hydrogen and nitrogen are missing from the moon and will have to be transit ported from earth. The slag left over after refining can be used for radiation protection by forming a coat on the outside layer of the space colony. The refined materials will need to be melted into a billet of suitable size for subsequent processing into a plate, extrusion or drawn wire. This melting can easily be accomplished by focusing the sun's rays on the material. Availability of varying degrees of "gravity" depending on location of the equipment within the manufacturing facility and the presence of high amounts of power and vacuum should enable the purely manufacturing processing (rolling, drawing, shearing) to be accomplished readily, although no doubt special handling methods will need to be developed. Fabrication of the completed piece parts into a Model 1 configuration may prove to be more difficult. It would seem that as early as possible, some rotation of the evolving structure should be imparted, in order to eliminate the extremely flexible, non-rigid assembly resulting from zero gravity. This, of course, will cause added difficulties in the subsequent assembly. Productivity in the manufacturing and assembly processes is assumed to be on the order of 10 to 30 tons (9,000 to 27,000 kg) per person per year. This is comparable to similar figures on earth. The difficulties in assembling the colonies and power stations will be quite similar to those discussed in Section IV.5, but heightened considerably by the necessity for extending the process right back to the raw materials. The difficulties may be eased somewhat by avoiding constraints placed on earth materials by the launch vehicle. That is, in orbit one is not constrained to maximize density for launching nor are the overall weight constraints as severe. A symposium held at Princeton University on space manufacturing * There is the possibility that carbonaceous chondritic type asteroids contain these elements in abundance and would therefore provide a cheaper source for them than the earth.
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