On the Architecture of Asgard

In this piece I'd like to discuss the details of the Asgard phase and habitat and how, much like Aquarius, the architecture of the orbital colony must evolve over time from a small and unmanned structure to a kind of colony that can become the basis of an on-orbit civilization.

The space station has probably been the subject of more speculation among futurists, engineers, writers, artists, and architects than the marine colony yet, in spite of a vast and colorful spectrum of different proposed space structures, no space system or facility deployed to date has actually been designed with the practical situation of space settlement in mind. Space agencies seem to forever miss the point when it comes to space station design. The structures they deploy never make the leap from ultimately disposable Earth-dependent prefab structures to structures with a potential for practical on-orbit fabrication, let alone evolution to permanent self-supporting settlement. Space habitat design is still stuck in the 'outpost' mode of thinking.

The priorities of exploration are rooted in the priorities of survival with life-support at the top of the list. The harsher the environment being explored the more one is compelled to bring along to maintain survival with the environment of space so harsh that, as it has often been assumed, everything needed for survival must be sent with explorers, thus necessitating the ultimate in what is essentially 'camping' gear. But the priorities of settlement are rooted in the priorities of subsistence -a very different equation. The outpost is, by definition, a temporary habitat. It is meant to function only for a short time. To provide life-support until its supplies run out or until its lines of re-supply are abandoned. The settlement is permanent and to be permanent it must cultivate an infrastructure which exploits local resources for its subsistence, turning those resources into goods to support local life-support and turning what surpluses it can generate into product it can trade for those goods it needs but cannot locally produce. Thus the tree of priorities of settlement derives not out of life-support but out of industry. Living in space means learning how to make as much as you can in space, and that includes -if not starts with- the habitat itself. Thus what is needed in the practical architecture of the permanent orbital settlement is systems of structure and construction designed specifically for on-orbit fabrication and the perpetual evolution of structure to suit changing needs. Most everything one can make needs to be made in a different way in the orbital environment and not enough -if any- effort by the government space agencies has been put into the necessary technologies of subsistence. When they think about 'subsistence' they resort to the priorities of basic survival and so devise elaborate systems for the production of food, air, and water yet overlook the even more critical and fundamental question of how one builds those systems from stuff there in the environment. THAT is the difference between temporary survival and permanent subsistence.

In my previous articles on Asgard I presented the concept of the EvoHab as an evolution of orbital settlement architecture beginning with the MUOL -the modular unmanned orbital habitat. In that material I discussed how the settlement of orbital space must begin with the establishment of an industrial infrastructure which, at first, has no strict need for human settlers until the scale and complexity of systems precludes the cost-effectiveness of the common satellite's scheme of planned obsolescence, thus necessitating on-demand maintenance down to a level of intricacy too elaborate for teleoperated robotics to handle. Thus the beginning of the Asgard phase is based on an unmanned teleoperated facility focused not on the priorities of human survival but on the research and development of the technologies of orbital industrial production intended to produce products for terrestrial consumption. It is from this one develops the technology and capability to produce goods for local subsistence, and thereby enables perpetual life-support for permanent human settlement. We will not rehash that previous article here. Instead, I would like to discuss in more detail the specific designs of habitats and their systems and how they must evolve over time to support the development of large orbital settlements. I envision Asgard as moving through a series of development phases not dissimilar to that of Aquarius -following a progression toward great industrial self-sufficiency but with the key difference of beginning with an unmanned automated facility, a step unnecessary for Aquarius. We'll start with the MUOL and a look into the core of its design concepts, as it originates a series of architectural ideas that will carry through the whole of the Asgard stage of TMP and into other stages as well.

MUOL phase

The modular unmanned orbital laboratory is actually a very simple facility in design and concept and yet it is fundamentally different from any space structure currently or previously deployed. The basic premise of the MUOL is that of a perpetually expandable and upgradeable teleoperated orbital laboratory located either in LEO, GEO, or at the GEO up-station position of a first generation Space Elevator system. The MUOL functions as a modular 'backplane' like the passive backplane of an industrial computer, hosting laboratories and small factories in the form of self-contained modules which plug into both the frame structure of the station and a standardized service backplane which provides isolated interfaces to power, thermal management, and Internet Protocol based data communications. The MUOL structure consists of a cubic geometry space frame in the form of a box truss with a one-to-several meter module size that can expand in any direction. It uses special quick-connect frame node components which provide the primary attachment points for all other components using mechanically locked bayonet connectors. The struts of the truss provide a secondary series of attachment points, using clamp connectors. With its nodes as the primary attachment points, the geometry of the space frame defines the basic geometry of all the components attaching to it, the majority taking the form of simple boxes slightly smaller in base dimensions than the cubic module size of the space frame.

The initial configuration of the station would be based on a simple box truss beam growing from seed package consisting of a carrier pallet with initial core command/communication module, a service robot, and a container of truss components. This initial beam would establish a basic organization with service systems using the perimeter edge and space-facing side of the station while 'client' modules use the earth-facing side of the station. As the station grows the initial beam form would be expanded into a plane truss in both length and width with axial truss booms to host solar, radiator, and telecom modules.

Service of the station is performed using a series of 'inch worm' telerobotic arms which have dual modular end-effectors and connect to the station using modular anchor and tool pallet modules that plug into the station service backplane. Both ends of these 'inch worms' are identical, their end effectors plugging into either anchor modules or tools. This allows the robot arm to reposition about the station by 'walking' from one anchor module to another. In addition to these robots, the station is maintained by several 'service' modules which include deployable solar panel, radiator, thruster, telecom/network management, component storage, and shield panel modules.

Being a non-pressurized station there is no need for special docking structures to provide access to it. All transit to the station would be provided by the use of 'carrier pallets' which are simple chassis structures which components plug-into for transport and which have their own propulsion systems, embedded flight control intelligence, and teleoperated control systems. These pallets would be designed to fly to within reach of the station robots which capture them, plug them temporarily into the station frame, and remove their cargo then later reuse them for de-orbit of waste material. Some may be designed for recovery, featuring reentry shield and recovery components. In some cases specialized recovery units may be supplied to the station or even attached to specific modules. This approach would allow the station to be serviced by any existing launch systems without adaptation, affording it the largest diversity of potential transit systems it is likely to have in it history. Even modest systems such as today's Pegasus launch systems could effectively support this station. And it is well suited to the very modest carrying capacities of first generation Space Elevator climber systems.

The station's command and control architecture mirrors its physical architecture. The station is essentially a completely homogeneous network structure based on an Internet Protocol WAN and the use of embedded intelligence in all modules. There is no central control computer on the station. It's core computer systems serve primarily for network support services and latency-sensitive higher-level control and are integrated in its telecommunications service modules. Everything else relies on web-controllers -embedded computers which feature a simple built-in web server that hosts a virtual control panel for manual operation of the module and which can also communicate in byte-code commands which can be managed and generated by other higher-level programs called 'sequencers' which can control groups of modules collectively and can run either in systems on the ground or on the station's core computers -and in some situations on the service module's own web servers. In this way control and management of the station is distributed across systems both on board the station and on the ground and with the IP network infrastructure providing innumerable options for redundant communications and control routing. A much more reliable and efficient architecture than the more typical systems in spacecraft which rely on too much dedication and centralization of discrete control links, leading to concentrated failure points, a waste of much physical mass, and control network hardware that is very difficult to service. This architecture would also be employed for all the lab and factory modules installed on the station, using a second isolated IP network which hosts VPN channels for the individual station clients. This command and control architecture is a very important feature. I envision it being mirrored in the utilities infrastructure of just about every kind of facility employed in the overall scheme of TMP (I've already described how it would be used for the utilities systems of Aquarius) and being the basis of launch control systems and the command and control systems of later spacecraft.

