Skylab

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.

Component data

Component	Mass[5][6][4]		Habitable volume		Length		Diameter		Image
	lb	kg	ft3	m3	ft	m	ft	m
Payload shroud	25,600	11,600	—		56.0	17.1	21.6	6.6
Apollo Telescope Mount	24,500	11,100	—		14.7	4.5	11.3	3.4
Multiple Docking Adapter	12,000	5,400	1,140	32	17.3	5.3	10.5	3.2
Airlock module	49,000	22,000	613	17.4	17.6	5.4	10.5	3.2
Saturn V instrument unit	4,600	2,100	—		3.0	0.91	21.6	6.6
Orbital Workshop	78,000	35,000[4]	9,550	270[4]	48.1	14.7	21.6	6.6
Total in orbit	168,750	76,540	12,417	351.6	82.4	25.1	21.6	6.6
Apollo CSM	31,000	14,000	210	5.9	36.1	11.0	12.8	3.9
Total with CSM	199,750	90,610[4]	12,627	357.6	118.5	36.1	21.6	6.6

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,[7] 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,[8] 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.[9] 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.

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,[12] an idea used for Skylab.[13] 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,[14] or a Gemini capsule[15] 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.[16] 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.[17]

The Department of Defense (DoD) and NASA cooperated closely in many areas of space.[18] In September 1963, NASA and the DoD agreed to cooperate in building a space station.[19] The DoD wanted its own crewed facility, however,[20] and in December 1963 it announced Manned Orbital 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.[21][22][23] MOL competed for funding with a NASA station for the next five years[24] and politicians and other officials often suggested that NASA participate in MOL or use the DoD design.[20] The military project led to changes to the NASA plans so that they would resemble MOL less.[19]

NASA management was concerned about losing the 400,000 workers involved in Apollo after landing on the Moon in 1969.[25] A reason von Braun, head of NASA's Marshall Space Flight Center during the 1960s, advocated for a smaller station after his large one was not built was that he wished to provide his employees with work beyond developing the Saturn rockets, which would be completed relatively early during Project Apollo.[26] NASA set up the Apollo Logistic Support System Office, originally intended to study various ways to modify the Apollo hardware for scientific missions. The office initially proposed a number of projects for direct scientific study, including an extended-stay lunar mission which required two Saturn V launchers, a "lunar truck" based on the Lunar Module (LM), a large, crewed solar telescope using an LM as its crew quarters, and small space stations using a variety of LM or CSM-based hardware. Although it did not look at the space station specifically, over the next two years the office would become increasingly dedicated to this role. In August 1965, the office was renamed, becoming the Apollo Applications Program (AAP).[27]

As part of their general work, in August 1964 the Manned Spacecraft Center (MSC) presented studies on an expendable lab known as Apollo X, short for Apollo Extension System. Apollo X would have replaced the LM carried on the top of the S-IVB stage with a small space station slightly larger than the CSM's service area, containing supplies and experiments for missions between 15 and 45 days' duration. Using this study as a baseline, a number of different mission profiles were looked at over the next six months.

An early "wet workshop" version of Skylab

In November 1964, von Braun proposed a more ambitious plan to build a much larger station built from the S-II second stage of a Saturn V. His design replaced the S-IVB third stage with an aeroshell, primarily as an adapter for the CSM on top. Inside the shell was a 10 feet (3.0 m) cylindrical equipment section. On reaching orbit, the S-II second stage would be vented to remove any remaining hydrogen fuel, then the equipment section would be slid into it via a large inspection hatch. This became known as a "wet workshop" concept, because of the conversion of an active fuel tank. The station filled the entire interior of the S-II stage's hydrogen tank, with the equipment section forming a "spine" and living quarters located between it and the walls of the booster. This would have resulted in a very large 33 by 45 feet (10 by 14 m) living area. Power was to be provided by solar cells lining the outside of the S-II stage.[28]

One problem with this proposal was that it required a dedicated Saturn V launch to fly the station. At the time the design was being proposed, it was not known how many of the then-contracted Saturn Vs would be required to achieve a successful Moon landing. However, several planned Earth-orbit test missions for the LM and CSM had been canceled, leaving a number of Saturn IBs free for use. Further work led to the idea of building a smaller "wet workshop" based on the S-IVB, launched as the second stage of a Saturn IB.

