Small space stations are launched fully assembled, but larger stations are sent up in modules and assembled in orbit. To make the most efficient use of its carrier vehicle's capacity, a space station is launched vacant, and its crew members—and sometimes additional equipment—follow in separate vehicles. A space station's operation, therefore, requires a transportation system to ferry crews and hardware and to replenish the propellant, air, water, food, and such other items as are consumed during routine operations. Space stations use large panels of solar cells (solar cell) and banks of storage batteries as their source of electrical power. They also employ geostationary relay satellites for continuous communication with mission controllers on the ground and satellite-based positioning systems for navigation.

Between 1952 and 1954, in a series of articles in the popular magazine Collier's, the German-American rocket pioneer Wernher von Braun (Braun, Wernher von) presented his vision of a space station as a massive wheel-shaped structure that would rotate to generate “artificial gravity” from centrifugal force, sparing its crew of 1,000 scientists and engineers the drawbacks of weightlessness. It would be serviced by a fleet of winged spaceships employing nuclear engines. One of the station's primary tasks would be to assemble vehicles for expeditions to the Moon. That concept remained a popular portrait of humankind's future in space as late as 1968, when the American motion-picture director Stanley Kubrick (Kubrick, Stanley)'s classic science-fiction film 2001: A Space Odyssey depicted a spinning double-wheel station under construction above Earth. On a regular schedule, a fleet of commercial space planes flew people up to the station, from which they could catch a ferry to the Moon.

In Braun's day, the development of a space station was thought to be a preliminary stepping-stone to the Moon and planets, but, when Cold War politics prompted President John F. Kennedy (Kennedy, John F.) in 1961 to commit the United States to landing a man on the Moon before the decade was out, there was no time to pursue this logical route. Rather, a single spacecraft would be obliged to ride an expendable rocket into orbit and fly directly to its goal. Nevertheless, even as the National Aeronautics and Space Administration (NASA) plunged deeply into the Apollo program, it studied several space station strategies as part of an Apollo Applications Program, which would exploit vehicles built for the Moon race for more general orbital activities.

Even as 2001 was restating Braun's ambitious vision to the public, it already was obvious to space engineers that the first real space stations would have to be much simpler than their fictional counterparts. One NASA plan was to have an Apollo spacecraft dock with a spent rocket stage, whereupon its crew would pressurize the rocket's empty hydrogen-propellant tank with air and install scientific equipment that would turn it into a laboratory for several weeks of occupancy. The U.S. Air Force had its own plan to operate a Manned Orbiting Laboratory fitted with an advanced camera to facilitate military reconnaissance activities. In 1969, however, just as NASA attained Kennedy's goal of a manned lunar landing, President Richard M. Nixon (Nixon, Richard M.) canceled the Manned Orbiting Laboratory and restricted the Apollo Applications Program to a single station.

Like the U.S. military, the Soviet Union (Union of Soviet Socialist Republics) had a plan to put a series of reconnaissance stations in orbit by the 1970s. In 1969, with development running late for the large spacecraft that was to ferry crews and supplies to the station, Soviet officials decided to accelerate the program by employing the Soyuz spacecraft that had been developed during the failed attempt to win the Moon race. Moreover, because some of the systems needed for a military reconnaissance platform were not yet available, it was decided to initiate the program with a station equipped as a scientific laboratory.

The Soviet Union's space station design took the form of a stepped cylinder 14.6 metres (48 feet) long, with its widest section 4.25 metres (13.9 feet) in diameter. Although the station could be oriented arbitrarily, its maneuvering engine was located immediately behind the wide section, which thus came to be defined as the rear of the station. At the front end was a docking system for Soyuz ferries. Internally, apart from an air lock in the cylindrical front section, the station formed a single rectangular room. The name of the station program, Salyut (Russian: “salute”), was chosen to honour cosmonaut Yury Gagarin (Gagarin, Yury Alekseyevich)'s historic first orbit of Earth a decade earlier.

Salyut 1, which was launched April 19, 1971, atop a Proton rocket, was outfitted from the start to support two three-man crews for a total of two months over a six-month period. Although its first designated crew docked five days later in Soyuz 10, the cosmonauts could not open their ferry's hatch and had to return home. Once the fault had been rectified, the crew of Soyuz 11 spent 23 days aboard the station in June, although tragedy struck on the way home when a valve in the descent capsule allowed the air to leak out, and the three cosmonauts were killed. At that time, it was not Soviet practice for Soyuz cosmonauts to wear pressure suits. In redesigning the Soyuz to prevent such an accident from recurring, one seat had to be omitted to accommodate a life-support system for two pressure-suited cosmonauts.

After failed attempts to establish a second station, the U.S.S.R. managed a 16-day tour in July 1974 by a two-man crew aboard Salyut 3. This military reconnaissance variant resembled its predecessor but had its docking system at the rear rather than at the front.

The Soviet program achieved the operationally significant milestone of reoccupying a space station in 1975 when two crews lived aboard Salyut 4 for 30 and 63 days, respectively, and conducted scientific experiments. By the time Salyut 4 was abandoned, its environmental system had been exhausted, and the internal walls reportedly were laced with a smelly green mold. It served one final function, however, by receiving the prototype of an automated, unmanned form of the Soyuz spacecraft, called Progress, that was being developed to resupply future stations.

