University.
Abstract: This paper considers the structural design for a mass-driver reaction engine which uses steering forces applied to the buckets to provide active control of lateral guideway deflections. The length of the guideway is divided into stiff segments joined together by hinges at which electromagnetic steering forces may be applied. As the buckets oscillate laterally during their journey, the leave a resultant torque on each segment. The magnitude of this torque may be set, using steering forces, to correct for local dislocations thereby keeping the mass-driver aligned. [FIRST PAGE from AIAA website]
VI: NOVEL TECHNICAL CONCEPTS
Electromagnetic Propulsion Alternatives. Henry H. Kolm, Kevin Fine, Peter Mongeau, and Fred William, Massachusetts Institute of Technology.
Abstract: The electromagnetic mass driver, a linear synchronous motor which accelerates payloads in re-usable superconducting buckets, is well suited to launching lunar raw materials into precise orbits for interception at remote construction sites. Mass drivers can also serve to propel massive objects by expelling any available material as reaction mass. However, mass driver engines have several limitations such as relatively large payload size, the minimum caliber being about 10 cm, and they must be several km in length, which leads to dynamic stability problems. The bucket recirculation and servicing mechanism might be too complex for use in small reaction engines. A number of alternative acceleration mechanisms exist which offer advantages for certain applications, such as higher acceleration at a sacrifice in efficiency, smaller possible size and decreased complexity at a sacrifice in service life, etc. The alternative concepts include several variants of the railgun, a family of superconducting slingshot oscillators, a momentum transformer, an impulse induction motor, and a family of hybrid synchronous accelerators. A potential application of considerable interest is the earth-based launcying of space cargo or nuclear waste by using off-peak generating capacity to accelerate one ton cargo cylinders at intervals of several minutes. [FIRST PAGE from AIAA website]
Laser Propulsion from the Moon. Philip K. Chapman, Arthur D. Little, Inc.
Abstract: A preliminary investigation was undertaken of the potential performance of a launching laser on the lunar surface, for support of extensive lunar operations such as mining. It is found that the advantage of laser propulsion over conventional chemical propulsion for this application is not the higher specific impulse which is available, but that indigenous lunar propellants may be used. In comparison with the mass driver, laser propulsion offers larger unit payloads, simplified guidance, an initial capital investment which is almost certainly much lower, and the capability of using the system for landing on the moon as well as for launch from it.
Laser-Boosted Advanced LTAV as a Heavy Launch Lift Vehicle. L. N. Myrabo, W. J. Schafer Associates, Inc.
Abstract: The purpose of this paper is to introduce a heavy lift launch vehicle (HLLV) concept proposed for the large scale transport of materials into orbit for space industrialization and colonization. The prospective payoffs for the concept are a reduction in number and cost of shuttle launches, and an enhanced payload/vehicle structural configuration. Whereas conventional rocket boost vehicles disgard the lightweight, high volume fuel tank enclosures along the launch trajectory in order to place a small volume, high-density pay-load into orbit, the HHLV concept suggested here inserts the entire primary vehicle structure into orbit. The launch vehicle design is based upon lighter-than-air-vehicle (LTAV) structures technology and innovative variable-cycle laser propulsion engines (VCLPE’s) which are attached to the vehicle ends as removable power-heads. The HLLV contains a portion of the propulsive fuel in gaseous form and flies on direct-beamed laser power from orbital solar power stations. The vehicle centersections serve as ground mass-produced construction modules for use in assembling large space structures in orbit. A catalog of ten promising propulsion systems (for use in propelling the modules into orbit) are presented in this paper. Neutral buoyancy at sea level altitudes facilitates pre-launch preparation and handling. Air-breathing pulsejet engines augmented by aerostatic, aerodynamic, and free-vortex-induced lift propell the vehicle out through the dense portion of the atmosphere. Within this flight regime, low flight velocities permit the attainment of high ratios of impulse-to-laser-power while keeping aerodynamic drag to a minimum. Two of the promising space applications for the construction modules necessitate the use of superconducting cables which are housed within the primary longeron structure. Several additional high specific impulse electrically-augmented thruster concepts suggested which incorporate the superconductors into the overall systems design to attain a positive propulsive advantage over unaug-mented rocket systems. These propulsion modes are portrayed for use in at higher flight velocities within the rarefied upper atmosphere and ionosphere where the HLLV is no longer aided by aerostatic lift. The physics of a suitable variable cycle engine, vehicle and engine performance, and laser power requirements are assessed for each of several speculative propulsive modes and vehicle sizes.