A number of S-IVB-based stations were studied at MSC from mid-1965, which had much in common with the Skylab design that eventually flew. An airlock would be attached to the hydrogen tank, in the area designed to hold the LM, and a minimum amount of equipment would be installed in the tank itself in order to avoid taking up too much fuel volume. Floors of the station would be made from an open metal framework that allowed the fuel to flow through it. After launch, a follow-up mission launched by a Saturn IB would launch additional equipment, including solar panels, an equipment section and docking adapter, and various experiments. Douglas Aircraft Company, builder of the S-IVB stage, was asked to prepare proposals along these lines. The company had for several years been proposing stations based on the S-IV stage, before it was replaced by the S-IVB.[29]

On April 1, 1966, MSC sent out contracts to Douglas, Grumman, and McDonnell for the conversion of an S-IVB spent stage, under the name Saturn S-IVB spent-stage experiment support module (SSESM).[30] In May 1966, astronauts voiced concerns over the purging of the stage's hydrogen tank in space. Nevertheless, in late July 1966, it was announced that the Orbital Workshop would be launched as a part of Apollo mission AS-209, originally one of the Earth-orbit CSM test launches, followed by two Saturn I/CSM crew launches, AAP-1 and AAP-2.

The Manned Orbiting Laboratory (MOL) remained AAP's chief competitor for funds, although the two programs cooperated on technology. NASA considered flying experiments on MOL or using its Titan IIIC booster instead of the much more expensive Saturn IB. The agency decided that the Air Force station was not large enough and that converting Apollo hardware for use with Titan would be too slow and too expensive.[31] The DoD later canceled MOL in June 1969.[32]

Design work continued over the next two years, in an era of shrinking budgets.[33] (NASA sought US$450 million for Apollo Applications in fiscal year 1967, for example, but received US$42 million.)[34] In August 1967, the agency announced that the lunar mapping and base construction missions examined by the AAP were being canceled. Only the Earth-orbiting missions remained, namely the Orbital Workshop and Apollo Telescope Mount solar observatory. The success of Apollo 8 in December 1968, launched on the third flight of a Saturn V, made it likely that one would be available to launch a dry workshop.[35] Later, several Moon missions were canceled as well, originally to be Apollo missions 18 through 20. The cancellation of these missions freed up three Saturn V boosters for the AAP program. Although this would have allowed them to develop von Braun's original S-II-based mission, by this time so much work had been done on the S-IV-based design that work continued on this baseline. With the extra power available, the wet workshop was no longer needed;[36] the S-IC and S-II lower stages could launch a "dry workshop", with its interior already prepared, directly into orbit.

A dry workshop simplified plans for the interior of the station.[37] Industrial design firm Raymond Loewy/William Snaith recommended emphasizing habitability and comfort for the astronauts by providing a wardroom for meals and relaxation[38] and a window to view Earth and space, although astronauts were dubious about the designers' focus on details such as color schemes.[39] Habitability had not previously been an area of concern when building spacecraft due to their small size and brief mission durations, but the Skylab missions would last for months.[40] NASA sent a scientist on Jacques Piccard's Ben Franklin submarine in the Gulf Stream in July and August 1969 to learn how six people would live in an enclosed space for four weeks.[41]

Astronauts were uninterested in watching movies on a proposed entertainment center or in playing games, but they did want books and individual music choices.[39] Food was also important; early Apollo crews complained about its quality, and a NASA volunteer found it intolerable to live on the Apollo food for four days on Earth. Its taste and composition were unpleasant, in the form of cubes and squeeze tubes. Skylab food significantly improved on its predecessors by prioritizing palatability over scientific needs.[42]

Each astronaut had a private sleeping area the size of a small walk-in closet, with a curtain, sleeping bag, and locker.[43] Designers also added a shower[44][45] and a toilet[46][47] for comfort and to obtain precise urine and feces samples for examination on Earth.[48] The waste samples were so important that they would have been priorities in any rescue mission.[49]

Skylab did not have recycling systems such as the conversion of urine to drinking water; it also did not dispose of waste by dumping it into space. The S-IVB's 73,280 liters (16,120 imp gal; 19,360 U.S. gal) liquid oxygen tank below the Orbital Work Shop was used to store trash and wastewater, passed through an airlock.

