By the early 1970's the U.S. public had grown disillusioned - with the war in Viet Nam, the war on poverty, the cold war, the space race, and government in general. Space travel was seen as extravagant and wasteful. People asked, if we can send a man to the Moon, why can't we rebuild our cities or clean up the environment? As early as 1971, Arthur Kantrowitz felt compelled to editorialize in defense of "the relevance of space", harking back to the days when people dared to dream of the possibilities of large space colonies .
The last Skylab crew returned to Earth on February 8, 1974. More than a year elapsed before the next U.S. manned mission: the Apollo-Soyuz Test Project of July 1975. Following this was a period of nearly six years during which the U.S. launched no manned missions. Meanwhile, the Soviet Union launched a series of Salyut space stations, and eventually surpassed the U.S. in total man-hours of space flight. The American absence finally ended with the launch of the space shuttle Columbia on April 12, 1981.
The end of project Apollo, the abandonment of the Saturn launch vehicle, and the six-year Soviet monopoly on manned space flight were demoralizing to NASA and the U.S. aerospace industry. From the mid-1960's to October 1973, NASA employment fell from more than 34,000 to about 25,000; contractor employment fell from 377,000 to 100,000 . While NASA wrestled with shuttle development and seemingly interminable space station studies, individuals and small groups working independent of official space policy once again assumed the leadership in visionary work on space habitation.
The seeds of a movement were planted in 1969 when Princeton professor Gerard K. O'Neill posed a question to his freshman physics seminar: "Is a planetary surface the right place for an expanding technological civilization?" Their conclusion was "No." After several years, this line of inquiry led to an article in Physics Today , and inspired the first Princeton Conference on Space Colonization, which convened on May 10, 1974. The second Princeton Conference on Space Manufacturing Facilities (Space Colonies) convened one year later, on May 7, 1975, cosponsored by the American Institute of Aeronautics and Astronautics (AIAA) and NASA. Later that year, space settlements were the focus of a ten-week summer study in engineering systems design sponsored by NASA and the American Society for Engineering Education (ASEE). O'Neill and colleagues began to distribute The High Frontier Newsletter in 1975. His book The High Frontier was first published in 1976. The Space Studies Institute (SSI) was founded at Princeton in 1977 to promote scientific research leading to space resource utilization and space settlement. Princeton, SSI, and AIAA have continued to host biennial conferences on space manufacturing and colonization; the eleventh such conference convened May 12-15, 1993. NASA sponsored further studies on space manufacturing and settlement in 1976 and 1977.
Meanwhile, several populist pro-space organizations have evolved. Wernher von Braun founded the National Space Institute (NSI) to focus public attention on space exploration. Participants in the 1975 Princeton conference returned home to form local chapters of the L5 Society. NSI and L5 merged in 1987 to form the National Space Society (NSS). The Space Frontier Foundation (SFF) was established in 1991 "to shoulder the public relations responsibilities which parallel SSI's 'quiet' research mission" [63, 64, 65].
The space settlements considered by the L5, SSI, and NASA studies are almost unimaginably large by today's technological standards: typically hundreds of meters in diameter, supporting entire communities with populations of thousands. They would be easy to write off as fantasies if not for the scientific credentials of the participants and their careful technical treatment of the subject. They have studied the chemistry of the Moon and asteroids, the physics of electromagnetic mass drivers and other propulsion systems, the dynamics of orbit, the ecology of closed life support systems, and the economics of getting it all off the ground. While NASA has shown some interest, this sort of colonization calls for a scale of development far beyond its present means or ends. Its immediate interest is in identifying technologies that could be applied to more modest projects, such as the space station Freedom planned for the late 1990's, and outposts on the Moon and Mars.
Artificial gravity has continued to dominate the designs of the large-scale long-term space colonies: the studies take for granted that gravity will be required to maintain the health of populations over lifetimes. Such colonies remain in the indefinite future, however, and their study at this time seems largely academic. (Many members of SSI, SFF, and NSS would probably disagree.) On the other hand, as noted earlier, artificial gravity appears to be at odds with the missions of near-term space stations such as Freedom, in which access to zero gravity is a top priority. NASA's current interest in artificial gravity is primarily in relation to manned interplanetary missions - particularly Mars missions. In these, long periods of weightless coasting through empty space are an annoyance, not an objective. Because the crew must be physically fit to survive the return to gravity upon arriving at the alien planet, mission planners look for ways to minimize their exposure to weightlessness, by shortening transit times or providing artificial gravity.
Of the 1974 Princeton Conference, Gerard K. O'Neill wrote :
This meeting has been convened by individuals interested in exploring the possibilities of space colonization: the construction of habitats in space which could be richly productive and delightful to live in. The time scale is not the distant future, but now, using the science and engineering of the 1970's ... The basic notion in accomplishing this goal ... is to treat space as a culture medium which is rich in matter and energy ripe for exponential growth. To live, people need energy, land, water, air and gravity.
He proceeded to describe his concept for a first-generation space colony, to accommodate ten-thousand people. "Model 1" would be constructed at the L5 libration point  in Earth orbit, from material mined from the Moon. It would be a cylinder 200 meters in diameter and 1000 meters in length, rotating 3 times per minute. The circumference would be divided into six arcs, alternating between strips of land and window areas running the length of the cylinder; each of the three land areas would see a window directly above. Outside each window would be a full-length (1000-meter) cable-supported mirror to reflect sunlight into the interior. The angle of these enormous mirrors would be varied to simulate the rising and setting of the sun. Their mass would be very small relative to the total mass of the colony, so that tilting them would have a negligible effect on inertia and rotation; the biggest problem would be in keeping them sufficiently flat and stiff. (Although the original write-up implies that the entire reflector would tilt as a unit, some subsequent illustrations show a large array of small adjustable mirrors mounted on fixed "wings".) The axis of the cylinder would be kept aligned with the sun, meaning that over the course of a year it would need to precess through 360 degrees. This would be accomplished by pairing it with an identical but counter-rotating cylinder (reminiscent of Noordung's "wheel station"). A compression tower connecting the cylinders at one end, and a tension cable connecting them at the other, would provide the torque to precess both cylinders; a mere sixty pounds of force would be sufficient.
Descriptions of life in the colony are rife with the incongruity of trying to transplant middle-class America into the concave environment of a self-contained spinning cylinder :
It would be like urban living in this country, with the difference that instead of looking into the windows of another apartment one would be looking out onto farmland. One could imagine having a promenade of shops, cinemas, restaurants, markets, and libraries, extending all the way around the cylinder.
Notwithstanding the prosaic arrangement of the circumference, the low-gravity environment near the spin axis would offer novel forms of recreation, such as human-powered flight and low-gravity swimming. (What fate might befall a human-powered flyer that strayed too far from the axis, O'Neill does not say.)
"Model 1" would be only the first step in space colonization. One of its principal functions would be to construct "Model 2", which could be ten times larger. Each step could produce a tenfold increase in the size of the next colony. This would culminate in "Model 4", "the largest colony that could be built within the limits of the strengths of conventional materials" (as of 1974), with a diameter of 4 miles (more than 6400 meters) :
As a matter of interest, the Model 4 space colony could enclose more than half of the island of Bermuda in one of its valleys. Or, if you prefer, the pleasant residential community of Los Altos Hills, near San Francisco, or perhaps Carmel Bay, from Pebble Beach down to Point Lobos.
O'Neill went on to suggest that after 150 years the total land area of space colonies could exceed the total land area of Earth, and that space colonization could ultimately support a human population twenty thousand times its present size.
O'Neill has continued to develop his ideas on space colonization, and to popularize them through his book The High Frontier, which was first published in 1976. "Island One" is now described as a sphere, 1 mile in circumference (512 meters in diameter). The troublesome, tilting, 1000-meter mirrors are gone - replaced by rings of smaller mirrors that reflect sunlight into annular windows surrounding the poles. The concept is based on the "Bernal Sphere" configuration considered briefly during the 1975 Summer Study (summarized later in this chapter). The colony would be placed in high Earth orbit, but L5 per se has lost some of its earlier glamour. O'Neill has upped his estimate of the largest possible colony: "Island Three", 4 miles in diameter and 20 miles in length, would be only "moderate" in size. "The largest communities that could be built, within the limits of ordinary, present-day structural materials like iron and aluminum, and with oxygen pressures equal to 5,000 feet above sea level on Earth, could be as much as four times as long and wide, with a land area half the size of Switzerland" [70, 71]. In a 1988 interview he reaffirmed his convictions, stating that the new edition of The High Frontier would be virtually unchanged from the earlier edition (except for a new preface and an additional chapter at the end). He felt "virtually certain" that a substantial fraction of the human population may be living in space colonies by the year 2081 . The Space Studies Institute, which he founded in 1977, continues to provide a forum for both theoretical and applied research in issues related to space manufacturing and colonization.
