In earlier chapters conservative projections are made on the possibilities of space colonization. The view is that the potentialities of the concept are substantial even if no advanced engineering can be employed in its implementation. Abandoning a restriction to near-term technology, this chapter explores long-term development in space, mindful of a comment made many years ago by the writer Arthur C. Clarke. In his view, those people who attempt to look toward the future tend to be too optimistic in the short run, and too pessimistic in the long run. Too optimistic, because they usually underestimate the forces of inertia which act to delay the acceptance of new ideas. Too pessimistic, because development tends to follow an exponential curve, while prediction is commonly based on linear extrapolation.
What might be the higher limits on the speed and the extent of the development in space? First, consider some of the benefits other than energy which may flow from space to the Earth if the road of space colonization is followed. To the extent that these other benefits are recognized as genuine, space colonization may take on added priority, so that its progress will be more rapid.
From the technical viewpoint, two developments seem almost sure to occur: progress in automation, and the reduction to normal engineering practice of materials technology now foreseeable but not yet out of the laboratory. Those general tendencies, in addition to specific inventions not now foreseen, may drive the later stages of space colonization more rapidly and on a larger scale than anticipated in the other chapters of this report.
Space colonization is likely to have a large favorable effect on communication and other Earth-sensing satellites. Already communication satellites play an important role in handling telex, telephone, computer, and TV channels. They provide data-links and track airplanes and ships as well as rebroadcast TV to remote areas. In the future even more of these data-link applications can be expected. Not only will planes and ships be tracked and communicated with by using satellites, but trains, trucks, buses, cars, and even people could be tracked and linked with the rest of the world continuously. Currently, the main obstacle blocking direct broadcasting of radio and TV to Earth from orbit is the lack of low-cost power in space. SSPS's would produce such power. In addition, their platforms could be used to provide stability. Currently, up to 40 percent of the in-orbit mass of communication satellites consists of equipment used to provide power and maintain stability. Finally, colonists could carry out servicing and ultimately build some of the components for such satellites.
Space manufacturing such as growing of large crystals and production of new composite materials can benefit from colonization by use of lunar resources and cheap solar energy to reduce costs. Space manufactured goods also provide return cargo for the rocket traffic which comes to L5 to deliver new colonists and components for SSPS's.
Within the past half-century many of the rich sources of materials (high-grade metallic ores in particular) on which industry once depended have been depleted. As the size of the world industrial establishment increases, and low-grade ores have to be exploited, the total quantity of material which must be mined increases substantially. It is necessary now to disfigure larger sections of the surface of the Earth in the quest for materials. As both population and material needs increase, the resulting conflicts, already noticeable, will become more severe. After the year 2000 resources from the lunar surface or from deep space may be returned to the Earth. Much of the lunar surface contains significant quantities of titanium, an element much prized for its ability to retain great strength at high temperature, and for its low density. It is used in the airframes of high performance aircraft, and in jet engines. Given the convenience of a zero-gravity industry at L5 a time may come when it will be advantageous economically to fabricate glider-like lifting bodies in space, of titanium, and then to launch them toward the Earth, for entry into the atmosphere. The transportation of material to the Earth in this form would have minimum environmental impact, because no rocket propellant exhaust would be released into the biosphere in the course of a descent. (Some oxides of nitrogen would be formed as a result of atmospheric heating.) Titanium may be valuable enough in its pure form to justify its temporary fabrication into a lifting-body shape, and its subsequent retrieval in the ocean and towing to port for salvage and use. If such lifting-bodies were large enough, it might be practical to employ them simultaneously as carriers of bulk cargo, for example, ultra-pure silicon crystals zone-refined by melting in the zero-gravity environment of the L5 industries. It has been suggested that the traditional process of metal casting in the strong gravitational field of the Earth limits the homogeneity of casts because of turbulence due to thermal convection. Quite possibly, in space, casting can be carried out so slowly that the product will be of higher strength and uniformity than could be achieved on Earth. A titanium lifting-body might carry to the Earth a cargo of pure silicon crystals and of finished turbine blades.
