When what would become the International Space Station (ISS) was first conceived of several decades ago, it was envisioned as the first in a long line of orbital outposts that would lead humanity into a permanent future in space. The United States's first true space outpost would be a test bed for bigger and better things to come.
Unfortunately, politics and budget pressures have forced the ISS to become an end in itself. No replacement for it is ever mentioned anymore, and it will remain NASA's lone orbital facility for long into the foreseeable future.
However, forces are at work that may alter this equation. China is now a manned space power and the European Space Agency is making strides in that direction. Russia's economy is slowly recovering from its sluggishness, and may resurge independently into space in the next decade or two. And perhaps far more significantly, private enterprise has made its first ventures toward orbit with the likes of Burt Rutan's SpaceShipOne, with many more firms likely to follow.
All may seek their own outposts in space, independent of NASA, which may in the long run be forced to alter its own space station policies to keep up.
In this article we'll examine the character of future space stations beyond the ISS. First we'll look at some major requirements all space station builders must take into consideration, then examine types of stations that may come online by the end of the century.
Power
Near-future space outposts will have four major options for generating power: nuclear generators, solar cells, electrodynamic tethers, and solar boilers.
Dozens of different designs have been created for spaceborne nuclear reactors, from simple radiative heating with radioactive materials to more advanced direct-power designs such as particle bed reactors. While such generators can provide large amounts of power quickly, nuclear reactors are generally no longer considered for manned space stations due to the added complications and cost of shielding the crew from the radiation.
Solar cells are a long-proven technology, and have undergone steady incremental improvement over the decades. They are typically deployed on large booms away from the main body of a station, in order to maximize available surface area for energy gathering.
Electrodynamic tethers have so far only been used experimentally on Space Shuttle missions. Basically, the station would deploy a kilometers-long electrically conductive cable. As this cable moves through Earth's electromagnetic field, electrical current is generated along its length, providing power the station can use.
A fourth type of space station power plant was originally proposed by Werner von Braun at the dawn of the space age, and occasionally sees the light of day in science fiction: the closed-system solar boiler.
Basically, this is an advanced, space-going steam engine. Large, gimbaled mirrors concentrate sunlight on a fluid medium (usually water, though mercury was also mentioned as a possibility in von Braun's original proposal) pumped through pipes on the sunward side of the station. This water is turned into high-pressure steam which is used to power electrical turbines. The steam is then pumped into pipes under shadow, where in space it cools very quickly and condenses back into liquid water. The liquid is then pumped back to the mirror focus for reheating to complete the cycle.
The solar boiler scheme has a number of disadvantages, such as its many complicated moving parts and the high-pressure cycling system that would need constant monitoring and maintenance. It would, however, be able to generate more power more quickly than either solar cells or electrodynamic tethers as those technologies currently stand, but without the radiation hazards associated with nuclear plants.
Supplementary power systems are also currently present aboard the ISS and will be available to future stations, such as fuel cells and chemical batteries. As space stations become more advanced, chances are they will use a combination of some or all of the above techniques.
Life Support
The first concern in supporting a human crew is providing them with a breathable, healthy atmosphere. It is of course extremely impractical to constantly haul up fresh air from Earth, so a means of generating and/or recycling it is needed.
Means of removing excess carbon dioxide, cleansing the air of particulates, replenishing the oxygen supply, maintaining proper gas pressure, and more all have to be taken into account in order to provide a comfortable environment for the station's inhabitants. Currently on the ISS, oxygen primarily comes from splitting water molecules by electrolysis. Water is easier to handle and store than tanks of pressurized oxygen, though those are also present for redundancy. The leftover hydrogen from the electrolysis is combined with excess carbon dioxide in the air from CO2 scrubbers in a chemical reaction that creates water and methane. The water is then fed back into the loop while the methane is released into space as a waste gas. Eventually engineers want to reuse the methane as well, perhaps as fuel for maneuvering jets, so that nothing goes to waste in the system.
On more advanced future stations, large amounts of plants or algae, grown and tended in low-gravity hydroponic modules, may be used aboard to help fully recycle the atmosphere, much as they do in Earth's biosphere. As our understanding of biological ecosystems grows, our ability to replicate them in small, enclosed environments through purely technological means will grow as well.
Water recycling is a bit more straightforward, using an advanced three-step filter process that, respectively, removes particulates, eliminates organic and inorganic impurities, and kills any microorganisms present. The systems on board the ISS reclaim waste water from fuel cells, from urine, from oral hygiene and hand washing, and by condensing humidity from the air. Every drop counts. Without such careful recycling, 20,000 pounds per year of water from Earth would be required to resupply a minimum of two crew members for the life of the ISS. At current launch costs that would be about $200 million dollars per annum just for hauling up the station's water.
