While there are a variety of designs for space habitats, such as the Bernal sphere, the O'Neill cylinder and the Stanford torus, the Japanese L-4 colony uses the banded torus design with slight modifications. As the standard torus holds approximately 10,000 people, two groups of torus structures have been stacked to form a dumbell-like arrangement. The two groups of torus structures rotate in opposite directions to prevent precessing. Minor corrections are made by pumping the water which forms the shielding along the circumferential walls.

A toroidal shape makes it possible to control the radius of the pressurized ring separately from the radius of rotation. Moreover, the torus can distribute its habitable area in a large ring. To provide a 0.85-gravity pseudo-acceleration at the circumference, a single torus offers about 670,000 m2 of usable area, with a mass of only about l50 kt, a major radius (the radius of the torus itself) of 830 m, and a minor radius (the radius of the pressurized donut) of 65 m. This mass figure excludes non-structural masses such as buildings, atmosphere, equipment and such. Each unit of usable floor area in a torus can be obtained with only about a quarter of the mass required for a cylindrical or spherical structure. so that it offers excellent economy of structural mass.

The torus is intentionally designed to provide physical and psychological spaciousness, to compensate for the artificial and crowded nature of the habitat.

SHIELDING

Living organisms must be shielded adequately from the ionizing radiations of space. The ideal shield would bring the radiation dosage below 0.5 rem/yr cheaply and without impairing the contact of the colonists with their environment. There are a variety of active shields which electromagnetically trap, repel or deflect incident particles, but considerations of cost (both initial and running) and long-term reliability instead led the Japanese to select a passive shield which simply absorbs the particles in a thick layer of matter. In this case, the shield mass was provided from Moonbase One and the asteroid belt, in the form of metals and water.

In Shintenchi, each single torus requires 9.9 Mt of shield. Due to constraints imposed by engineering materials in the late 20th century, the original specifications for the Stanford torus claimed that such a mass could not be rotated at the same angular velocity as the habitat because the resultant structural stresses would exceed the strength of the materials of the shield. Consequently, the shield would have had to be separate from the habitat itself and either rotated with an angular velocity much less than 1 rpm (the velocity of the habitat) or not rotated. To minimize the mass required, the shield would be built as close to the tube of the torus as possible, and therefore the rotating tube would be moving at about 90 m/s past the inner surface of the shield from which it is separated by only a meter or two. In engineering terms, even from the standpoint of engineering in the 20th century, this was not a difficult task.

Significant improvements in materials since then, however, made it possible to include approximately half of the shield mass within the rotating habitat itself, with the remaining mass situated some distance away in the form of asteroidal iron. The mass within the habitat is primarily constituted of water derived from lunar ice mines, and in addition to providing a radiation shield also plays an important part in the closed environment of the colony. In addition to the use of carbon nanotube materials for various structural members, the skin of the habitat was also gradually improved to provide significant reductions in particle penetration.

The majority of particle screening, however, comes from the "tetsutate," an enormous iron slab positioned between Shintenchi and the sun. The tatsutate (literally, iron shield) was shaped from an iron asteroid, and is roughly conical. It throws a shadow over the L4 colony, protecting it. The colony, meanwhile, is suspended from the tetsutate by several carbon nanofiber cables which are connected to its central shaft (the axis of the rotating habitats). At the ends of the central shaft, which does not rotate, several solar cell arrays are positioned in such a way that they not only reflect sunlight onto the outside two habitats (one at each end), but also pull the entire L4 colony assembly backwards, away from the tetsutate, under the force of the solar wind and sheer photo pressure. The mass of the asteroid is sufficient to prevent from wandering out of the L4 stability region, and the entire structure automatically maintains its orientation by acting as a huge, self-adjusting weathervane.

CONSTRUCTION OF THE L4 COLONY

Standard technology for hot and cold working metals was sufficient to form the sheet, wire and structural members needed. An extensive space machine shop was provided so that many of the heavy components of a rolling mill, extrusion presses, casting beds and other equipment could be made on-site rather than have to be brought from Terra or the moon.

Assembly of the habitat from titanium plate and ribs began with the spherical hub (including docking facilities), extending gradually outward through the spokes to start each torus shell. Both the spokes and shell were constructed using a technique called "space tunneling," where movable end caps are gradually advanced along the tube as construction proceeded. This allowed "shirt-sleeve" conditions for workmen as they position prefabricated pieces brought through the spokes and made the necessary connections. In parallel with the construction process, large masses of asteroidal material were shaped and positioned around the colony to provide shielding, reducing incoming radiation significantly. Much of the metal required for construction also came from these convenient floating stockpiles, although certain metals (titanium, aluminum, magnesium, copper) had to be imported from the moon or even earth. The plan called for workers to complete the basic shell and the first layer of shielding as quickly as possible so that spin-up could begin, provided simulated gravity to allow additional construction crew and even colonists to move into the colony and begin work on bootstrapping life support functions.

Lunar material availability and processing

The colony utilized a variety of materials extracted from the lunar soil. Silica was used in windows and solar cells. Oxygen is the major component of the colony atmosphere and is required for manufacturing water, as well as being used as rocket propellant. On the moon, titanium is available in the form of a magnetic mineral (ilmenite) which can be easilyi separated from the bulk of the lunar ore. The use of titanium for structure (as opposed to steel or even aluminum) provided significant savings in the total amount of refined material because, although more difficult to form and fabricate, its strength-to-mass ratio is greater than that of the other metals available. Since ilmenite is basically FeTi03, significant amounts of iron and oxygen were also extracted as byproducts.

