Mankind has been inhabiting the earth for millions of years and its resources are being used continuously which has lead to major crises, among them one is Power. In order to overcome the crises of power, world is shifting from hydro to solar power. The solar panels are capable to provide solar energy, but to fulfill the demands of energy of the entire world, one needs a big structure of solar panels. In 1968 Dr. Peter Glaser suggested [1] to construct Solar Power Satellite (SPS) containing solar panels. The points in its favor as compared to the big structure on Earth are:
Excess amount of solar energy in Earth's orbit i.e. 144% more than the energy reaching on the surface of Earth.
Solar exposure time is much more as compared to Earth.
There is no effect of weather in the orbit.
The structures of such satellites will be very large, generating giga watts of electric energy. For such large structures human attention is required.
The other consideration is requirement of minerals. In our space there is a large number of asteroids, having lot of (PGMs) Platinum group metals [2] are less in concentration on the surface of Earth but these are very useful. Several design groups at NASA and elsewhere determined that there is an ample quantity of all the necessary materials on Moon and Near Earth Asteroids.
These are the main reasons to construct space settlement as compared to other ones like safety and healthy environment. The first time NASA sponsored studies [3] were conducted in the field of space settlements consecutively during the summer of 1975 and 1976 at Stanford University. Then the publication of books and articles in this field boosted the idea in the aerospace technical community. Princeton physicist Gerard O'Neill [4] with the help of NASA Ames Research Center and Stanford University in the 1970's showed that we can build giant orbiting spaceships to live in. A few years later, they showed that orbital space colonies were physically possible (Johnson and Holbrow, 1975) [5] (O'Neill, 1977).In same year 1977, T. A. Heppenheimer [6] wrote a book "Colonies in Space". Louis J. Halle [7], formerly of the United States Department of State, wrote in Foreign Affairs (Summer 1980) that the colonization of space will protect humanity in the event of global nuclear warfare. In September 2005, NASA, Chief Michal Griffin [8] has identified space colonization as the ultimate goal of current space flight programs. Physicist Stephen Hawking [9] has said:"The long-term survival of the human race is at risk as long as it is confined to a single planet. Sooner or later, disasters such as an asteroid collision or nuclear war would wipe us all out. But once we spread out into space and establish independent colonies, our future will be safe.
In 1869, it was the beginning of space colonization when first time Edward Everett [10] used the world inhabited artificial satellite. Then in 1920 various writers like Bernal, Oberth, Von Pirquet, and Noordung used the word space colonies in their books. The idea of space colony continued in the publications by Wernher von Braun [11] in 1952. In 1961, Sagan was the first scientist to hypothesize about terraforming [12] in a technical journal. By 1976 NASA had published the proceedings of a workshop examining the feasibility of terraforming Mars [13]. Space settlements are big closed air tight structures in space, in which everything is artificially created like surface (hills and valleys), atmosphere, ventilation, day and night cycle, and so on. In short, we can say that Space settlement is the home away from home. A self-sustainable space settlement must fulfill all the fundamental needs of people living in space mentioned below:
i. Location of the settlement
ii. Design Considerations (includes exterior and interior)
iii. Life support system
iv. Technology used to construct such a huge structure and the technology for running the various functions of the settlement
i. Suggestive location of settlement:
There are lots of options available for the settlement location, such as:
a) On the surface: - It can be surface of Moon, Mars, of some big asteroid (for habitat). Generally we don't consider other planets since Venus is far away from Earth and too hot. Mercury is too hot during the day and too cold at night, (as the days and nights are so long). Jupiter, Saturn, Neptune, and Uranus have no solid surface. Pluto is very far away. While considering surface colonies we are left with two options Mars and the Moon. Mars [14] has all the materials required for colonization: oxygen, water, metals, carbon, silicon, and nitrogen. We can even generate rocket propellant from the atmosphere. The Moon [15] has almost everything needed, the exceptions being carbon and nitrogen; water is only available at the poles. However, the idea of constructing a big structure on the surface is greatly inferior in comparison to orbital space colonies in every way except for access to materials.
b) In orbit: - One of the logical place for these structures is at one of the Earth-Moon libration points [16] (There are five Lagrange libration points in a system where a large body is in orbit around a much larger body (e.g., the Moon in orbit around the Earth); these are the places where the gravitational forces due to both objects "balance out" in a way that enables a settlement to stay in one spot relative to both objects. "L4" and "L5" is the most stable. In fact, a large number of space facilities could be placed a "L4" and "L5". The other three libration points are unstable. If an object is perturbed and drifts a small distance, it will tend to continue drifting away until it assumes a more normal orbital path. Objects could, however, be kept in these locations with the use of their engines to continually elbow them back where they belong.