Arthur C. Clarke published his novel Rendezvous with Rama in 1973 - several years after O'Neill began his work on space colonization but a year before the first Princeton Conference . Rama is described as an alien technological marvel utterly beyond human capabilities. Yet in its essentials, it appears strikingly similar to O'Neill's "Model 4". The interior of Rama is a hollow cylinder, roughly 50 kilometers long and 16 kilometers in diameter, spinning once every four minutes, yielding about 0.6 g at ground level. It is illuminated by gigantic strip lights that run the length of the cylinder. The lights are spaced at 120° intervals around the circumference, so that each light shines down across the diameter to a strip of land below.
One of the more interesting aspects of Rama is its weather. Sea ice melts from the bottom up as heat from the sun penetrates the cylinder walls. Storms occur when warm air rises from the spinning ground toward the axis of rotation. Waterfalls bend sideways under the Coriolis effect. It's another perspective on the oddities of life in a large rotating space habitat.
Gerald W. Driggers of the Southern Research Institute presented a "Baseline L5 Construction Station" at the 1975 Princeton Conference . Essentially, this would be the "Model 0" that would provide the living and working environment for the builders of "Model 1". While "Model 1" was conceived as a permanent self-sufficient colony, Driggers' station was conceived as temporary accommodations for construction workers. He estimated an active work force of two-thousand people for a period of ten years, with crew rotations based on twelve-month assignments. Fifty percent of the crew - one thousand people - would be replaced every six months.
Much of Driggers' study was concerned with general problems of space base design, independent of artificial gravity: volume requirements, power requirements, atmospheric pressure, radiation protection, closed-loop life support versus periodic resupply from Earth, and so on. He identified three principal elements of the construction station: the habitat, the construction sphere, and the power station. He endeavored to show the feasibility of his scheme by extrapolating from the space base studies of 1969-1970, and by designing modules with an eye to current launch capabilities. He claimed a great deal of confidence in his conclusions - even his uncertainty factors were carried to two or three significant digits.
The decision to design for artificial gravity was motivated by the assumption of twelve-month stays at L5. Shorter stays under zero-gravity conditions did not appear to offer any significant advantage: the potential savings in station mass and complexity would be offset by increased costs in transportation, operations, and retraining. Consequently, artificial gravity considerations dominated the design of the habitat and strongly influenced its relationship to the other two elements.
For the sake of safety and flexibility, Driggers proposed a "dual" design of two autonomous stations, with either station capable of supporting the entire crew for a short time in an emergency. After comparing the relative efficiencies of a variety of monolithic and modular geometries, he selected the "Double-Spoked Parallel Cluster Station" (DSPCS). Each station would be a "barbell", consisting of two clusters of cylindrical modules connected by double spokes to a cylindrical hub. The mean radius from the hub to the center of each cluster would be 60 meters. Each cluster would consist of nine modules arranged in two layers. Each module would be a cylinder, 15 meters in diameter and 12 meters high, divided into three decks. The floor-to-floor height between decks would be 3 meters; the remaining 1.5 meters in the top and bottom end caps would accommodate storage and utilities. The configuration would rotate 3.86 times per minute to produce 1 g at the 60 meter radius; the gravity on the various decks would range from 0.85 to 1.15 g. Driggers hypothesized that, in order to facilitate readaptation to Earth gravity over the course of a year, it might be advisable for crew members periodically to change living quarters between decks. Nevertheless, the fifteen percent variation from Earth-force gravity seems like a minor distortion compared to the 3.86 rotations per minute that would be common to all decks. Furthermore, movement between the artificial-gravity habitat and the zero-gravity construction sphere would be part of the daily routine.
Movement between the habitation modules and the station hub would be facilitated by an elevator in each of the four spokes. Each elevator would be 4 meters in diameter and 6 meters long, divided into three 2-meter decks. (Two 3-meter decks would align better with the habitat. Perhaps this is an editing error in the original paper.)
The rotating hub would be attached to the 100-meter-diameter non-rotating construction sphere. Driggers opted for a mechanical connection rather than separate free-flying elements for two reasons: to minimize the need for extra-vehicular activity; and to provide the habitat with access to a large air volume and, consequently, a sufficiently long evacuation time in the event that a compartment became exposed to vacuum. The construction sphere would provide a zero-gravity shirt-sleeve work environment for the materials processing and sub-assembly of Model 1. No details were provided regarding the interior, but it seems virtually certain that elaborate scaffolding or a web of guide wires would be required to enable the workers to move within this enormous volume.
The atmosphere in the combined personnel-construction station would be oxygen and nitrogen at 8 pounds per square inch (slightly more than half of normal sea-level pressure), with 30 percent oxygen. The reduced pressure would save mass in both the structure and in the atmosphere itself.
A single solar power station would serve the pair of personnel-construction stations. Because the power station would need to remain continually pointed toward the sun, Driggers felt that it would be "ill-advised" to attach it to the other elements. Instead, power would be transmitted to the personnel-construction stations via microwave links. This strategy determined the "constellation" of the various elements: The rotation axis of each habitat would be parallel to the Earth-orbital axis. The north pole of each habitat would be connected to the south pole of its construction sphere. The north pole of each sphere would be covered with a microwave collector 50 meters in diameter. The power station would maintain position above the north poles of the spheres. In this configuration, the power station would always be approximately aligned with the rotation axes of the personnel-construction stations; nevertheless, the absence of a hard mechanical connection would allow independent adjustments in the attitudes of the various elements.
Ludwig Glaeser of the Museum of Modern Art presented "Architectural Studies for a Space Habitat", also at the 1975 Princeton Conference . His aim was to explore the role of architectural design in space habitats, as a prerequisite to developing any specific design concepts. He was particularly interested in the relationship between environmental design and psychological stress, and in the capacity of a wholly artificial environment to sustain the colonists' "non-heroic day-to-day existence" for years at a time. He felt that it would be "premature and even misleading" to present any particular image of space habitat architecture before investigating the more fundamental issues of stimulus, perception, interpretation, and response. He did not discuss artificial gravity in particular. Instead, he discussed the broader issue of simulating Earth in an extraterrestrial, man-made environment :
Must we simulate it literally, with real spreading oaks where the scenario calls for it, or can we substitute a Styrofoam oak or perhaps a stylized one made of titanium mined on the Moon; perhaps a hologram of an oak will do? ... It may well be that the very artificiality of such a simulation approach would be a source of stress in itself - the stress of a schizophrenic existence in which the inhabitant is asked to accept the illusory as the real.
Glaeser casts doubt on the simple transplanting of Earth architecture into an artificial-gravity environment - a process taken for granted in many artificial-gravity design studies.
The 1975 Summer Faculty Fellowship Program in Engineering Systems Design, sponsored by NASA and the American Society for Engineering Education (ASEE), convened at Stanford University and the NASA Ames Research Center. Nineteen professors of engineering, physical science, social science, and architecture, three volunteers from academe, industry, and government, six students, a technical director, and two co-directors worked for ten weeks to design a system for the colonization of space. The technical director was Gerard K. O'Neill. Their final report appeared in 1977 as NASA Special Publication SP-413 . Their prototype colony is known as "the Stanford Torus".
This study drew heavily from the earlier work by O'Neill and the Princeton Conferences of 1974 and 1975. While previous efforts had aimed to show the basic feasibility of space colonies, this new study aimed at determining an optimum configuration for such a colony.
The colony was conceived as an industrial town with a population of ten-thousand. Forty-four percent of the population would be employed in export labor, producing goods and services to be marketed to Earth. Twenty-nine percent would be employed in internal and support labor. Twenty-seven percent (primarily children and retirees) would not be in the labor force. These assumptions were admittedly arbitrary, and compared to typical Earth communities the percentages were biased significantly in favor of high productivity. Nevertheless, the goal was to design not a space station, but a space colony - the first of many - to support permanent extraterrestrial emigration.