The design of client modules -those modules which are provided by lease space tenants of the station- starts with the geometry of the space frame and the interface to the service backplane. Sizes of modules are limited to multiples of the space frame module size and can assume most any shape within the bounds of this grid, but must be unified rather than sprawling shapes. As noted before, the service backplane provides 'isolated' interface to power, communications, and thermal regulation. This is performed through optical, inductive, and passive thermal exchange interfaces which insure that the internal plumbing and electronics of the client modules remain isolated from that of the station. This is done to insure that power overloads in client modules and other types of failures do not cause further damage to the station or adjacent client modules. Client modules may be disposable (having a fixed duty life and a fixed built-in reserve of supplies), recoverable (being returned whole to the earth for product collection, analysis, and refurbishing), or perpetual using modular cartridges for resupply and recovery of product. There are three basic forms of client modules; rack modules, open pallets, and enclosed modules.

Rack modules are the smallest of client modules and would rely on a special rack carrier pallet module designed to host them. They would be similar in form to rack mount electronic equipment but use a quick-connect interface and a standard end-effector attachment point to make installation and removal by the service robots easier. Rack carrier pallets would offer bay space that is either partially enclosed or fully open, depending on the need for access to the space environment or views of the earth.

Open pallet modules are those full modules which are intended primarily for access to the space environment -most likely for experiments in materials that need space exposure. They would tend to be relatively rare.

Enclosed modules would be the primary form of full module and would generally consist of simple containers that house self-contained teleoperated labs and, later, factories. Their enclosure is chosen as a way to either protect the internal systems from the space environment or to allow for a pressurized environment -in most cases, pressurized units likely to employ a nitrogen-only atmosphere. Though not allowed to sprawl in shape individually, a lab can employ multiple modules with transfer ports between them, allowing for the creation of a sprawling complex of individual modules which each house an individual workstation. In this way later factories could develop assembly lines of specialized process workstation modules. The internal configuration of these modules is likely to follow a simple pattern. If dedicated to a relatively simple task based on simple systems of high reliability, a more monolithic internal structure may be used. If based on more complex systems or designed for a broad spectrum of activities, a more open-interior design would be used to accommodate one or more internal service robots which would be used to perform internal module maintenance and move supplies and product around between different points inside the module. This would tend to be common with larger modules, using central face mounted fixed position robots or, as the modules get progressively larger, single or multiple robots on centralized transverse rails running through the core of the module. Normally, the internal maintenance of any module would be left to its internal systems. The exterior service robots would be used only for the exchange of external plug-in supply and recovery cartridges. But it is possible that some modules may feature access ports to allow the external service robot access to the interior through the use of a special micro-manipulator tool head.

One of the interesting aspects of the MUOL concept is that proof of concept for the facility doesn't require orbital deployment. A largely functional mock-up of the station can be readily constructed on the ground, making it relatively easy to engineer its systems, test their performance, demonstrate the station operations, and generally greatly reduce the cost of R&D for the project. The one limitation in this ground based testing is the inability to support high-mass object manipulation due to the complications of gravity but low-mass dummy objects can still be used as analogs with high mass characteristics simulated in software, along with the effects of communications latency. Much PR value can be obtained from this functional mock-up as it makes both the station itself tangible and can perfectly simulate the installation, service, communications conditions of operation clients can expect for their lab projects.

MUOF phase

Since the ultimate goal of the MUOL is the development of industrial techniques resulting in the on-orbit manufacture of products it must ultimately evolve into a robust factory facility -or more precisely an industrial 'campus' playing host to a number of manufacturing operations. Initial orbital factory systems will be quite similar in nature to the laboratory systems first deployed on the MUOL but more dedicated in function with the option to spread serial production processes among multiple modules or to deploy modules of progressively larger scale. However, the initial MUOL systems -though easily replaceable and upgradeable- are still based on a duty life model similar to that of telecommunications satellites where planned obsolescence is based on statistical mean-time between failures and where whole module replacement is considered a more practical option than on-site repair -if not the only option for very small components. But as production systems increase in scale and the products produced on-orbit become commodities this approach becomes less cost-effective. Larger facilities have longer periods of amortization for their investment yet at the same time mean-time between failures shrink. Downtime periods become increasingly costly the larger the production volume and the more narrow the profit margin on goods produced. Planned obsolescence of whole facilities becomes impractical and, instead, the factory must increase in serviceability in order to allow perpetual sub-component level maintenance.

This situation is likely to result in the evolution of a new kind of orbital factory; a built-up structure that employs the basic components of the MUOL to create larger enclosures hosting full scale MUOL modules as modular subsystems within the factory structure. This Modular Unmanned Orbital Factory or MUOF would be sheltered behind a hull of modular plug-in shield panels or, if pressurized, employ a TransHab style pneumatic hull system inside which the structural space frame is assembled. Non-pressurized enclosures would tend to take on rectilinear or prismatic polygon shapes and have their components arrayed primarily along their inner walls. Transhab units would be cylindrical in shape with smaller units concentrating factory component modules along a central core truss and larger ones replacing the core truss with a perimeter space frame allowing for an inner-wall arrangement similar to the unpressurized factories. New node components, accommodating new structural geometries, would be employed to support these new larger enclosure shapes. The MUOF would employ its own internal robots similar to the external service robots in their scale and multi-function capability along with higher precision robots intended for a much smaller scale of inspection and servicing. It may also have its own docking ports for access by more specialized reusable service vehicles.

The MUOF frame structures would be a direct extension of the original MUOL frame structure, though utilities would still be isolated from that of the station at large. Individual MUOF structures would be arranged on the station much as the earlier individual MUOL modules were, with the client facilities on the Earth-facing side and the service facilities on the space facing side. However, as MUOF facilities eventually dominate the mass of the station the primary structure of the station will begin to see a transformation. Openings through the primary plane truss would be used to allow MUOFs to exchange containerized goods through the truss rather than over and around the Earth-facing side. This would become necessary as the surface area of the station's plane truss becomes quite large and the space-facing side of the structure becomes used increasingly as a mustering area for cargo, dedicating robots to primarily cargo handling and edge space to the docking of reusable vehicles in addition to the disposable carrier pallets. The wider the area of the plane truss the longer transit spans become and the more robots that become engaged in cargo handling. This may eventually result in a 'folding' of the station into a radial prismatic structure with a square, hexagonal, or other polygonal section to make cargo transit easier, giving the station an overall form akin to that of individual MUOF facilities but on a larger scale and creating a large hollow interior bay used primarily as a storage and transit corridor for product and a docking facility for spacecraft. This would become important with the advent of reusable service spacecraft of progressively larger scale, these requiring progressively more specialized docking structures. This shift to a radial station form could also result in a shift in orientation of the station with one bay end Earth-facing and the other space-facing. solar arrays congregating to the space-facing end and telecom structures congregating on the Earth-facing end.

This MUOF phase may also see the first experiments in the acquisition and processing of materials in space for on-orbit supply of feed stocks for factory production or the fabrication of structural components. A logical choice of initial source of raw material for such experimentation is the earth orbit itself, with its swarms of debris from past space activity. The clearing of this debris is also a valuable commercial service by itself, which the MUOL operation can profit on in service charges to governments while obtaining these raw materials. Two strategies for the recovery of this orbital debris are likely. One is to adapt MUOL carrier pallet vehicles into reusable inter-orbit vehicles equipped with capture mechanisms and manipulators and which use solar powered tether booster systems for primary propellent-free propulsion. The recent advent of nanofiber mats as a non-chemical adhesive may allow for the creation of new capture methods for small debris based on large flexible paddles or petals. Another approach is beam steering, where a constellation of orbital lasers or particle beams is used to target and alter the trajectory of debris (using their own molecular ejecta as propellant) in order to guide it to large capture stations. Sophisticated new recycling techniques would be needed to convert this material into usable feedstocks, though a high proportion of aluminum alloys among space debris may make this a bit easier. And new kinds of storage structures based on the MUOF approach to construction would be needed. In any case, these experiments would provide very useful experience for later development of orbital mining systems.