In fact, orbital location has nothing except few atoms per cubic centimeter. Near Earth Objects [17] (NEOs), (which include asteroids and comets with orbits near Earth's) are very accessible from Earth; include water, metals, carbon, and silicon nearly everything we need. NEOs can be mined and the materials transported to early orbital colonies near Earth. From Moon we can also supply metals, silicon, and oxygen in large quantities.
The other options can be in the orbit near Mars [18] or inside the asteroid belt [19]. While selection of the location in asteroid belt needs specific attention, it should have less amount of debris, as compared to rest of the asteroid belt. Such location provides an easy access to various asteroids.
Hence, it can be concluded that orbital settlements are only one potential target for early space settlement. Al Globus [20] in his book Anon,orbital space colonies, emphasized on the same point. O'Neill also emphasized that a colony in space is more practical than one on the surface:
Consideration of gravity: The moon has one-sixth and the mars has 0.037 of the earth's gravity, thus while constructing the settlement we can't ignore it. This value is very less as far as the physical aspect of human beings is concerned but very high as far as inter orbital motion is concerned. To increase the value of g on the surface of moon or mars is very tedious and expensive process. While on the free space, we can simply produce 1g (pseudo-gravity) by using the concept of centripetal force.
Solar energy: The moon has a 14-days night, thus there is a serious problem of obtaining energy. Same problem is on the surface of mars as it has --------days night. Therefore, low-cost solar power is not available for these days and storage of energy for such a long period is difficult. That is on the surface some alternative method of power generation is required. On the other hand, on an orbital colony continuous, ample, and reliable solar energy can be obtained.
3. Approaching surface is More Expensive: To reach the moon from Earth, firstly we will have to use energy in order to escape from gravitational pull of Earth and then again in the second phase while reaching near Moon. The transport costs to get to the moon are about twice as high as they are to go out into free space. Similar is the case of Mars.
4. Growth Potential: The settlement on the surface will easily get materials. On the other hand, in the orbital settlements we can easily get materials from NEO's, which are more in variety as compared to the surface of Moon or Mars.
ii. Design Considerations
The main consideration of designing the residential area is acceleration due gravity. In other words 1g (9.8m/s2) is required in order to fulfill the physiological needs of human beings [21]. There are various methods of production of an artificial gravity [22]:
a) Straight line accelerated motion of the settlement can produce gravity.
b) Heavy mass at the center of the settlement can produce gravity.
c) Tidal forces can produce gravity, by placing heavy mass on the settlement.
d) Electromagnetic field can produce gravity on the surface.
e) Method of Rotation can produce gravity on the surface.
One of the effective and efficient methods to produce 1g can be by rotating the settlement symmetric around an axis. The value of the rate of rotation with the radius of the big structure can be verified by using gravity calculator [23].
Similarly, while designing the interior of the space settlement we take care of the Human factors, which includes Physiological and Psychological aspects, [5, 24] e.g:
a) Line of sight
b) Day and night cycle
c) Tree lining
d) Good community design
e) Per person enough space for luxury.
f) Comfortable modern community
g) Access of entertainment and
h) Safety criteria etc.
iii. Life support system [25]: In the life support system, we can include pressurized and unpressurized areas covering all physiological and psychological needs. The various operations and infrastructure required for the establishment of the space settlement can be:
a) Power generation
b) Atmospheric effect
c) Temperature control
d) Humidity control
e) Waste management
f) Plantation
g) Day and night cycle
h) Communication and
i) Transportation corridors etc.
iv. Technology: We will have to consider the construction sequence of the settlement. For which we will split the construction process into various phases, e.g.
First phase
Survey, Planning and selecting the location for the space settlement.
Second phase
Establishment of mining base near the location [26] so that the material can be obtained easily for the construction. For this we can send Inflatable habitat to the location.
Third Phase
Initializing the process of construction of the proposed settlement at the desired location.
Fourth Phase
Establishment of life support systems.
For the completion of above phases automated construction and assembly devices are required. For the construction of the space settlement high-tech robotic systems are needed with contingency plans. While designing the settlement emphasis should be taken on security aspect, it may be in terms of security of the habitats or security of stored data. The port facilities should be taken into consideration as it is the only mean through which the settlement will remain in connection with rest of the universe. Business development of the settlement will be based upon the technology used inside it.
Basic designs of space settlements: Initially three feasible shapes for orbital settlements were proposed:
a) Stanford Torus
b) Bernal Sphere and
c) O Neill Cylinders.
All of these settlements featured natural sunlight directed into the settlement by mirrors, rotation rates and radii consistent with 1g pseudo-gravity.