The Space Shuttle External Tank as a Re-Entry Module. Stanley Kent, Aerojet Liquid Rocket Company.
Abstract: The concept of re-entering the space shuttle’s external tank has received, small scale study aimed at placing estimates upon the many factors influencing the safe recovery of the external tank. This study expands the scope of previous research through the use of a computer model of the re-entry process. This model allows a quick comparison of re-entry configurations and the effect of flight path angle, lift to drag ratio, ballistic coefficient, and rotation of the tank upon stagnation point skin temperature. The results of a series of re-entry simulations support the optimistic claims of previous research, as well as providing much needed data with which to analyze the economics of re-entering the external tank.
Use of a Duct-Burning Turbofan for an Earth-to-Orbit Vehicle Booster. James A. Martin, NASA Langley Research Center.
Abstract: The space shuttle will soon reduce the cost of transportation to orbit. The resulting traffic growth will lead to economic pressure for a new vehicle. Several vehicles have been studied. A supersonically staged system with twin turbojet boosters may be attractive but the development of a large supersonic turbojet engine would be required. Subsonic staging with a reasonable number of turbojet engines does not provide the desired pay load capability with existing engines. The addition of a duct burner to an existing large turbofan engine provides sufficient thrust to allow the design of an attractive system with twin boosters staged subsonically. [FIRST PAGE from AIAA website]
VII: CONTROLLED ECOLOGICAL LIFE SUPPORT SYSTEMS
Systems Integration in the Development of Controlled Ecological Life Support Systems. J. L. Carden, Georgia Institute of Technology.
Abstract: This paper presents a brief summary of the conclusions of the systems engineering group at the NASA workshop, “Guiding the Development of a Controlled Ecological Life Support System (CELSS), held at the Ames Research Center between January 9-12, 1979. [FIRST PAGE from AIAA website]
Nutrition, Diet and Food Processing in Controlled Environment Life Support Systems. J. Peter Clark, ITT Continental Baking Company.
Abstract: The status and research requirements for the nutrition, diet and food processing components of a controlled environment life support system have recently been evaluated. The nutritional requirements for man, even in space, can be specified in terms of chemical composition rather than traditional foods. The dietary requirements are less well-known in terms of acceptability, variability and other psychological factors. Food processing requirements refer both to preservation of stored foods and processing of recycled nutrients. There are novel constraints imposed by space conditions and by various potential raw materials. The key need is for efficient means of providing the organoleptic characteristics found necessary for an acceptable and nutritional diet.
Agriculture and Food Production. J. M. Phillips, Arizona Research Associates.
Abstract: The human settlement and industrialization of space may be viewed as an idea whose time is coming. Controlled Ecological Life Support Systems (CELSS) must be developed if humans are to function properly during long duration missions such as will be conducted at Space Manufacturing Facilities. Food production systems for CELSS perform the crucial function of recycling wastes into usable components. Agricultural systems for Space Manufacturing Facilities will be very intensive and highly responsive to human demands and manipulations. In large space habitats with access to extraterrestrial resources, particularly carbonaceous chondritic asteroids, food production may be achieved using a wide variety of biological components. These include traditional crop and livestock species, as well as novel food sources such as aquatic plants and animals and microorganisms. To evaluate potential biological components of CELSS, research must be conducted both in the space environment and in ground-based facilities. This research effort will return significant new knowledge concerning basic biological processes as well as information on intensive food production systems. Synergistic interfaces potentially exist between terrestrial controlled environment agriculture and space industrialization activities. These include multiple-use scenarios for Solar Power Satellite rectennae facilities which are also controlled environment farms.