In this phase, or shortly before it, we can also expect to see the MUOL technology begin to be employed in the on-orbit construction of new larger satellites and interplanetary exploration craft -and possibly manned craft as well. The MUOL's technology offers the option to deploy spacecraft of any scale regardless of the launch capability one has available because it becomes so easy to assemble spacecraft on-orbit from the very same kinds of components used to build the MUOL itself. The variety of spacecraft that could be built in this way is unlimited and by the addition of transhab habitat modules -as well as a new manned-system component rating- it is a simple matter to make them manned spacecraft. While the unmanned spacecraft might assume a vast assortment of configurations the manned spacecraft are likely to have their configurations dominated by the transhab units with the most likely forms based on truss beams passing through the transhab units and with propulsion and functional systems on booms at either end. By varying the scale of the core truss and the habitat modules, a quite diverse range of manned mission vehicles could be realized with this simple configuration. Thus in this phase we may expect the MUOL to spawn quite an impressive menagerie of systems and vehicles with applications ranging from telecommunication platforms to prototype space solar power systems to manned missions to the Moon and Mars and unmanned missions to the asteroids and outer planets -all using essentially the same technology and all assembled on orbit at the MUOL and similar facilities.

Asgard Outpost phase

As orbital factories continue to grow in scale and complexity they begin to approach the limits of telerobotics to perform efficient repair and maintenance. LEO facilities actually have a worse time of it as they suffer much greater and varying latency in their telecommunications links than GEO facilities because of the movement of LEO objects relative to operations centers on the ground. Clever engineering to match the task to the tools along with steady advances in robotics will allow for practical teleoperation for a long time but at a certain point it is likely that the cost of manning the station with technicians will start to become nominal compared to the economic losses incurred due to the limitations of telerobotics. It is at this point the MUOL will begin its evolution toward a manned orbital colony, starting with an outpost facility. This would be an 'outpost' facility because, at this stage of development and available technology, it is unlikely to be able to host a permanent resident population. It has not yet established the orbital resource exploitation infrastructure necessary to independently support a resident population -and the medical technology to allow this in the absence of artificial gravity may not yet be available. So at the beginning of this phase, at least, the station will be home to human crew on rotation, usually spending no more than weeks at a time and totally dependent upon the earth for its continual re-supply much like today's existing primitive space stations. This community would be akin to that of oil rig workers, though these would be much more highly trained individuals. They would most likely be housed in Transhab-style station modules, probably using the same structures developed for pressurized MUOF use. Current experimental Transhab modules -originally developed with the International Space Station in mind- have tended to be over-complex and over-massed in their design. They are based on the use of a 'deck' division which have never made a lot of sense in a microgravity environment. Thus the pneumatic habitat enclosure likely employed here would be much simpler in design, using a MUOL truss core with end-cap bulkheads that plug directly into the external truss structure as well as connector modules similarly matched to the truss geometry. All internal systems and fixtures would be retrofit to the truss core, some structures branching off of it. If it becomes necessary to employ the area of the inner wall of the pneumatic hull for equipment mounting, a perimeter internal space frame structure would be employed for equipment attachment, just as in pressurized MUOFs.

These outpost worker's primary job would be MUOF maintenance and, for the most part, this would still be done telerobotically relying simply on the technician's proximity to eliminate telecom latency. However, they would also be able to physically enter MUOF facilities when needed or could have components removed to the interior of their habitat for hands-on repair in a shirt-sleeve environment.

The initial manned habitat would be quite crude due to the need to maximize the amount of utility from habitat modules which may be quite small and be completely devoid of actual windows -relying on much cheaper and safer video windows. Crews would be small and there would be minimal privacy -but, thankfully, duty periods would be relatively short and technicians would spend most of their time waiting for failures -most scheduled maintenance still being done from the ground. The habitat modules would interface to the station essentially the same way as MUOF facilities would with workers using the service face or bay of the structure during the rare EVA or possibly deploying pneumatic access tunnels to pressurized MUOF structures.

This phase could also see the introduction of a tourism facility based on similar habitat systems -or the development of a largely separate tourism outpost. This would most certainly be limited to a LEO position at this phase. Most tourism station proposals have tended to be rather elaborate but, realistically, they would tend to have to start out with the same kind of simple habitat technology employed for MUOF maintenance workers. Thus I envision a simple facility consisting of a simple 'star' shaped array of Transhab modules around a central docking hub with the possible addition of large window hatches at the module ends and perhaps a large specially constructed observation cupola in the form of a large transparent semi-sphere. That cupola would be one of the most expensive (perhaps as much as the whole rest of the tourism facility combined) features of the tourist facility -both in terms of its fabrication and transportation. But it would be a key attraction because it would offer a space-walk experience with a shirt-sleeve environment. Another possible feature would be a 'space sphere' where a conventional but large and spherical transhab module with no central core truss is deployed as a large open recreational space.

With the addition of manned habitats comes a need for new pressurized docking structures which will mean a specialization in the service vehicles for the manned portions of the outpost. These would tend to rely on docking structures on the TransHab ends, rather than the usual edge or bay docking structures used for automated cargo handling.

We must also consider in this phase the possibility of the beginning of a diversion in the path of space development in favor of transhumanist colonization. The odds seem low that a truly sentient AI would appear at this stage of development, though it may in the long-term be an inevitability. If AI appears at all by this time, it will initially take the form of fairly specialized computer programs engineered to perform fairly specific tasks and relying more heavily on sophisticated vision processing systems than on the AI 'decision making' itself. But this would still be enough to allow a more diverse range of higher precision robots operated from station-based computers to overcome most of the latency problems associated with teleoperation. There would still be a heavy reliance on ground control but now the systems of the station could be assigned complex tasks to do independently with the AI systems only needing human consultation in exceptional situations and steadily acquiring accumulated experience to incrementally extend this task-independence. This technology could see the MUOL/MUOF facility achieve extremely large scales without any need for a human presence -unless introduced solely for tourism and scientific purposes.

Now, while this would seem to present a possible delay in the manned settlement of space, there is a factor in this which could result in the reverse. Overcoming the obstacles of latency without the overhead of human worker support means an accelerated pace of exploitation of orbital resources. The technologies of human life support are of secondary importance to the technologies of resource exploitation -since the former cannot be sustained without the latter. To support safe and rapid human settlement in space one must establish a broad infrastructure with very long lines of communication (materials, goods, and information) due to the fact that resources are not homogeneously distributed in the solar system. The sooner that kind of infrastructure can be established -even if it's to support the use of an unmanned facility- the sooner human settlement for its own sake becomes practical. Latency limits the reach of resource exploitation and development from any given location and if it requires the establishment of progressively more far-flung human settlements to reach far enough into the solar system to access the full spectrum of materials a high standard of living in space requires then the pace of progress for human settlement would be fairly protracted. But AI not only overcomes the latency problem in Earth orbit, it overcomes it for every part of the solar system. It becomes as practical to operate mining plant in the Oort Cloud as it does to operate a station in LEO. Thus one has the means to rapidly establish a solar-system-wide infrastructure long before anyone thinks of leaving the Earth.

However, while sentient AI is not necessary for this development, the technology necessary to host sentient AI is little different from that needed to host its non-sentient cousins. Thus in the process of establishing this resource utilization infrastructure one would be simultaneously creating the very same advance settlements an AI civilization would ultimately use. Even long before the first sentient AI is created, an AI civilization would have a head-start on colonization!

Asgard Settlement phase

The transition of Asgard from an outpost to a true settlement would be marked the establishment of local industries producing components for the construction and maintenance of the station and life support goods made from materials sourced in space. With this capability the station has the ability to establish permanent populations supported primarily by space resources -assuming that by this time a clinical solution to the problem of physical deterioration in microgravity is at least partially realized. This phase would also see the development of mass exploitation of orbital solar power as a primary industry, relying on space resources for the cheap production of vast solar power arrays providing beamed power to not only the Earth but multiple points in Earth orbit.

