On May 14, 1973, the final Saturn V rocket lifted off to deploy Skylab, America’s first space station. Despite a troubled start to its life, Skylab pioneered long-duration spaceflight, and fifty years later lessons learned in this program are still contributing to the success of the International Space Station.
With Apollo 11’s successful landing on the Moon in July 1969, NASA had met President Kennedy’s challenge of landing a man on the moon and returning him safely to the Earth by the end of the 1960s. While six more crewed lunar missions would follow over the next three and a half years, the objective had been achieved and funding was cut. With missions past Apollo 17 canceled, NASA’s attention returned to low Earth orbit, and the development of a space station using Apollo hardware. This would allow the agency to continue scientific research and study the effects of spaceflight on crews over the course of long-duration missions.
The launch of Skylab came two years after the Soviet Union had deployed the world’s first space station, Salyut 1. This station only hosted a single crew, which arrived aboard the ill-fated Soyuz 11 mission in June 1971, the earlier Soyuz 10 mission having failed to dock. After three weeks aboard Salyut 1 the Soyuz 11 crew returned to Earth, but were killed when their capsule depressurized during re-entry.
Although the Soviet Union would launch three more space stations in 1972 and early 1973, the first of these failed to reach orbit and the next two malfunctioned shortly after deployment, with no Soyuz missions being launched to any of them. Skylab was, therefore, the second space station to be crewed in orbit, but as of 2023 it remains the only crewed station to have been operated exclusively by the United States.
Design and Launch
With a mass of 76,500 kilograms (168,800 lb) and a length of 25 meters (82 feet), Skylab was considerably larger than Salyut and still remains the largest monolithic space station (i.e. deployed as a single unit) ever launched and one of the largest spacecraft ever to operate in Earth orbit. Only modular space stations assembled in orbit (specifically Mir and the International Space Station), the Space Shuttle orbiters, and Buran have surpassed it in mass. The Soviet Polyus experimental military platform also had a greater mass than Skylab, however it failed to reach orbit.
Skylab’s large size was a result of its use of surplus hardware from the Apollo program. Its Orbital Workshop (OWS), the main section of Skylab, was converted from an S-IVB stage that had been intended for a Saturn IB rocket. A Saturn V rocket left over from the cancellation of the last three planned lunar Apollo missions would be used to place it into orbit.
Alongside the lunar program, NASA had planned the Apollo Applications Programme (AAP) to take advantage of hardware and technologies developed during Apollo for a range of other missions. The Orbital Workshop had originated from this project, with two concepts being considered: the “Dry Workshop” – which would see the stage converted before launch – and the “Wet Workshop” which would convert a live S-IVB stage from a Saturn IB launch on-orbit.
As a dedicated station, the Dry Workshop – which would later evolve into Skylab – would provide more space for astronauts, be able to carry more equipment, and not require as much of their time to convert – allowing for more time on-orbit to conduct experiments. Since its main propulsion system and propellant tanks were removed, the station would not be able to contribute to its own ascent to orbit, so a modified Saturn V would be needed to deploy it.
McDonnell Douglas, the manufacturer of the S-IVB, would be responsible for converting the stage into Skylab’s Orbital Workshop, under a contract signed in August 1969.
As well as the Orbital Workshop, Skylab incorporated a Multiple Docking Adapter (MDA) with two ports for Apollo spacecraft to dock, an airlock, and the Apollo Telescope Mount (ATM) observatory which carried instruments to study the Sun. Although the station had two docking ports, the second was intended as a backup, rather than a means of having two spacecraft docked simultaneously – although this could have been supported if necessary. Power was to be generated by two main solar panels attached to the OWS, with four additional panels mounted at right angles on the ATM.
Skylab’s launch would mark the thirteenth and final flight of the legendary Saturn V rocket, which had powered previous Apollo missions on their journeys to the Moon. For the Skylab mission, the rocket flew in a two-stage configuration, with the Orbital Workshop taking the place of the S-IVB third stage. The rocket’s instrument unit (IU) was incorporated at the forward end of the OWS, with a four-sector payload fairing encapsulating the airlock, MDA and ATM.