Citing the detrimental effects of prolonged weightlessness as demonstrated by Skylab, and the lack of knowledge regarding the minimum gravity necessary to maintain health, the study assumed that humans permanently in space should live between 0.9 and 1.0 g. The ratio was termed the "habitability parameter", and was used to compute the habitable volumes of various basic geometries. In all cases, it was assumed that 1 g of artificial gravity would be provided at the maximum radius , and that radii less than were essentially "uninhabitable".
Citing problems of adaptation to rotating environments - due to Coriolis forces and cross-coupled rotations - and the necessity for a large segment of the population to commute between zero gravity and artificial gravity on a regular basis, the study specified a maximum of 1 rotation per minute. Taking this as the actual rotation rate, along with the habitability parameter defined above, this limited the habitable radius to the range of approximately 806 to 895 meters.
In establishing requirements for projected area and habitable volume, the study cited a number of "traditional" sources: for residences - the Uniform Building Code; for shops, offices, light industry, and storage - the Town Planning Committee of South Australia; for hospitals, schools, assembly halls, and open space - Planning Design Criteria by Joseph DeChiara and Lee Koppelman. The total requirement for living and working space was estimated to be 47 square meters of projected area and 823 cubic meters of volume per inhabitant. Agriculture would require an additional 20 square meters and 915 cubic meters per inhabitant. The total for ten-thousand inhabitants would then be 670,000 square meters of projected area and 17,380,000 cubic meters of volume.
The study was somewhat inconsistent in its measurement of "projected area". For the torus, it was measured at the major radius. Depending on the aspect ratio of the torus and the habitability parameter, this might or might not intersect the habitable volume. For the sphere and cylinder, the projected area was measured at the minimum radius of the habitable volume. In all cases, the area was measured on an imaginary cylindrical surface at the designated radius. It could be argued that a more accurate measurement might be obtained by integrating cylindrical elements along the actual geometric surface. Nevertheless, any improvement in the accuracy of the measurement would probably be meaningless in proportion to the uncertainties of the requirement.
The internal atmosphere was to be a nitrogen-oxygen mix at a pressure of 51.7 kilopascals (approximately 1080 pounds per square foot) - about half of normal sea-level pressure. The superimposed load from internal mass was estimated to be 7.66 kilopascals (160 pounds per square foot). Thus, even at the reduced pressure, the principal structural load would be atmospheric rather than gravitational.
With all these assumptions, the optimum colony geometry was determined to be a torus with a major radius of 830 meters and a minor radius of 65 meters. (Thus, the maximum radius would be 895 meters, as determined above.) "Relative to the sphere and cylinder [the torus] is economical in its requirements for structural and atmospheric mass; relative to the composite structures it offers better esthetic and architectural properties" . The tensile stress in a round pressure vessel (such as a sphere, cylinder, or torus) is directly proportional to the radius. With the torus, the radius that withstands the atmospheric pressure is significantly less than the radius of rotation; yet the torus provides a continuous volume with long sight lines.
Using aluminum as the structural material, a torus skin designed to support both atmospheric and gravitational loads would be 2.08 centimeters thick. Alternatively, a 1.68-centimeter skin could support the atmosphere alone, and hoops could be incorporated into the substructure to support the gravitational loads. The internal structure would be a modular, tubular, load-bearing frame with lightweight wall, floor, and ceiling panels. A typical bay size would be 4 6 meters.
The torus would be divided into six sectors, alternating between residential and agricultural uses. Six 15-meter-diameter spokes would extend from the torus to the 130-meter-diameter spherical hub. A non-rotating docking module would extend 60 meters from the north pole of the hub. A non-rotating fabrication sphere, 100 meters in diameter, would connect to the south pole. Extending from the sides of this would be a 200-megawatt solar furnace and a 490,000-square-meter radiator. A transport tube would extend 10 kilometers (!) from the south pole of the fabrication sphere to the extraction facility, where useful minerals would be refined from raw Lunar and asteroidal material.
The torus would rotate in the orbital plane, edgeways to the sun. A large, free-flying primary mirror would be positioned high above the north pole and tilted at 45 degrees to reflect sunlight down the axis. The hub would be surrounded by a large disk of north-facing solar cells, and these in turn would be surrounded by a ring of secondary mirrors tilted to reflect sunlight from the axis out to the torus. In this orientation, the solar cells would receive energy from the primary mirror, but would be protected from the degrading effects of the solar wind. The inner or "top" third of the torus skin would be a ribbed skylight illuminated by the secondary mirrors; the outer two-thirds would be opaque.
Active (magnetic, static electric, or plasma) radiation shielding for the torus was ruled out due to large energy requirements and "speculative" technology. Passive (bulk mass) shielding was estimated to require 4.5 metric tons per square meter of surface area. This would consist of a 1.7-meter-thick layer of "bricks" made of fused undifferentiated Lunar soil. Such a shield could not support its own weight; therefore, it could not rotate with the torus. (If rotated once per minute it would fly apart.) Instead, it would be a hollow structure separated from the torus by a distance of one or two meters. It would be either non-rotating, or, if used as reaction mass when spinning-up the torus, counter-rotating at 0.07 rotations per minute. Above the torus skylights, the shield would be cast in the form of chevrons with mirrored surfaces that reflect visible light but block cosmic rays. Apparently, there would have to be a 15-meter gap in the shield to allow the passage of the rotating spokes; the report does not mention this.
The total cost of the system was estimated to be $190.8 billion (1975). To justify the expense, "no alternative at all was found to the manufacture of solar satellite power plants as the major commercial enterprise of the colony" .
The study also examined the impact of loosening some of the design parameters. It determined that if the minimum gravity level was reduced from 0.9 to 0.7 g and the maximum rotation rate was increased from 1.0 to 1.9 per minute, then the optimum geometry would be a sphere with a radius of 236 meters. The study provided a schematic design of such a colony, which it dubbed the "Bernal Sphere" in honor of J. D. Bernal. In this scheme, much of the agriculture was moved from the spherical habitat into a "banded" torus. The environment in the torus would be optimized for plants rather than people, with different requirements for atmospheric composition, gravity level, and radiation shielding.
This study is notable for its concern with quality of life as well as quantity of material. Long sight lines, large overhead clearances, and reconfigurable interior structures were considered essential for the psychological well-being of the inhabitants. The report warned against "motel banality" and solipsism that could result from long-term confinement in a small, isolated, inflexible, totally-artificial environment. Colonies composed of multiple small structures were rejected due to their fragmentation of internal space. Nevertheless, the report stated that such structures were worthy of further consideration, since they appeared to offer substantial mass savings and the ability to build up a colony gradually.
Conceptual problems arise in the habitat's interior architecture. The proposed modular system is rigidly rectangular, and there is no discussion of how it would be fitted to the curvature of the torus. The report describes a village of apartments, shops, and offices of modular construction arranged on multiple levels between 0.9 and 1.0 g. But a ten percent variation in artificial gravity implies a ten percent variation in radius and circumference as well. Thus, assuming "vertical" (radial) continuity of structure, a module of 4 6 meters at the bottom level would shrink to 3.6 6 meters or 4 5.4 meters at the top level, depending on its orientation. Forgoing strict vertical continuity, the error in a single 4-meter story would be on the order of 1 to 3 centimeters, which could be made up by offsetting the columns slightly at each level. Alternatively, the internal construction could be truly rectangular - this is implied by the illustrations - but then the error would accumulate along the horizontal dimension. Eventually, the floor would have to curve up, and the longer this was put off, the more noticeable would be the floor's perceived deviation from "level" and the kink required to correct it. These issues were simply not dealt with. In fact, interior perspectives of the torus were drawn with straight horizontal lines vanishing to infinity. It is difficult to say whether these long straight chords were a drafting error or a design error. In any case, the interior of the torus would probably appear much less placid if its curvature was accurately represented.
Ten years after the completion of this study, architect Patrick D. Hill stated :
I think I am the only person from the visual arts or design fields who came or even applied to join the 1975 Summer Study ... The designs in the 1975 Summer Study were strictly a one-night sketch problem during the tenth week of the Study. Something was needed. I got my drawing instruments out and did an all-nighter. Unfortunately, that design probably has been looked at by many people out of context. It was never meant to be a definitive solution.