With a steady diversification of industrial, commercial, and recreational activity on the station the resulting steady increase in on-orbit population would require new kinds of habitat structures to support a living environment comfortable for progressively longer duration. Simple Transhab style habitat modules would be insufficient in scale to provide a comfortable private residence unless entire modules were turned over to individual habitation. That approach is inefficient because it requires a constant increase in the number of pressure hatch connections in the overall structure, increasing the number of potential failure points and resulting in portions of structure that become progressively more difficult to reach for maintenance and module replacement. As long as the station must rely on some components of large scale that are being made and supplied from Earth -such as pneumatic habitat enclosures- it must remain topologically simple. Thus the support of increased populations would favor the development of shared habitat structures of progressively larger scale. The need to create an attractive environment for tourism purposes and psychological well-being also favors structures which can provide some generous open space to create the illusion of an easily accessible 'outdoors'. The first wave of space tourism has been driven primarily by Space Age nostalgia and the fantasy/mystique of the astronaut lifestyle. 'Roughing it' has been an accepted part of that experience -much as it is for 'dude ranches' simulating the cowboy life. But the second wave of space tourism is likely to be about accessibility -about the 'packaging' of the space experience in a way that the maximum diversity of patrons can enjoy it. So just as the tents and foot trails into the wilderness give way to the rustic lodge hotels and tour-bus roadways of the national park so too would space tourism shift toward the presentation of a progressively more comfort-oriented environment that builds attractions on top of those basic novelties of the space environment. Comfort and luxury become increasingly important to the equation of permanent settlement as well because for a large number of people to want to live in space they have to be able to perceive it as a place where they can realize all their desires -and ideally with greater ease than on Earth. That's a challenge in an environment that imposes very great and critical demands on the physical shape and performance of shelter and with the cultural legacy of over a century of very fanciful portrayals of life in space.

The likely initial approach to new larger habitat modules would be to improve upon the existing Transhab technology by finding ways to increase the internal volume supported by a pneumatic hull of roughly the same transport mass. The obvious solution is to decouple the previously built-in shielding from the pneumatic hull structure so as to allow more mass of structure to be dedicated to pressure containment. This would be achieved with the approach already developed for MUOF structures; using a space frame enclosure to provide a mounting frame for modular shield panels inside of which the new larger scale pneumatic hull can be inflated. The next improvement on this, allowing habitat enclosures of potentially any scale, would use of the isolated and controlled -if unpressurized- environment provided by the MUOF style enclosure to provide a working environment for the application of plastic materials to create a composite pressure hull. The outer shielding panel system would be complemented by an inner panel which provides the foundation for the application of sprayable plastic materials allowing for a pressure hull to be built-up in layers, using the core truss running through the enclosure as the platform on which all the fabrication equipment is mounted. In both these instances the end result is essentially the same; a large space frame enclosure of spherical or cylindrical form covered in modular panels with a pressure hull inside it and -at least initially- a core truss running through a polar axis.

How then do we use such structures to create a comfortable and attractive habitat for a large community? The basic architecture of the original MUOL and the Transhab are our guide. The core truss is the heart of the physical structure of the habitat -its primary structural element- and thus everything in it would tend to have to be attached to it. We have three possible configurations of the core truss. It can run down the polar axis of the structure as in the original Transhab, it can run along the inner surface of the hull structures, or it can do both as would be common with pressurized MUOF structures. The choice depends on how one wishes to use the space. A pressurized MUOF needs to maximize the use of the internal area while providing a common 'track' for service robots. It only needs as much open space as is necessary to move product around and service factory modules. Using both a truss frame to mount components along the inner hull and a truss through the center to mount more components and the robots to service them all makes sense. Human habitation needs open space to fight claustrophobia, distribute light, and provide a reserve of air to buffer thermal changes and pressure losses from punctures. One also needs speedy access to the surface of the pressure hull to effect repairs. This tends to favor an approach that concentrates the functional structures of the habitat on a central axis core truss -at least until one gets to extremely large scales where the relative thickness of habitat structures on the inner surface -plus a hull and utility access space behind them- is thin relative to the overall volume of the habitat. Thus we can anticipate that the likely architecture for these habitats would be what I call a 'tree-form' habitat with the core truss as the primary 'trunk' and the habitat branching out from it into the open space of the hull whose surface is intended to simulate a kind of sky or virtual window by serving as a light diffuser and video projection surface. In this way we can virtually create the kind of transparent hull enclosure envisioned by Marshal Savage, but using a composite 'video window' to present images of the exterior and heliostat systems to pipe safe UV and IR filtered sunlight into the habitat. Indeed, the larger and more robust this kind of habitat becomes the more the central habitat structure can be made to resemble a living tree by virtue of hosting hydroponics planting systems within its structures and perhaps ultimately hosting a cultured spherical tree grown about a hydroponic life support structure.

We thus arrive at a permanent settlement form that is quite similar to that envisioned by Marshal Savage only with its hull a composite geodesic hull structure. A great sphere or domed end cylinder inside which resides a great 'urban tree' and with large MUOF style radial structural extensions housing factories, farms, and docking structures at both its poles, large solar and radiator arrays congregating at the space-facing pole and telecom congregating at the Earth facing pole. The same mix of specialized passenger service vehicles and predominately automated service vehicles would be employed with specialized docking terminals employed for the manned spacecraft. If the settlement is not already in a GEO location it is likely in this phase to start becoming a staging area for GEO facilities which may ultimately replace LEO facilities as permanent settlements. This will mean supporting a new generation of inter-orbital shuttle vehicles.

Let's consider this urban tree, how it works, and the kind of living accommodations it might offer. There's a specific path of evolution from the simple Transhab-style structures of the outpost phase to this urban tree concept. The first Transhab habitats would lack much in the way of privacy and amenities being focused instead on minimizing mass and maximizing utility within a small space. The core truss of the Transhab would be the mounting structure for most everything in the habitat and privacy would be provided by fabric and foam partitions where necessary -though for the most part personal space would amount to a storage locker and a fabric and foam enclosure sleeping 'pod' strung on cables or placed within the hollow of the core truss. (if long enough) Since the outpost would only be hosting people for short duty cycles this need to deal with camping-style accommodations would not be a particular problem. The tree-like nature of the central truss structure would be apparent even in this stage. All the equipment used in the habitat is attached to, branches from, and radiates around this core structure, its interior space housing airlocks and the few features requiring a bit more privacy.

As larger size Transhab structures are used we can expect the simple sleeping pod to evolve into progressively larger and more elaborate personal dwelling units, though still based primarily on the same fabric and foam composition as this offers comfort and low mass. Additionally, many workstations would evolve into more enclosed structures such as cages, platforms, or capsule rooms. I envision a progression of these sleeping pods evolving toward a full 'dwelling pod' deriving in design from the example of the Japanese capsule hotel module which has been around since the 1960s and proven pretty functional. While having large open spaces that can emulate an 'outdoors' environment is important psychologically, too much space in a private dwelling is an inconvenience in microgravity because easy mobility requires an environment where there are plenty of hand-holds and such within arm's reach. Thus personal dwellings may normally not use large spans and spaces would always confine at least one dimension to about one to -at most- a few meter span. Thus the example of the capsule hotel module is a logical choice as the basis of microgravity dwelling design. Though the design of dwelling pods would tend to focus on maximizing the multifunctionality of a single personal space, they would be usable in clusters of rooms with an eventual evolution into a kind of three room apartment (main room, sleeping, and bathing/toilet) likely. Like the original sleeping pods they would consist primarily of fabric, foam, or high pressure rigidized pneumatic panels over a light frame of aluminum or pultruded fiberglass or carbon fiber composite As with the capsule hotel module, most fixtures would rely on built-ins and concealed compartments with light items attaching by velcro. With a high reliance on foams and fabrics, optical power distribution and the use of fiber optic ambient lighting may become standard as a means to minimize fire risks as well as reduce latent heat. Windows would be few and where they are used they would be dual-use, serving as exit portals as well as window openings. As they develop more sophistication in features, they may eventually support their own emergency life support, being sealable like a space suit in the event of a hull failure. Indeed, many features of these dwelling pods may mimic the engineering of space suit design, especially for ventilation and climate control.

