Skylab was the United States' first space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three trios of astronaut crews: Skylab 2, Skylab 3, and Skylab 4.
The manifestation of the Apollo Applications Program, Skylab was constructed from a repurposed Saturn V third stage (the S-IVB), a dry workshop, and took the place of the stage during launch. Operations included an orbital workshop, a solar observatory, Earth observation, ten spacewalks, and hundreds of experiments. NASA's station concept had competed with the Department of Defense's Manned Orbiting Laboratory concept from 1963 to 1969.
Skylab was the world's second space station, after the Soviet space program's Salyut 1. Its pressurized volume of over 350 m3 was not rivalled until the completion of the first modular space station, Mir, in 1996; the central Orbital Workshop, at 270 m3, dwarfed subsequent station modules, including the largest on the International Space Station (ISS), Kibō, at 150 m3.
A successor station, Skylab B, was concurrently constructed in 1970 and considered for integration with Salyut and Soyuz, but cancelled in 1973. Plans for Skylab to be boosted and reused with the Space Shuttle were abandoned after it became clear that the shuttle would not be ready by 1979. Skylab's orbit eventually decayed and it disintegrated in the atmosphere on July 11, 1979, scattering debris across the Indian Ocean and Western Australia. Unity became the second US-built space station element, joining the ISS in 1998.
Skylab had a mass of 199,750 pounds (90,610 kg) with a 31,000-pound (14,000 kg) Apollo command and service module (CSM) attached and included a workshop, a solar observatory, and several hundred life science and physical science experiments. It was launched uncrewed into low Earth orbit by a Saturn V rocket modified to be similar to the Saturn INT-21, with the S-IVB, with Skylab itself being the repurposed third stage. This was the final flight for the rocket more commonly known for carrying the crewed Apollo Moon landing missions. Three subsequent missions delivered three-astronaut crews in the Apollo CSM launched by the smaller Saturn IB rocket.
Skylab included the Apollo Telescope Mount (a multi-spectral solar observatory), a multiple docking adapter with two docking ports, an airlock module with extravehicular activity (EVA) hatches, and the orbital workshop, the main habitable space inside Skylab. Electrical power came from solar arrays and fuel cells in the docked Apollo CSM. The rear of the station included a large waste tank, propellant tanks for maneuvering jets, and a heat radiator. Astronauts conducted numerous experiments aboard Skylab during its operational life.
Rocket engineer Wernher von Braun, science fiction writer Arthur C. Clarke, and other early advocates of crewed space travel, expected until the 1960s that a space station would be an important early step in space exploration. Von Braun participated in the publishing of a series of influential articles in Collier's magazine from 1952 to 1954, titled "Man Will Conquer Space Soon!". He envisioned a large, circular station 250 feet (75 m) in diameter that would rotate to generate artificial gravity and require a fleet of 7,000 short tons (6,400 metric tons) space shuttles for construction in orbit. The 80 men aboard the station would include astronomers operating a telescope, meteorologists to forecast the weather, and soldiers to conduct surveillance. Von Braun expected that future expeditions to the Moon and Mars would leave from the station.
The development of the transistor, the solar cell, and telemetry led, in the 1950s and early 1960s, to uncrewed satellites that could take photographs of weather patterns or enemy nuclear weapons and send them to Earth. A large station was no longer necessary for such purposes, and the United States Apollo program to send men to the Moon chose a mission mode that would not need in-orbit assembly. A smaller station that a single rocket could launch retained value, however, for scientific purposes.
In 1959, von Braun, head of the Development Operations Division at the Army Ballistic Missile Agency, submitted his final Project Horizon plans to the U.S. Army. The overall goal of Horizon was to place men on the Moon, a mission that would soon be taken over by the rapidly forming NASA. Although concentrating on the Moon missions, von Braun also detailed an orbiting laboratory built out of a Horizon upper stage, an idea used for Skylab. A number of NASA centers studied various space station designs in the early 1960s. Studies generally looked at platforms launched by the Saturn V, followed up by crews launched on Saturn IB using an Apollo command and service module, or a Gemini capsule on a Titan II-C, the latter being much less expensive in the case where cargo was not needed. Proposals ranged from an Apollo-based station with two to three men, or a small "canister" for four men with Gemini capsules resupplying it, to a large, rotating station with 24 men and an operating lifetime of about five years. A proposal to study the use of a Saturn S-IVB as a crewed space laboratory was documented in 1962 by the Douglas Aircraft Company.
The Department of Defense (DoD) and NASA cooperated closely in many areas of space. In September 1963, NASA and the DoD agreed to cooperate in building a space station. The DoD wanted its own crewed facility, however, and in December 1963 it announced Manned Orbiting Laboratory (MOL), a small space station primarily intended for photo reconnaissance using large telescopes directed by a two-person crew. The station was the same diameter as a Titan II upper stage, and would be launched with the crew riding atop in a modified Gemini capsule with a hatch cut into the heat shield on the bottom of the capsule. MOL competed for funding with a NASA station for the next five years and politicians and other officials often suggested that NASA participate in MOL or use the DoD design. The military project led to changes to the NASA plans so that they would resemble MOL less.
For the final two crewed missions to Skylab, NASA assembled a backup Apollo CSM/Saturn IB in case an in-orbit rescue mission was needed, but this vehicle was never flown. The station was damaged during launch when the micrometeoroid shield tore away from the workshop, taking one of the main solar panel arrays with it and jamming the other main array. This deprived Skylab of most of its electrical power and also removed protection from intense solar heating, threatening to make it unusable. The first crew deployed a replacement heat shade and freed the jammed solar panels to save Skylab. This was the first time that a repair of this magnitude was performed in space.
The Apollo Telescope significantly advanced solar science, and observation of the Sun was unprecedented. Astronauts took thousands of photographs of Earth, and the Earth Resources Experiment Package (EREP) viewed Earth with sensors that recorded data in the visible, infrared, and microwave spectral regions. The record for human time spent in orbit was extended beyond the 23 days set by the Soyuz 11 crew aboard Salyut 1 to 84 days by the Skylab 4 crew.
Later plans to reuse Skylab were stymied by delays in the development of the Space Shuttle, and Skylab's decaying orbit could not be stopped. Skylab's atmospheric reentry began on July 11, 1979, amid worldwide media attention. Before re-entry, NASA ground controllers tried to adjust Skylab's orbit to minimize the risk of debris landing in populated areas, targeting the south Indian Ocean, which was partially successful. Debris showered Western Australia, and recovered pieces indicated that the station had disintegrated lower than expected. As the Skylab program drew to a close, NASA's focus had shifted to the development of the Space Shuttle. NASA space station and laboratory projects included Spacelab, Shuttle-Mir, and Space Station Freedom, which was merged into the International Space Station.