Skylab, a versatile scientific and engineering laboratory (fig. 1-1), was launched into Earth orbit on May 14, 1973, on a Saturn V rocket. Three crews of three men each subsequently visited the space station during extended missions for a total of 171 days of manned occupancy. A Saturn IB rocket launched each three-man crew in an Apollo spacecraft that was also the reentry vehicle. The manned missions lasted 28, 59, and 84 days (fig. 1-2). During the flight out of the Earth's atmosphere, Skylab's micrometeoroid and heat shield was unexpectedly torn off. A specially built parasol to shield the space station was therefore sent up with the first crew, whose launch was consequently delayed until May 25, 1973. The second crew was launched on July 28, and the third on November 16. The last mission was originally planned for 56 days, but because the...
....overall performance had been very successful, the final mission was extended by 28 days, the entire length of the first mission. The third crew splashed down on February 8, 1974.
Circling 50° north and south of the Equator at an altitude of 435 km, Skylab orbited the Earth every 93 min and completed some 3900 orbits from launch to the end of the third manned mission. The zone containing the spacecraft's track includes 75 percent of the Earth's surface, 80 percent of its developed land, and 90 percent of its population.
An extensive tracking network kept radio and television contact between ground controllers and the Skylab crews. During some orbits (fig. 1-3) contact was maintained almost continuously (track A); during others, there were periods of up to 90 min without contact with the crew (track B).
Radiant energy at some wavelengths passes through the Earth's atmosphere and reaches the ground. At other wavelengths, the atmosphere is opaque, and the only way to make astronomical observations at these wavelengths is to lift the instrument above the Earth's atmosphere. Skylab made observations in two such wavelength bands, the ultraviolet and the X-ray.
The astronomy experiments included measurements of the ultraviolet brightness and spectral lines of stars and galaxies. Galactic X-ray sources were observed with an instrument attached to the third stage of the Saturn V rocket, which briefly orbited behind and below Skylab. Measurements were also made at visible wavelengths of low-light phenomena such as the zodiacal light (light scattered from dust in the solar system). These measurements are difficult to make from the ground  because of interference from airglow in the atmosphere, even on clear nights when viewing conditions are optimal.
Above the Earth's atmosphere, it is possible to see regions near the Sun without the interference of scattered light. Only during an eclipse of the Sun can these regions be seen from the Earth's surface. By artificially eclipsing the Sun, Skylab cameras photographed Comet Kohoutek near the Sun when it could not be detected by Earth-based cameras. The astronauts were also able to observe and sketch the comet at this time. In addition, Skylab provided an excellent vantage point for studying the upper atmospheric airglow, the aurorae, and the ozone layers.
The atmosphere shields the Earth from cosmic ray particles, which are high-energy atomic nuclei traveling through space at speeds near that of light. When they collide with the atmosphere, they cause showers of secondary particles. These showers can be studied by instruments in balloon flights and sounding rockets, but the analysis of significant numbers of the primary cosmic ray particles, especially the rarer ones, requires long-duration observations above the atmosphere. Skylab experiments were able to record tracks of these cosmic rays.
Near the northern and southern extremes of its orbit, Skylab passed through portions of the Van Allen belts and through a portion of the South Atlantic anomaly, off the coast of South America. The latter is a region of the radiation belts that dips toward the surface because Earth's magnetic axis is displaced from the planet's center. The charged particles trapped in the belts were measured from Skylab at high latitudes as well as in the South Atlantic anomaly.
Other studies made by Skylab included observations of neutrons, micrometeoroids, and materials emanating from the space station itself. The neutrons inside the Skylab vehicle came mainly from the interaction of Van Allen belt protons with Skylab's structural materials. Special detection devices were used to measure neutrons, to gather micrometeoroid particles, and to measure contamination from the space station itself.
Several Skylab experiments viewed the sky in the all-important ultraviolet wavelengths. Since this region of the electromagnetic spectrum is absorbed by optical glasses, the windows and lenses for these experiments were fabricated of quartz and certain metal fluorides, which transmit ultraviolet light.
Two small airlocks were installed through Skylab's workshop wall, one on the side facing the Sun and one pointed in the opposite direction. They provided direct access to space and eliminated the need to depressurize the workshop. Each instrument using an airlock was first sealed to the inner port of the airlock, which was then evacuated and the outer port opened to space. When instruments were not occupying an airlock, a quartz window for specialized photography or a metal plate was installed.
In the weightlessness of free flight, man can handle heavy pieces of equipment with ease. Owen K. Garriott of the second crew had no trouble maneuvering the large Earth-terrain camera with three fingers, as shown in figure 1-4. The small airlock that was used for the Earth-terrain camera and other instruments is on the wall at the left of Garriott's right hand.
Figure 1-5 shows the exterior of the Sun-facing airlock; the photograph was taken by the first Skylab crew...
 ....before docking. The blistered appearance of the workshop was due to the loss of the micrometeoroid and heat shield, which caused bonding material to be irradiated by the Sun for several days. The parasol flown up with the first crew was deployed through this airlock. The workshop cooled down, but the airlock was blocked from further use in scientific experiments. Some instruments that would have used it were redesigned for spacewalks during later visits; others were used in the remaining (antisolar) airlock.
The antisolar airlock is shown in figure 1-6 from the inside of the workshop with a window installed and the outer door closed. The vacuum hose, wrapped around the airlock, was used for equipment depressurization. Film canisters, for example, were evacuated before they were stored in film vaults. The airlock was used sequentially for several instruments. Scheduling to obtain optimal data and to take advantage of targets of opportunity, such as Comet Kohoutek, was a continuing challenge to the Skylab astronauts and the ground planning teams, especially with the solar airlock occupied by the parasol.
A mirror with ultraviolet-reflecting surfaces was used in the ultraviolet stellar astronomy experiment (fig. 1-7) and with other instruments. It was mounted in the antisolar airlock and extended beyond the spacecraft wall where it could be tilted ±15° and rotated through 360° to scan the sky. Because of this maneuverability, instruments pointed at the mirror from inside Skylab could view a selected target on the celestial sphere without having to move the entire space station.
The flat mirror was elliptical and measured 19 by...
....38 cm. Its reflective surface had an aluminum coating with a thin magnesium fluoride overcoating. In figure 1-7, the ultraviolet stellar astronomy experiment's optical canister and film canister are attached to the mirror mechanism.
The airlock, vacuum hose, maneuverable mirror, and other multiple-use equipment provided a flexibility used repeatedly throughout the Skylab mission. Their adaptability allowed rescheduling experiments displaced by the emergency use of the solar-facing airlock and accommodated unanticipated comet observations.