Initial dwelling pod designs would tend to favor an industrial style functionalism but as they become more sophisticated and large we may see a small number of aesthetic styles develop. The three most likely are 'cellular' structures, 'modular curved space systems', and all-in-one dwelling pods.

Cellular designs derive directly from the capsule hotel model and would consist of a catalog of cellular room modules designed to 'pack' together sharing common geometrical faces. They would tend toward prismatic polygon shaped rooms; boxes, hexagonal and octagonal cells with triangular cell interstitial space for utilities. This form would be very popular where the interior of the core truss is used for personal dwelling space, but ultimately there would be insufficient volume in that region of the structure and its use as a transit via would become more important.

Modular curved space systems are something a bit newer and more interesting. Derived from the geometry of saddle polyhedra, curved space systems use modular components to produce flowing curved spaces and interconnecting tunnels that form a more organic structure. They've even been developed for children's play labyrinth structure. One promising type uses a combination of saddle-pentagon, saddle-hexagon, plane-hexagon, and plane-square panel shapes to make up the curved space network. The primary room shape is, of course, spherical and the number of flat plane-hexagon and plane-square panels in a structure would tend to be rather small, complicating the use of built-in fixtures. One advantage of this type of system is that it not only serves for individual dwellings but for large complexes of spaces and can become a primary surface structure for the entire habitat, concealing most of the frame structures used.

The all-in-one dwelling would employ a more free-form architecture to create a dwelling pod of complex internal shape and potentially large size -almost as large as conventional Earth homes. It could be more organic or use a hybrid of shapes, though it would tend to employ curved edges and corners and fairly simple exterior forms capable of nesting or clustering using connecting straps or cables. Many interesting external features are possible, such as cable-stayed terraces and built-in garden and aquarium units. These would be much larger than the other types of pods and their component systems and so would tend to rely on pneumatic rigidization structures inside them and the use of foam, alloy, or composite inserts which would allow the main sewn-fabric structure to be completely rolled-up or folded into a small package. They would have some issues of wear and obsolescence because of their uniform fabric material skins but they would also tend to be more creatively designed, offer better sound proofing, and be better suited to use as a whole as an emergency shelter. Such structures would also feature for mass solar flare shelters, featuring insert rad-shield panels and woven wire fabrics. Some of these may be deployable structures.

Initial use of dwelling pods would tend to be based on positioning them within the space of the core truss or immediately outside and attached to it. These early dwelling pods will usually have a single entrance. Ultimately the role of the core truss as a transit via would supersede most other uses, leading to dwelling pods which are arrayed radially outside it and use the space between truss chords to access entry portals. This generation of dwelling are likely to use at least two primary entrances; one facing the core truss and the other facing the exterior and serving as a main view window. As the Asgard settlement approaches the use of its full size habitats and full size urban tree structures, radiating branches from the core truss would be used to host clusters of dwelling pods instead. These dwellings may be arrayed around a common central space serving as a kind of neighborhood center with the branch truss running through it as a transit via and attachment point for shared facilities, gardens, and service systems. For large habitats where a high population density is an issue, we may see a cellular dwelling approach become predominate with these attached to a space frame paralleling the surface of the inner hull -leaving a gap large enough for each air flow and access to the inner hull for speedy repairs. The use of the hull surface as projection display and light diffuser is eliminated by this approach. Instead, the use of small lights along the inward facing side of the cellular dwellings would illuminate the interior space and obscure the sight of the cellular dwellings creating a sky dome effect. The central core would then tend to be dominated by commercial and recreational activity.

Decorative gardening would be a popular feature of the settlement, helping to compensate for the industrial feel of things. Most intensive farming would be done in a MUOF setting where conditions of light, humidity, and growing density can be optimized to make the most of limited space. But in the rest of the settlement gardening would serve functions of aesthetics, supplemental air purification, and humidity moderation. Due to the microgravity environment, all such gardening would be hydroponics based and tend to rely on techniques of capillary delivered nutrient film or aeroponic methods. Most planter units would take the form of self-contained planter capsules with elastic membrane plant-holding ports which can be mounted on or within other structures, rigid capillary tubing loops and columns to whose surface plants are attached to by their roots all along their length by an elastic webbing, and free-floating planters which host spherical clusters of plants attached to a computer-controlled feeder ball and which drift about the open spaces on the internal air currents using thin transparent elastomeric fan-like sails much in the manner of a Man-O-War jellyfish. Species with climbing tendencies and tolerance of low diffuse light would be preferred, though supplemental lighting could be used for some planters. The most sophisticated of gardening systems likely to be employed on Asgard would be the 'cybertree' system which cultivates an enormous living tree about the core truss structure. This is done by exploiting a phenomenon called 'inosculation' where trunks or branches of like plant species (and ideally genetically matched by propagation of cuttings) will fuse and grow together into one to create a single larger plant. Long toyed with as the basis of decorative gardens in centuries past using a technique called 'pleaching', the technique has in recent years been explored by sustainable and avant-garde architects as a means for the cultivation of living houses. On Asgard this technique could be used to turn the urban tree of a habitat into a living tree by applying an array of capillary feeder panels to the outside of the core truss and a series of truss branches, cultivating a continguous fused tree-trunk on the feeder panels using large numbers of saplings generated by tissue culture cloning. The process would take some decades to complete but the end result would be a core structure completely enclosed by living plant material with its center still hollow and usable as transit vias and a conduit for utilities servicing dwelling pods clustered like fruit among this cybertree's branches.

Mobility throughout the settlement would be based primarily on human power but as everything grows in scale the need for powered transit assistance would be necessary. Of particular concern are the large open spaces about the core tree which would be very popular for recreation but offer no hand-holds within easy reach. The solution may be the use of a kind of hand-held ducted fan or jet unit that works similarly to the small hand-carried tow motors used by scuba divers and which can be kept attached to oneself on a tether. A similar device may find use for quick tracked transit along the core truss or across large open spaces using a tether or track called a 'zip cord'. The zip cord would basically be one half of a linear motor with the handhold serving as the other half. Clamping the handhold to the track and pulling a trigger engages the linear motor, allowing one to be pulled along at speeds varied by a thumb-wheel. Breaking is automatic with the release of the trigger and releasing one's hold on the handhold would completely deactivate it. Sensors on the zip cord also send a feedback signal to engage automatic breaking when two units are in proximity along the line. Small payloads can be attached to the handhold as well using straps with carabiner locks. A variant of the zip cord system may use a simple elastic single-strap harness on a rigid shuttle attached at two points to the zip cord. The user would wrap one leg and arm around the harness strap and use a control pallet on the strap to activate the unit. This unit would be limited to a fixed speed and have a feature that can read codes embedded in the zip cord to allow one to travel long distances automatically by typing in a destination code on the control pallet. A similar device would be used strictly as a payload carrier pallet and allowed to travel unattended.

A number of fully and semi autonomous service robots would also be likely on the settlement with some used primarily for the transport of large bulky high mass items. Key among these would be a carrier pallets and containers similar in design to those used for the MUOL's module transport but relying on small ducted fans for propulsion. Other free-roaming robots would include scrubbers which scour the air for free-floating liquids and debris, limpet cleaner robots which clean the flat surfaces of the structure using non-chemical adhesive tracks to haul them along surfaces they are cleaning, and small inspection and voice-activated personal computer robots much like those NASA has already developed for ISS use. Ducted fans and, in come cases, nitrogen gas jets would be common propulsion for such free-floating robots, affording movement at a modest pace around the station rather like deep-sea submersibles and marine robots.