The 1976 Summer Study also convened at the Ames Research Center, with support from the NASA Office of Aeronautics and Space Technology and the NASA Office of Space Flight. Gerard K. O'Neill again served as study director. Whereas the 1975 study produced a single, large, integrated report describing the group's overall concept of a space colony, the 1976 study produced a series of smaller technical papers that addressed various aspects of spaced-based manufacturing. The collection was published in 1977 as Volume 57 of "Progress in Astronautics and Aeronautics", by the American Institute of Aeronautics and Astronautics .
Most of the commentary on habitat design was authored by O'Neill himself. In his introductory article, he stated that there appeared to be "substantial economic benefit associated with a relatively early transition to a shielded habitat with artificial gravity" . In a paper on maximum-strength minimum-mass structures, he noted that a complete program of space manufacturing would require a variety of structures optimized for different purposes. As an example of a high-efficiency enclosure in which aesthetics were secondary, he elaborated the concept of the "banded torus" proposed for the agricultural areas of the Bernal Sphere colony (studied briefly during the previous summer). He dubbed this light-weight toroidal structure the "crystal palace", for its passing resemblance to Sir Joseph Paxton's iron-and-glass pavilion . O'Neill also cited the significant decrease in structural and shielding mass permitted by higher angular velocity (due to reduced radius for a given gravitational force). The Stanford Torus had been limited to 1 rotation per minute, but O'Neill and Gerald W. Driggers cited evidence that "a healthy person can adapt readily to 5 rpm or more" . They performed a cursory analysis of stair-climbing in a rotating environment, and argued that the side-effects of rotation up to three times per minute would be negligible.
The 1977 Summer Study convened once again at the Ames Research Center, sponsored by the NASA Offices of: Space Science; Aeronautics and Space Technology; and Space Transportation Systems. Once again, Gerard K. O'Neill served as study director. Forty senior research workers and ten students participated in the six-week study devoted to space resource utilization and space settlement. The participants were organized into five task groups, each of which produced one or more technical papers: regenerative life-support systems; parametric studies of habitat efficiency; detection and analysis of special classes of asteroids suitable as material resources; electromagnetic mass drivers; and chemical processing of nonterrestrial material in space. The collection of papers was released in 1979 as NASA Special Publication SP-428 . Two of these papers are discussed in the following two sections.
As part of the 1977 Summer Study, Edward Bock, Fred Lambrou Jr., and Michael Simon studied the "Effect of Environmental Parameters on Habitat Structural Weight and Cost" . The purpose of this paper was to quantify the weight and cost penalties associated with the "conservative" design objective of providing "Earth-normal" conditions. It considered five environmental dimensions:
three population sizes:
and four basic configurations:
The authors estimated fixed values for certain environmental parameters. They assumed that the smaller habitats could deviate farther from Earth-normal conditions, due to their relatively-small, carefully-selected populations and shorter mission durations. The larger habitats, intended to support larger, less-selective populations for longer periods, would require closer adherence to Earth-normalcy. After establishing these constraints, the authors conducted a series of sensitivity analyses: they allowed other environmental parameters to vary, and studied the impact on structural mass and cost. For example, citing past research on human adaptation to rotation, the authors estimated the maximum habitable angular velocity to be: 3 rpm for the 102 population, 2 rpm for the 104 population, and 1 rpm for the 106 population. Taking these values as constants, they varied the minimum habitable gravity, and studied the influence on mass and cost for various combinations of population and configuration. Similar analyses were conducted for volume, atmosphere, and radiation.
The mass of atmosphere diluent imported from Earth had the greatest potential influence on habitat cost. This quantity was influenced by total volume, atmospheric pressure, and the type of diluent used (nitrogen or helium). The total volume, in turn, was determined by the volumetric requirements and the geometry. The geometry was influenced by the basic configuration and the minimum habitable gravity. Lowering the gravity requirement increased the volumetric efficiency of the structural shell - especially for spheres and cylinders - by considering a greater portion of the enclosed volume to be "habitable".
Admitting the limitations of their analysis, the authors refrained from recommending an overall best habitat configuration. Nevertheless, they noted that the torus appeared to offer the lowest costs for both the basic structure and the atmosphere: the torus encloses only habitable volume, while much of the sphere's volume is uninhabitable due to minimum gravity requirements. Unfortunately, several parts of the torus structure were not included in the analysis: spokes, hub, shielding alignment, and extra stiffening required in the compressively-loaded inner (ceiling) surface. Also, illumination of the torus appeared "cumbersome". The sphere appeared easier to illuminate, and did not require extra structure for shielding alignment. Furthermore, its "uninhabitable" low-gravity volume might be utilized for space manufacturing.
The authors felt that one of their most important discoveries was that, regardless of configuration, the cost per inhabitant appeared to minimize for habitat populations in the 105 to 106 range. While they acknowledged that this particular result was based on their initial assumptions and limited analysis, they felt that a more extensive analysis with different initial assumptions should also point to some minimum-cost-per-capita population. They recommended that further research be concentrated in areas where non-Earth-normal conditions appeared to offer the greatest cost savings, particularly: rotation rates, g-level variations, low-g tolerance, low atmospheric pressure, and low-density atmospheric diluent (such as replacing nitrogen with helium).
J. Peter Vajk, Joseph H. Engel, and John A. Shettler studied the "Habitat and Logistic Support Requirements for the Initiation of a Space Manufacturing Enterprise" , as part of the 1977 Summer Study. As in the 1975 (Stanford Torus) Summer Study, the ultimate goal was the manufacture of solar power satellites. To support that enterprise, the authors developed a scenario that called for the establishment of two Lunar bases and three orbital bases, as well as the deployment of an array of launchers, landers, and transfer vehicles. They placed particular emphasis on the use of Lunar materials and salvaged Shuttle external tanks. Although they acknowledged the assistance of Bock, Lambrou, and Simon in developing the scenario, their adaptive reuse of fuel tanks as habitat modules led to a design unlike any of the configurations considered in the previous paper.
In configuration, the orbital bases described here are reminiscent of the habitat proposed by Driggers two years earlier: clusters of cylinders connected by spokes in a "dumbbell-like" arrangement. In the use of spent fuel tanks, they are reminiscent of the Douglas Aircraft proposal of 1959. Previous proposals to retrofit the fuel tanks of Saturn rockets had been rejected, in part because the cryogenic insulation was on the inside, where it would tend to outgas fuel vapors into the habitat for prolonged periods. (The tank used for Skylab was never fueled: it was converted to a habitat on the ground and launched "dry".) The insulation of the Shuttle tank is on the outside, where it poses no problem with outgassing.
What is commonly known as "the Shuttle external tank" is actually a set of two tanks - hydrogen and oxygen - with some additional connecting structure. In this proposal, only the hydrogen tank would be used in the habitat; the rest of the tank set would be ground into pellets, for use as reaction mass by mass driver reaction engines (MDRE) in the fleet of transport vehicles. Each hydrogen tank is 8.5 meters in diameter and 31 meters long. Several of these would be clustered together in what the authors described as a "condominium tower" configuration, with several residential tanks surrounding a communal tank. Each tank would be divided into ten circular levels, with access between levels provided by a 1.8-meter-diameter central shaft containing an elevator and ladders. Seven of the levels in each residential tank would be subdivided into three "studio apartments" per level, providing accommodations for 21 people per tank. The area of each apartment would be 17.5 square meters (188.5 square feet). The other three levels in the residential tanks would be devoted to: storage; toilet, bath, and laundry; and leisure and social areas. The communal tank would house: storage; gymnasium and game room; library; music room; pantry and galley (two levels); dining rooms (three levels); and EVA preparation. An emergency air lock would be provided at the top of each tank. The top level and one (unspecified) lower level of each tank would be connected to the corresponding levels of each adjacent tank. Environmental control and life support systems (EC/LSS) would be nestled between adjacent tanks.
Two identical tank clusters would be connected to opposite ends of a 140-meter tunnel (measured to the tops of the tanks), and the configuration rotated three times per minute to provide 0.7 g at the top level and "Earth-normal" gravity at the bottom. (The radii of the various levels would range from 70 to 100 meters.) One of the six "dining room" levels among the two clusters would be used instead as a medical clinic. An additional tank, aligned with the rotation axis at the center of the connecting tunnel, would serve as a docking hub. Cables extending out from the hub would carry the weight of the tank clusters - the tunnel itself would be non-load-bearing. No details are provided for the locations of solar collectors or the orientation of the rotation axis.