Waste Treatment Options for Use in Closed Systems. M. L. Shuler, Cornell University.
Abstract: The key requirements placed on waste processing in a closed ecological life support system (CELSS) are discussed. Candidate systems are delineated. Wet oxidation processes (representing a physical chemical approach) are contrasted with non-photosynthetic aerobic microbial waste treatment (representing a biologically-centered option) using the key requirements generated in the beginning of the paper. Too many unknowns exist to select a “best” option. Generally wet oxidation processes might be anticipated to be more efficient, but the form of the outputs from biological processing may be more compatible with objectives such as providing a hydroponics solution for plant growth. Substantial experimental programs will be necessary to insure selection of a “best” choice. [FIRST PAGE on AIAA website]
VIII: FABRICATION AND PRODUCTS
Development of Space Manufacturing Systems Concepts Utilizing Lunar Resources. E. H. Bock, General Dynamics/Convair.
Abstract: This paper presents results of a study sponsored by the National Aeronautics and Space Administration (NASA) to evaluate the relative merits of constructing solar power satellites (SPS) using resources obtained from the earth and from the moon. Three representative lunar resources utilization (LRU) concepts are developed and compared with a previously defined earth baseline concept. Major system hardware elements, steady state logistics, start-up logistics, and personnel requirements are defined. Results indicate that LRU for space construction is competitive with the earth baseline approach for a program requiring 105 metric tons per year of completed satellites. Results also show that LRU can reduce earth launched cargo requirements to less than 10 percent of that needed to build satellites exclusively from earth materials. A significant percentage of this reduction is due to the use of liquid oxygen derived from lunar soil as a space vehicle propellant. All LRU logistics techniques investigated offered earth launched payload reductions, but a concept that uses the mass driver to catapult lunar material into space was superior to the others considered. [FIRST PAGE on AIAA website]
Lunar Resources Utilization — An Economic Assessment. Robert C. Risley, General Dynamics/ Convair.
Abstract: In this paper, a solar power satellite (SPS) program scenario was analyzed to determine the economic viability of using lunar resources for manufacturing and constructing large structures in space. Three concepts featuring lunar resource utilization (LRU) were derived and compared with a NASA/JSC-furnished earth baseline concept. The economic assessment of the alternatives included cost determination, economic threshold sensitivity to manufacturing cost variations, cost uncertainties, program funding schedule, and present value of costs. It was found that LRU is potentially more cost-effective than earth-derived material utilization, depending upon such factors as the efficiency of the facilities and the manufacturing chain as well as the type of ownership and organization of the earth-based and lunar-based scenarios. Because of the uncertainties, cost-effectiveness cannot be ascertained with great confidence. The probability of LRU attaining a lower total program cost within the 30-year program appears to range from 57 to 93 percent. [FIRST PAGE from AIAA website]
Design of a Space Manufacturing Facility to Use Lunar-Material Inputs. D. B. S. Smith, Massachusetts Institute of Technology.
Abstract: The design procedure for a space manufacturing facility (SMF) is outlined, and the design of a reference SMF is described. This SMF receives 1unar material and converts them to components of large space structures, such as solar power satellites. Examples of space-specific machinery designs are presented. A costing technique is discussed, and tentative conclusions are presented. Space has some beneficial characteristics which may simplify manufacturing processes. Technology demonstration programs are needed, but prototypes of SMF equipment can be small devices.
Is Lunar Material Use Practical in a Non-SPS Scenario? Gerald Driggers, Southern Research Institute.
Abstract: The issue of material quantity requirements in Earth orbit which make lunar material use competitive with classical transport techniques is examined. Scenarios of future raw and finished material needs in orbit as a function of Earth-based market potential are presented. Cost of transportation from Earth and cost of a lunar-based industry to satisfy these markets are addressed and compared. An absolute minimum mass requirement and lunar materials implementation cost are not identified; however, the thresholds are shown to be between 10 and 100 times less than previously believed. The key technology needs over the next decade and possible scenarios leading to use of lunar materials in the 1990’s are discussed.