Water is still lost in several ways: the water recycling systems produce a small amount of unusable brine; the oxygen-generating system consumes water; air that's lost in the air locks takes humidity with it; and the CO2 removal systems leach some water out of the air. Engineers hope to get water recycling up to 95% efficiency or higher, the point at which the water in the astronaut's normal food supply would be sufficient to replace the water lost.
Fire suppression, gas mixture control, pressure management, leak detection, solid waste recovery, food storage, and more are all considerations life support engineers of future stations will have to take into account when building their orbital outposts.
Radiation Shielding
The space around Earth is constantly bombarded by radiation from the sun and more distant cosmic sources, radiation that would fry most life on the planet were it not for the double protection of Earth's atmosphere and magnetic field. One of the main reasons the ISS was built in such a low orbit was so it could remain under the protective blanket of the planet's magnetic fields, greatly reducing the potential hazard to its crew.
However, future stations, depending on their purpose, may not have the luxury of such a location, and much greater attention will have to be paid to radiation shielding as a result.
Radiation shielding is traditionally heavy, composed of metal or composite material plates, and therefore can greatly add to a station's cost. NASA's Johnson Space Center is currently sponsoring the Space Radiation Health Project, designed to develop means of protecting future astronauts from spaceborne radiation hazards. The scientists working at the project have developed polyethylene fiber bricks, which are very effective at scattering and absorbing stray particles but are twice as light as aluminum of the same volume. The same technology is used to provide armor for combat helicopters, and the bricks can double as a micrometeoroid shield.
Another means of providing radiation protection is to design a station's layout to provide maximum shielding in certain areas just from its configuration. This is one of strategies used aboard the ISS. In the event of intense solar storm activity, astronauts retreat to a portion of the station where the outer modules help to provide additional barriers against exposure. Solar cells, water tanks, fuel tanks, and so on can be placed strategically around a station to provide increased protection for its human crew. In fact, fuel and water storage can be designed specifically as shielding, and be built in large narrow tanks that could completely encircle modules of the station.
Active magnetic shielding has also been proposed as an alternative means of protection. Since both particles from the solar wind and cosmic radiation are charged, a strong magnetic field could theoretically deflect them. We know this works on a large scale, as Earth's own electromagnetic field diverts most of the incoming radiation from space.
The primary means of creating an active electromagnetic barrier on a station would be to use loops of superconductor coils wrapped around the outer hull of the station. High-temperature ceramic superconductors can be held at their nominal operating temperatures by simple radiative cooling into space. In the short term, active magnetic shielding may force a redesign of some station systems, especially electronic, in order to accommodate the presence of a constant magnetic field. In the long term, the effects of these powerful fields may eventually have some adverse effect on the crew's health.
Artificial Gravity
One of the most perplexing problems involved with space stations is weightlessness. Prolonged exposure to microgravity can lead to decreased bone density, fluid loss, osteoporosis, and a host of other complications. Dietary supplements and rigorous daily exercise can mitigate many of the effects, but can't arrest them completely.
The solution to this problem has been known for almost a century: artificial gravity through centripetal acceleration. In other words, simulating gravity by rotating a body fast enough to provide a continuous acceleration force along its inner surface. Just like on whirling carnival rides, spinning constructs exert continuous G-forces that feel indistinguishable from true gravity.
In the fifties and sixties, extensive studies were carried out by NASA and the US Air Force on the feasibility and comfort factors of rotating habitats. However, as human experience in space mounted, it seemed less of an immediate concern and research was halted in favor of other projects. Recently, as the possibility of prolonged exposure to microgravity loomed on the horizon for interplanetary missions, interest in the subject revived.
Creating a comfortable artificial gravity environment can be tricky. Research has shown that the human "comfort zone" for gravity, the range in which people can live and work without adverse effects, is between 0.35 G and 1.0 G. A certain minimal rotational velocity is needed to provide this, usually starting around 6 meters per second for projected stations. But the complication here is that the station cannot complete more than four revolutions per minute or else the inhabitants on board will invariably start feeling very dizzy from motion sickness. Motion sickness is a very real concern, especially if one is going to spend weeks or months on board.