The rationale for an emergency air lock in the top of every tank is not elaborated. Once the configuration is spun up, it is difficult to imagine a situation in which it would be safer to exit through one of these than through the connecting tunnel and central hub. A decision by the crew to "abandon ship" through these air locks would scatter them in all directions with a relative velocity of 22 meters per second.
An illustration shows not one, but two connecting tunnels, reinforcing the similarity to Driggers' "Double-Spoked Parallel Cluster Station". The tunnel diameter is dimensioned as 2.5 meters. The length of a hydrogen tank is shown as 29.7 meters, rather than the 31 meters specified in the text.
The establishment of the low-Earth-orbit (LEO) station would be the "zero-th" milestone. Its mission would be to provide a staging base for the assembly of Lunar orbit payloads. It would accommodate a (relatively) small crew, permanent occupation, minimal radiation protection (acceptable because of its location below the Van Allen belts), possible rapid return to Earth via the Shuttle, and frequent visitors. The tour of duty would be one year. The first phase would be minimum habitability for 5 people in a single hydrogen tank under zero-gravity conditions. The crew would soon grow to 24 as more hydrogen tanks were converted to habitats (with the rest of each tank set being ground into pellets for the MDREs). After about eighteen months, the crew would be expanded to 48 workers. In the station's final configuration, two residential tanks would flank one communal tank at each end of the dumbbell, providing accommodations for 84 people. The paper does not explain the apparent excess capacity of 36, except to say that it leaves room for further expansion. (Perhaps, not all of the people aboard are externally productive workers: some may be devoted entirely to the internal operation of the station itself; others may be new arrivals not yet acclimated to the space environment; still others may be "visitors" en route to or from the high-Earth-orbit station or the Moon. But these are my explanations, not the original authors'.)
The high-Earth-orbit station would be located at the space manufacturing facility (SMF), in a two-day circular orbit more than 60,000 kilometers above the Earth. (Low Earth orbit is typically about 350 kilometers.) Its mission would be to manufacture solar power satellites from Lunar materials. It would accommodate a large crew, permanent occupation, full radiation protection (required because of its location above the Van Allen belts), possible (but expensive) rapid return to low Earth orbit via orbital transfer vehicles, and no visitors. In the initial configuration, six residential tanks would surround one communal tank at each end of the dumbbell. The initial crew size would be 150 (including administrative, maintenance, galley, and medical personnel). At the rate of 21 people per residential tank, this station could accommodate 252 people. However, the authors acknowledged that this scheme would result in less communal space per capita than in the low-Earth-orbit station, and that some reallocation might be necessary to create more communal space in the residential tanks. Ultimately, the target production rate of 2.4 solar power satellites per year would require a crew of 3000 - twenty times the initial crew size. Furthermore, the authors proposed that the tour of duty for this station be extended to two years, because of its larger physical size, and because the larger community would provide "more opportunity for each person to establish satisfactory social interactions" . They do not describe the final configuration, but imply that it is one large integrated station rather than a loose collection of twenty smaller ones. Given its significantly larger size, its greater exposure to radiation, and its crew's longer tour of duty, a re-examination of its basic configuration is in order. The two clusters of seven tanks in the initial dumbbell would subtend about 38° of arc. A station twenty times that size, with forty clusters arranged around the plane of rotation, would subtend nearly 760° - more than enough for two complete rings. The final configuration would be a toroid, not a dumbbell, and should be designed accordingly. Perhaps the continuity of the interior space would be improved if the tanks were turned "horizontally" and connected end-to-end or side-by-side in a circle, with an interior layout similar to the "bungalow" that the authors proposed for the Lunar bases. A horizontal orientation would also eliminate the low-g levels and concentrate the habitat at the one-g radius, permitting a larger radius and a smaller angular velocity. This would conform to the hypothesis of Bock, Lambrou, and Simon, that larger habitats should provide more Earth-normal environmental conditions.
The third orbital base would be located near the L2 libration point, approximately 63,000 kilometers behind the Moon on the Earth-Moon axis. Its mission would be to perform routine monthly maintenance on the "catcher" - a device that would collect the material launched from the Lunar surface and send it to the space manufacturing facility. This station would accommodate short monthly visits by 12 Lunar workers. It would consist of a single tank, arranged like a "tower" with a mixture of residential and communal levels, but operated as a zero-gravity environment. The consistent up-down orientation of the interior might help the crew to adapt, but it seems likely that the interior furnishings of this zero-gravity station would differ substantially from those of the two pseudo-gravity stations.
In 1976, during its bicentennial year, the United States soft-landed two Viking spacecraft on the surface of Mars - the first successful spacecraft to land on that planet. Yet within a few years, NASA had discontinued funding for Mars studies. To encourage continuation and expansion of these studies, a mix of Mars enthusiasts - including students, professors, and professionals - convened at the University of Colorado at Boulder in April 1981 to present "The Case for Mars". The conference was cosponsored by, among others, the National Space Institute and the Planetary Society. The papers covered a range of topics, including mission strategy, spacecraft design, life support, surface activities, materials processing, and social and political considerations. The conference was well received, and subsequent "Case for Mars" conferences have convened at three-year intervals.
Daniel Woodard and Alcestis R. Oberg presented a paper on the medical aspects of a flight to Mars. They stated that bone demineralization was a serious problem in micro-gravity environments that had yet to be solved :
The question of whether or not bone demineralization is completely reversed upon return to Earth remains open ... It may well turn out that much of the loss of trabecular bone is irreversible.
Surprisingly, the conference summary, authored by the organizing committee (which included neither Woodard nor Oberg), adopted an opposing point of view :
Although there are medical effects of zero gravity on humans, they are not serious enough to warrant the expense and difficulty of providing artificial gravity on the trip ... Bone demineralization will be reversed once the crew is on the surface and under the influence of Mars' 1/3 g pull.
We are left to speculate on how such a dichotomy could exist between a conference summary and one of the "summarized" papers. In any event, the summary seems not to have inspired much confidence: artificial gravity concepts have been prominent at subsequent "Case for Mars" conferences.
The "Case for Mars II" conference convened at Boulder in July 1984. A workshop conducted during the conference resulted in "Mission Strategy and Spacecraft Design for a Mars Base Program" . This paper describes one possible scenario for sustained human presence on Mars, including the design of an artificial-gravity "interplanetary habitat" for ferrying people between the planets. In this scenario, Mars missions are launched at roughly 26-month intervals - a period determined by celestial mechanics and the relative motions of the planets. Each mission begins with the departure of four vehicles from Earth orbit: one unmanned "cargo assembly", and three manned "interplanetary spacecraft". Each of the three interplanetary spacecraft carries a crew of five, and consists of: two Space Station habitability modules; two solar thermal collectors; a life support system (closed loop on water and air, open loop on food); a communication antenna; maneuvering engines, tanks, and propellant; and a two-stage Mars surface shuttle. These elements are mounted to a box beam, 2 meters wide by 37 meters long, that encloses a tunnel providing crew access between the habitability modules, the Mars shuttle, and a docking port at the far end of the beam. Three days out from Earth, the three interplanetary spacecraft dock nose-to-nose to form the interplanetary habitat. The habitat is spun-up to 3 rotations per minute to provide Mars-level gravity (about 0.38 g) at the 37-meter radius for the duration of the six-month transit time to Mars . In this configuration, the habitability modules are oriented "vertically" (radially) as 14-meter towers, rather than horizontally. Thus, the only thing they have in common with the Space Station is the outer pressure shell; the interior layout must be completely redesigned. The report does not provide any detail regarding the interior layout, but each tower could accommodate four floors. If the gravity at the lowest floor (37-meter radius) is 0.38 g, then the gravity at the highest floor (approximately 26-meter radius) is only about 0.26 g.
Each of the three arms of the habitat accommodates a nominal crew of five, with fifty cubic meters per person, but is designed to accommodate ten in an emergency. Because each arm is an autonomous spacecraft, if any arm becomes uninhabitable, its crew can evacuate to the other two through the connecting tunnels. The report does not discuss the dynamic effects of such a mass shift.
The habitat's rotation axis is directed toward the sun; propellant is required to provide the necessary precession. On each arm, the Mars surface shuttle is mounted to the box beam in the shadow of the solar thermal collectors, habitability modules, and massive propellant tanks. In the event of a solar flare, the shuttle's crew cabin would serve as a storm shelter.