New Methods for the Conversion of Solar Energy to R. F. and Laser Power. John W. Freeman, William B. Colson, and Sedgwick Simons, Rice University.
Abstract: This paper discusses two new devices which may have application to space deployed solar energy conversion and transmission systems, the photoklystron and the free electron laser. The photokly-stron converts solar energy directly to R.F. radiation. It operates on the principle of the klystron with the cathode replaced by a photoemitting surface. We have tested a model which oscillates at 30 MHz. This laboratory model requires two low-voltage bias voltages which can be supplied by D.C. solar cells. Concepts for a self-biasing device are also being considered. The photoklystron is expected to be easier and less expensive to manufacture than solid state solar cells. A photoklystron array could replace the high voltage solar cell array, slipring and klystron transmitter in the SPS. The second device, the free electron laser (FEL), converts energy from a relativistic electron beam to narrow band electromagnetic energy, tuneable from the infrared to the ultraviolet. Because the lasing electrons are not bound in atomic energy levels the ultimate efficiency of the FEL is expected to exceed that of conventional lasers, possibly making lasers a practical means of energy conversion and transmission in space systems. [FIRST PAGE from AIAA website]
High Performance Solar Sails and Related Reflecting Devices. K. Eric Drexler, Massachusetts Institute of Technology.
Abstract: High performance solar sails are light tension structures bearing space-manufactured, thin-film reflecting elements. They offer thrust-to-mass ratios 20 to 80 times those of proposed deployable sails. Development costs and risks appear modest. The low cost expected for sail production promises to make these sails more cost-effective than solar electric propulsion for most missions of interest. Applications to near-Earth orbital transfers, deep space scientific missions (some unique), and non-terrestrial resource recovery are examined and found attractive. In the latter application, sails permit recovery of asteroidal resources with a very low initial investment. The promise of high performance, low cost, and great versatility recommend this system for further study. [FIRST PAGE from AIAA website]
IX: ENVIRONMENTAL EFFECTS
An Environmental Assessment of the Satellite Power System Reference Design. Nathaniel F. Barr, U.S. Department of Energy.
Abstract: The Department of Energy (DOE) and the National Aeronautics and Space Administration (NASA) are conducting a joint Concept Development and Evaluation Program (CDEP) which will provide by June of 1980 an improved basis for national decisions regarding Satellite Power Systems (SPS). An important element of this program is an Environmental Assessment which will identify and define environmental issues associated with the installation and operation of SPS and assess what is known, unknown, and uncertain regarding these issues. The CDEP will also provide a plan for ground based research and development activities required to further reduce uncertainties regarding potential environmental impacts. The CDEP has passed the mid-point of its schedule. Environmental issues fall into four major categories: I) potential impacts of microwave exposures on human health and ecosystems; II) other potential health and ecosystem impacts of SPS construction and operation; III) potential impacts of SPS launch and heat insertions on the atmosphere; and IV) impacts of SPS operations on electromagnetic systems and use of the radio spectrum. The status of environmental issue identification, definition, and assessment to date in each of these areas are summarized along with research and development efforts which will contribute to the June 1980 Environmental Assessment. Activities required to reduce uncertainty after June 1980 are identified and discussed.
Solar Power Satellites: The Ionospheric Connection. Lewis M. Duncan and John Zinn, Los Alamos Scientific Laboratory.
Abstract: This paper reviews the ionospheric effects and associated environmental impacts which may be produced during the construction and operation of a solar power satellite system. Propellant emissions from heavy lift launch vehicles are predicted to cause wide-spread ionospheric depletions in electron and ion densities. Collisional damping of the microwave power beam in the lower ionosphere will significantly enhance the local free electron temperatures. Thermal self-focusing of the power beam in the ionosphere will excite variations in the beam power flux density and create large-scale field-aligned electron density irregularities. These large-scale irregularities may also trigger the formation of small-scale plasma striations. Ionospheric modifications can lead to the development of potentially serious telecommunications and climate impacts. A comprehensive research program is being conducted to understand the physical interactions driving these ionospheric effects and to determine the scope and magnitude of the associated environmental impacts.