Recreation on the settlement would be critical to both general well-being and the commercial viability of tourism but presents complications because one must incorporate every form of recreation offered within the structures of the settlement and a great many activities aren't possible in the microgravity environment. (there won't be a lot of bicycling and snow-boarding on Asgard...) New forms of recreation must be devised to compensate for those which must be left back on Earth. The microgravity environment itself will probably feature as the basis of many recreational activities with new games, dance forms, and types of theatre devised and, of course, needing their own permanent or deployable structures to accommodate them. Spectator sports, of course, present certain design complications in a microgravity environment because of the tendency of all enclosure surfaces to double as vaulting surfaces. This makes the design and placement of windows tricky, perhaps calling for the use of netting structures or new soft transparent materials superior to the transparent vinyl common today.

Observation chambers and lounges will be a likely feature of the settlement, elaborating on those employed in early space tourism facilities. Some may be designed to focus on views of the Earth and others on the immersion into the space environment. Suited space walks -though perhaps becoming a form of recreation- will always be inconvenient and hazardous even with the advent of technology such as mechanical counter-pressure suits. Children especially will be barred from such activity in general. So for the casual space-walk experience of the observation sphere or dome will probably be preferred. As the settlement becomes larger these too would increase in size as the techniques used to build the settlement afford the option of fabrication of these transparent shells on orbit rather than having them imported from Earth. However, they are unlikely to ever be suitable as habitat hulls due to their susceptibility to debris impact and lack of radiation shielding. They may never be usable at all outside of LEO locations. Advancing video technology may ultimately compete with this technology, allowing the virtual transparency offered by video displays on the habitat hull to progressively improve to where it becomes truly indistinguishable from a natural windowless view.

The use of water as the basis of recreation in space has often been proposed but requires the use of new technologies for the containment of large volumes of fluid in microgravity. A popular concept has been the low-g swimming pool which uses either a rotating structure or a series of peristaltic pumps to contain water by establishing a flow or current with just enough centrifugal force to keep the water contained, fans being used to drive stray water droplets to the main water surface. Superdroplet pools have also been proposed, consisting of gigantic droplets of water entrained by fans and acoustic waves.

As very large habitats are realized the zip cord system could become the basis of a recreational activity in the form of various styles of racing or perhaps thrill rides. Tow thruster racing might also become a form of recreation. Solar sailer racing has, in recent years, been proposed as a commercial sponsored professional sport and any established orbital settlement of scale is likely to be a logical location to host such activities, though this would not generally lend itself to any large participation, serving more as a spectator sport.

Spiritual pursuits may see some significant application as a form of recreation on the settlement, especially for those practices which have potential health maintenance benefits as well. For instance, meditation and yogic practices would find a suitable environment in microgravity, especially when one considers the potential for sensory deprivation afforded by carefully designed dwelling pods and the high freedom of physical movement one has when free of the forces of gravity. Whole new systems of yogic and tai chi exercise might be developed for use in the space environment. Most overt religious practice, however, would tend to be problematic in such small communities. Due to their inherent cultural, racial, gender, and class prejudices, there really are no religions in the world today that are psychologically healthy and safe in such dense culturally diverse communities -as much as contemporary theologians would deny this fact. Their introduction would invariably lead to social conflict and so social codes should probably confine their practice to private activity. However, conflicts over seasonal religious and psuedo-religious displays -the Christmas season decorations in particular- are probably unavoidable.

It is possible that the dominant form of recreation in the space settlement will be media based. The use of digital text, audio, video media, the many forms of computer games, and the many forms of computer based arts and crafts will present a great economy in terms of space and material when providing recreation. But the sedentary nature of such activities does present important health ramifications in an environment where physical activity is critical in fighting the negative biophysical effects of microgravity. And telecommunications latency will, again, become an issue in participation in many forms of interactive Internet based entertainment such as games and live chat venues. Most media use would tend to be personal and rely on personal media appliances -mostly likely employing multi-use devices as the cost for highly specialized media devices would be high. But with the inner hull of the habitats being able to double as display screens, their use as public video theater displays could result in a lot of media oriented social events. This would be especially appropriate with spectator sports.

A diversity of hobbies are also likely and, owing the the demographics of the majority of initial settlement inhabitants, these are likely to be technically oriented and have a practical as well as entertainment aspect. The most obvious of such hobbies, of course, would be gardening with both its aesthetic value and its value as the basis of cultivating new techniques to increase the spectrum of plant species that can be cultivated, the performance of hydroponics systems, and the products one might produce with plant materials. The special conditions of microgravity pose great complications for the cultivation of plants and the design of both industrial and decorative planter systems, offering a new field of infinite possibilities for the inventive gardener. Pets are a possibility, though very few animals would be practical companions in the microgravity environment. Marshal Savage envisioned birds as popular pets for both marine and space settlers, though we are more likely to see fish, a few crustaceans like hermit crabs, large tropical insects, and insectivorous or herbivorous climbing reptiles as more likely pets. Not exactly the most cuddly collection of creatures, but their care is better suited to the environment in space would produce techniques useful for mariculture and agriculture. Of course, a home on orbit is the amateur astronomer and HAM radio enthusiast's ultimate dream and so these are likely hobbies as well, though the external space to mount equipment for these activities may be in limited supply. Various forms of recreational robotics may become common, ranging from such things as racing of miniature fan thruster vehicles or fanciful model sailing vessels that cruise the internal air currents of the station, to the creation of fanciful robotic pets to compensate for the impracticality of most living animals as pets, to the development of automated solar sail spacecraft. It is possible that robotics enthusiasts might turn hobbies into vocations, developing their own robotics systems for the exploration, prospecting, and teleoperated base settlement of asteroids, the Moon, and other planets -a far more rewarding hobby than the usual model train layout. Media production is another likely hobby and one could expect some residents to turn their spare time to the creation of news blogs, amateur TV programs, and the like using life on the orbital settlement as their primary subject. These activities might later form the basis of the settlement's own mass-media industry as the community grows in scale. (indeed, one can only account for NASA's failure to have already established a regular weekly TV program broadcast from the ISS to their fundamental lack of imagination. Can one possibly imagine a better venue for a global science and technology news program?)

Sexual activity should not be overlooked as an important form of recreation as well as a normal aspect of human daily activity -though it has tended to for most of the history of the first space age thanks to the infantile mock prudishness and misogyny of academics, government bureaucrats, senior engineers, and corporate executives. To date, few scientists, engineers, physicians, or futurists have had the bravery and adult sensibility to give this activity serious consideration but it is most definitely going to be a key factor in people's well-being and for the bottom-line of tourism. While the first wave of space tourism will be largely male dominated and focused on nostalgia for the anachronisms of first space age with a necessary lack of privacy precluding any intimate activity, later tourism must accommodate a much broader spectrum of amenities and this includes the need to accommodate sexual activity. There is simply no denying that for a great many potential space tourists the imagined novelty of sexual intercourse in a microgravity environment is going to be a key attraction and accommodating and enhancing the experience for that activity will be important to the success of tourism in general. Indeed, it amazes me that the porn industry -forever in search of new novelties for its media and no stranger to SciFi themes no matter how cheesy- has not as yet even explored the techniques of current space theme film making to try and emulate this activity in its media. Strange as it sounds, they might actually be able to contribute more in the way of practical R&D relating to this than any government space agency.