The transit time from Earth to Mars is approximately six months. Five days before Mars encounter, the habitat is de-spun and the three shuttles depart for separate landings. Meanwhile, the previous mission - which has been at Mars for twenty-six months - departs the surface in its three shuttles. The habitat, now de-spun and unmanned, performs an automated maneuver to enter an Earth-return trajectory. Two days after Mars encounter, the three shuttles of the returning mission rendezvous and dock with the habitat. The habitat is again spun-up to 3 rotations per minute for the duration of the thirty-month return trip to Earth. At Earth, the habitat is de-spun; the shuttles separate, enter low Earth orbit, and rendezvous with the Space Station. The unmanned habitat enters a high elliptical Earth orbit, and is later retrieved, broken down into its three constituent spacecraft, and refurbished for another mission.
The total mission time for an individual crew member would be five years, virtually all of it in Mars-level gravity (except during relatively brief periods of weightlessness in planetary orbit). Two years would be spent on the surface of Mars, and three years would be spent in artificial gravity at 3 rotations per minute. The report further proposed that, in order to build up the Mars base, some crew members might remain on the Martian surface for two intervals (more than four years), bringing their total mission time to more than seven years.
In October 1985, Colgate University and Hobart and William Smith Colleges sponsored a conference titled "Space Colonization: Technology and the Liberal Arts". The proceedings were published in 1986 by the American Institute of Physics . The "call for papers" asked how a liberal arts curriculum could address matters of technology - in particular:
The sponsors noted that technology is more than hardware; technology is inseparable from the society that creates, maintains, and operates it. They saw the space colony as a microcosm of society, and a metaphor for real-world socio-technical problems :
How are we to understand the Challenger tragedy? Is it to be seen as a profound failure of our technology? Is it to have been expected? Or what of the Chernobyl disaster? Is it planet threatening? And if it is, what are our alternatives? ... Very similar questions can be raised in connection with space colonies. An advantage is that they can be raised in a context less emotionally and politically charged than that which surrounds the events of the evening news.
As it turned out, the conference itself was, at times, "emotionally and politically charged". It attracted twenty-seven participants from a broad spectrum of disciplines, including religious studies, philosophy, psychology, sociology, anthropology, and economics, as well as physics, engineering, and architecture. Ten of the twenty-seven were alumni of the 1975 "Stanford Torus" Summer Study - here reunited ten years later. Just as significantly, the other seventeen had not participated in the 1975 study; they brought new perspectives to the problem of space colonization. The ten Stanford Torus alumni served as panel members for a closing discussion, in which "endorsers of the efficacy of technology are closely questioned, and a deep division on the issue becomes explicit" . This occurred several months before the Challenger and Chernobyl disasters (January 28 and April 26, 1986), though the proceedings were published after.
Of the eight years that had passed since the 1977 Summer Study, Thomas A. Heppenheimer wrote :
After 1977 things began to settle down. During the years 1974 through 1977 the ideas were sufficiently new and sufficiently fresh, there was enough new stuff to be learned and to be developed, that it was worth people's time to look into these matters. After 1977 things came somewhat to a halt. Since then, for the most part, people have been quoting and citing references from those earlier years ... O'Neill had another conference in '81 which has led to some sort of proceedings volume. He had another conference in '83, still another in '85. But how often can you reheat the same soup; how often can you rewrite the same material?
Presumably, Heppenheimer saw in this conference the opportunity to add some new ingredients to the old soup.
In "Baseball in Space: Space as a Unifying Theme in Physics for non-Science Majors", David Van Blerkom described how he used the concept of space colonies in teaching Newton's Laws. First, he analyzed the behavior of a ball when dropped from the Tower of Pisa (as in Galileo's fabled experiment) and when thrown from a pitcher's mound. Then, using a computer simulation that he developed, he illustrated the odd behavior of the ball when dropped or thrown in a rotating space colony. A game of baseball in space would be significantly influenced by the orientation of the diamond relative to the colony's axis of rotation; the players would have to reacquire basic skills in throwing, batting, and catching .
In "Space in the Classroom", William E. MacDaniel described his travails in developing a new course in sociology :
Essentially, the concept of living in space was to be used as a medium through which to emphasize the impact of social and cultural forces upon human behavior and well-being ... The extent to which gravity is unobtrusively imbedded in Earth norms and behavior patterns would be emphasized, and students would be assisted in developing ideas of how norms would differ in an environment of which gravity was not a part.
Though MacDaniel was primarily concerned with the absence of gravity, a thorough treatment of the subject would have to consider artificial gravity as well.
In "Limitation and Life in Space", Marvin Israel and T. Scott Smith asked whether Earth's limits are an essential part of being human :
It might be argued that we are able to produce gravity artificially by spinning the structures in which people would be living in space. But ... the very knowledge that it is our will which decides whether or not there will be gravity (or how much) - the knowledge that what we formerly took to be a parameter of our experienced reality is really relative to our inclinations and therefore arbitrary - makes our experience of gravity-related phenomena essentially meaningless.
With regard to the potential benefits of solar power satellites (the major commercial enterprise of the Stanford Torus colony), Israel and Smith noted :
Some conservationists and environmentalists, coming to the problem of scarcity from the humanities, believe that we have more energy now than is good for us because it has served to encourage gluttony and sloth.
In "Technological Possibility and Public Policy", Steven Lee and Scott Brophy pointed out :
While it is a reliable principle in ethics that "ought implies can" (that if something ought to be done, it can be done), the converse proposition is ludicrous - not everything that can be done ought to be done.
Despite the serious questions raised by several of the other papers, this was the only one to specifically conclude that space colonization should not be adopted as public policy: Lee and Brophy argued that the sacrifices required of the present generation would be morally unjustifiable.
Gonzalo Munévar, on the other hand, argued that space colonization was justifiable. In "Space Colonies and the Philosophy of Space Exploration", he cited recent developments in the philosophy of science (in particular, the work of Paul Feyerabend, Thomas Kuhn, Imre Lakatos, and Karl Popper) and reasoned that the beneficial spin-offs of space research were not merely fortunate accidents, but were direct consequences of scientific inquiry :
The essential feature of science is not merely the addition of a few, or even many, interesting facts but the transformation, perhaps radical, of our views of the world. This essential feature turns serendipity into a natural consequence of science.
The most militant, controversial, and perplexing remarks came from Arthur Kantrowitz and Thomas A. Heppenheimer, in the panel discussion that concluded the conference. Kantrowitz, who had been editorializing against "technological pessimism" since 1971, rebuked a skeptical sociologist :
I would assert that just as technological optimism can do most anything, technological pessimism can destroy, can create these instances of failure to which you refer. Twenty years ago we knew how to make nuclear power plants. Today we don't. And they did not hurt anybody; they were safe. Today we don't. That is the difference between optimism and pessimism.
Heppenheimer - an aerospace engineer, a participant in the 1975 Summer Study, and the author of two books on space colonization - intensified the attack :
I want to comment on this famous cliche which we hear so often, "Oh, technology has got such horrible side effects. Technology has been so destructive." ... We have had people, mostly academics, mostly liberal-arts types, who were envious of the attention paid to their engineering and physicist colleagues, who proceeded to invent, largely out of old cloth, terrible disasters due to nuclear energy. Terrible threats due to waste which you know we could easily dump in the ocean and get rid of it. I think that all of this stuff about the threat of technology, at least 95 percent of it, is nothing more than a put-up job by envious people who resent the attention paid to those who can do rather than merely talk.
He lambasted proponents of ecological equilibrium and limited growth confined to Earth :
The people who speak of ecological harmony and equilibrium, in fact, are speaking of death sentences for 97 percent of humanity. Considering that their close ideological allies did just that in Cambodia, I'm not surprised.
Yet, he discredited solar power satellites as an economic justification for space colonies, maintaining that terrestrial coal and nuclear power were more viable sources of energy :
Economically, the power satellite isn't going to cut it, and technically we don't know how to do it. Maybe space tourism, maybe orbiting hotels in lower-Earth orbit are going to be an important entering wedge. That's an interesting speculation. But on the grounds of economics and technology we are farther away now than we were ten years ago.