The foundries of the Earth fabricate heavy machinery in an intense gravitational field simply because there is no other choice. The ideal location for the construction of a very large object is almost certainly a zero-gravity region. The L5 colonies, furnished with abundant solar power, relatively conveniently located for access to lunar materials, and with zero gravity at their "doorsteps" will very likely become the foundries of space, manufacturing not only satellite power stations but also radio telescope antennas many kilometers in dimension, optical telescopes of very large size, and vessels intended for scientific voyages to points farther out in space. Research probes to the asteroids and to the outer planets could be built, checked out and launched gently from L5 colonies and with none of the vibration which attends their launch from the surface of the Earth. Once the principles of closed-cycle ecology have been worked out thoroughly, as they almost surely will be during the first few years of colonization, a vessel large enough to carry a "laboratory village" of some hundreds of people could be built at L5 and sent forth on an exploratory trip of several years. On Earth, villages of smaller size have remained stable and self-maintaining over periods of many generations, so there seems no reason why a trip of a few years in the spirit of one of Darwin's voyages could not be undertaken in deep space.
Lunar resources, when available, will have a profound impact on the cost of travel between low Earth orbit (LEO) and L5. Indeed, an ordinary chemical rocket, able to reload with liquid oxygen at L5 - and to carry only hydrogen as a propellant component from the Earth, would perform as a LEO-to-L5 shuttle. For a trip from L5 out to the asteroids, it may be that eventually each exploratory ship will carry enough propellant for only a one-way trip, relying on the carbonaceous chondritic asteroids as inexhaustible "coaling stations" for hydrogen and oxygen, thereby making longer voyages possible.
For operations from Earth a rocket engine has to be compact and very strong, capable of withstanding high temperatures and pressures. For voyages between L5 and the asteroids there is no need for rapid acceleration, and in zero gravity there is no reason why an engine could not be many kilometers in length and quite fragile. One obvious candidate for a deep-space rocket engine is the "mass-driver" which would, presumably, be proven and reliable even before the first space colony is completed. For deep-space use a solar-powered mass-driver could be as much as 50 km in length, made with yard-arms and guy-wires, much like the mast of a racing vessel. Note that on the surface of the Earth, in one gravity, it is possible to build very lightweight structures (television towers) with a height of 500 m. For deep space an acceleration of l0^-4 g would be sufficient, so it should be possible without excessive structure to build something much longer.
A mass-driver optimized for propulsion rather than for materials transport would have a lower ratio of payload mass to bucket mass than is baselined for the Moon. For a length of 50 km an exhaust velocity of as much as 8 km/s (in rocketry terms, a specific impulse of 800) should be possible without exceeding even the present limits on magnetic fields and the available strengths of materials. A mission to the asteroids, with an exhaust velocity that high, would require an amount of reaction mass only a little more than twice as large as the final total of payload plus engine.
A mass-driver with a length of 50 km could hardly be made in a miniature version; it would probably have a mass of some thousands of tonnes, a thrust of about 10,000 newtons, and would be suitable as the engine for a ship of several tens of thousands of tonnes total mass.
In the course of the first decades of colonization it seems likely that solar-cell power-plants for space vehicles will decrease in mass, ultimately becoming very light. It will not be economically reasonable to continue using rocket engines which exhaust hydrogen, scarce as it is on the Moon. The rocket engines of that period will very likely be solar-powered, and must exhaust as reaction mass some material that appears naturally as a waste-product from the processing industries in space; further, that material must not be a pollutant. One good candidate may be oxygen; it constitutes 40 percent by weight of the lunar soils.
At least two types of rocket engines satisfying these conditions seem good possibilities: the mass-driver, used with liquid or solid oxygen payloads for reaction mass, and the colloidal-ion rocket, which would accelerate electrically small micropellets having a ratio of charge to mass which is optimized for a particular mission. The mass-driver, as a rocket engine, only makes sense for large vehicles or loads; its length would be comparable to that of an SSPS, and its thrust would be several thousand newtons. The colloidal-ion rocket would have much lower thrust but could be compact.