Solar Power Satellite Beam Disturbance of the Upper Ionosphere. James E. Drummond, Power Conversion Technology.
Abstract: Interaction of the microwave beam from a Solar Power Satellite with earth’s ionosphere produces, according to theory, an amplifying system. What is amplified is a pattern of ripples which may exist in ionization density at the top of the ionosphere. Such ripple patterns are formed evidently in association with aurora. The question is, how does the gain of this system depend upon ripple wave length and power in the beam. So far a fairly broad maximum has been found at a ripple period of about 70 meters. The amplification of an auroral “curtain” extending from 230 to 330 km would be about 8000. This would turn a barely detectable 0.01% ripple into an 80% modulation of electron density at the bottom of the ionosphere if the non-linear growth followed the linear pattern. Such a modulation would produce a Fresnel lens which would cause east-west deflections of the beam by several kilometers. Laboratory experiments to test the basic mechanisms have begun at PCT. [FIRST PAGE from AIAA website]
Atmospheric Attenuation of Centimeter Microwaves. W. Ziegler, Sigma Data Services Corporation at the Goddard Institute for Space Studies.
Abstract: The purpose of this paper is to review the literature on absorption and scattering of centimeter microwaves in the troposphere. Theoretical and measured values for absorption and scattering are applied to the case of a Microwave Power Transmission System to determine possible atmospheric heating and loss of power from the beam. [2ND PAGE from AIAA website]
X: LUNAR MATERIAL TRAJECTORIES
Guidance, Trajectory and Capture of Lunar Materials. T. A. Heppenheimer, Center for Space Science.
Abstract: A trajectory to a given target point is achromatic if the miss distance is proportional not to launch velocity error but to (launch velocity error). An evaluation of over 48,000 test trajectories yields all achromatics from the Moon to any of the Earth-Moon libration points L1, L2, L4, L5, these being candidate sites for a mass-catcher. There are ten such achromatics; their characteristics are studied and a photographic atlas of their launch sites is given. The best transport mode involves launch from Mare Tranquillitatis to L2, as treated in previous work. The mass-catcher is considered to maneuver near L2; general equations for its motion are given, incorporating the elliptic restricted three-body problem along with the requirement that it always be reachable via achromatics, and that it undergo acceleration due to the incoming mass-stream. Discussion is given of optimized solutions for catcher motion found elsewhere, which involve solution of an approximate linearized literal form of the equations. Required acceleration levels and delta-Vs are found, and assessment is given of three propulsion modes: ion-electric, Rotary Pellet Launcher, and Advanced Space Engine. A reference catcher design is proposed, and discussion given of its operational use as well as of an overall system for lunar mass transport. [FIRST PAGE from AIAA website]
Optical Scanning of Mass Driver Payloads. Rainer M. Malzbender, Princeton University.
Abstract: This paper addresses the question of mass driver payload trajectory measurements in lunar material transfers to L2. A design for a laser scanning system is presented which will provide the required resolution. An experimental determination of this resolution is also discussed; using relatively crude methods a resolution close to that required for 1 meter dispersion at L2 has been obtained. [FIRST PAGE from AIAA website]
Light Pressure and Solar Wind Perturbations to Payload Trajectories. B. P. Von Herzen, Princeton University.
Abstract: Dispersions of the lunar mass-driver payload stream are calculated from the predominant forces in the near-lunar environment. The forces included are the solar radiation pressure, solar wind, trace lunar atmosphere drag, magnetic forces, and micro-meteorite impacts on the payloads. The light pressure and solar wind forces are found to be much more significant than the other forces mentioned. Dispersions can be kept to within a few meters if the following conditions are met: (1) the payloads should be spherical and have uniform albedoes; (2) the payloads should have constant mass to within a few tenths of a percent. [FIRST PAGE from AIAA website]
XI: SPACE HABITATS
Design Opportunities — Zero Gravity Versus One Gravity Environments. Charles A. O’Donnell, M. Rosenblatt & Son, Inc.