Apparently, Heppenheimer advocated large-scale space colonization primarily as a relief valve for terrestrial population pressure. Yet his concept of the colony is a far cry from the utopian city-states described by O'Neill and others. In his book Toward Distant Suns, Heppenheimer predicted that space colony society would resemble the Panama Canal Zone: a bureaucratic enclave with no independent political substance, administered by an appointed governor, where everyone is employed by the government (and non-employees are expelled), where the government owns and operates every aspect of commerce - hardly a vision to entice massive migration [107, 108]. But then, this was based on his earlier belief in solar power satellites as the principal enterprise of the colony.
In reviewing the proceedings, Michael L. Smith characterized the remarks of Kantrowitz and Heppenheimer as "breathtaking departures from balanced judgment" :
One of the main points underlying Kantrowitz's and Heppenheimer's remarks is a good one: that applied science and public policy occur in a social context, and that sound technological objectives are often distorted by impassioned partisans. Unwittingly, these space advocates are prime examples of the dangers they warn us against.
The National Commission on Space was "appointed by the President of the United States and charged by Congress to formulate a bold agenda to carry America's civilian space enterprise into the 21st century." It solicited expert testimony and public opinion at Commission meetings, workshops, and public forums held throughout the country during 1985. It also received the input of many individuals through letters and electronic (computer) mail. Pioneering the Space Frontier: The Report of the National Commission on Space  was published in May 1986. It was dedicated to the crew of the Space Shuttle Challenger, Flight 51-L, which had met with disaster the previous January.
Although the report was published several months after the Challenger Disaster, the attitudes that informed it were acquired before; it is unremittingly optimistic in its view of America's future in space:
Our Vision: The Solar System as the Home of Humanity ...
Our Purpose: Free Societies on New Worlds ...
Our Ambition: Opening New Resources to Benefit Humanity ...
Our Method: Efficiency and Systematic Progression ...
Our Hope: Increased World Cooperation ...
Our Aspiration: American Leadership on the Space Frontier ...
Our Need: Balance and Common Sense ...
Our Approach: The Critical Lead Role of Government ...
Our Resolve: To Go Forth "In Peace for All Mankind" ...
It characterized "the recent tragic loss of shuttle Mission 51-L" as "a temporary interruption" .
The report proposed a "Highway to Space" and a "Bridge Between Worlds" - an interdependent system of next-generation launch vehicles, orbital transfer vehicles, Earth-Mars cycling spaceships, space stations, and burgeoning bases on the Moon and Mars. Among its many recommendations, three were particularly relevant to artificial gravity :
In a commentary on the report, Von R. Eshleman - the Director of the Center for Radar Astronomy at Stanford University - asserted that the end should be not space colonization per se, but exploration and commercial and scientific use of the space environment. As a means of achieving those ends, he argued against manned space flight in general, in favor of robotics and remote sensing and manipulation :
In the report, for example, it is the dream of colonization that determines the proposed infrastructure of spaceports, cycling spaceships, and lunar and martian bases ... I submit that space colonization is a dream whose time has not yet come, and that the benefits of the space frontier beckon us to realize them in the most direct and effective ways possible.
The third Case for Mars conference convened at Boulder, Colorado in July 1987. David N. Schultz, Charles C. Rupp, Gregory A. Hajos, and John M. Butler presented "A Manned Mars Artificial Gravity Vehicle" . This paper describes a preliminary concept for a piloted artificial-gravity vehicle for use in a split-mission Mars scenario . The vehicle concept was developed through a NASA multi-center in-house study: Marshall Space Flight Center led the study; Ames Research Center, Johnson Space Center, Kennedy Space Center, Langley Research Center, Lewis Research Center, and Headquarters participated. Their concept differs in almost every detail from the scenario developed three years earlier at "The Case for Mars II".
This new mission scenario called for a crew of 6 and a round-trip time of 420 days: the flight from Earth to Mars (orbit to orbit) would take 224 days; the return flight would take 166 days; time spent at Mars would be 30 days. The study examined several key system options:
Citing a lack of data regarding long-term exposure to fractional gravity, the study specified a conservative design of 1 g at 2 rotations per minute; this implied a rotational radius of 734 feet. Rather than balancing the habitat modules on opposite sides of the rotation axis (separated by 1468 feet), life support considerations favored aggregating the modules. This implied a two-body configuration, with the habitat counterbalanced by all available remaining mass. Using only required elements, the estimated division of mass was approximately 2/3 for the habitat (179,990 pounds) and 1/3 for the counterbalance (94,849 pounds). The 734-foot habitat rotational radius and 2-to-1 (approximate) mass ratio implied a total separation of 2058 feet. The study concluded that adding junk mass to the counterbalance would not be cost-effective, even though it would reduce the separation distance.
The vehicle would be assembled in Earth orbit. Initially, the habitat and counterbalance would be docked together and non-rotating. After departure from orbit, the two bodies would separate and the configuration would be spun-up. Shortly before arrival at Mars orbit, the configuration would be spun-down and the two bodies would re-dock. The process would be repeated for the return to Earth.
The connecting structure would have to meet several requirements: extend 2058 feet; survive the space environment; deploy and retract without extra-vehicular activity; withstand static and dynamic loads; and stow behind the aerobrake shield. The tether was selected as the only viable solution. Because over-design can cause failures to propagate, a single tether was selected, rather than multiple tethers. But, the study noted that more research was needed to determine the best tether design to survive micrometeoroid impacts. If the tether broke, the propellant required to halt the separation of the two bodies and re-dock would be about the same as that required for a normal spin-down; sufficient propellant would be available. After re-docking, if the tether could not be repaired, the vehicle would have to continue in a zero-gravity mode.
The paper does not discuss the expected performance of the crew when dropped suddenly from Earth weight into weightlessness in the event of tether failure. The NASA Man-System Integration Standards state :
Severe disorientation and performance degradation have been experienced by air and space crew members during random tumbles. Serious problems persist through the period of tumbling causing disorientation, reach and manipulative performance degradation - ultimately interfering with the ability to make corrective actions.
It seems likely that a quick and effective response to stabilize and rendezvous the two bodies would require an automated system.
The habitat would consist of two shortened Space Station habitability modules, four Space Station nodes, and an air lock. One of the nodes would serve as a solar flare storm shelter. The modules would be arranged "horizontally" rather than "vertically", in a plane parallel to the rotation axis. The orientation of the modules within this plane - that is, their rotation about the tether axis - was another issue. The preferred orientation placed the module axis parallel to the rotation axis, to reduce the Coriolis effects on the movements of the crew within. Unfortunately, an end-body dynamic analysis showed this orientation to be unstable: small rotations about the tether axis would tend to grow rather than dampen. Consequently, the perpendicular orientation was selected. (The module axis, tether axis, and rotation axis would be mutually perpendicular.)
The counterbalance would consist of the aerobrake, a propulsion stage, propellant tanks, thrusters, and the Earth return capsule.
A solar collector array would hang below the habitat. If the rotation axis was kept aligned with the sun, in the orbital plane, the array could be arranged to face the sun continuously. However, the study found that precession of the rotation axis for solar pointing would require an inordinate amount of propellant. With the rotation axis inertially fixed, perpendicular to the orbital plane, the array would rotate with respect to the sun; it would have to be larger, and active on both sides. Nevertheless, the savings in propellant mass would more than compensate for the additional complexity and mass of the solar power system. The inertially-fixed axis orientation was selected.
In comparing the artificial-gravity concept with a similar zero-gravity concept, the study estimated that artificial gravity would require a 26 percent increase in mass and a 10 percent increase in cost.
In another paper, Robert L. Staehle of the Jet Propulsion Laboratory described "Earth Orbital Preparations for Mars Expeditions" , based on the mission scenario developed three years earlier at "the Case for Mars II". One such preparation would be the establishment in Earth orbit of a variable gravity research station: to test the efficacy of artificial gravity in preserving the health of astronauts, as well as to explore the boundaries of the "comfort zone" for rotation. "Before committing a crew to years in a potentially nauseating environment, these parameters need to be tested." He proposed to connect one section of an interplanetary habitat (two habitability modules, arranged vertically) and a counterweight to opposite ends of a tether. The length of the tether would vary from 20 to 1000 meters, and the spin rate would vary from 0 to 5 rotations per minute. The counterweight could be a spent Shuttle external tank, or - at greater cost - another habitable facility. By placing a habitable facility at each end of the tether and transferring ballast mass (such as water) between them, the center of rotation could be shifted, and the two habitats could operate at different rotational radii and gravity levels while at the same rotational rate, enhancing the station's research potential. Staehle also noted that spin-up propellant could be saved by first extending the tether to its maximum length, then firing the spin-up thrusters to impart a slow rotation, and finally drawing the tether in to its preferred length (with a consequent increase in rotational speed). In this way, part of the energy of rotation would be provided by the electric motors that draw the tether; this energy would be replenished by the solar panels.