When traffic between the Earth and the colonies becomes great enough, the most economical system may consist of a single-stage-to-orbit shuttle between Earth and LEO. Because there would be no need for the transport of large single structures, shuttles of that kind could be sized for optimum efficiency. From LEO to L5 the transport problem is entirely different, transit times are several days rather than a few hours, and high thrusts are not required. The most economical vehicles for that part of space may be large ships built at the colonies. These ships, mass-driver powered through solar energy, could carry a round-trip load of oxygen as reaction mass when they leave the colonies, and could then rendezvous with shuttles in low orbit. The outbound trip would be faster than the inbound.
Construction methods which are now only at the stage of laboratory test may be practiced only in the space environment. In zero gravity and with a good vacuum, it may be practical to form a shell by using concentrated solar heat to melt aluminum or another metal at the center of a thin form. Evaporation over a period of months or years would build up on the form a metal shell, for which the thickness at each point would be controlled by masking during the evaporation. This process would lend itself well to automation.
Alternatively, or in addition, habitat sections could be constructed of fiber-composites. On Earth, the most familiar example of such a material is fiberglass, a mixture of glass threads in an organic matrix. Boron filaments are used in place of glass for high strength in aerospace applications. Glass fiber could easily be made from lunar materials. As a matrix, a silicon compound might be used in the space environment similar to a corresponding carbon-based organic. Such a compound might be attacked by the atmosphere if it were used on Earth, but could be quite stable in vacuum.
In the long run, as colony size approaches diameters of several kilometers and individual land areas of more than 100 km^2, the cosmic-ray shielding provided by the colony land area, structure, and atmosphere becomes great enough so that no additional shielding need be added, allowing the development of large-size colonies earlier than can otherwise be justified on economic grounds. Mankind's descendents who may live in space during the next century will probably be far more adventurous in their choice of styles of habitation than can now be projected, and in the spirit of this section, a relaxation of strict choices of physiological parameters seems permissible.
The assumption that the retention of artificial gravity in the living habitat continues to be necessary may be rather conservative. This assumption is based on human nature. Most people do not keep in good physical condition by self-imposed exercise. Return to Earth, whether or not occurring, must remain an option with strong psychological overtones. To rule it out, as might be the case if bones and muscles were allowed to deteriorate too far by long habitation in zero gravity, would be to make of the colonists a race apart, alien to and therefore quite possibly hostile to those who remain on Earth.
Habitation anywhere within a range of 0.7 to 1.0 g is assumed to be acceptable, and in the course of a normal day a colonist may go freely between home and zero gravity work or recreation areas.
As colony size increases, the rotation-rate criterion ceases to be a design limit. Atmospheric pressure is important to large colonies. With increasing experience in an environment of very large volume, with an abundant source of water, and with artifacts made for the most part of minerals rather than organics, fire protection is expected to be practical in an atmosphere having a total pressure of 36 kPa, of which half is oxygen. The oxygen at Denver, Colorado (which is 18 kPa), is normal for millions of human beings in that area. It is no great leap to assume an atmospheric mix of 50 percent oxygen, 50 percent nitrogen with appropriate amounts of water vapor.
From the esthetic viewpoint, people might prefer an "open" nonroofed design habitat (sphere or cylinder) when it is available to one of the more mass-efficient roofed designs. It may be possible to get some better information about public preference after further exposure of the ideas to the public. Architectural design competitions could be a means to yield valuable new ideas. It seems certain that over a time-span of several decades new designs will evolve. Some may combine mass-efficiency, achieved by optimizing the shape of the pressure shell and the cosmic-ray shield, with visual effects which are tailored to meet the psychological needs of the colony's people. The ways in which sunlight is brought into a habitat may be adjusted to suit psychological needs which we on Earth do not yet appreciate. Similarly, the degree of visual openness of a habitat may be separated from the structure itself; it is possible to divide an open geometry into visual subsections, and to provide visual horizons in a variety of ways, though a closed geometry cannot easily be opened.
