Modular Space Stations

The space stations of the modern age (Mir, ISS, Tiangong) use a modular design, with several segments (or modules) launched from the Earth and assembled in space. Modules are typically cylindrical in shape, allowing them to be launched inside conventional rockets. It seems likely that all space stations will be constructed in this way for some time to come as it is the most practical solution.

Of course, a modular design doesn’t necessarily mean that a space station is limited in size and complexity. Space stations like the ISS could develop into huge structures, with bigger rockets allowing the addition of bigger modules.

Inflatable Space Stations

One way in which the traditional modular space station design might change over the next few decades is the introduction of inflatable modules. This might sound like a crazy idea, but inflatable designs have already been tested and may soon be put into operation.

The idea of creating inflatable segments was first proposed by NASA as part of their concept for a manned flight to Mars. The TransHab concept consisted of a traditional cylindrical module with an inflatable exterior shell, providing extra living space. The module would have been launched inside a rocket as usual, and would then have expanded once in orbit, inflated by the breathable atmosphere within. The fully inflated TransHab module would be 8.2 meters in diameter (spacious compared with other craft) and contain three levels of leg-stretching luxury.

The TransHab concept was reluctantly dropped by NASA in 2000, but the concept was bought by Bigelow Aerospace, who continued to develop the design. The company has since launched the Genesis I (2006) and Genesis II (2007) modules to test the viability of inflatable space modules. They plan to build their own TransHab space station – referred to as Space Complex Alpha – as early as 2014.

Orbital Tether

Another ambitious idea that has been explored by NASA is the building of an ‘orbital tether’, also known as a ‘space elevator’, ‘space ladder’, ‘space tether’ or ‘skyhook’. This idea has actually been around for a very long time (hence its many names) and has long been considered a practical solution to the problem of transporting materials and personnel into orbit, and refueling spacecraft.

The idea is to connect (or tether) an orbiting space station to the ground. The space station would act as a counterweight, keeping the tether upright. The space station could then be reached by climbing the tether in an airtight elevator car.

Many science fiction writers have played with this idea, including Arthur C. Clarke (in The Fountains of Paradise), but the original credit goes to Russian scientist Konstantin Tsiolkovsky who, in 1895, was inspired by the Eiffel Tower to conceive a tall tower connected to a “castle” in a geostationary orbit.

Orbital Drydock

The building of an orbital ‘drydock’ (or ‘space dock’) like those seen in Star Trek would seem to be a practical necessity of any space-faring civilization.

The need to build and repair space ships in orbit is likely to grow as we venture farther out into space, as building large craft on the surface and launching them into space becomes impractical. Even modular space craft require some assembly in space. This means more spacewalks and a greater need for robotic arms. A design that surrounds the craft allowing access and inspection from all angles would therefore be advantageous.

An orbital space dock would not only allow the construction of larger spaceships, but of larger space stations too.

Habitation Wheel

This design was first used in 1929, when Austro-Hungarian scientist Herman Potocnik proposed its use for long space voyages. The spinning wheel has since become a staple of science fiction, and is still considered to be the most practical solution to the problem of artificial gravity.

An enclosed  spinning wheel creates centrifugal force, pushing its contents outward and creating the illusion of gravity. A spinning segment does not need to be a complete circle for the effect to work, but this is the most practical design in terms of floor space. The purpose of the wheel is generally to provide a comfortable living space for the crew.

The idea of using rotation to create artificial gravity dates back to 1903 and Russian scientist Konstantin Tsiolkovsky and it has been something of a scifi revolution. The simple spinning wheel concept has since led to many more interesting designs.

Stanford Torus

The Stanford torus takes the idea of a spinning wheel and applies a much large scale. The design was developed at Stanford University in 1975 as part of a study into potential long-term habitation of space. Unlike smaller wheel designs, the Stanford torus would be a permanent, self-sufficient orbital habitat. Sunlight would be provided by a series of mirrors on the inside of the wheel (shielding against radiation from direct sunlight).

Like other wheel-shaped space stations, the torus features spokes leading to a central ‘hub’ section. As this section would experience the lowest ‘gravity’, it would be the ideal place for ships to dock.

O’Neill Cylinder

Another spinning space station concept is the O’Neill cylinder. The idea was first proposed by American physicist Gerard O’Neill in his 1976 book “The High Frontier: Human Colonies in Space”.

The O’Neill cylinder works in much the same way as the Stanford torus, except that it is cylindrical rather than wheel-shaped. This provides a larger surface area and more living space within.

Light enters the habitat via three large windows running the length of the cylinder. Three large shutters, one for each window, can be closed to protect the habitat from radiation. While in an open position, the mirrored inner-surface of these shutters reflects light into the habitat, thus protecting it from the harmful effects of direct sunlight. In the picture (right), you can see the sun reflected in the uppermost shutter.

Probably the most famous O’Neill cylinder in science fiction is space station Babylon 5. However, Babylon 5 is not a true O’Neill cylinder as it lacks the reflective radial shutters.

Bernal Sphere

Similar to the O’Neill cylinder, the Bernal sphere is a large rotating habitat with added protection from radiation. If the O’Neill cylinder is Scooby Doo, the Bernal sphere is Scrappy Doo – smaller but twice as tough. While it may not be as efficient in terms of providing comfortable living space, it is much more efficient in protecting against radiation from solar flares. Unlike Scrappy Doo, however, the Bernal sphere is much older than its cousin. It was first proposed in 1929 by British scientist John Bernal. It was later used by Gerard O’Neill in his study at Stanford University, alongside the O’Neill cylinder and the Stanford taurus.

The central habitat would be roughly spherical in shape to help deflect radiation. This would create a valley of habitable space inside. Light would enter the habitat through windows at either end of the sphere’s rotational axis, reflected by a series of mirrors. This enclosed design would be much less vulnerable to radiation and impact damage than the O’Neill cylinder, though it does lack the design elegance of its counterpart.

As the habitat would be much smaller, a series of agriculture rings were added to either end. These greenhouses would be insulated against radiation by the large amounts of soil they would contain, but they would be considerably more vulnerable than the sheltered habitat of the sphere.

Ringworld

Imagine a giant Stanford torus encircling a star, with a radius equal to the distance between the Earth and the Sun. This is the concept of the ‘ringworld’, first envisioned by Larry Niven in his 1970 novel, “Ringworld“. Niven’s ringworld had an inner surface several million times greater than that of the Earth. The illusion of night and day was created by a series of huge shadow squares positioned closer to the star.

Dyson Sphere

Another idea, similar to that of the ringworld, is the Dyson sphere. In most fictional portrayals, the Dyson sphere takes the ringworld concept and encloses it as a complete sphere, creating a giant hollow globe around the sun. The purpose being to harness vast amounts of solar energy, and to provide huge amounts of living space.

Freeman Dyson, for whom the concept is named, had a different vision however. His concept did not involve building a single spherical mega-structure, but encircling the sun with a series of smaller space stations; a ‘swarm’ or ‘shell’.