6.18 Pathways through the shield ("cutouts") would also cause structural and fabrication problems. To bring agricultural sunlight in through the shield, the ES group had two options: a window cutout, or a windowless cutout. A selective window--allowing only visible light--would have to meet the 5000 kg/m area density requirement to stop harmful radiation. Crystal glass has a density of 3000 kg/m 3 (6.2), and would therefore have to be 1.67m thick. This posed serious fabrication problems as compared to a shield made from whatever loose material might be at hand. The other option was to bring the light through the shield via a windowless hole and to hide the hole from space radiation with baffles. The harmful radiation would also be removed from the light going in. Such an arrangement is shown in Figure 6.9; the selective mirror would reflect only the desired wavelengths,letting the dangerous radiation travel on. The disadvantage of this option was that it required extra shielding and a more complicated fabrication. Thus, both windowed and windowless cutouts created problems which suggested keeping the shield cutouts few and small. Convinced by the arguments above, the ES group decided to concentrate the agricultural sunlight into a beam then pass this beam through one small shield cutout. Then the beam would enter the hull through one small window in the tip of one endcap (the tip of the other endcap being reserved for the cargo hatch). Once inside the hull, the light beam would be spread by a set of mirrors onto the agricultural area. No specific choice between windowed and windowless shield cutout was made pending development of a specific configuration. VI.4.4: Shield Configuration: The sunlight, windows, and cutouts issues affected the choice between the free and fixed relative orientation options. The free orientation design now required a variable-geometry pathway for the light beam. Furthermore, moving cargo into the colony now required bringing it through a small shield cutout, around to the hull's cargo hatch, and into the hull. Figure 6.10 shows the effect of these complications on the free orientation design proposed in Figure 6.8.
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