Editing Skylab (section)

== Plans for re-use after the last mission ==
[[File:SkylabRescue.jpg|thumb|upright|Skylab Rescue vehicle Apollo CSM being removed from its [[Saturn IB]] rocket after the last Skylab mission]]
{{blockquote|Calculations made during the mission, based on current values for solar activity and expected atmospheric density, gave the workshop just over nine years in orbit. Slowly at first – dropping 30 kilometers by 1980 – and then faster – another 100 kilometers by the end of 1982 – Skylab would come down, and some time around March 1983 it would burn up in the dense atmosphere.{{r|benson361}}}}

After nearly 172 days, Skylab considerably exceeded its planned 140 day habitation. The station had held up relatively well, but its onboard supplies were low and its systems were beginning to degrade. One of the three control-moment gyroscopes (CMGs) failed 8 days into Skylab 4,<ref>{{cite news |author=<!--not stated--> |date=November 24, 1973 |title=A Skylab Gyroscope Fails, Leaving Only 2 for Control |url=https://www.nytimes.com/1973/11/24/archives/a-skylab-gyroscope-fails-leaving-only-2-for-control.html |work=The New York Times}}</ref> and by the end of the mission another was showing signs of impending failure.<ref>{{cite web |url=http://spaceflighthistory.blogspot.com/2015/11/reviving-reusing-skylab-in-shuttle-era.html |title= Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters |last=Portree |first=David |date=November 14, 2015 |website=No Shortage of Dreams |accessdate=June 27, 2022 |quote=At the time the Skylab 4 crew departed, one CMG had already failed and another showed signs of impending failure.}}</ref> With just a single CMG Skylab would be unable to control its attitude, and it was not possible to repair or replace one of the broken gyroscopes on-orbit. Virtually all of the prepackaged food launched with the station had been consumed, Skylab 4's mission extension from 56 to 84 days required the crew take an extra 28 days worth of food with them,<ref>{{cite book |last1= Kilka |first1=Mary |last2=Smith |first2=Malcolm |date=April 1982 |title= Food for U.S. Manned Space Flight |page=24 |url=https://apps.dtic.mil/sti/pdfs/ADA118316.pdf |quote=The Skylab 3 crew launched with a 28-day supply of formulated nutrient-defined, high-density food bars which enabled the extension of their flight from the planned 56-day mission to 84 days.}}</ref> but there was still enough water to support three men for 60 days, and enough oxygen/nitrogen to support the same for 140 days.<ref>{{cite web |url=http://spaceflighthistory.blogspot.com/2015/11/reviving-reusing-skylab-in-shuttle-era.html |title= Reviving & Reusing Skylab in the Shuttle Era: NASA Marshall's November 1977 Pitch to NASA Headquarters |last=Portree |first=David |date=November 14, 2015 |website=No Shortage of Dreams |accessdate=June 27, 2022}}</ref>

A fourth crewed mission using an Apollo CSM was considered, which would have used the launch vehicle kept on standby for the Skylab Rescue mission. This would have been a 20-day mission to boost Skylab to a higher altitude and do more scientific experiments.<ref name=astronautixskylab5>{{cite encyclopedia|last=Wade|first=Mark|title=Skylab 5 |encyclopedia=Encyclopedia Astronautica |url=http://www.astronautix.com/flights/skylab5.htm|url-status=dead|archive-date=2017-05-20|archive-url=https://web.archive.org/web/20170520162643/http://www.astronautix.com/s/skylab5.html}}</ref> Another plan was to use a [[Teleoperator Retrieval System]] (TRS) launched aboard the Space Shuttle (then under development), to robotically re-boost the orbit. When Skylab 5 was cancelled, it was expected Skylab would stay in orbit until the 1980s, which was enough time to overlap with the beginning of Shuttle launches. Other options for launching TRS included the [[Titan III]] and [[Atlas-Agena]]. No option received the level of effort and funding needed for execution before Skylab's sooner-than-expected re-entry.<ref name= oberg199202/>

The Skylab 4 crew left a bag filled with supplies to welcome visitors, and left the hatch unlocked.<ref name= oberg199202/> Skylab's internal systems were evaluated and tested from the ground, and effort was put into plans for re-using it, as late as 1978.<ref>{{Cite report |title=Skylab reactivation mission report |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800012907.pdf |date=March 1980 |last1=Chubb |first1=W. B.}}</ref> NASA discouraged any discussion of additional visits due to the station's age,<ref>{{harvp|Benson|Compton|1983|pp=335, 361}}.</ref> but in 1977 and 1978, when the agency still believed the Space Shuttle would be ready by 1979, it completed two studies on reusing the station.<ref name="oberg199202"/><ref>{{harvp|Martin Marietta|Bendix|1978|p=3-1}}.</ref> By September 1978, the agency believed Skylab was safe for crews, with all major systems intact and operational.<ref>{{harvp|Martin Marietta|Bendix|1978|p=3-2}}.</ref> It still had 180 [[man-days]] of water and 420-man-days of oxygen, and astronauts could refill both;<ref name="oberg199202"/> the station could hold up to about 600 to 700 man-days of drinkable water and 420 man-days of food.<ref>{{harvp|Martin Marietta|Bendix|1978|p=2-7}}.</ref> Before Skylab 4 left they did one more boost, running the Skylab thrusters for 3 minutes which added 11&nbsp;km in height to its orbit. Skylab was left in a 433 by 455&nbsp;km orbit on departure. At this time, the NASA-accepted estimate for its re-entry was nine years.{{r|benson361}}