For launch, the ATM stowed forward of the main docking port. Once in orbit, it was relocated to its operational position perpendicular to the rest of the station. The Skylab launch was the only time a Saturn V would fly with a payload fairing. Like earlier Saturn V missions, the rocket was assembled in the Vehicle Assembly Building at the Kennedy Space Center, with build-up commencing on Aug. 2, 1972. The rocket was stacked atop Mobile Launcher 2, which was then moved to Launch Complex 39A (LC-39A) by Crawler Transporter.
Saturn V SA-513, the rocket tasked with deploying Skylab, would have an eventful climb toward orbit. Liftoff occurred at 1:30:00 p.m. EDT (17:30:00 UTC) on May 14, 1973, with the five F-1 engines of the S-IC first stage powering Saturn aloft. After clearing the tower the rocket began a pitch-and-roll maneuver to establish itself on its planned trajectory, building up speed as it climbed and reaching Mach 1, the speed of sound, just over a minute after liftoff.
About 63 seconds into the flight, the Orbital Workshop’s micrometeoroid shield – which was also designed to act as a critical part of the station’s thermal control system – suffered a structural failure and detached. Debris from this damaged tie-down fittings holding the No.2 solar array in its launch position, jammed the No.1 solar array and also damaged the separation systems on the interstage between the first and second stages of the rocket. These events occurred about ten seconds before the rocket passed through Max-Q, or the area of maximum dynamic pressure.
The launch continued, with the S-IC burning all five engines until T+140 seconds, when its center F-1 was shut down as planned to reduce acceleration. The remaining engines shut down in pairs, about 18 seconds later, before the first and second stages separated. Retro-rockets pushed the S-IC away from the S-II second stage, whose five J-2 engines ignited to take over the job of boosting Skylab towards orbit.
A little under 20 seconds after S-II ignition, the interstage should have been jettisoned from the aft end of the stage. This did not occur as scheduled, with a post-flight analysis determining that one of the shaped charges responsible for separating the interstage had most likely been damaged by debris from the micrometeoroid shield. Instead, the interstage remained attached all the way to orbit, with higher temperatures being recorded around the base of the stage as a result. The rocket had sufficient reserve propellant to make up for any underperformance as a result of its additional mass.
Five minutes and 14 seconds into the flight, the S-II’s center engine was commanded to shut down with the four outboard engines continuing to fire until the nine-minute, 48-second mark. About two seconds after shutdown, Skylab was released from the second stage, which then fired retro motors to maneuver itself away and reduce the risk of recontact between itself and the space station. The No.2 solar array, whose tie-downs had been severed by the initial debris incident earlier in the launch, was caught in the exhaust of the retro rockets and ripped away from the station.
Despite this incident, Skylab’s deployment sequence continued as scheduled, with the payload fairing separating at 15 minutes, 20 seconds mission elapsed time. The station then re-oriented itself to the planned sun-angle for solar array deployment, and shortly afterward began to deploy the ATM to its post-launch position.
With this in place, the ATM’s solar panels were deployed successfully, however the one remaining solar panel on the OWS failed to open. This left the station with insufficient power to support long-duration missions beyond the lifespan of the fuel cells on visiting Apollo spacecraft, while the loss of thermal protection from the lost micrometeoroid shield meant that temperatures aboard the station were uninhabitable.
The launch of the first crew to Skylab was delayed while NASA evaluated data from the launch to devise a rescue plan. On May 16, the National Reconnaissance Office (NRO) launched a KH-8 Gambit spy satellite, OPS-2093, from Vandenberg Air Force Base, tasked with capturing images of the crippled Skylab to help assess the damage. One of the satellite’s two film-return capsules was used to send these back to Earth on May 21, by which point plans for a repair mission were in an advanced stage of planning, with launch four days away.