Launch of the modified Saturn V rocket carrying the Skylab space station

On August 8, 1969, the McDonnell Douglas Corporation received a contract for the conversion of two existing S-IVB stages to the Orbital Workshop configuration. One of the S-IV test stages was shipped to McDonnell Douglas for the construction of a mock-up in January 1970. The Orbital Workshop was renamed "Skylab" in February 1970 as a result of a NASA contest.[50] The actual stage that flew was the upper stage of the AS-212 rocket (the S-IVB stage, S-IVB 212). The mission computer used aboard Skylab was the IBM System/4Pi TC-1, a relative of the AP-101 Space Shuttle computers. The Saturn V with serial number SA-513, originally produced for the Apollo program – before the cancellation of Apollo 18, 19, and 20 – was repurposed and redesigned to launch Skylab.[51] The Saturn V's third stage was removed and replaced with Skylab, but with the controlling Instrument Unit remaining in its standard position.

Skylab was launched on May 14, 1973, by the modified Saturn V. The launch is sometimes referred to as Skylab 1. Severe damage was sustained during launch and deployment, including the loss of the station's micrometeoroid shield/sun shade and one of its main solar panels. Debris from the lost micrometeoroid shield further complicated matters by becoming tangled in the remaining solar panel, preventing its full deployment and thus leaving the station with a huge power deficit.[52]

Immediately following Skylab's launch, Pad 39A at Kennedy Space Center was deactivated, and construction proceeded to modify it for the Space Shuttle program, originally targeting a maiden launch in March 1979. The crewed missions to Skylab would occur using a Saturn IB rocket from Launch Pad 39B.

Skylab 1 was the last uncrewed launch from LC-39A until February 19, 2017, when SpaceX CRS-10 was launched from there.

Skylab 3's Saturn IB at night, July 1973

Skylab in orbit in 1973 as flown, docking ports in view

Three crewed missions, designated Skylab 2, Skylab 3, and Skylab 4, were made to Skylab in the Apollo command and service modules. The first crewed mission, Skylab 2, launched on May 25, 1973, atop a Saturn IB and involved extensive repairs to the station. The crew deployed a parasol-like sunshade through a small instrument port from the inside of the station, bringing station temperatures down to acceptable levels and preventing overheating that would have melted the plastic insulation inside the station and released poisonous gases. This solution was designed by NASA's "Mr. Fix It" Jack Kinzler, who won the NASA Distinguished Service Medal for his efforts. The crew conducted further repairs via two spacewalks (extravehicular activity or EVA). The crew stayed in orbit with Skylab for 28 days. Two additional missions followed, with the launch dates of July 28, 1973, (Skylab 3) and November 16, 1973, (Skylab 4), and mission durations of 59 and 84 days, respectively. The last Skylab crew returned to Earth on February 8, 1974.[53]

In addition to the three crewed missions, there was a rescue mission on standby that had a crew of two, but could take five back down.

• Skylab 2: launched May 25, 1973[54]
• Skylab 3: launched July 28, 1973
• Skylab 4: launched November 16, 1973
• Skylab 5: cancelled
• Skylab Rescue on standby

Also of note was the three-man crew of Skylab Medical Experiment Altitude Test (SMEAT), who spent 56 days in 1972 at low-pressure on Earth to evaluate medical experiment equipment.[55] This was a spaceflight analog test in full gravity, but Skylab hardware was tested and medical knowledge was gained.

