limit of the Shuttle payload bay. Accordingly, study and technology development to allow assembly of larger habitat vessels is recommended. Such study should assume availability of a conventional shuttle-launched modular station as a work base and living quarters. [2] Mass-driver research should continue. In my view, a mass-driver capable of launching relatively large payloads, e.g., 1 ton, direct to geosynchronous orbit from the Moon, should be considered. These 1-ton payloads could be simple vehicles with some guidance and control, and would avoid trajectory precision issues as well as those associated with trying to design a “mass catcher.” [3] Basic processes for production of engineering materials from lunar ones should be researched, including laboratory-scale process experimentation. None of these steps would be expensive. The potential payoff is quite large. A definitive evaluation of the economic benefits derivable from space manufacturing cannot be made without results from such research. CONCLUDING REMARKS This paper has presented a conceptual approach to utilization of space settlements and extraterrestrial resources that is evolutionary in nature. Developments, risks, and benefits are incremental. Each step has a payoff that seems commensurate with the potential risks and costs. The evolutionary approach, of course, presupposes an existing large-scale space industrial enterprise developed from Earth-derived resources. The evolutionary approach may be contrasted with the “bootstrap” approach, conceived by O'Neill and O'Leary et al. The latter begins with small-scale lunar resources development and attempts to bootstrap its way to large-scale space industry through buildup of facilities, equipment, and operational capabilities almost entirely from extraterrestrial resources. In this scenario, the initial investment is argued to be small, but a great many technical innovations must be developed to operational maturity before any economic benefit is realized. Problems that would result in severe schedule slippage or cost overruns, or total failure, must be regarded as likely. The bootstrap approach for space manufacturing of SPS proposes that an initially small investment in space factories can result in a self-contained “economy” (that is what it really is) with very high annual growth. The high growth is necessary for the bootstrap approach to grow from a small resource recovery and factory system to a large one capable of SPS production in a few years. Such high growth has never been observed in historical economies. The best that I know of is the roughly ten percent per year exhibited by the Japanese economy in recent years, and some economists argue that this has been possible only because the Japanese did not have to invent the technology that permitted this rapid growth, they only had to invest in it. The growth of economies seems to be limited by people productivity. The space manufacturing advocates will create reasons why space manufacturing “is different,” but I doubt if it is. The rapid growth is a far more complex issue than calculating the length of time it takes a refining process to create its own mass in refined product. This is where the evolutionary approach comes in. We propose a space manufacturing economy that is tightly coupled to the U.S. economy here on Earth. Rapid growth is indeed observed in sectors of an economy. A rapidly-growing sector siphons resources from other sectors by attracting investment, it does not bootstrap.
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