Artificial Gravity
The consequences of extended exposure to weightlessness are undesirable physiological adaptations that increase the difficulty of returning to an environment with gravity. The longer the period of weightlessness, the greater the difficulty.
Although countermeasures such as daily periods on a spinning wheel, diet and exercise can reduce these physiological adaptations, they are not entirely effective. The perfect solution would be to create artificial gravity, enabling humans to better maintain their health in space.
Studies show that people get motion-sick in centrifuges with a small rotational radius (generally less than 100 meters) or with a rotation rate above 2 rotations per minute (rpm). To generate a rate of spin of 2 rpm or less and produce a gravitational force equivalent to the surface of the earth, the radius of rotation would have to be 224 meters (735 ft) or greater. For the same gravitational force, a rate of spin of 1rpm would virtually eliminate motion sickness and require a radius of rotation of 894 meters (2933 feet). By comparison, the ultimate dimensions of the ISS will be 108 by 74 meters. Obviously, future space habitats able to generate artificial gravity would need to be much larger than space stations constructed to date.
This calculator determines the radius and rotation speed necessary to create various levels of artificial gravity.
Wheel
The image to the right is the classic space habitat from the movie 2001:A Space Odyssey, directed by Stanley Kubrick in 1968. The movie based its space station concept on former German scientist Wernher Von Braun's model. Kubrick's station in the movie was 900 feet in diameter, orbited 200 miles above Earth, and was home to an international contingent of scientists, passengers, and bureaucrats.
The spoke design shown in the movie was a popular concept at the time. The shape combined with the revolution speed created simulated gravity in space. The more scientists learned, however, the more they became aware of the physical hazards and the costs necessary to avoid those hazards.
As indicated above, to create an artificial gravity similar to earth's and turn at a speed slow enough to not trigger motion sickness would require a very large wheel. Large meant expensive, which gave pause to the politicians. The design difficulties worried the engineers. Unsurprisingly, no space habitats (stations) actually built include artificial gravity.
Torus
The most extensive study on the options for space habitations grew out of a 10 week program held in California at Stanford University and NASA's Ames Research Center during the summer of 1975. The report on this summer program set forth in great detail the potential for creation of space habitats given technical knowledge at that date.
The torus, although not the best design in many respects, seems to give the most desirable balance of qualities. Relative to the sphere and cylinder it is economical in its requirements for structural and atmospheric mass. Relative to the composite structures it offers better esthetic and architectural properties. Because of its good habitability properties, large volume, a variety of possible internal arrangements, the possibility of incremental construction, a clear circulation pattern, access to zero gravity docks and recreation at the hub, agriculture as an integral part of the living area, and a clear visual horizon for orientation, the torus was adopted as the basic form of the habitat.
The Sun's rays in space are deflected by a large stationary mirror suspended directly over the hub. This mirror is inclined at 45 degrees to the axis of rotation and directs the light onto another set of mirrors which, in turn, reflect it into the interior of the habitat's tube through a set of louvered mirrors designed to admit light to the colony while acting as a baffle to stop cosmic radiation.
The shielding masses required cannot be rotated at the same angular velocity as the habitat because the resultant structural stresses would exceed the strength of the materials from which the shield is to be built. Consequently the shield must be separate from the habitat itself and either rotated with an angular velocity much less than 1 rpm or not rotated.
The space habitat image on the right above illustrates one way to construct the space habitat laid out in the parameters described above. It is called a Stanford Torus. On the left is shown a cut away view into the interior of the space habitat. To the lower right we see a possible interior view.
The outer "tire" is a radiation shield built of compressed cinder-block-like lunar material or the remains of an asteroid. The central hub contains the docking station and communications antenna; six spokes connect the hub with the ring-shaped outer wheel and provide entry and exit to living and agricultural areas.
To simulate Earth's normal gravity the entire habitat rotates at one revolution per minute about the central hub. The burnished disc that hangs suspended above the wheel is a floating mirror panel which reflects sunlight down onto the slanted panels and into the chevron shields that screen out cosmic rays.
Future Prospects
In 1991, Al Globus wrote in an article entitled, "The Design and Visualization of a Space Biosphere", There has been little published work on space settlement design in the last decade. Things did not change much in the next 15 years. The most recent online source is by the selfsame author and is a 2003 excerpt form his proposed book, Orbital Space Colonies. As of late 2006, there was no evidence that the book had been published.
In 1992, NASA did publish a summary report related to the benefit from the use of natural resources found in space: (1) energy from the Sun, (2) certain properties of space environments and orbits, and (3) materials of the Moon and near-Earth asteroids. This paper was based on the activity of a study group convened for 10 weeks in the summer of 1984 at the California Space Institute at the University of California at San Diego.
This hiatus in study of and planning for space habitats likely has one principal cause; the bad taste left by experience with the International Space Station.
All material in the ISS was transported into space at great expense from the earth's relatively high gravity surface. Further progress on development of space habitats will be dependent on progress in two areas: (1) establishment of permanent bases on the lower gravity moon or asteroids which can provide material that can be transported more cheaply to potential space habitat orbits and (2) discovery of space-based technology that can process these materials into useable products in a cost effective manner.
Space Habitat Links
The Space Settlements web page provides a wealth of links to resources relating to space habitats. The Space Studies Institute, founded by Gerard O'Neill, funds research to achieve the productive use of the abundant resources in space. PERMANENT is an acronym for Program to Employ Resources of the Moon and Asteroids Near Earth in the Near Term. It discusses a number of subjects related to space habitats. Mike Combs has provided an excellent FAQ related to our subject.
Life in habitats (includes the terms space colonies, settlements or stations) floating in space has been visualized by scientists and in science fiction for a good part of the 20th Century.
The current International Space Station has been under construction since 1998 - far longer than contemplated (originally to be complete in 2004) - at a cost ($100 billion?) far beyond original projections and at the loss of attaining alternative paths (moonbase, etc.) into space that this money could have financed.