To estimate the total resources of land area which could ultimately be
opened by space colonization requires a model. An example from what might
be a class of geometries is the "Bernal sphere" discussed in
If automation permits a moderate increase of productivity to a value of
100 t/person-year, which is twice the value now appropriate for
processing and heavy industries on Earth, the large Bernal sphere could
be built for an investment of 50,000 man-years of labor. That is
equivalent to the statement that 12 percent of the maximum population of
one such sphere, working for 3 yr could duplicate the habitat. Automation
is much better suited to the large scale, repetitious production
operations needed for the habitat shell than to the details of interior
architecture and landscape design. It seems quite likely, therefore, that
the construction of new habitats will become an activity for specialists
who supply closed shells, ready for interior finishing, to groups of
prospective colonists.
From the viewpoint of economics, the logical site for colony
construction is the asteroid belt itself. The construction equipment for
colony-building is much smaller in mass than the raw materials it
processes. Logically the optimum method is to construct a new habitat
near an asteroid, bring in its population, and let them use the colony
during the period of about 30 yr it takes to move it by a colloidal-ion
rocket to an orbit near L5. They may, however, prefer to go the other
way, to strike out on their own for some distant part of the solar
system.
At all distances out to the orbit of Pluto and beyond, it is possible to
obtain Earth-normal solar intensity with a concentrating mirror whose
mass is small compared to that of the habitat.
If the asteroids are ultimately used as the material resource for the
building of new colonies, and if by constructing new colonies near
asteroids relatively little reaction mass is wasted in transportation,
the area of land that is made available on the space frontier can be
estimated. Assuming 13 km of total area per person, it appears that space
habitats might be constructed that would provide new lands with a total
area some 3,000 times that of the Earth. For a very long time at least
mankind can look toward resources so nearly inexhaustible that the
current frustration of limits to growth can be replaced by a sense of
openness and the absence of barriers to further human development.
Size of an Individual Habitat
The structural shell which contains the forces of atmospheric pressure
and of rotation need not, in principle, itself be rotated. In the very
long term, it may be possible to develop bearings (possibly magnetic)
with so little drag that a structural shell could be left stationary
while a relatively thin vessel containing an atmosphere rotates inside
it. Such a bearing cannot be built now, but does not seem to violate any
presently known laws of physics. An invention of that kind would permit
the construction of habitats of truly enormous size, with usable areas of
several thousand km^2.
Even in the absence of a "frictionless" bearing, the size possibilities
for an individual habitat are enormous. As an example, a large titanium
sphere seems technically feasible of a diameter of 20km. It would contain
an atmosphere at about 18 kPa pressure of oxygen and be rotated to
provide Earth normal gravity at its equator. The usable land area is
several hundred km^2, comparable to the size of a Swiss canton or to one
of the English shires.
The Speed of Growth
It may be that the residents of space, enjoying a rather high standard
of living, will limit their population growth voluntarily, to zero or a
low value. Similar populations, on Earth, underwent a transition of that
kind in passing from an agrarian to an affluent industrial society.
Economic incentives for having a substantial workforce in space may,
however, drive the rapid construction of new industries and new
habitations there. An upper limit to the speed of growth of space
colonization is estimatedby assuming 3 yr for the duplication of a habitat by a workforce
equivalent to 12 percent of a habitat's population. Only 56 yr are
required at this rate for the construction of communities in space
adequate to house a population equal to that of the Earth today.