Owen Garriott performing an EVA in 1973

Skylab orbited Earth 2,476 times during the 171 days and 13 hours of its occupation during the three crewed Skylab expeditions. Each of these extended the human record of 23 days for amount of time spent in space set by the Soviet Soyuz 11 crew aboard the space station Salyut 1 on June 30, 1971. Skylab 2 lasted 28 days, Skylab 3 – 56 days, and Skylab 4 – 84 days. Astronauts performed ten spacewalks, totaling 42 hours and 16 minutes. Skylab logged about 2,000 hours of scientific and medical experiments, 127,000 frames of film of the Sun and 46,000 of Earth.[56] Solar experiments included photographs of eight solar flares and produced valuable results[57] that scientists stated would have been impossible to obtain with uncrewed spacecraft.[58] The existence of the Sun's coronal holes was confirmed because of these efforts.[59] Many of the experiments conducted investigated the astronauts' adaptation to extended periods of microgravity.

A typical day began at 6 a.m. Central Time Zone.[60] Although the toilet was small and noisy, both veteran astronauts – who had endured earlier missions' rudimentary waste-collection systems – and rookies complimented it.[61][45][62] The first crew enjoyed taking a shower once a week, but found drying themselves in weightlessness[62] and vacuuming excess water difficult; later crews usually cleaned themselves daily with wet washcloths instead of using the shower. Astronauts also found that bending over in weightlessness to put on socks or tie shoelaces strained their abdominal muscles.[63]

Breakfast began at 7 a.m. Astronauts usually stood to eat, as sitting in microgravity also strained their abdominal muscles. They reported that their food – although greatly improved from Apollo – was bland and repetitive, and weightlessness caused utensils, food containers, and bits of food to float away; also, gas in their drinking water contributed to flatulence. After breakfast and preparation for lunch, experiments, tests and repairs of spacecraft systems and, if possible, 90 minutes of physical exercise followed; the station had a bicycle and other equipment, and astronauts could jog around the water tank. After dinner, which was scheduled for 6 p.m., crews performed household chores and prepared for the next day's experiments. Following lengthy daily instructions (some of which were up to 15 meters long) sent via teleprinter, the crews were often busy enough to postpone sleep.[64][65] The station offered what a later study called "a highly satisfactory living and working environment for crews", with enough room for personal privacy.[66] Although it had a dart set,[67] playing cards, and other recreational equipment in addition to books and music players, the window with its view of Earth became the most popular way to relax in orbit.[68]

Riccardo Giacconi shared the 2002 Nobel Prize in Physics for his study of X-ray astronomy, including the study of emissions from the Sun onboard Skylab, contributing to the birth of X-ray astronomy.[72]

Concept for proposed Skylab re-boost

The reactivation would likely have occurred in four phases:[110]

1. An early Space Shuttle flight would have boosted Skylab to a higher orbit, adding five years of operational life. The shuttle might have pushed or towed the station, but attaching a space tug – the Teleoperator Retrieval System (TRS) – to the station would have been more likely, based on astronauts' training for the task. Martin Marietta won the contract for US$26 million to design the apparatus.[130] TRS would contain about three tons of propellant.[131] The remote-controlled booster had TV cameras and was designed for duties such as space construction and servicing and retrieving satellites the shuttle could not reach. After rescuing Skylab, the TRS would have remained in orbit for future use. Alternatively, it could have been used to de-orbit Skylab for a safe, controlled re-entry and destruction.[132]
2. In two shuttle flights, Skylab would have been refurbished. In January 1982, the first mission would have attached a docking adapter and conducted repairs. In August 1983, a second crew would have replaced several system components.
3. In March 1984, shuttle crews would have attached a solar-powered Power Expansion Package, refurbished scientific equipment, and conducted 30- to 90-day missions using the Apollo Telescope Mount and the Earth resources experiments.
4. Over five years, Skylab would have been expanded to accommodate six to eight astronauts, with a new large docking/interface module, additional logistics modules, Spacelab modules and pallets, and an orbital vehicle space dock using the shuttle's external tank.