Much with this issue would fall under the province of the design of dwelling pods which would -because of the potentially different mechanics involved- need some rooms to be specifically engineered to accommodate this activity for maximum convenience and pleasure. However, specially engineered 'sex rooms' would tend to be unpopular because their special addition to a dwelling would incur some embarrassment as it openly communicates one's private intentions. So accommodating this activity would have to be a standard element in dwelling design and features associated with it subtly included in the design. The designers of hotel accommodations understand the logic of this quite well -or at least the more successful ones do. The space habitat would tend to offer fewer venues for personal privacy than is typical on Earth until it achieved very large scales and great economy in the construction of space. Thus the places where people might be able to engage in sex would be few and the use of private dwellings for this would predominate -though human beings have proven quite inventive and adaptive in this regard. But as most adults well know, the long popularity of hotels, automobiles, and the like as locations for sexual rendezvous is based on the desire to keep such rendezvous discreet, especially when couples are not married or of the same sex. This becomes a tricky issue in an environment where unobserved access even to one's own dwellings may be difficult. Variations in location are also an important factor in the enjoyment of sex and we must take into account that the essential fantasy associated with sex in space is based on the vision of sex 'in space' -in some analogous environment to the open space environment of EVA. It would be rather difficult to employ public observation lounges in this way or to make numerous small domes like this for this activity so some inventiveness would be needed here. And there are medical issues to address when it comes to pregnancy and birth control with pregnancy probably meaning forced eviction to Earth until the effects of microgravity on long-term child development are clearly determined. Contemporary culture -Western culture especially- remains rather primitive in its attitudes toward sexuality and in recent years we've seen quite a reversal in the state of sexual sophistication. This will definitely have its impact. Would the orbital settlement see a more progressive shift in its cultural attitudes to accommodate its different situation? Perhaps only time will tell.

Asgard Colonial phase

This phase of development would be marked by the creation of the largest of orbital settlements practical with the EvoHab technology and the creation of multiple such settlements as LEO locations are ultimately abandoned (or converted into transfer stations) for GEO locations and eventually Lagrange Points. This is the point where Asgard ends and Solaris begins; the transition from settlement of Earth orbit to the colonization of orbital space throughout the solar system. By this phase the infrastructure of remote resource utilization would be robust enough for the economy of the Asgard settlement (or settlements) to have begun a comprehensive shift from Earth market dependency to local market self-sufficiency. While the settlement phase was marked by the initial establishment of a space resource utilization infrastructure, limited industrial diversity would have maintained a strong reliance on the Earth economy, self sufficiency seen primarily as a means to improving the bottom-line of export goods production. But now the local industrial capability of Asgard should be sufficient to support a completely independent civilization with virtually all goods produced by domestic industry. Asgard would enter a phase of escalating import deficit with ramifications in terms of the economic and political leverage that represents.

LEO is ultimately an impractical location for settlement due to the instability of orbits there. No settlement built in that location is likely to be sustainable long term or be particularly useful when deploying spacecraft to other parts of the solar system. Indeed, the development of a comprehensive Space Elevator system may result in an abandonment of most LEO satellite use in order that the threat of tether impacts be eliminated. But the reliance on GEO positions and beyond presents new challenges in the design of the habitat and the spacecraft that serve it due to the loss of protection from the Earth's magnetosphere. Radiation exposure now becomes a much more critical issue in the lives of permanent orbital settlement inhabitants and new technologies will be needed to address this. One likely development in this respect is the creation of plasma shield systems for use as artificial magnetospheres, both for habitat structures and spacecraft. In its infancy today, such technology could potentially allow for the continued use of transparent observation domes on settlements, keep physical shielding masses to volumes only sufficient for impact shielding, and eliminate the need for special heavily shielded mass shelters for solar flare emergencies. They may also serve extra duty for attitude control through solar wind deflection. Nanotechnology is likely to emerge as a key factor in this phase but will likely be in the NanoChip and NanoFoundry phases of its development in this period and so will not likely impact structural design except in terms of component performance and fabrication technique.

Scale is the primary change for Asgard architecture in this phase with individual habitat enclosures reaching their largest practical size using EvoHab components and new clustered habitat structures -forming fractal branching arrays of such habitat spaces each with their own urban tree into a single complex- allowing for truly vast communities. Not much else in terms of habitat design may change unless -as is likely to be finally determined by this time- it becomes clear that there is no clinical solution to microgravity degeneration of the human body. If this proves to be the case than in this phase a radical shift in habitat design would take place beginning with the creation of hybrid artificial gravity habitats as described in my previous descriptions of the Asgard stage.

Deriving from systems for precision controlled gravitation in MUOF facilities and possible exercise facilities on earlier settlements, this hybrid rotating habitat would incorporate the use of a gravity deck ring or cylinder supported by magnetic bearings which would rotate within, and independently of, the usual microgravity EvoHab structure. Used primarily for residential space as habitation there would represent the majority of the daily activity cycle, the design of structures would be radically different from that of the dwelling pods of the urban tree habitat. Though still relying on a materials composition of fabric, foam, and light alloy or composite components, the design of dwellings would tend toward a one or two storey townhouse style of configuration relying on long circumference avenues or series of sunken atriums to create an 'outdoor' space providing light and transit between the dwellings. Where the gravity deck is sufficiently wide, a reef-like arrangement of sprawling avenue networks or varied shaped sunken atriums may be employed to allow for the roof space of dwellings to double as recreational garden space -though the need to minimize structural mass on the gravity deck would limit the scale of this gardening. Views of the surrounding structure -and possibly the central core as well- would have to be obscured to prevent vertigo and thus the habitat may have to employ light diffusing screen structures to create translucent skylights over the gravity deck. This would offer some useful experience for the design of later subterranean habitats for lunar and planetary settlement. These structures would generally follow a style of interior design consistent with Earth dwellings, will full storey height and normal portals. But they would also need to be designed to accommodate both microgravity and gravity conditions since, early on, the gravity deck is likely to see frequent slows and stops as its technology is refined and as it deals with periodic transfers of large bulk materials and components during phases of construction. This would call for a high reliance on built-in fixtures, positive connection of all structure, and relatively small spaces that can be easily moved about in during microgravity conditions.

Transition between the rotating and non-rotating portions of the structure would be accomplished through the use of a shuttle system on the edge of the gravity deck that would employ alternate braking on tracks connected to the gravity deck and the outer structure to alternately decelerate and accelerate a shuttle car. The shuttle car would then dock at a stationary microgravity station where access to the core structure would be provided by zip cord.

With the availability of gravity many traditional forms of terrestrial recreation may become available but limited space would still limit the range of activities that can be supported. But small court sports activities and other types of recreation common to urban settings may prove practical.

The use of such hybrid artificial gravity habitats would not be practical at very large scales, the use of gravity decks employed primarily as a retrofit to existing settlements. Rotating habitats cannot use volumetric space as efficiently as microgravity habitats and thus real estate on these gravity decks would be costly. They would not support extensive gardening because of mass limitations and so might actually present less of a naturalistic feel to their living environment than the microgravity habitats. Thus as new artificial gravity based habitats are built it would be much more practical to employ whole-rotating structures simply to increase their potential economy, usable in-gravity surface area, and mass capacity -though their cost would still be quite high in comparison to microgravity structures. As previously described, these habitats would employ the EvoHab structural technology as the basis of a wound-hull system, using the enclosure space frame as a winding form for laminate tapes of nanofiber which ultimately bear the structural loads and provide pressure containment while providing matrixes of connector sockets for the attachment of replaceable shield panels and other exterior fixtures and simple structural columns and decking on the interior. Using strictly cylindrical shapes and with spherical end-caps akin to conventional EvoHab hulls (as they would not bear the loads of the primary cylinder walls) these habitats would feature a reduced core truss structure serving primarily as a support for a membrane light diffuser system for sunlight collected by exterior heliostat arrays and piped into the habitat by vacuum core fiber optic tubing or rigid light guides -a simpler, safer, and more economical solution than the vast window structures proposed with previous orbital colony designs. It may also host some microgravity facilities along the core truss but would need to keep them within very close proximity of the polar axis of the structure to maintain the microgravity conditions -and even then it would still tend to be low-g. This type of structure would be designed to support expansion in all dimensions but with easiest expansion along its polar axis through incremental extensions of the underlying winding form truss structure at the perimeter of the main cylinder walls followed by migration through concentric layers of the end-caps.

