ORBITAL GREENHOUSES

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A planet is a good place to visit, but the middle of nowhere is a better place to live and work. Abundance of raw materials, vacuum, cheap solar energy, and weightlessness make the outer space a perfect environment for mining, chemical processing, metallurgy, construction, agriculture, and transportation. A metallic asteroid can be easily processed into a steel shell of an orbital greenhouse. The shell is fitted with small windows and filled with soil, air, water, flora, fauna, and people. Artificial gravity (pseudogravity) is generated by the centrifugal force of the spinning greenhouse. Although it is not known how much passive shielding is needed to absorb deep space radiation, 1 ton of soil per square meter seems adequate. The inner surface of the shell is protected from moisture by a layer of aluminum and cathodic protection. Molten silicate is splattered on the aluminum, so it looks and feels like the natural rock.

Unlike the Earth, the greenhouse is under human control and can sustain most terrestrial ecosystems, as well as exotic ones, such as a low-gravity rainforest awash in perpetual sunlight. A perfect beach, free of flies, mosquitoes, and ultraviolet radiation, does not exist on the Earth, but it can exist in the greenhouse.

Windows are expensive and vulnerable to collisions with space junk and meteoroids. To reduce the cost of construction and maintenance, the windows should be shielded by mirrors, and the window area should be minimized. Surprisingly, most published images depict windows taking up half the surface area of the greenhouse!

The minimum window area is determined by the transparency of the glass pane and the intensity of heat removal from the pane. Fused silica and its cheap substitute, Pyrex glass, are the best materials for the pane. The pane is supported by ribs to reduce its thickness and cost. To match the coefficient of thermal expansion of the pane, the ribs are made of a glass matrix reinforced with carbon fibers. Unless the ribs are coated with a reflective layer of aluminum, the carbon fibers will absorb sunlight and overheat the window. Heat absorbed by the pane is removed by submerging it in water. Boiling water rises as fog, spreads horizontally, precipitates on trees, drips down as rain, and flows in a stream back to the window. The fog also disperses sunlight and generates wind which is needed for healthy growth and seed dispersal of many plants. To reduce the volume of fog, the flow of water above the window is accelerated by placing the mouth of the stream close to the window. When a speckle of dust or a microorganism drifts above the window, it absorbs sunlight and floats away like a hot air balloon.

NASA