The first three phases would have required about US$60 million in 1980s dollars, not including launch costs. Other options for launching TRS were Titan III or Atlas-Agena.[110]

Skylab captured this view of the Sun

Solar prominence recorded by Skylab on August 21, 1973[138]

British mathematician Desmond King-Hele of the Royal Aircraft Establishment (RAE) predicted in 1973 that Skylab would de-orbit and crash to Earth in 1979, sooner than NASA's forecast, because of increased solar activity.[134] Greater-than-expected solar activity[139] heated the outer layers of Earth's atmosphere and increased drag on Skylab. By late 1977, NORAD also forecast a reentry in mid-1979;[133] a National Oceanic and Atmospheric Administration (NOAA) scientist criticized NASA for using an inaccurate model for the second most-intense sunspot cycle in a century, and for ignoring NOAA predictions published in 1976.[140]

The reentry of the USSR's nuclear powered Cosmos 954 in January 1978, and the resulting radioactive debris fall in northern Canada, drew more attention to Skylab's orbit. Although Skylab did not contain radioactive materials, the State Department warned NASA about the potential diplomatic repercussions of station debris.[141] Battelle Memorial Institute forecast that up to 25 tons of metal debris could land in 500 pieces over an area 4,000 miles (6,400 km) long and 1,000 miles (1,600 km) wide. The lead-lined film vault, for example, might land intact at 400 feet per second.[9]

Ground controllers re-established contact with Skylab in March 1978[142] and recharged its batteries.[8] Although NASA worked on plans to reboost Skylab with the Space Shuttle through 1978 and the TRS was almost complete, the agency gave up in December 1978 when it became clear that the shuttle would not be ready in time;[130][143] its first flight, STS-1, did not occur until April 1981. Also rejected were proposals to launch the TRS using one or two uncrewed rockets[110] or to attempt to destroy the station with missiles.[9]

Equirectangular projection relief map of the Skylab re-entry site and final orbits, as predicted by NASA

Fragment of Skylab recovered after its re-entry through Earth's atmosphere, on display at the U.S. Space & Rocket Center

Skylab's demise in 1979 was an international media event,[144] with T-shirts and hats with bullseyes[9] and "Skylab Repellent" with a money-back guarantee,[145] wagering on the time and place of re-entry, and nightly news reports. The San Francisco Examiner offered a US$10,000 prize for the first piece of Skylab delivered to its offices; the competing San Francisco Chronicle offered US$200,000 if a subscriber suffered personal or property damage.[8] A Nebraska neighborhood painted a target so that the station would have "something to aim for", a resident said.[145]

A report commissioned by NASA calculated that the odds were 1 in 152 of debris hitting any human, and odds of 1 in 7 of debris hitting a city of 100,000 people or more.[146] Special teams were readied to head to any country hit by debris.[8] The event caused so much panic in the Philippines that President Ferdinand Marcos appeared on national television to reassure the public.[134]

A week before re-entry, NASA forecast that it would occur between July 10 and 14, with the 12th the most likely date, and the Royal Aircraft Establishment (RAE) predicted the 14th.[134] In the hours before the event, ground controllers adjusted Skylab's orientation to minimize the risk of re-entry on a populated area.[8] They aimed the station at a spot 810 miles (1,300 km) south-southeast of Cape Town, South Africa, and re-entry began at approximately 16:37 UTC, July 11, 1979.[7] The station did not burn up as fast as NASA expected. Debris landed about 300 miles (480 km) east of Perth, Western Australia due to a four-percent calculation error,[7] and was found between Esperance, Western Australia and Rawlinna, from 31° to 34° S and 122° to 126° E, about 130–150 km (81–93 miles) radius around Balladonia, Western Australia. Residents and an airline pilot saw dozens of colorful flares as large pieces broke up in the atmosphere;[9] the debris landed in an almost unpopulated area, but the sightings still caused NASA to fear human injury or property damage.[147]

The Shire of Esperance light-heartedly fined NASA A$400 for littering.[148] (The fine was written off three months later, but was eventually paid on behalf of NASA in April 2009, after Scott Barley of Highway Radio raised the funds from his morning show listeners.[149][150])

Stan Thornton found 24 pieces of Skylab at his home in Esperance, and a Philadelphia businessman flew him, his parents, and his girlfriend to San Francisco where he collected the Examiner prize and another US$1,000 from the businessman.[7][9] The Miss Universe 1979 pageant was scheduled for July 20, 1979 in Perth, and a large piece of Skylab debris was displayed on the stage.[151] Analysis of the debris showed that the station had disintegrated 10 miles (16 km) above the Earth, much lower than expected.[9]

After the demise of Skylab, NASA focused on the reusable Spacelab module, an orbital workshop that could be deployed with the Space Shuttle and returned to Earth. The next American major space station project was Space Station Freedom, which was merged into the International Space Station in 1993 and launched starting in 1998. Shuttle-Mir was another project and led to the US funding Spektr, Priroda, and the Mir Docking Module in the 1990s.