Three long-duration crews, each consisting of three astronauts, visited Skylab aboard Apollo Command and Service Modules (CSMs). All three missions were launched in 1973, with each setting a new record for the longest-duration human spaceflight up to that time. The crewed missions launched atop Saturn IB rockets, flying from the modified Mobile Launcher 1 (later MLP-3) at LC-39B using a “milkstool” adaptor which enabled the smaller Saturn IB to make use of the Launch Umbilical Tower (LUT) that had been built for the Saturn V.
Skylab’s first crew was commanded by Gemini and Apollo 12 veteran Pete Conrad, who was joined by rookies Joseph P. Kerwin and Paul J. Weitz. Designated Skylab 2, their mission lifted off on May 25, 1973, having been delayed from May 15 due to the damage Skylab had sustained during launch. While the astronauts had trained for a mission of scientific research, they found themselves responsible first for putting NASA’s rescue plan into effect to save the space station that was to become their home for the next few weeks.
After rendezvous with Skylab, the crew performed a flyaround of the outpost to inspect the damage. After the CSM was maneuvered close to the stuck solar array, Weitz performed a stand-up spacewalk from the Command Module hatch in an attempt to free the panel using tools mounted on a three-meter (10-foot) long pole, but was unsuccessful as a strip of metal blocking its deployment was wrapped around a beam and could not be freed with the available tools.
The crew proceeded to dock with the station, however their spacecraft’s docking system failed to engage. After some troubleshooting, including depressurizing the Command Module again and removing the back plate of the docking probe, a successful docking was achieved.
When the crew boarded Skylab the following day, one of their first tasks was to deploy a replacement sun shield which had been rapidly improvised on the ground. This consisted of a “parasol” device in a canister which could be deployed from a small science airlock on the Orbital Workshop. Once this was completed, temperatures aboard the station began to stabilize and the crew were able to begin their planned science mission.
Owing in part to concerns around the reliability of Skylab’s batteries, which could have jeopardized future missions to the station, another attempt to free the jammed solar panel was made on June 7. Conrad and Kerwin performed a three-hour, 25-minute EVA to cut away the debris and attach a tether that was then used to pull the solar array free. With this accomplished successfully, concerns about Skylab’s ability to generate electrical power became less acute.
A further EVA was conducted on June 19 by Conrad and Weitz, the first of several to change out film cassettes in the Apollo Telescope Mount. The crew returned to Earth on June 22, completing their 28-day mission.
The second crewed mission – Skylab 3 – was commanded by another former member of the Apollo 12 crew in Alan Bean. Bean was joined by rookies Owen Garriott and Jack Lousma for the mission, which lifted off on July 28, 1973, and splashed down 59 and a half days later on Sept. 25. The final mission was launched on Nov. 16, with an all-rookie crew: Gerald P. Carr commanding with Edward G. Gibson and William R. Pogue.
Skylab 4’s stay aboard the station would be the longest, at over 83 days. The crew returned to Earth on Feb. 8, 1974, after which no more astronauts would set foot aboard the Skylab station.
Although its three expeditions are short by modern standards – the astronauts of Dragon Freedom‘s Crew-4 mission last year spent roughly the same time aboard the International Space Station as Skylab was crewed across all three missions – these were the longest human spaceflights in history for their time. As such, medical research was a significant focus for studies to ensure crews could remain fit and healthy during extended stays in space.
A bicycle ergometer was provided for crew exercise, with the metabolic responses of the astronauts being monitored to track their physiological condition. Other experiments included a rotating chair that was used to test the astronauts’ sensitivity to motion sickness and pre-and-post-flight x-rays to look for mineral loss – combined with the examination of urine samples that showed the astronauts were excreting greater amounts of calcium and hydroxyproline than expected.
The only research that took up more time than life science research was solar physics. The Apollo Telescope Mount contained instruments to study the Sun in detail, with the crew able to fine-tune the observations. A key objective achieved by the Skylab 2 mission was to record a solar flare in progress. Paul Weitz accomplished this on June 15, 1973, tracking a flare for two minutes as it rose and fell.
Throughout the lifetime of the station, crews made a number of spacewalks to collect film cassettes from the telescope mount and to install replacements. A total of 127,000 frames of film were captured by the Skylab’s telescopes.