Abstract: The weightless environment produces physical changes in man, alters his modes of perception, and presents a new interface with his physical surroundings. These changes suggest that the eventual shape and form of habitat interiors will differ from many present concepts. This paper presents selected examples of physical and perceptual changes in zero gravity and raises issues which must be incorporated into the design process to provide space habitats which will meet man’s new needs in a changed environment. It also suggests that methods must be developed to measure the qualitative effects of design on persons in confined environments.
Aesthetic Considerations in Bernal Sphere Design. Marjorie L. Stuart.
Abstract: The interior of a sphere is not suitable for raising children because of the variable gravity. This can be corrected by placing a cylinder inside the sphere. Concentric cylinders would more efficiently use the entire volume of the sphere. Buildings should not rest their weight on the outer shell of the sphere; instead they should be part of the structure of the sphere.
XII: ANTHROPOLOGY AND PSYCHOLOGY
The Value of Anthropology for Space Settlements. Darlene Thomas, Lock Haven State College.
Abstract: This paper points out the unique aspects of anthropology and suggests the application of its perspectives, theories and methodology to space settlement. The role of the anthropologist as a trainer of settlers is discussed. Speculations are illustrated by responses having occurred in two seminars on space anthropology. The role of the anthropologist as mediator among international personnel is also examined. Spin-offs to anthropology are briefly indicated in the conclusion of the paper.
Consciousness Alteration in Space. B. J. Bluth, California State University at Northridge.
Abstract: It is suggested that astronauts, not trained in the disciplined, stress resistant military tradition, may experience altered states of consciousness as a result of space and become dangerously disoriented. Research on altered states of consciousness, ethnomethodology, and intuition show such states are not random, spontaneous, or caused by external environmental stimuli. Disorienting experiences come from breakdown in the flow of reality maintenance interaction which can be facilitated in pre-flight training. [FIRST PAGE from AIAA website]
The Art of Living in Space: A Preliminary Study for the Local Government of a Space Community. Patricia M. Sterns and Leslie I. Tennen, Attorneys and Counselors at Law
Abstract: Mankind’s ancient dream of living in space may soon become a reality. Permanent space settlements are being considered for a variety of specialized programs to improve the human condition. The environment of space offers the promise of limitless experimentation, and the potential to dramatically improve the quality of life. Although each extraterrestrial settlement may present a unique societal structure, a primary consideration necessarily common to all settlements will be the need for a form of limited home rule. Throughout history, philosophers have considered alternatives for community governments. These alternatives strive to attain the virtuous state, where the interests of the society and the individual are in harmonious accord with their nature. As mankind moves into space, community organization, philosophical goals and overall ensocialization will be in the control of the settlers. Thus, a space settlement may provide an appropriate forum in which to apply human ideals to reality. This study describes the existing legal and social principles that will greatly influence the establishment and life of a space settlement. Philosophical tenets are examined and applied, to develop a plausible alternative for the foundation for societal organization of a community in space. The concept of limited home rule is then examined and explained by consideration of the traditional areas of local concern that will be present in the context of the space habitat.
XIII: ASTEROIDAL RESOURCES
Asteroid Prospecting and Retrieval. Brian T. O’Leary, Princeton University.
Abstract: Recent concepts of near-Earth asteroid prospecting and retrieval are reviewed. An expanded search program and diagnostic tests for the presence of water of hydration are among the more promising near-term research activities. Unmanned precursor missions to Earth-approaching asteroids are not in NASA’s current mission plan. Cost-effective low delta-v asteroid retrieval missions include double lunar gravity assists, planetary gravity assist, inter-asteroid collisional velocity change, and grazing reentry Earth-orbital injection. [FIRST PAGE from AIAA website]
Low Thrust Alteration of Asteroidal Orbits. David Justin Ross, Stanford University.