To provide window areas for the space structure, glass was manufactured from lunar materials. Silica (SiO2), the basic ingredient in glassmaking, is found in abundance on the moon. In Terran glassmaking sodium oxide (Na20) is used in the most common flat plate and sheet glass industrially produced to lower the required melting temperature, but since the solar furnaces used to process lunar material are capable of generating temperatures considerably higher than those needed for glassmaking, no Na20 is required - a fortunate thing, as it is only available from earth..

Transport of Lunar Material

The construction of the colony depends critically on the capability of transporting great quantities of lunar material from the Moon to the colony without large expenditures of propellant. There are three parts to this problem: launching the material from the Moon, collecting it in space, and moving it to the colony. The construction of Shintenchi was made possible by an electromagnetic mass accelerator constructed near Moon Base One.

The stream of material launched by the electromagnetic mass driver was intercepted and gathered by an active device which tracked the incoming material with radar and moved to catch it. The momentum conveyed to the catcher by the incident stream of matter was balanced out by ejecting a small fraction of the collected material in the same direction as, but faster than, the oncoming stream.

An arrangement of catching nets tied to cables running through motor-driven wheels made possible rapid placement of the catcher anywhere within a square kilometer. There were a number of problems with mechanical reliability of the system, but on-site maintenance kept them within design parameters. For three reasons, however, this catcher was deployed at L2, and not L4.

First, the stream of payloads presented an obvious hazard to navigation, posing the danger of damage if any of the payloads struck a colony or spacecraft. This danger was particularly acute in view of the extensive spacecraft traffic in the vicinity of the colony. The payloads, like meteoroids, were quite difficult to detect.

Second, L2 is one-seventh the distance of L5, so that the catcher could be made smaller and the mass driver less accurate, representing significant cost savings in both cases..

Third, shooting the lunar material directly to L5 would have required the launch facility to be constructed on farside, and it was deemed that communication with earth was important.

Once the lunar material was captured at L2, it was propelled to L5 with mechanical pellet ejectors powered by an onboard nuclear system of 25 MW. This same system was used to offset the momentum transferred to the catcher by the payloads arriving at speeds of up to 200 m/s.

Modular construction

In the construction of Shintenchi, a modular system was used involving a light, tubular structural frame (composed of modular column and beams) in combination with walls that are nonload bearing and with prepackaged, integrated subsystems (such as bathrooms) where needed. This system provides light weight, excellent modularity, good spanning capabilities, adequate structural rigidity, and a short assembly time because all labor-intensive mechanical systems are prefabricated off-site (in this case, at Moon Base One).

LIFE SUPPORT

Food

Food supplies could of course be supplied from Terra, but transportation costs are relatively high. In addition, general self-sufficiency precautions call for local production. Various forms of algae have been developed, especially in the difficult period immediately following the Twilight War, but they have been relegated to the position of emergency rations, and are considered unpalatable by the majority of the population. As a result, food production is handled through a terrestrial type of agriculture: land and sea crops, fish, poultry, and meat-bearing animals.

This form of agriculture has the advantage of depending on a large variety of plant and animal species with the accompanying improvement in stability of the ecosystem that such diversity contributes, although species have been selected to provide optimal food yield. In addition, photosynthetic agriculture also serves as an important element in regeneration of the habitat's atmosphere by conversion of carbon dioxide and generation of oxygen, as well as providing a source of pure water from condensation of humidity produced by transpiration.

Recycling Wastes

High costs of transportation place great emphasis on recycling all the wastes of the colony. Crucial elements fundamental to agriculture - carbon, nitrogen, and hydrogen - must be imported from distant (and therefore expensive) sources when required. To avoid this expense and further contribute to self-sufficiency, all wastes and chemicals are recycled with as small a loss as possible.

Wastewater is treated through a combination of microbiological digestion and wet oxidation (the Zimmerman process). Operating at a pressure of 10^7 MPa and a temperature of 260 degrees C, wet oxidation with a total process time of 1-1/2 hr produces a reactor effluent gas free of nitrogen, sulfur and phosphorous oxides, and high-quality water containing fine phosphate ash and ammonia. Both the reactor gas and the water are sterile. Chemical constituents are extracted through various downstream processes for reinjection into the system as required, while water is returned to the circumferential lake.

Composition and Control of the Atmosphere

The atmosphere has a normal terrestrial partial pressure of oxygen, a partial pressure of carbon dioxide somewhat higher than that of Terra to enhance agricultural productivity, and a partial pressure of nitrogen about half of that at sea level on earth. Nitrogen is included to provide an inert gaseous buffer against combustion, and for medical reasons. The total atmospheric pressure is about half that of earth at sea level.

Atmospheric oxygen regeneration and carbon dioxide removal are by photosynthesis using the agricultural parts of the life support system. Humidity control is achieved by cooling the air below the dewpoint, condensing the moisture and separating it. The separation of condensate water is handled in the zero-gravity hub through the use of a combination of hydrophobic and hydrophilic materials, thereby minimizing the resultant pressure drop and improving reliability by eliminating of moving parts.

COLONY LAYOUT

The NGK-Froust Corporation and SpaceGlass