Launch vehicles:[152]

• SA-206 (Skylab 2)
• SA-207 (Skylab 3)
• SA-208 (Skylab 4)
• SA-209 (Skylab Rescue, not launched)

5-person Apollo Command module for the Apollo Rescue mission

SA-209 served on standby for Skylab 4 and ASTP, and has been preserved at the Kennedy Space Center rocket garden.

There was a Skylab Rescue mission assembled for the second crewed mission to Skylab, but it was not needed. Another rescue mission was assembled for the last Skylab and was also on standby for ASTP. That launch stack might have been used for Skylab 5 (which would have been the fourth crewed Skylab mission), but this was cancelled and the SA-209 Saturn IB rocket was put on display at NASA Kennedy Space Center.[152]

Skylab 5 would have been a short 20-day mission to conduct more scientific experiments and use the Apollo's Service Propulsion System engine to boost Skylab into a higher orbit. Vance Brand (commander), William B. Lenoir (science pilot), and Don Lind (pilot) would have been the crew for this mission, with Brand and Lind being the prime crew for the Skylab Rescue flights.[109] Brand and Lind also trained for a mission that would have aimed Skylab for a controlled deorbit.[147]

The mission would have launched in April 1974 and supported later use by the Space Shuttle by boosting the station to higher orbit.[153]

In addition to the flown Skylab space station, a second flight-quality backup Skylab space station had been built during the program. NASA considered using it for a second station in May 1973 or later, to be called Skylab B (S-IVB 515), but decided against it. Launching another Skylab with another Saturn V rocket would have been very costly, and it was decided to spend this money on the development of the Space Shuttle instead. The backup is on display at the National Air and Space Museum in Washington, D.C.

The Skylab Medical Experiment Altitude Test or SMEAT was a 56-day (8-week) Earth analog Skylab test.[159] The test had a low-pressure high oxygen-percentage atmosphere but it operated under full gravity, as SMEAT was not in orbit. The test had a three-astronaut crew with Commander Robert Crippen, Science Pilot Karol J. Bobko, and Pilot William E. Thornton;[160] there was a focus on medical studies and Thornton was an M.D.[161] The crew lived and worked in the pressure chamber, converted to be like Skylab, from July 26 to September 20, 1972.[55]

Mission	Emblem	Commander	Science Pilot	Pilot	Start date	End date	Duration
Skylab Medical Experiment Altitude Test (SMEAT)		Bob Crippen	Karol Bobko	William Thornton	July 26, 1972	September 20, 1972[55]	56 days

• NASA History Series Publications (many of which are on-line)
  • SP-4011 Skylab, a Chronology (1977) 1
  • SP-401 Skylab, Classroom in Space (1977)
  • SP-399 Skylab EREP Investigations Summary (1978)
  • SP-402 A New Sun: Solar Results from Skylab (1979)
  • SP-404 Skylab's Skylab's Astronomy and Space Sciences (1979)
• Skylab: Four Rooms Earth View on YouTube NASA Educational Film
• Airlock Module under construction (1971) (Medium)
• Airlock and Docking Module together (1972) (Medium)
• Skylab Crew Quarters Illustration Archived September 27, 2011, at the Wayback Machine
• Apollo (in foreground) and Skylab space food Archived September 27, 2011, at the Wayback Machine (M487)

• Skylab Collection, The University of Alabama in Huntsville Archives and Special Collections
• Leland F. Belew Collection, The University of Alabama in Huntsville Archives and Special Collections Files of Leland Belew, Skylab's project manager.
• eoPortal: Skylab
• Historic Spacecraft: Skylab
• Skylab reboost module
• Skylab Reentry (Chapter 9 of SP-4208)
• Skylab cutaway drawing from Encyclopædia Britannica Archived July 8, 2011, at the Wayback Machine
• Cutaway line drawing of Skylab
• Skylab "Christmas tree"