Salyut 5, occupied by two crews in 1976 and 1977, was another reconnaissance platform. Salyut 6, launched in September 1977, introduced the second generation of Soviet space stations. It had a docking system at each end, which permitted Progress resupply ferries to link with an occupied station—i.e., while the crew's Soyuz was docked at the opposite end. The primary objective of this phase of the program was to extend crew endurance gradually in order to demonstrate that humans could survive in weightlessness for the length of time required for a round-trip journey to Mars, which was considered to be as long as 18 months.

A major problem facing the endurance program was that the contemporary variant of the now-venerable Soyuz was limited to a two-month stay in space; beyond this time, its propellant system would deteriorate. This meant that, for a Salyut crew to remain aboard longer, its ferry would have to be replaced periodically. As a solution, endurance crews not only received Progress ferries but also hosted a succession of visitors who would arrive in a fresh Soyuz, stay for a week or so, and then leave in the older Soyuz so that a usable crew-return craft was always available. By supporting crews for 684 days over a four-year period (with five long-duration crews unloading a dozen resupply craft and hosting 11 visiting crews), Salyut 6 greatly advanced the state of the art in space station operations. One indicator of the rapid maturation of the Soviet program came when the two resident cosmonauts made an impromptu space walk in August 1979 to cut away the frame of a radio telescope that had become fouled at the rear of the station.

Unfortunately, whereas Salyut 6 had had a run of exceptionally good luck, Salyut 7's very ambitious program was frustrated by a series of operational problems. Nevertheless, crews successfully undertook repairs of malfunctions so serious that, until they actually had been faced and overcome, they would have been considered cause to vacate the station. After a pipe of the space station's engine developed a propellant leak, specialized tools were fabricated and sent up, and members of the station's third main crew spacewalked in May 1984 to install a bypass. In February 1985, while temporarily vacant, Salyut 7 suffered a total power failure and started to drift. Four months later, after making a rendezvous without the assistance of the station's radar docking system, the fourth main crew managed to link their Soyuz T-13 with the inert station and restore it to life. Various setbacks had caused the first three crews to leave the station unattended, and the objective of an in-space handover was not achieved until the Soyuz T-14 crew arrived in September 1985 with the Soyuz T-13 cosmonauts still in residence. Salyut 7's bad luck continued to the end; when one of the Soyuz T-14 cosmonauts fell ill, the crew was recalled to Earth.

Mir's base block, launched in February 1986, was placed into the same orbital plane as Salyut 7. This allowed Mir's commissioning crew to shuttle between the two stations in their Soyuz T in order to wrap up Salyut 7's program and salvage usable apparatus from the older station. Like Salyut 7, Mir was intended to be occupied on a continuous basis, but delays in building the first of the add-on modules led to the station's being vacated for a time. When the second crew set off for Mir in February 1987, it was in a new form of Soyuz, called Soyuz TM. In April the cosmonauts received the first of the expansion modules, Kvant 1.

In 1969, NASA proposed to President Nixon that a reusable space shuttle be developed to serve as the basis for post-Apollo activities in Earth orbit. At that time, still pursuing Braun's vision of a space plane and complementary orbiting base, the agency had hoped to use the shuttle to build a space station, but the high cost of funding the first project precluded starting the second. Once the shuttle was in service, however, the agency again began lobbying for a station. In 1984, President Ronald Reagan (Reagan, Ronald W.) ordered that a space station be developed within a decade and that it involve international participation. Unfortunately, because of legislative disputes over the station's design and cost and various setbacks to NASA's overall space program, no actual construction was begun during the assigned period.

In 1993, as the prospect of cancellation loomed, President Bill Clinton (Clinton, Bill) ordered NASA to widen the multinational partnership by including Russia, with which relations had dramatically improved following the end of the Cold War. In an extension of that spirit of cooperation, the United States and Russia agreed to merge their respective space station developments into a program for a single orbital structure. The first phase of the integrated program called for a series of shuttle missions to Mir to allow NASA to gain practical experience operating in conjunction with a station and to make an early start on microgravity research. With this renewal of effort, Russia launched its last two modules, named Spektr and Priroda, to Mir. In 1996, a decade after its assembly began, Mir was finally completed.

NASA's interest in Mir was strictly as a stepping-stone, and it intended soon after the final shuttle-Mir mission in early 1998 to put into orbit the first element of its multinational project, which had come to be called the International Space Station (ISS). Launched by Russia atop a Proton rocket in late 1998, the initial module, called Zarya, was designed to provide attitude control and solar power arrays for the nascent station. Shortly afterward, space shuttle astronauts ferried up and attached the first U.S.-built element, named Unity, a connecting node with multiple docking systems.

The overall plan for the ISS called for assembly of a complex of habitable modules crossed by a long truss that supported four large solar power arrays. Among major elements scheduled for later installation were specialized laboratories supplied by the European Space Agency and the National Space Development Agency of Japan. Much of the early scientific work aboard the station focused on life sciences and materials sciences research in the weightless environment. Although ungainly compared with the elegant space wheels envisioned at the start of the space age, the ISS was expected to serve as the basis for human operations in Earth orbit for at least the first quarter of the 21st century.