In March 1988, the NASA Ames Research Center sponsored a "Workshop on the Role of Life Science in the Variable Gravity Research Facility" . Thirty-one life scientists and six systems engineers were chosen to participate, based on their current involvement in research relevant to artificial gravity or gravitational biology. The participants were divided into three science work groups: physiology, performance, and habitability. Each of these groups was led by two co-chairs: one from Ames, and one from outside NASA. Additionally, an engineering sub-group was formed, whose members were distributed among the three science groups for most of the workshop.
Given a "straw-man" design of a variable gravity research facility (VGRF) , the participants were asked to describe a hypothetical set of experiments that would be appropriate for the VGRF and important to their group. These descriptions were to include specific values for the various parameters of VGRF operation: g-level, angular velocity, rates of change, and so on. The straw-man design would then be evaluated with respect to experiment requirements, leading to redefinition of both the VGRF and the program of experiments appropriate to it. The three science groups worked separately for most of the first two days. On the third day, the co-chairs from each of the science groups and the engineering sub-group met to draft the set of summaries to be incorporated into the final report.
The physiology group focused on issues related to the neurovestibular, cardiovascular, and musculoskeletal systems. Items of particular interest were: potential g-level / time profiles for the maintenance of health; the character of transient and steady-state responses and the time required to reach the steady state; the relationship of steady-state response to g-level (linear or non-linear) and the threshold g-level for minimum response; the relationship of activity level to g-level response; and the relationship of age to g-level response.
The performance group was concerned with perception, cognition, communication, and motor behavior. They proposed a suite of experiments to examine a wide variety of performance parameters in an artificial-gravity environment. Some of these - particularly in the area of cognition - would be generic experiments in environment and behavior. Others would be more focused on artificial gravity - especially its effects on: vestibular function; skin sense; proprioception; spatial orientation; visual perception; time perception; fine and gross motor muscle control; posture control; locomotion; reflexes; and hand-eye coordination.
The habitability group reiterated the objective of the "Artificial Gravity Experiment Definition Study" of 1970: "to define a nominal design with adequate variations which will provide the most comfortable and convenient crew accommodations for a range of radii, rotation rates, and gravity levels." To this, they added an emphasis on crew safety and rescue. They proposed a set of experiments to investigate psychological responses, physiological functions, and design implications. While these necessarily overlapped the interests of the other two groups, the emphasis of the habitability group was on developing design guidelines for the environment itself. They proposed experiments in: the use of lighting, color, and texture, as visual cues to spatial orientation; layout of controls, aisle ways, and sleeping facilities, to minimize cross-coupled head rotations; requirements for radial translation through the center of rotation, especially in support of ingress, egress, and extra-vehicular activity; human waste collection and management; spatial implications of posture and body motion envelope under various gravity conditions; and the location and design of windows for rotating spacecraft.
The straw-man design proposed a single cylindrical habitation module, 30 feet long and 14.5 feet in diameter, arranged horizontally, tethered to an attitude control module that would also serve as a counterweight. The tether would be 4,441 feet long and 1.6 inches in diameter, with a safety factor of 10. When fully extended, the rotational radius to the floor of the habitat would be approximately 750 feet (735 feet of tether from the center of rotation to the outside top of the habitation module, plus 11.75 feet to the inside floor surface). The initial design accommodated a first-guess set of requirements:
As intended, the life science experiments proposed during the workshop suggested several modifications and articulated additional requirements:
The engineering sub-group reviewed these expanded requirements, and observed that the strongest feature of the straw-man VGRF design was the ability to treat angular velocity and gravity level as independent variables over a very large dynamic range. Its weakness was in underestimating the crew size and the mass and volume of support equipment. They concluded that the requirements could be met through straightforward modifications, at greater cost of mass in orbit. For a given gravity level, cost would increase approximately linearly with crew size, but non-linearly and rapidly with decreasing angular velocity (due to increases in radius, tangential velocity, and propellant for spin-up and spin-down). The most difficult experiments to accommodate were among those proposed by the habitability group, which requested a Mars-mission-size crew, actual or simulated extra-vehicular activity, and access to the center of rotation for ingress and egress without interrupting the rotation.
The final report noted the "impressive enthusiasm in the life science community for a space-based VGRF", and described the workshop as "highly productive". Nevertheless, it recommended that the workshop not be repeated in the near future; instead, efforts should be applied to defining the required facility equipment to a greater level of detail.
The idea of space colonization has begun to enter the public consciousness, and concepts pop up in unexpected places. In 1983, a book on atriums in architecture included an illustration of an "orbital city" .
In 1985, Jesco von Puttkamer of NASA Headquarters described his vision of the long-range future. He asserted that humanity on Earth would benefit from the progressive development of new worlds in space - "in the course of natural evolution." The technological prerequisites included a 100-man space base in low Earth orbit, a geosynchronous space station (high Earth orbit), and a Lunar-orbital station, as well as surface bases on the Moon and Mars. Robotics, artificial intelligence, and remote sensing and manipulation would support - not replace - the growing human presence in space. The requirement for artificial gravity remained a "major question" .
In 1986, NASA reaffirmed that a major goal of national space policy was to establish a permanent human presence in space. One of the objectives of its life sciences program was to "develop effective means to maintain spacecraft crews in good health, treat illnesses and injuries that occur in space, and enable crew members to achieve the highest work productivity of which they are capable." A "possible initiative" for fiscal years 1991-1995 would be a flight-qualified "vestibular and variable gravity research facility", incorporating a centrifuge capable of exposing test subjects to gravity fields of variable intensity between 0 and 1 g .
On December 21, 1988, cosmonauts Vladimir Titov and Moussa Manarov returned from the Mir space station after 366 days in weightless orbit.
Also in 1988, Japan's National Aerospace Laboratory completed a concept study for a space station, 200 meters in diameter, to accommodate 16 crew members. The station would rotate to provide between 0.2 and 0.6 g. Electromagnetic bearings would support a non-rotating center section, using technology similar to that of "mag-lev" trains. The study projected that such a station could be built in orbit by the year 2015, assuming that a heavy-lift launch vehicle was available. The electromagnetic bearing technology was tested in space aboard a satellite launched by Japan's H1 rocket in 1986 .
In 1990, the fourth "Case for Mars" conference convened in Boulder, Colorado. (Previous conferences were held in 1981, 1984, and 1987.) Manned Mars expeditions have been the principal focus of artificial gravity concepts in recent years. Because a round trip to Mars will take more than a year to complete, there is little doubt that unprecedented measures will be required to prevent serious harm to the crew. There remains significant uncertainty regarding the requirement for artificial gravity, and whether it should be provided through periodic use of an on-board centrifuge or through continuous rotation of the entire spacecraft.
In 1993, the eleventh conference of the Space Studies Institute (SSI) convened in Princeton, New Jersey. (Previous conferences were held in 1974, 1975, and biennially since 1975.) The first two conferences were, in the words of SSI founder Gerard K. O'Neill, "purely, blatantly and simply, one-hundred percent on space colonies." Subsequent conferences have stressed the "semi-orthodox" themes of space manufacturing and space resource utilization . Nevertheless, the SSI conferences continue to provide a forum for advocates of large-scale long-term space colonization.
Although the United States and the (former) Soviet Union are still the only two nations with the requisite technology for manned space flight, astronauts and cosmonauts now hail from many lands: Canada, Cuba, Czechoslovakia, France, Germany, India, Japan, Mexico, the Netherlands, Saudi Arabia, Switzerland, and Syria have all been represented on American or Soviet space flights. Spacelab D-1, which flew with the shuttle Challenger in 1985 (mission 61-A, October 30 - November 6, 1985), was virtually a German mission, with ground control passed from Houston to Oberpfaffenhofen (near Munich). Europe, China, and Japan have their own space agencies. Britain, France, and Germany have each developed preliminary concepts for their own manned space shuttles: HOTOL, Hermes, and Sänger. For whatever reason, manned space flight has transcended the old Soviet-American rivalry; it is becoming a policy of industrial civilization.