The world's first space stations
The Salyut series was a series of seven Soviet space stations, deployed 1971-1982, which was eventually replaced by the larger and more advanced Mir in 1986. That was before spit, duct tape and good will became the primary bonding elements of Mir. The Salyuts were humanity's (and the Soviet Union's) first space stations, and each of the stations weighed approximately 18 tons, had an interior compartment of approximately 100 cubic meters, and were powered by solar panels. If there ever was a jinxed project in Soviet space engineering, the Salyut series was it. Both the Salyuts themselves and the Soyuz spaceships used to ferry people and cargo between the station were plagued by frequent accidents, usually completely unpredictable ones. One Salyut, launched in 1973, was "posthumously" named Cosmos 557, in an attempt by the Soviet leadership to avoid losing prestige over the ambitious Salyut project. All were launched from Baikonur cosmodrome.
The Salyuts were mostly scientific stations, although two of them apparently had military application, functioning as space-based observation posts. They pioneered work in space stations, and indeed much knowledge acquired in the Salyut project was used by the Soviet Union itself in its later Mir project, the Americans in their (equally luck-challenged) Skylabs, and by the international community with the International Space Station (ISS).
Salyut 1 (April 9th, 1971)
The stated purpose of Salyut 1 was to test the components of a space station, to perform research into whether space stations were actually viable. It had three pressurized compartments, two of which could be entered by the station's crew. The most pioneering feature of Salyut 1 was its docking system, the first such system ever built, making it capable of connecting with a Soyuz-class spaceship to transfer crew or cargo. Salyut 1 contained, among other things, seven work consoles for the crew, a large supply of oxygen and water, 20 portholes giving the crew a view of space (a few of which they could actually use; most of them were obstructed by equipment) and some of the most advanced communication equipment of its day. The first attempt to actually send humans into the thing was when Soyuz 10 docked with it on April 23th 1971, although this met with failure due to problems with the entry hatch. After having been docked with the station for five hours, Soyuz 10 returned to Baikonur. On June 7, Soyuz 11 docked with Salyut 1 and its crew successfully entered the station. Their stated mission was to check and test the onboard equipment, and study the effects of such long stays in space. On June 29th, the Soyuz 11 crew transferred back to their ship and returned to Earth -- unfortunately, their ship lost pressure during re-entry, killing the entire crew. On October 11, after 175 days in space, Salyut 1 was guided back into the atmosphere to burn up in re-entry, which it did over the Pacific Ocean. The Salyut 1 weighed 18425 kilograms, making it a big fucker for its day.
Salyut 2 (April 4th, 1973)
Slightly heavier than its predecessor, at 18500 kilograms, Salyut 2 was mainly meant as a test flight of onboard systems. On April 11th 1973, an unexplainable accident ripped off four of its solar panels and cost the station its power. On the 28th of May, it exploded in orbit, for unknown reasons.
Salyut 3 (June 24th, 1974)
The world's first military space station, Salyut 3 was semi-automatized. It performed various surveillance tasks on its own while it wasn't manned, and transferred data back to Earth by means of a detachable recovery module which it could launch into the Earth atmosphere for pickup in the Soviet Union. It was operated by the cosmonauts of Soyuz 14 for its only period as a manned station; Soyuz 15 was intended to dock with the station in late 1974, although problems with their craft made it impossible for them to dock with the station. The station re-entered the atmosphere and was incinerated on January 24th, 1975. It weighed 18500 kilograms.
Salyut 4 (December 26th, 1975)
Another 18.5 ton Salyut, this was a scientific station. It contained an advanced X-ray instrument named the Filin telescope, and was used for observation of stars. Soyuz 17 cosmonauts spent 23 days on board, and the crew of Soyuz 18 had 63 days of service in the station. Muscle atrophy and various other medical complications posed serious problems for the crew, proving that the human body simply isn't designed for zero gravity, especially not lengthy stays (well, duh). Soyuz 20 was also sent to Salyut 4 -- this was an unmanned craft, which performed the first automated ship-to-station docking. It re-entered the atmosphere and burned on February 2nd 1977.
Salyut 5 (June 22nd, 1976)
Salyut 5 was structurally similar to Salyut 3, but weighed a full 19000 kilograms. It contained the same recovery module used in that station, although in Salyut 5's case it was used for transferring scientific research data rather than top secret military data. It was inhabited by the Soyuz 21 cosmonauts (staying there from July 7th to August 24th), and later the Soyuz 24 crew who worked there from February 8th to 25th. The Soyuz 23 had been meant to dock with it in October 1976, but was unable to enter the station. It was sent into decayed orbit and burned upon re-entry on August 8, 1977.
Salyut 6 (September 29th, 1977)
Salyut 6 was the first of the second generation in Soviet space stations, meant to stay in space for a long duration. It weighed 18900 kilograms. It had dual docking mechanisms which were a large factor of its success. While much smaller than the American Skylab-class stations, it was the most technically advanced space station of its time. The idea behind the dual docking systems were to allow one long-duration crew to be present at the station at all times, while other Soyuz craft could fly in short-duration visitors (typically cosmonaut scientists and researchers), or accomodate the newly developed automated Progress-class freighters to refuel and resupply the craft. During its time in space, it received a total of 16 Soyuz crews of which six were long-duration crews. Notable Salyut 6 accomplishments include beating the "space endurance" world record for longest stay in space (set by the 84-day stay of the last American Skylab station) -- its first crew stayed for 96 days. Just to drive the point home, a later crew served at the station for 185 days (the poor fools had apparently agreed to this, returning to Earth in such a condition cannot be a very pleasant experience). The first ever space traveler not a citizen of the United States or the Soviet Union stayed at the Salyut 6; that was Vladimir Remek of Czechoslovakia. Indeed, Salyut 6 hosted cosmonauts from Hungary, Poland, Romania, Vietnam, Mongolia, East Germany and Cuba during its time in space. Finally, a large transport logistics spacecraft (Cosmos 1267) was automatically docked with Salyut 6 in 1982, proving that docking of large modules to space stations was viable, and thus paving the way for the modular architectures of both Mir and the International Space Station. It was decommissioned in 1982.
Salyut 7 (April 19th, 1982)
Largely similar to Salyut 6, this was the second and last Soviet second-generation space station (the third generation was Mir, of which only one was built). It was built to further test the viability of automatically docking very large modules, and one of its docking ports was modified specifically to accept a heavy Cosmos module (in its case, the Cosmos 1443 and Cosmos 1686). Most of the internal re-designs (except for replacing the telescope with an X-ray detector) was about comfort and making very long stays safe. Its portholes were modified to allow in ultraviolet light, in order to kill infections of the crew. It also had its command seats replaced with ones that looked more like bicycle seats, to make for more open space in the crowded station. Further, it featured improved medical, biological and exercise sections as well as a refrigerator and stove to allow the crew to eat food that was not some revolting paste. It had a total of six long-duration crews, one of them staying there a record-breaking 211 days. The last Soyuz to visit it was Soyuz T15, which had launched from Mir, to which it returned after a few days. Salyut 7 was decommissioned in 1986, at which point it had been superseded by Mir.
Superseded by Mir. Bizarre words to utter in 2003.