Exploiting the virtues of nanofiber materials, these structures could readily achieve circumferences orders of magnitude greater than anything imagined by the classic space colony visionaries of the 1970s -as NASA engineers have already surmised. And even as they reach their maximum circumference, they longitudinal expansion would be unlimited. Indeed, it is not infeasible to consider the use of these habitats ringing entire Earth orbits. (or, as I proposed for the Solaris stage, entire solar orbits) Yet they would still face a problem with the efficient use of their interior space because, as vast as they would be, little of their interior volume would actually be usable. All functional space is at the cylinder surface. Thus there would be a constant competition for space between human habitat and surface area for the cultivation of gardens and parkland. Early smaller habitats of this type may deal with this problem by confining residence to a select handful of sunken avenues and atriums employing a few levels of terracing akin to the 'tectonic' architecture employed on the Aquarius marine colonies. Deeper interior space would thus be relegated to more industrial uses. However, at the maximum habitat scale we can expect the number of deck levels supported by the hull structure to approximate the height of contemporary super-skyscrapers. This would result in the creation of a vast intricately articulated tectonic landscape of vast terraced valleys and mountain forms very much like those of the Aquarius marine colonies and offering an effectively identical style of residential architecture based on terrace-edge dwellings built up of modular partition components within a 'wedding cake' style deck system. Some of these terraced mountain forms may be used to create periodically spaced access towers to the central core, allowing both human traffic to the facilities there but also providing conduits for the light from the external surface mounted heliostat arrays and a system of MagLev docking shuttles used to provide spacecraft access along the sides of the habitat rather than just the ends.

One key problem with such habitats is that they would not be able to internally host the full compliment of microgravity industrial activities that are vital to their subsistence. These would have to be hosted on separate structures using the original structural approaches of MUOL and MUOF. The use of large counter-rotating pressure-sealed hubs to transition between full-g and zero-g regions of a habitat, as has been a mainstay of the space stations featured in classic space futurism and sci-fi, is likely to prove very problematic in reality as the hub structures must continually rotate without wear for an indefinite duty life. The larger both full-g and zero-g portions become relative to each other the more problems any temporary halt of the rotating portions due to hub failure becomes. For the large habitat this could be quite the disaster. Non-contact counter-rotating hubs based on simpler magnetic bearings -derived from the same technology employed for the hybrid rotating habitat systems- could be employed to provide a true microgravity environment for structures within the enclosure of the full-g structure at the habitat core or outside at the center of the end caps. This may also be employed to provide a non-contact counter-rotation interface to radiating structures about the outer cylinder walls. Transfer capsule shuttles -likewise on magnetic bearings- would then be used to transfer between regions. However, the simplest, and perhaps most likely strategy may be to employ a toroidal topology in the configuration of the full-g hull structure so that one has two structural cores, the inner-one non-pressurized and using magnetic bearings to isolate it from the wall of the outer one. This arrangement may allow for the simplest design of transfer capsules to move people and goods between the two regions while maintaining a structural design most consistent with the previous types of EvoHab structures.

Future nanotech materials may solve this problem in the form of 'psuedosolids'; materials formed of precision interlocking molecular structures that are rigid and can contain pressure yet will allow their molecular components to roll or slide with little friction along a specific axis almost like gears while maintaining a strong rigid molecular bond along all other axis. This would allow one to create pressurized tunnels or hub sleeves that can twist infinitely without ever coming apart. Today, though, such materials are pure speculation.

Asgard and the Space Elevator

Let us now consider the effect of Space Elevator development on the course of Asgard development. As I described previously, the MUOL and the first generation Space Elevator would be logical companions as the MUOL's initial payload needs are within the likely capacity of early tether climber systems. For the MUOL to accommodate SE use its structure must maintain a position adjacent to the tether within reach of its robot arms and may employ robots with a longer reach than usual for this purpose. This would favor an orientation where the service-side of the station plane truss faces the tether and, when the MUOF stage is reached and the station scale is sufficient, its folds into a bay wrapping around the tether. This may require a very large bay section in order to accommodate the passing of tether climbers, since they must also deliver payloads to other higher altitude points along the tether length. There would be no other particular change in the design of the MUOL except that, in order to be a part of the SE system, it would be compelled to employ a GEO location along the tether from the start. Without the SE GEO locations are more costly to support because of the larger launch systems needed reach them and the possible need for LEO stations as inter-orbit transfer stations. But the SE would effectively eliminate that extra cost making its use no more expensive -if not actually cheaper. However, this economy would not extend to passenger transit for a very long time due to the initial inability of the SE system to support payloads of such size or provide high transit speeds.

This would be an ideal situation for Asgard development as communications latency is low and constant for GEO locations, whereas the LEO facility would see latency which continuously varies over a very large range from very short to very long. GEO locations are also much more useful for transit to the rest of the solar system and much more stable. But GEO locations are beyond the protection of the Earth's magnetosphere and this presents complications for initial deployment of manned habitats as they must employ much heavier radiation shielding. This may compel the acceleration of the development of plasma shield systems or the use of EvoHab structures whose modular external shield panels would allow for much thicker shielding. It may also mean that tourism must do without the novelty of transparent observation domes unless plasma shield technology is realized before-hand.

Another advantage to SE use is that an Aquarius colony becomes the logical location for the Downstation facility supporting the SE and this means an acceleration in the exploitation of industrial capability on the MUOL as the marine colony would be able to host complimentary portions of production with a direct link between Earth and space. This means that in the production of products using space produced materials or components the space production facilities can be seen as an integrated component in a surface-based production system, though transit times on the tether would initially be some weeks long. As Asgard and the SE grow so too would Aquarius in the traditional coastal urban role of nexus for intermodal transport. Aquarius -and any subsequent SE equipped marine colonies- would become Earth's premier interface to space, resulting in its development to a scale perhaps never imagined by Marshal Savage.

Current SE project schemes seem to have put little thought into the long term development of the technology. I remain puzzled by how their project developers and advocates seem to perceive any suggestion of the eventual development of systems beyond the scale of their initial thin tape traction climber based technologies as pure science fiction. There simply is no question that for the SE to become a vital transportation technology in competition with other launch systems it must pursue a continual expansion in scale and capability, which would imply a relatively straightforward strategy based on incremental tether expansion by laminate addition of nanofiber tapes. As I've described in previous articles, I envision SE expansion to be initially based on a simple thickening of the tether and then a transition into a polygonal section corrugated structure with a series of channels or tubes which would eventually host different functions including MagLeg driven elevators and data, power, and possibly molar materials transportation using channels as waveguides for laser and maser beams. At terminal points the sectional volume of the tether would be expanded around truss forms to allow the internal channels to be individually accessed through exterior portals while at the same time not causing any reduction of the mass of tether laminate structure. These terminal point portals may ultimately host MUOF structures and Transhab habitats using the tether as a primary attachment point, though until the tether was quite large an outer truss structure is more likely to increase attachment surface area. This could lead to the development of the use of the tether as a direct replacement for the core truss of large habitats, an urban tree habitat being physically attached to it.

Some visionaries have suggested the ultimate development of a vast GEO urban ring as a result of the development of multiple SE systems and their link-up along the GEO path. It has been suggested that in this way the stability of individual SE tethers would be further increased allowing for even larger scale expansion. Considering the potential of the SE tether as a replacement for a core truss in an EvoHab structure, we can easily imagine how this GEO urban ring could be created simply by fabricating a tether along the GEO trajectory and enclosing it in an EvoHab hull to allow it to host that urban tree all along its length. Artificial gravity habitats would need to employ either the hybrid gravity deck strategy previously described or use a toroidal topology hull with magnetic bearings around the tether core. The potential carrying capacity of such a structure would be truly vast -yet insignificant when one considers that the exact same approach could be employed for solar orbits! We will look further into that possibility in a later article on the Solaria stage.

Eric Hunting

05/06