Skylab was America's first experimental space station. Designed as a precursor to what many hoped would be a larger, more extensive space station, Skylab set out to prove that humans could live and work in space for extended periods of time and also to expand knowledge of solar astronomy well beyond that available from Earth-based observations. Successful in all respects despite early mechanical difficulties, three crews of three persons each occupied the Skylab workshop for a total of 171 days, 13 hours. The orbiting laboratory was the site of nearly 300 scientific and technical experiments: medical experiments on humans' adaptability to zero gravity, solar observations, and detailed Earth resources experiments.

The National Aeronautics and Space Administration (NASA) had studied concepts for space stations, including an inflatable donut-shaped station, since the earliest days of the space program. However, it was not until the Saturn rocket was developed in the mid-1960s that the Skylab program was born. Initially called the Apollo Applications Program, Skylab planned to use leftover Apollo lunar hardware on its missions.

At first there were two competing concepts. The first was called a “wet” workshop, in which the smaller Saturn 1B rocket would be launched, fuelled, and its S-IVB upper stage vented and refurbished in orbit. The second was the “dry” workshop, whereby an empty S-IVB stage was outfitted on the ground before launch, then launched on the massive Saturn V rocket. In July 1969, NASA selected the “dry” workshop concept. The crews visiting the orbiting S-IVB, given the name Skylab, would travel to and from the orbiting laboratory in modified Apollo command and service modules launched by Saturn 1B rockets.

This 100-ton (91-metric ton) structure was 118 feet (36 meters) high, 22 feet (6.7 meters) in diameter, and flew at an altitude of 270 miles (435 kilometres). It had a habitable volume of about 10,000 cubic feet (283 cubic meters) and was divided into two levels separated by a metal floor—actually an open grid into which the astronauts' cleated shoes could be locked. The upper floor had storage lockers and a large empty space for conducting experiments, plus two scientific airlocks, one pointing toward the Earth and the other toward the Sun. The lower floor had compartmented “rooms” with a dining room table, three bedrooms, a work area, a shower, and a bathroom.

 
Astronaut Charles Conrad Jr., Skylab 2 commander, poses after a hot bath in the shower facility in the crew quarters of the Orbital Workshop.

The largest piece of scientific equipment, attached to one end of the cylindrical workshop, was the Apollo Telescope Mount (ATM), used to study the Sun in different wavelengths with no atmospheric interference. The ATM served as the platform for eight experiments: two X-ray telescopes; an X-ray and extreme ultraviolet camera; an ultraviolet spectroheliometer; an extreme ultraviolet spectroheliograph and an ultraviolet spectroheliograph; a white light coronagraph; and two hydrogen-alpha telescopes. The unit had its own electricity-generating solar panels.

Skylab also had an airlock module for spacewalks (required for repairs, experiment deployments, and routine changing of film in the ATM). The Apollo command/service module remained attached to the station's docking adapter while the astronauts were on board.

The Skylab space station was launched May 14, 1973, from the NASA Kennedy Space Centre by a huge Saturn V launch vehicle. It was planned that a crew would be launched from Earth the next day to inhabit the space station. Sixty-three seconds after lift-off, however, a critical meteoroid shield ripped off, taking one of the craft's two solar panels with it and preventing the other from deploying properly. Ground command manoeuvred Skylab so its solar panels faced the Sun to provide as much electricity as possible. But because the meteoroid shield was gone (which also operated as a sun shield), temperatures inside the workshop rose to 126˚F (52˚C). The launch of Skylab 2 (the first crew to inhabit the space station) was postponed for 10 days while scientists, engineers, astronauts, and management personnel at NASA's Marshall Space Flight Centre in Huntsville, Alabama, and elsewhere developed procedures and trained the crew to make the workshop habitable. At the same time, engineers "rolled" Skylab to lower the temperature of the workshop.

 
Parking places for two Apollo spacecraft were provided by the docking module of Skylab. The circular port on the right end was used as the primary one. The port on the bottom would have been used had a rescue become necessary.

On May 25, 1973, the crew of Skylab 2, Charles Conrad, Jr., Paul Weitz, and Joseph Kerwin, finally began their 28 days aboard Skylab. Their first task was to make substantial repairs. These included positioning of a parasol sunshade that cooled the inside temperatures to a more comfortable 75˚F (23.8˚C). By June 4, 10 days after launch, the workshop was fully operational, and the crew began to conduct solar astronomy and Earth resources experiments, medical studies, and student experiments. In the period up to June 22, when the crew left for home, they circled the Earth 404 times, completed 392 experiment hours, and carried out three “spacewalks” (extravehicular activities—EVAs) totalling six hours, 20 minutes.

The Spacelab 3 crew, consisting of Alan Bean, Jack Lousma, and Owen Garriott, arrived on July 28 for a 59-day stay. After an early bout of motion sickness, they continued the work of the previous crew. Garriott and Lousma deployed a second sun shield during a spacewalk that lasted six-and-a-half hours. This crew completed 858 Earth orbits and 1,081 hours of solar and Earth experiments. Their three EVAs totalled 13 hours, 43 minutes.

 
Astronaut Jack R. Lousma, Skylab 3 pilot, is seen outside the Skylab space station in Earth orbit during the August 5, 1973, Skylab 3 extravehicular activity (EVA) in this photographic reproduction taken from a television transmission made by a colour TV camera aboard the space station.

At 84 days and 1 hour, Skylab 4 remains the longest U.S. spaceflight to date. Its crew members—Gerald Carr, William Pogue, and Edward Gibson-used a treadmill in addition to the on-board bicycle-like ergometer to help stay in shape. The last of the Skylab missions, its experiments  included observations of the Comet Kohoutek. The crew completed 1,214 Earth orbits and four EVAs totalling 22 hours, 13 minutes.

After all crew activities had been completed and the crews had returned to Earth, Skylab was positioned into a stable attitude and systems were shut down. It was expected that Skylab would remain in orbit for eight to ten years. However, in the fall of 1977, Skylab was no longer flying in a stable attitude as a result of greater than predicted solar activity. On July 11, 1979, the empty Skylab spacecraft returned to Earth, scattering debris from the south-eastern Indian Ocean across a sparsely settled region of western Australia. NASA and the U.S. space program were criticized for allowing this to happen-ranging from the sale of hardhats as "Skylab Survival Kits" to serious questions about the propriety of space flight altogether if people were likely to be killed by falling debris. It was an inauspicious ending to the first American space station, not one that its originators had envisioned. Nevertheless, the experiment had whetted the appetite of NASA leaders for a permanent presence in space.