It works out that with a minimal rotational velocity of six meters per second, the radius of any rotating station has to be a minimum of about 15 meters, with a human crew becoming more and more comfortable the larger the spin radius becomes. In other words, the bigger the station, the easier it is to minimize motion sickness and accommodate a human crew comfortably through spin gravity.
Whether designers will incorporate spin gravity into future stations remains to be seen, and will probably depend greatly on the purpose of the station.
Space Hotels
The next stage in space station evolution will most likely be a technological step backward. The success of Burt Rutan's SpaceShipOne in capturing the Ansari X Prize has opened up the very real possibility of near-future space tourism. While it will at first be nothing more than a ride up and back from the edge of space, no one doubts that the industry will eventually upgrade into orbital flights, and from there to runs to "orbital hotels" where passengers can spend a day or more in weightlessness.
However, in order to make such ventures as profitable and as safe as possible, space tourism companies will most likely at first rely on small, low-cost, well-proven designs. This means building stations that would hark back to the basic "tin can" configurations of the first three decades of spaceflight—Salyut, Skylab, and Mir. While most companies talking about space hotels bandy about plans for luxurious, high-tech orbital facilities, it's almost certain they will start the same modest way the national space agencies did. Also, it seems likely that space hotels may come online within twenty years or so, and if so will probably beat into orbit any official NASA successor to the ISS.
Private companies will probably at first need to rely on the heavy boosters of the national space agencies to put their first facilities in orbit. A space hotel will need at least four major components: a power plant, a life support and maintenance module, a section dedicated to crew and passenger accommodations, and a "play" space.
Unlike current space facilities, the individual accommodations aboard a space hotel will need to be partitioned for privacy, and will probably be made modular in order to accommodate singles, couples, and families. Also, whereas past and present space stations had a lot of their volume dedicated to either scientific or observational equipment, a space hotel could dedicate that room to large, open spaces where passengers can play and experiment with a microgravity environment.
Just because space hotels have simple needs does not mean there's no room for innovation. Las Vegas entrepreneur Bob Bigelow is working in cooperation with NASA in order to develop inflatable space station modules that would cost a fraction of the ISS modules and be able to sport much more room. NASA had its own such program years ago but it was cancelled in one of many waves of belt-tightening. Bigelow took up where the NASA program ended, and despite a stormy initial relationship, Bigelow Aerospace and NASA are now fully integrating their efforts to create viable inflatable modules, whose first applications may well be to establish space hotels.
Tank Stations
Tank stations are an idea almost as old as NASA's Space Shuttle itself, perhaps best detailed in David Brin's short story, "Tank Farm Dynamo."
The huge External Tanks (ETs) used by the Space Shuttle are dumped after use, to burn up in the atmosphere. They are the one major part of the system that aren't reused. However, it would cost the agency little extra money, and the Space Shuttle no real loss in orbital velocity, to carry a tank with it all the way into orbit. Small thruster modules—either attached before launch or mounted in orbit via the shuttle's manipulator arm or a spacewalking astronaut—could help boost the tank into a higher orbit where it can be "stored" for future use.
Each tank is over ten stories tall and contains over fifty thousand cubic feet of volume. Plus, they are already reinforced structurally to be launch- and vacuum-proof. Perfect, really, to use as a base for simple space stations. NASA could make the tanks into a profit-making venture, storing them in high orbit against future use, and then either selling or renting them to future space entrepreneurs who want a space hotel but don't have the capital to boost a full one into orbit. The space hotelier would still need to provide the operational equipment for the station, life support, power, and so on, but that should prove minor compared to the thirty-five-ton launch mass of the tank itself.
This late in the shuttle program, it is doubtful that the traditionally conservative NASA will alter its policy on its own regarding its Shuttle ETs. However, private investors willing to pay NASA the extra expense for orbiting and storing the tanks could still see this scheme come to fruition.
Zerovilles
As space tourism increases in the coming decades, space hotels will likely become larger and far more elaborate. Even when stations with spin gravity come online, the novelty of living in microgravity conditions will still be a powerful draw for many curiosity seekers and adventurers.
Zerovilles are either large stations or tightly knit groups of smaller stations designed to hold hundreds if not thousands of people seeking the thrill of weightlessness for a week or more at a time. This would of course be a colossal engineering challenge, especially for environmental systems, as how to regulate a large-scale microgravity environment for such things like air filtration and waste management is a quandary no one has yet tackled.