The Decrease of Population Density With Time
Here on Earth it seems impossible for the population to increase without
a corresponding increase in crowding because economics force
concentrations into cities. It is expected that with the passage of time
the population density must almost certainly decrease, irrespective of
the total number of colonists. It is fundamental to the colonization idea
that productivity can continue to grow in the colonies. As a consequence
there is a continued growth of energy usage per person. As an example,
suppose that there is a real (noninflationary) productivity growth rate
of 2.5 percent per year and a 1:1 relationship between this productivity
growth rate and the increase in energy usage. That implies a growth of a
factor of 24 in total energy usage over a 128-yr period (1976 to 2104).
Space colonization appears to offer the promise of near-limitless
opportunities for human expansion, yielding new resources and enhancing
human wealth. The opening of new frontiers, as it was done in the past,
brings a rise in optimism to society. It has been argued that it may also
enhance the prospects of peace and human well-being. Just as it has been
said that affluence brings a reduction in the struggle for survival, many
have contended that expansion into space will bring to human life a new
spirit of drive and enthusiasm.
Space community economies will probably be extensions of those of Earth
for some time to come. There is, however, room to speculate as to how
locational differences may enhance organizational differences. There is,
for example, some evidence that the societies in the lands settled in
recent periods have tended to display differences from those settled
further in the past; for example, one can compare the U.S. society and
economy to that of the British, or the economy and society of California
with that of Massachusetts.
It may be that the shared circumstances of risk associated with early
colonization will bring the earlier settlers into a close relationship.
This, and the problems of access to Earth-produced products, may foster a
sense of sharing and of cooperation, more characteristic of a
frontier than of a mature Western society. With the increase of colony
population, the impersonality characteristic of modern terrestrial
societies would be expected to emerge.
Ownership and proprietary rights may be somewhat different from those
found on Earth in part because the environment within space habitats will
be largely man-made. The balance of privately-controlled vs.
publicly-controlled space may be significantly influenced by these closely
similar environmental experiences. On the other hand, the cultural
inheritance of social forms from the Earth will serve to inhibit utopian
impulses toward leaving the ills of human life behind. For example, it is
unlikely that any serious development toward egalitarianism in the
personal distribution of income will be found to arise in the colonies of
the future. That boundless energy may lead to boundless wealth is a belief which
will doubtless be tested in such future developments. Successful
exploitation of the extraterrestrial environment is expected to enhance
the standard of living not only of the population in space but the
population remaining on Earth as well.
With the advent of the era of extraterrestrial communities, mankind has
reached the stage of civilization where it must think in terms of
hitherto unknown cultural options. In the extraterrestrial communities,
many of the constraints which restrict the life on the Earth are
removed. Temperature, humidity, seasons, length of day, weather,
artificial gravity and atmospheric pressure can be set at will, and new
types of cultures, social organization and social philosophies become
possible. The thinking required is far more than technological and
economic. More basically it is cultural and philosophical.
This new vista, suddenly open, changes the entire outlook on the future,
not only for those who eventually want to live in extraterrestrial
communities but also for those who want to remain on the Earth. In the
future, the Earth might be looked upon as an uncomfortable and
inconvenient place to live as compared to the extraterrestrial
communities. Since a considerable portion of humanity - even most of it -
with ecologically needed animals and plants may be living outside the
Earth, the meaning, the purpose, and the patterns of life on Earth will
also be considerably altered. The Earth might be regarded as a historical
museum, a biological preserve, a place which contains harsh climate and
uncontrolled weather for those who love physical adventure, or a
primitive and primeval place for tourism. This cultural transition may be
comparable to the transitions in the biological evolution when the
aquatic ancestors of mammals moved onto land or when Man's quadrupedal
ancestors became bipedal and bimanual. The opportunity for human
expansion into space is offered; it needs only to be grasped.AUTOMATION AND PRODUCTIVITY
As the space community produces increased revenue, the standard of
affluence is expected to increase. Increased use of automation and
adjustment of levels of employment may permit the construction of
habitats with a greater amount of area per person. Also, esthetic
considerations will have greater impact on habitat design and
architecture as habitat construction continues and per-capita wealth
increases.LIMITS TO GROWTH
SOME ECONOMIC CONSIDERATIONS