Other research carried out aboard the station included Earth observation, using photography and multispectral imaging, and microwave measurement of soil moisture. During Skylab 4, the crew was tasked with observing designated areas and briefed on what to look for and photograph, in an attempt to assess the utility of using crew to target observations.
Crew time was also given over to technology development research – including tests of an astronaut maneuvering device aboard the station – materials science and manufacturing, astrophysics, and student experiments.
While crewed Skylab operations ended with the departure of the Skylab 4 crew, NASA had hoped to reactivate the station once the Space Shuttle became operational. Ultimately this was prevented by a combination of increased atmospheric drag causing Skylab to decay from orbit more quickly than expected, and delays to the Shuttle program.
Under early plans, the third Space Shuttle mission would have been flown by astronauts Jack Lousma and Fred Haise, who would have deployed a module called the Teleoperator Retrieval System (TRS). TRS would have then been docked with Skylab via remote control from the Shuttle. Once docked, its thrusters would have been used to raise the space station into a higher orbit — or to deorbit it to a controlled re-entry if deemed necessary. TRS was later moved to the second Shuttle mission as the program began to fall behind schedule, but would be abandoned when it became clear that Skylab was going to re-enter before the Shuttle would be ready to fly.
Skylab re-entered the atmosphere over the Indian Ocean on July 11, 1979, with some debris reaching the coast of Australia.
When the Space Shuttle finally flew in 1981, NASA gained a spacecraft capable of carrying out significant amounts of on-orbit research independently – particularly when equipped with a Spacelab or later Spacehab, module in its payload bay. As such, the agency pivoted away from space stations and instead focussed on the Shuttle. A new station called Freedom was proposed in the mid-1980s, using the Space Shuttle to assemble multiple modules in orbit. While this was never built as designed, its design would later form the basis for the International Space Station which began on-orbit construction in 1998.
The end of the Space Shuttle program has seen NASA’s primary focus shift back towards missions of exploration, with the Artemis program aiming to return humans to the Moon, while crewed missions in low Earth orbit are being outsourced to commercial operators. The advent of commercial human spaceflight has seen a renewed interest in space station development, with several companies investigating the development of new orbital outposts. These include Orbital Reef – a partnership between Blue Origin and Sierra Space – and Starlab, which is being built by a consortium including Lockheed Martin, NanoRacks, and Voyager Space.
Axiom Space has already flown one commercial mission to the International Space Station, with another planned for later this month. These serve as precursors to the company deploying its own space station modules – initially as an annex of the ISS, but these could lead to a free-flying station in the future.
Another startup, Vast, has announced plans in the last week to launch a small space station called Haven-1 in 2025 aboard SpaceX’s Falcon 9 rocket. This is planned to host a four-person crew for thirty days, launching aboard a Crew Dragon spacecraft.
Announcing the Haven-1 and Vast-1 missions to low-Earth orbit. Launched by @SpaceX, Haven-1 is scheduled to be the world’s first commercial space station and will be visited by a crew of four aboard a Dragon spacecraft during Vast-1 → https://t.co/ToxFSiyQJj pic.twitter.com/YSPrM9Krtr
— VΛST (@vast) May 10, 2023
The impact of SpaceX’s own Starship on the future of space stations is yet to be seen. With the ability to carry greater numbers of humans or larger amounts of cargo into orbit, it could vastly reduce the cost of transporting crew and equipment to outposts in low Earth orbit. Alternatively, it might be adapted as a research platform in its own right, much as the Shuttle was with the addition of research modules, taking the place of space stations.
No matter what the future holds, Skylab was one of the first steps on a journey towards understanding human reactions to long-duration spaceflight which has enabled space stations to flourish over the subsequent decades. Building on its legacy, and that of the Soviet Salyut and Mir stations, the ISS continues to deliver invaluable science. The experience with long-duration missions that has been honed aboard these outposts will also be key to future spaceflight beyond Earth’s orbit.
(Lead image: Skylab in orbit, seen from the departing Skylab 4 mission. Credit: NASA)