However, having much larger facilities to play around with will allow designers to go far beyond the small microgravity playrooms of first-generation space hotels. One can imagine large, open spaces in Zerovilles dedicated to a number of different purposes. For example, large-scale microgravity entertainment productions could be staged in the center while the audience floats around the edge. Or the space can be a high-pressure environment with a number of artificially generated air currents, allowing guests to strap on artificial wings for maneuvering in play or sports. Or such spaces can be made over into enormous "parks" with a number of genetically engineered plants filling their volume, creating three-dimensional forest the likes of which has never been seen on Earth.
Though operational crews will for the most part be rotated on and off, the Zeroville stations may nonetheless accumulate a permanent population over time. People who suffer from heart diseases, high blood pressure, and other select ailments fare better in a weightless environment. They may permanently move into space for health reasons, with the Zeroville stations providing them with all the necessities needed for a comfortable live. In fact, if space travel becomes commonplace, whole Zeroville "retirement" communities, unconnected to the tourist trade, may spring up in orbit.
Spin Modules
The first stations to use centripetal acceleration to simulate gravity most likely won't be huge or elaborate. In fact, they will most likely resemble dumbbells—two or more modules of about equal mass, connected to each other by a central corridor. The arrangement will rotate about its central axis, providing artificial gravity to both end modules. The perceived gravity at the center of the axis would be zero, and would increase incrementally the farther out one went. A person on board would always feel their sense of "down" to be toward the outer sections of the station.
The central axis of a spinning station is the most logical place for a number of systems because it is the one part of the structure that will always remain in microgravity. Docking rings and cargo transfer facilities are the most obvious, as are primary maintenance hubs; repairing and modifying large mechanical systems is easier when one can just float alongside.
However, a station with a single spin module presents a problem: the spinning of the station invokes the phenomenon of progression, which makes it gyroscopically unstable in the plane of its rotation. To put it simply, it will eventually drift in unwanted directions if left to spin on its own. This same phenomenon also perplexed early pioneers of the helicopter, and can be solved in one of the same ways: add another spin module right next to the first one but spinning in the opposite direction, both attached to the same central axis. If both sets of spin modules are of about the same mass and are spinning at the same rate, the effect cancels out. An example of this can be seen in the movie 2010 aboard the interplanetary craft Leonov, where two counterrotating spin modules provide living and working quarters for the crew.
Wheel Stations
Wheel stations are as old as the idea of space stations themselves. Early space visionaries like Werner von Braun and Konstantin E. Tsiolkovsky both promoted the design.
To optimize useful space aboard a spinning station without subjecting its crew to too many significant shifts in gravity, one needs a station with a circular cross section. This has traditionally taken the form of "wheel" stations, with a tubular rim fitted with living quarters and working facilities, and a central microgravity hub connected to the rim by two or more spoke-like causeways. Depending on its size, the wheel's rim may contain several decks, with a slightly different gravity level on each the farther out one goes.
As with spin modules, rotational progression may become an issue with a wheel station, so having two or more wheels attached to the same central hub, but rotating in different directions, would be a logical fix for that. This in fact was the type of space station depicted in the film 2001: A Space Odyssey.
With their greatly expanded interior volume and pseudogravity, wheel stations, more than any other station mentioned so far, will allow a true permanent manned presence in space. With the health hazards of microgravity and the psychological hazards of cramped quarters greatly diminished, people would be able to work and live in space for months or even years at a time in relative comfort.
Werner von Braun's vision of a wheel station, a design he updated from earlier work by Herman Noordung, remains for space visionaries the standard to this day. As outlined in articles for Collier's magazine in the 1950s, von Braun's station would measure over 76 meters across, with a ten-meter-wide rim with three separate decks, rotating once per minute. The station would hold a crew of up to several hundred personnel. Von Braun foresaw the station as a springboard for solar system exploration, a navigation beacon for ships and airplanes back on Earth, a meteorological observatory, and a military reconnaissance platform.
A much larger version of von Braun's wheel station was conceptualized in 1975 by NASA and Stanford University, as a means of housing both orbital factories and the personnel needed to run them. What resulted was the concept for the Stanford Torus, a wheel station nearly two kilometers in diameter, 200 meters wide, and capable of holding up to 10,000 permanent residents.
The innermost wall of the torus would be transparent, in order to allow sunlight to enter by means of giant louvered mirrors anchored to the wheel hub. These mirrors can be opened and closed, in order to allow an Earth-approximate day/night cycle. Below this inner-rim skylight would be large open areas holding very normal-looking houses, buildings, and even soil, parks, and hydroponic farms, all designed to provide its inhabitants with as familiar an environment as possible. Rotating approximately once per minute would provide Earth-normal gravity on the interior surfaces, with no great difference in gravity gradients between levels until one enters one of the spokes on the way to the hub. The outermost levels below the habitation level would be dedicated to maintenance, manufacturing, and storage.
