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| Asteroid habitat per Dandridge Cole |
Life in habitats (includes the terms space colonies, settlements or
stations) floating in space has been visualized by scientists and
science fiction for a good part of the 20th Century.
Some of these space structures were envisioned as being built
entirely out of manmade materials. Others were carved out of asteroids.
In 1963 Dandridge Cole
suggested hollowing out an ellipsoidal asteroid about 30 km long,
rotating it about the major axis to simulate gravity, reflecting
sunlight inside with mirrors, and creating on the inner shell a pastoral
setting as a permanent habitat for a colony.
Actual Space Station
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| Courtesy ESA, 2019 |
Other than the Soviet/Russian Space Station
Mir, which fell to earth in a planned de-orbit in 2001, the
International Space Station (ISS) is the only significant (weighing
more than 100 metric tons) space habitat ever constructed. Neither it
nor its precursors were designed to generate artificial gravity by
rotation.
The first ISS component was launched in 1998, with the first inhabitants arriving in November 2000. Since then, the station has been continuously occupied. The latest major pressurized module was fitted in 2011, with an experimental inflatable space habitat
added in 2016. Development and assembly of the station continues, with
several major new Russian elements scheduled for launch starting in
2020.
The ISS is probably the most expensive single item ever constructed. In 2010 the cost was expected to be $150 billion - far beyond
original projections. Probably most important, its construction resulted
in the relinquishing of alternative paths (moonbase, etc.) into space
that those billions could have financed. The ISS is funded until 2020,
and may operate until 2030.
Given the International Space Station experience, where do we go in
the future evolution of habitats in space? If the intention is for
humans to live in space for more than temporary periods of six months
or less, what will be the shape of future habitats designed to generate
artificial gravity?
Advantages of Space Habitats
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| Space habitat of a toroidal shape visualized in the 1970s, Courtesy NASA |
The advantages assume future habitats designed to make full use of
the unique capabilities associated with them. Such habitats were visualized by NASA in the 1970s,
back when visionaries were encouraged by the agency.
Artificial
habitats in outer space have a number of advantages over terraforming
planets and moons for human habitation. Some advantages are:
- Feasibility - Creation of space habitats is more feasible than terraforming planets and moons, given resources and technology reasonably available in the next 200 years.
- Gravity - Habitats could be created with artificial gravity equivalent to the surface of the earth. Terraforming of Venus, the only solar system world with a gravity equivalent to earth's, would not be possible at a cost and timescale we can contemplate in our current state of scientific and technical knowledge.
- Resource Availability - Access to vast resources, including the moon via mass drivers, asteroids and the energy resources of the Sun.
- Expandibility - Given the nature of outer space, there is no practical limit to the habitable space that could theoretically be created.
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.
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| Space station visualized under construction in the movie "2001 - A Space Odyssey" |
The image to the left is the classic space habitat from the 1968 movie "2001:A Space Odyssey" directed by Stanley Kubrick.
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 double wheel design was a popular concept at the
time. The shape combined with the revolution speed created simulated
gravity in space.
However, the more scientists and engineers studied the concept, the more they
became aware of the physical hazards and the costs necessary to avoid
those hazards when seeking to create artificial gravity.
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.
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 at maximum is about 100 meters. Obviously,
future space habitats able to generate artificial gravity would need to
be much larger than space stations constructed to date.
An artificial gravity calculator, created by Theodore Hall, determines the radius and rotation speed necessary to safely create various levels of artificial gravity.
Torus
The torus, although
not the best design for a space habitat in many respects, seems to give a desirable
balance of qualities. Relative to the sphere and cylinder it is
economical in its requirements for structural and atmospheric mass.
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| Stanford torus with sections removed to show the interior |
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 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.
Bernal Sphere to O'Neill Cylinder
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| Bernal Sphere |
Island One would be capable of
providing living and recreation space for a population of approximately
ten thousand people. Sunlight was to be provided to the interior of
the sphere using external mirrors to direct it in through large windows
near the poles. The form of a sphere was chosen for its optimum ability
to contain air pressure and provide radiation shielding.
O'Neill also proposed Island Two with a diameter of 1800
meters, which provided an equatorial circumference of 6.5 kilometers
(four miles). The habitat could house a population of some 140,000
people. This was a size considered economically feasible.
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| Interior view of an O'Neill Cylinder |
Lastly, we come to an O'Neill Cylinder. The third generation Island Three design, better known as the O'Neill Cylinder, consists of two counter-rotating cylinders, each five miles (8.0 km) in diameter, and capable of scaling up to 32 kilometers (20 miles) long.[5]
Each cylinder has six equal-area stripes that run the length of the
cylinder; three are transparent windows, three are habitable "land"
surfaces.
Furthermore, an outer agricultural
ring, 32 kilometers (20 miles) in diameter, rotates at a different speed to
support farming. The habitat's industrial manufacturing block is
located in the middle. To save the immense cost of rocketing the materials from Earth,
these habitats would be built with materials launched into space from
the Moon with a magnetic mass driver.
The National Space Society has presented a 3-D animation based on Rama, the cylinder space habitat in Rendezvous with Rama by Arthur C. Clarke. At a Blue Origin event in Washington on May 9, 2019 Jeff Bezos proposed building O'Neillian colonies rather than colonizing other planets.
Future Prospects
In 1991 it was written 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 have not changed much in the last 30 years.
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.
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