The NASA-sponsored studies developed the Stanford Torus [3] for the population of 10000 people. One rpm was provided to the structure of one mile in diameter, in order to produce one g. The cross-section of the torus interior is about 420 feet. Floating mirror was taken above the settlement to reflect the sunlight to solar panels for the generation of power and smaller mirrors around the hub that then directed the daylight into the living areas of the torus. The designers provided the low rise houses with green environment, open space with natural view.
The Bernal Sphere [27] was also intended for the same population as of Stanford Torus, who would live in the spherical volume in the middle. At either end of the spherical section Sunlight is directed into the interior by mirrors for day and night cycle. The agriculture area is designed in the banded torus shape on either sides of the sphere. In such shapes we can provide different climatic conditions in different torus for different crops.
The most ambitious of the space settlement designs was O'Neill cylinder also called Island Tree [28]. This design considered diameter of four miles and length twenty miles can accommodate population up to ten million people. To provide natural light to the three window areas of two miles width alternative to land areas of same width, mirrors were provided. The small pods ringed around the other end of each cylinder provide agricultural and manufacturing areas.
In the space settlement design scenario these designs are taken as basic designs, but they are having certain drawbacks like shielding mass, wobble control, rotational instability, natural light exposure etc. Alternately, hybrid designs may make sense. One of the hybrid designs was the Lewis One [29]. The basic design is a Sun synchronous cylinder, 1921m long 267m radius with flat end caps completely shielded for cosmic rays. Cylinder is rotating at 2 rpm to provide 1-g pseudo-gravity at the rim. There are two shielded, micro-g 100 m construction bays, one pressurized, the other not. Each is large enough to work on complete grav-modules with 10m radial and 20m axial clearance. Agriculture is on six levels within the grav-module, starting at 1/6 g and working outward in 4m increments. For more agriculture area, additional levels may be added without major design modifications. Like all the early settlements, an insufficient area was allocated in Lewis one for solar energy collection and hence it is also under-powered. Also, Lewis One kept the rotating habitat a few meters from non-rotating shielding. Contact between these is a disastrous failure mode. This is one of the major engineering problems of space settlements.
Similarly, an orbital settlement Kalpana One [30,31] was suggested by the group of scientists having shape of cylinder with radius 250m and length of 325m with rate of rotation 2rpm. The structure considered the wobbling effect remedy by attaching weights to cables on motorized winches under computer control. Internal cylinder arrangement having lower value of g is provided for industrial, storage and agriculture.
In 2010, a paper [32] was presented on the "Tesla" Orbital Space Settlement for mining the NEOs. The shape of the settlement is taken to be truncated torus having radius 397m for residential area (very much similar to torus shape, but structural mass decreases in truncated torus) and cylinder of height 500 m and radius 150 m at the middle for industrial activities and recreation zone. 1g is provided by rotating torus at 1.5 rpm. Active mass dampers are used for wobble and nutation control.
Considerations while designing the space settlements:
While designing the settlement one should consider its design which should provide residents a clean and healthy environment to live in. During the stay in space specially working in the low gravity, it is obvious for a person to face physiological and psychological problems but by improving such problems can increase productivity and efficiency. Certain precautions and preparations can be provided in the design of settlement.
1. The selection of the materials should be done in such a manner which is able enough to protect the settlement from radiations and debris.
2. Long line of sight (as minimum required is 64m), day & night cycle provision, space view, natural sunlight, view of sky, creation of natural views can prevent the residents from the psychological problems like Solipsism and Hysteria.
3. In order to reduce psychological cases a tree lining (of approximate 5 m each) would be provided on both the sides of the down surface area of residential torus which will help as the residents won't feel congested to a place.
4. Controlled environment (Temperature and humidity control etc.) can protect the residents from another psychological problem like Perceptual Alteration.
5. Intake of nutritious food, long line of sight, and recreational facilities can be remedy of the physiological problems like Change in immune system.
Conclusion: Although various proposals are there for first space settlement but while considering the hybrid designs of space settlement following steps are to be followed:
1. Check the requirement of settlement (whether it is for mining purpose or it is for tourism purposes). Design the structure with proper size (radius) to produce 1g.
2. There should be specific location of residential area, agriculture area, industrial area and storage area. The area requirements can be calculated by taking NASA summer study as base.
3. The location of docking ports should be specified in the structure. These will link the settlement with the earth and other mining bases.
4. Then the selection of materials can be done which depends upon the selection of location of the settlements. e.g.:
i) When the orbital settlement is at one of the libration points, the best choice is to get raw materials from moon.
ii) When the orbital settlement is near the mars, then the best choice is to get raw materials from the surface of mars.
iii) Similar is the case with the settlements designed in an asteroid belt.
5. Further the construction sequence is designed with proper time lining. The details will include exterior and interior designs with the considerations of life support systems.
6. The contemplation should be given to technology for automation and communication.
7. All the technical planning must have backup plan.