Bernal Spheres
A step up in sophistication and livable surface area from the Stanford Torus is the Bernal Sphere, conceived in its current form in the same 1975 study that produced its wheel-shaped cousin. Like the Stanford Torus, it crystallized from earlier science and science fiction sources.
Like the Stanford Torus, the Bernal Sphere is two kilometers in diameter and rotates about once per minute to provide Earth-like gravity. However, as its interior is a sphere rather than a simple ring, far greater surface area can be used by potential colonists. Large circular transparent sections near the rotational hub would allow sunlight to be directed into the interior by means of gimbaled mirrors. Like the Torus, the Bernal Sphere would have most of its internal volume completely open with very Earth-like communities, structures, and parks sculpted into its innermost surface. However, as one moves up the sphere from the rotational equator to the hub axis, perceived gravity would steadily decline. Thus this kind of colony would have varying high-gravity and low-gravity neighborhoods.
One of the more interesting concepts that arose for Bernal Sphere interior design is to have an open "river" ringing the entire length of the Sphere's equator, which would double as the construct's central water reservoir.
O'Neill Colonies
In the early 1970s, Dr. Gerard K. O'Neill, through college courses and his book The High Frontier, began promoting the idea of large-scale construction in space, and of a particular kind of gigantic space station that has since become known as an O'Neill Colony. O'Neill Colonies were included in the NASA/Stanford 1975 study on space colonization. O'Neill Colonies have also become one of the great enduring motifs of modern science fiction, having been seen in dozens of science fiction sources. The current generation probably knows them best from the various incarnations of the Gundam anime series, and from the title station in the TV series Babylon 5.
O'Neill's vision was of large rotating cylinders, from hundreds to thousands of meters across and many kilometers long. The interior would be open and pressurized, with the inner surface holding not only living and working quarters, but soil, forests, waterways, and so on, in essence becoming a large self-enclosed Earth-like ecology. Large gimbaled mirrors would direct sunlight into the interior along transparent strips running the length of the cylinder, closing for eight hours at a time to create an artificial night. Even more than the Stanford Torus or the Bernal Sphere, an O'Neill Colony would have the interior volume to become a miniature version of the homeworld, allowing people aboard to live, work, and even raise families in much the same manner as people on the ground. O'Neill colonies, once up and fully running, could hold hundreds of thousands of residents. The open park-like spaces could be turned into farms, and with strict recycling in place, the station could become virtually independent from Earth.
O'Neill Colonies are more than just outposts or way stations; they are true residences in the Great Dark. While other stations are usually mentioned as being in near-Earth orbit, O'Neill Colonies are often visualized as inhabiting locations much farther out—geosynchronous orbit, the Lagrange points, even orbiting the moon or other planets. In the centuries to come, they could well become the equivalent of the small towns of the solar system—the modest, sometimes isolated stopovers between the major population centers that would spring up on colonized worlds.
Conclusion
A permanent manned outpost in space was seen as the logical successor to the first manned spaceflights as far back as the 1960s. Unfortunately, politics, budget woes, and an apathetic public kept the dream from being realized until recently, and even that step, the ISS, is mostly a compromise to please myopic politicians.
But as more nations and private interests push for an expanded human presence in space, the need for permanent way stations will become paramount. By the end of the century, space stations in orbit and beyond may become commonplace, sparkling in the night sky like the campfires on a great frontier.
Further Information
"NASA Reveals New Plan for the Moon, Mars & Outward", by Leonard David [space.com]
"How Space Stations Work", by Craig Freudenrich, Ph.D. [howstuffworks.com]
Space Station Life Support:
"Designing For Human Presence in Space" [nasa.gov]
"Breathing Easy on the Space Station", by Patrick L. Barry [firstscience.com]
"Water on the Space Station" [nasa.gov]
Space Hotels:
"The ultimate public-private partnership", George Knapp [lasvegasmercury.com]
"Artificial Gravity and the Architecture of Orbital Habitats", by Theodore W. Hall [spacefuture.com]
Tank Stations:
"Tank Farm Dynamo", by David Brin [davidbrin.com]
"Geode Stations" [spaceislandgroup.com]
Wheel Stations:
"Collier's Station" [davidszondy.com]
Space Colonies:
"Space Settlements: A Design Study" [nasa.gov]