It may surprise you to know that in the 50 or so years of space flight, humans have built and launched at least a dozen space stations! The only space station currently in use is the International Space Station. Previous civilian stations were the Soviet Salyut series, the US Skylab and the Russian Mir.
The Russians also launched a series of Almaz military space stations, in response to the US "Manned Orbital Laboratory" program, that was also a military operation. The last military-use Russian space station was Salyut 5 in 1976 and 1977. The US program had five successful launches before being cancelled in 1975. Both the US and USSR by this time had spy satellites that were less expensive, more effective, and more discrete. The Astronaut Corps that was recruited for the MOL became the nucleus of pilots for the Space Shuttle program.
I find it a bit discouraging that the Chinese are embarking on a Space Station program of their own, rather than participating in the International Space Station. Theirs will be managed by the PLA and so it looks like that could be military as well.
Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even a bit longer. A space station is different from other manned spacecraft: they generally do not have major (or any) propulsion systems or landing facilities—instead, other vehicles are used as transport to and from the station, supplying the station, and using the ferry vehicles (the Shuttle and or the Soyuz) to maintain the station's orbit.
So far only low earth orbit (LEO) stations have been built and launched. These are known as orbital stations.
They are used to study the effects of long-term space flight on the human body as well as to provide platforms for greater number and length of scientific studies than available on other space vehicles.
Why is this important?
Well, while it takes only a few days to go to the moon, and even less to return, it takes -- depending upon planetary lineup, at least 9 months to get to Mars and about 15 months to return. With a handful of exceptions, even the fittest humans can currently can only spend about 6 months in space before physiological damage, specifically a drop in bone density, begins to occur. We also do not know the effect of really long term exposure to weightlessness, radiation and the impact on our psyches. So the Space Station offers us the ability to learn how to manage much longer duration space flights without undue risk.
Since the ill-fated flight of Soyuz 11 to Salyut 1, all manned spaceflight duration records have been set aboard space stations.
The duration record for a single spaceflight is 437.7 days, set by Valeriy Polyakov aboard Mir from 1994 to 1995. If you click on the image and look closely, you'll see his face in the window of Mir on the left. (It looks like he's trying to get out). As of 2008, only three astronauts have exceeded the six months mentioned above. Each completed single missions of over a year, all aboard Mir. However, even 14 months is less than half the time required (with today's propulsion systems) to have an adequate human mission to Mars.
The first conceptual design of a space station (left) was recorded in Hermann Noordung's "The Problem of Space Travel" (1929).
TYPES OF SPACE STATIONS
The space stations launched to date, have been of two types; the earlier stations, Salyut and Skylab, have been "monolithic", intended to be constructed and launched in one piece, and then manned by a crew later. As such, they generally contained all their supplies and experimental equipment when launched, and were considered "expended", and then abandoned, when these were used up.
Starting with Salyut 6 and Salyut 7, a change was seen; these were built with two docking ports, which allowed a second crew to visit, bringing a new spacecraft with them (for technical reasons, a Soyuz capsule cannot safely spend more than a few months in orbit, even powered down). This allowed for a crew to man the station continually. Skylab was also equipped with two docking ports, like second-generation stations, but the extra port was never utilized. The presence of a second port on the new stations allowed Russian "Progress" supply vehicles to be docked to the station, meaning that fresh supplies could be brought to aid long-duration missions. This concept was expanded on Salyut 7, which "hard docked" with a TKS tug shortly before it was abandoned; this served as a proof-of-concept for the use of modular space stations. The later Salyuts may reasonably be seen as a transition between the two groups.
The second group, Mir and the ISS, have been modular; a core unit was launched, and additional modules, generally with a specific role, were later added to that. (On Mir they were usually launched independently, whereas on the ISS most are brought by the Shuttle). This method allows for greater flexibility in operation, as well as removing the need for a single immensely powerful launch vehicle. These stations are also designed from the outset to have their supplies provided by logistical support, which allows for a longer lifetime at the cost of requiring regular support launches.
Space stations have various issues that limit their long-term habitability, such as very low recycling rates (air, water and unmentionables), relatively high radiation levels and a lack of gravity. Some of these problems cause discomfort and long-term health effects.
In the case of solar flares, the stations were somewhat protected by the Earth's magnetic field, being below the Van Allen radiation belts. However, when a large flare occurs, the astronauts have to enter the russian Soyuz transport vehicle till the flare passes. This is fine except when an EVA is underway.
Future space habitats may attempt to address these issues, and are intended for long-term occupation. Some designs might even accommodate large numbers of people, essentially "cities in space" where people would make their homes. No such design has yet been constructed, since even for a small station, the current (2009) launch costs are not economically or politically viable.
Possible ways to deal with these costs would be building a large number of rockets (economies of scale), reusable rockets, In Situ Resource Utilisation or space elevators if they are ever constructed.
Space Stations are complex systems with many interrelated subsystems, including:
Attitude determination and control
Orbital navigation and propulsion
Automation and robotics
Computing and communications
Environmental and life support
Crew and cargo transportation
The following pages outline each of the Space Station programs.