ORBITAL GREENHOUSES

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.

Water-cooled window

Water-cooled window

Greenhouses resemble living organisms. They consume sunlight, excrete waste heat, and cope with the force of artificial gravity. A small greenhouse shaped like a sphere can perform these tasks well, but a large greenhouse must have a complex shape and a complex system of mirrors guiding sunlight into its interior. When the diameter of the spherical greenhouse is doubled, its internal area is quadrupled, its mass is increased 8 times, and its cost per square meter of internal surface is doubled. A large greenhouse shaped like a torus, spiral, helix, or band can sustain a great diversity of species and commercial services, but is afflicted by human conflicts and pests.

A slender cylindrical greenhouse has a high ratio of internal horizontal surface to volume. On the other hand, it is unstable unless attached by a bearing to other spinning greenhouses. This instability is caused by the tendency of a freely spinning object to change its axis of rotation until it rotates about the axis having the greatest moment of inertia. A large cylindrical greenhouse fails catastrophically when its bearing malfunctions.

In my opinion, the most practical settlement is a cluster of small greenhouses docked with a stationary hub. The settlement is easy to build while providing lots of diversity, safety, environmental control, and freedom. A family living in a small greenhouse is self-sufficient, so it can sail away and join another settlement. Each greenhouse is shaped like a teardrop to reduce the slope leading to the docking port. In addition to providing an air-tight seal, the docking port acts as a journal bearing. Flora and fauna migrate between residential greenhouses through the hub. Seeds and small animals drift in the hub with a wind produced by fluctuation of air pressure in the greenhouses. Agricultural greenhouses are locked to keep pests away.

Exterior view of 
teardrop greenhouse

Exterior view of teardrop greenhouse (Can you see window?)

Interior view of 
teardrop greenhouse

Interior view of teardrop greenhouse (large image 217k)
(This family-size residential greenhouse has a diameter of 200 meters.)

Docked greenhouses

Docked greenhouses

Greenhouses docked with 
stationary hub

Greenhouses docked with stationary hub

Docking port profile

Docking port profile


A large reflecting telescope is an essential tool of a spacefaring civilization, both as a means of exploring the neighborhood, and as a component of wide-band communications across the solar system. To avoid the high cost of polishing the paraboloidal mirror, amateur astronomers sometimes use a mirror made of a glass plate deflected by air pressure. The same technique can be employed in outer space on a grand scale. Three fiberglass mats are launched into an eccentric Sun orbit. Each mat is spinning slowly about its center of mass. At the perihelion the mats melt and transform into thin glass disks. The edges of two disks are glued to each other. When air is pumped into the space between them, their flat surfaces become paraboloids. The third disk is placed in front of the pressurized contraption to protect it from punctures by micrometeoroids and space junk. A variety of aspherical surfaces can be produced by deflecting the surface of a molten glass with a stream of air.

Large 
reflecting telescope

Large reflecting telescope


Space settlement bibliography and images.

PS. IMAX 3D theaters play several films about space colonization. The most recent film "L5: First City in Space" is chockfull of computer renditions and factual lapses.

BIBLIOGRAPHY OF SPACE RADIATION SHIELDING

J. W. Haffner, Radiation and Shielding in Space, Academic Press, 1967.

Geoffrey A. Landis, "Magnetic Radiation Shielding: An Idea Whose Time Has Returned?," Space Manufacturing, Vol.8, AIAA, 1991, pp. 383-386.

John W. Wilson, John E. Nealy, Walter Schimmerling, Francis A. Cucinotta, and James S. Wood, "Effects of Radiobiological Uncertainty on Vehicle and Habitat Shield Design for Missions to the Moon and Mars," NASA Technical Paper 3312, 1993.

Kim Myung-Hee Y. et al., "Performance Study of Galactic Cosmic Ray Shield Materials," NASA Technical Paper 3473, 1994.