Abstract: The scenarios for low-thrust missions to retrieve asteroidal materials for mining purposes have been discussed in detail elsewhere and it is not necessary to rehash the mission plans here. The type of asteroid has also been discussed, but the selection of suitable targets by their orbital elements has so far only been looked at in a general way. The purpose of this paper is to discuss the dynamics of the optimal low thrust trajectories required to bring a portion of an asteroid from its solar orbit to intersection with the earth’s orbit. A companion paper in this volume by O’Leary and Ross discusses the capture of the incoming body into high earth orbit by gravity assist maneuvers near the moon. The first leg of the mission, from earth to asteroid, is assumed to be ballistic since the equipment required for asteroid retrieval is orders of magnitude less massive than the asteroid fragment itself and the same thrust is available on both legs. [FIRST PAGE from AIAA website]
Collisional Orbital Change of Asteroidal Materials. C. E. Singer, Princeton University.
Abstract: Earth-approaching asteroids have been suggested as a source of extraterrestrial materials for large scale space manufacturing. A major disadvantage of the direct retrieval of asteroidal material is the large deep-space propulsion system which is required for launching reaction mass from the asteroid to perturb its orbit. A possible solution to this problem is to use material from one asteroid to collisionally decelerate a second asteroid and perturb it onto a planet-intersecting orbit. [FIRST PAGE from AIAA website]
The Search for Asteroids in the L4 and L5 Libration Points in the Earth-Sun System. R. Scott Dunbar, Princeton University.
Abstract: The existence of Trojan-type aggregations of asteroids associated with Earth and other planets aside from Jupiter is a problem which has not been fully solved either theoretically or observation-ally. In this paper, the dynamics of libration orbits in the Earth-Sun system are discussed, using both numerical integrations and simple theoretical models to outline problems of stability and effects of perturbations. It is found that the greatest potential disturbance to the stability of these orbits is due to the 13:8 synodic Venus perturbation, which resonates with the libration frequencies of most Earth Trojan orbits. Problems of observation in searching for any existing asteroids include the distance, phase angle, and wide area of sky coverage corresponding to the possible orbits. The implications of the existence of Earth Trojan asteroids for space industrialization based on non-terrestrial materials are discussed, with the conclusion that the short mission times and low required delta-V values to reach and retrieve them make them a potentially important resource. [FIRST PAGE from AIAA website]
Terrestrial, Lunar and Asteroidal Materials Costs-Impact of Advanced Earth-to-Orbit Transportation. Robert Salkeld, System Development Corporation.
Abstract: Accumulation of enabling technologies indicating that uses of extraterrestrial resources will soon be feasible, has brought with it tacit acceptance among some that they are also obviously desirable as soon as possible, in view of limitations on terrestrial resources and projected requirements of large scale space activities. Indeed, use of lunar and particularly asteroidal resources appears economically attractive for certain assumed applications, because of the high costs of transporting terrestrial material to space with such launch systems as the first generation space shuttle and its potential derivatives. Even so, recent work shows that something like a lunar mining and processing operation would be economically justifiable only by a very large space project such as a system of solar power satellites, and then only with considerable uncertainty about its ultimate economic merit compared with use of terrestrial materials. Studies of advanced Earth-to-orbit transportation have begun to show the feasibility of fully reusable transporters which may be capable of airline-type operations from existing airbases thus obviating the high operational support costs apparently endemic to conventional space launch facilities and operations. If through some such approach Earth-to-space transport costs can be reduced to really low values (i.e.<1 $10/kg in orbit) then terrestrial materials are shown to be economically competitive or superior to extraterrestrial resources for a range of reasonable assumptions. Even if terrestrial materials are only roughly competitive in terms of life-cycle space system costs, the high “prices of admission” for the lunar and asteroidal options seem likely to delay their undertaking significantly in a real-world environment of limited budgets. These considerations should be explored in more depth, since comparative conclusions may differ according to various assumptions and scenarios, and because the possibility that terrestrial resources will be economically competitive or superior for some time to come, could impact fundamentally our approach to the space future. In a sense the Earth may remain “imperial” longer than some would presume.