The studies cited several benefits from reusing Skylab, which one called a resource worth "hundreds of millions of dollars"<ref name="MMB1-13">{{harvp|Martin Marietta|Bendix|1978|p=1-13}}.</ref> with "unique habitability provisions for long duration space flight".<ref name="ReferenceC">{{harvp|Martin Marietta|Bendix|1978|p=3-11}}.</ref> Because no more operational Saturn V rockets were available after the Apollo program, four to five shuttle flights and extensive [[space architecture]] would have been needed to build another station as large as Skylab's {{convert|12400|cuft}} volume.<ref>{{harvp|Martin Marietta|Bendix|1978|pp=1-12 to 1-13}}.</ref> Its ample size – much greater than that of the shuttle alone, or even the shuttle plus [[Spacelab]]<ref>{{harvp|Martin Marietta|Bendix|1978|p=2-8}}.</ref> – was enough, with some modifications, for up to seven astronauts<ref>{{harvp|Martin Marietta|Bendix|1978|p=2-31}}.</ref> of both sexes,<ref>{{harvp|Martin Marietta|Bendix|1978|p=3-14}}.</ref> and experiments needing a long duration in space;<ref name=MMB1-13/> even a movie projector for recreation was possible.<ref name="ReferenceC"/>

Proponents of Skylab's reuse also said repairing and upgrading Skylab would provide information on the results of long-duration exposure to space for future stations.<ref name="oberg199202"/> The most serious issue for reactivation was [[Spacecraft attitude control|attitude control]], as one of the station's gyroscopes had failed<ref name="benson361">{{harvp|Benson|Compton|1983|p=361}}.</ref> and the attitude control system needed refueling; these issues would need EVA to fix or replace. The station had not been designed for extensive resupply. However, although it was originally planned that Skylab crews would only perform limited maintenance,<ref>{{harvp|Belew|1977|p=34}}.</ref> they successfully made major repairs during EVA, such as the Skylab 2 crew's deployment of the [[Solar panels on spacecraft|solar panel]]<ref>{{harvp|Belew|1977|pp=73–75}}.</ref> and the Skylab 4 crew's repair of the primary coolant loop.<ref>{{harvp|Benson|Compton|1983|p=317}}.</ref><ref>{{harvp|Belew|1977|p=130}}.</ref><ref>{{harvp|Martin Marietta|Bendix|1978|p=3-21}}.</ref> The Skylab 2 crew fixed one item during EVA by, reportedly, "hit[ting] it with [a] hammer".<ref>{{harvp|Belew|1977|p=89}}.</ref>

Some studies also said, beyond the opportunity for space construction and maintenance experience, reactivating the station would free up shuttle flights for other uses,<ref name=MMB1-13/> and reduce the need to [[Extended Duration Orbiter|modify the shuttle for long-duration missions]].<ref>{{harvp|Martin Marietta|Bendix|1978|pp=2–9, 10}}.</ref> Even if the station were not crewed again, went one argument, it might serve as an experimental platform.<ref>{{harvp|Martin Marietta|Bendix|1978|p=2–61}}.</ref>

=== Shuttle mission plans ===
[[File:Teleoperator Retrieval System with Shuttle.jpg|right|thumb|Concept for proposed Skylab re-boost]]

The reactivation would likely have occurred in four phases:<ref name="oberg199202"/><ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |date=September 1978 |title=Skylab Reuse Study |url=https://archive.org/details/skylab-reuse-study/mode/2up?view=theater |publisher=Martin Marietta}}</ref> 
# [[Canceled Space Shuttle missions#STS-2A (Columbia)|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.<ref name="time19790101"/> TRS would contain about three tons of propellant.<ref>{{cite encyclopedia|url=http://www.astronautix.com/craft/skyodule.htm |title=Skylab Reboost Module |encyclopedia=Encyclopedia Astronautica |url-status=dead |archive-url=https://web.archive.org/web/20091231140817/http://www.astronautix.com/craft/skyodule.htm|archive-date=December 31, 2009}}</ref> 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.<ref>{{cite magazine |last=Dempewolff |first=Richard F. |date=August 1978 |title=Our Growing Junkyard in Space |magazine=Popular Mechanics |url=https://books.google.com/books?id=mM8DAAAAMBAJ&pg=PA57|page=57 |access-date=2020-07-19}}</ref>
# 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.
# 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.
# 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]].<ref name=oberg199202/>