''Skylab II'' was a

space station A space station is a spacecraft capable of supporting a human crew in orbit for an extended period of time, and is therefore a type of space habitat. It lacks major propulsion or landing systems. An orbital station or an orbital space station i ...

concept proposed in 2013 by the Advanced Concepts Office of NASA

Marshall Space Flight Center The George C. Marshall Space Flight Center (MSFC), located in Redstone Arsenal, Alabama (Huntsville postal address), is the U.S. government's civilian rocketry and spacecraft propulsion research center. As the largest NASA center, MSFC's first ...

, to be located at the Earth-Moon L₂

Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of ...

. Proposed by NASA contractor Brand Griffin, ''Skylab II'' would have been constructed as a "

wet workshop A wet workshop is a space station made from a spent liquid-propellant rocket stage. Such a rocket stage contains two large, airtight propellant tanks; it was realized that the larger tank could be retrofit into the living quarters of a space s ...

" using a spent upper-stage hydrogen fuel tank from the

Space Launch System The Space Launch System (SLS) is an American super heavy-lift expendable launch vehicle developed by NASA. As of 2022, SLS has the highest payload capacity of any rocket in operational service, as well as the greatest liftoff thrust of any r ...

(SLS), much as the ''

Skylab Skylab was the first United States space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three separate three-astronaut crews: Skylab 2, Skylab 3, and Skylab 4. Major operation ...

'' was originally planned to be built "wet" from the spent

bipropellant The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into ...

tanks of the

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; ...

S-IVB The S-IVB (pronounced "S-four-B") was the third stage on the Saturn V and second stage on the Saturn IB launch vehicles. Built by the Douglas Aircraft Company, it had one J-2 rocket engine. For lunar missions it was fired twice: first for Earth ...

upper stage. If constructed, ''Skylab II'' would have been the first crewed outpost located beyond the orbit of the Moon.

Space station design

''Skylab II'' would have orbited the Earth-Moon L₂ point, which revolves around the Earth at an approximate mean distance of from the surface, and stays approximately beyond the far surface of the Moon. Given its location far away from the nearest food, water, and air, the first iteration of the space station would have had to have been able to support a four-person crew for 60 days on one shipment of supplies, to be eventually improved to 180 days. As ''Skylab II'' would be built from an SLS hydrogen fuel tank, the costs of assembly were estimated at a relatively low $2 billion, a tremendous savings over a previously projected cost of $4.175 billion for a similar space station. The space station would have large modules with diameters of —much larger than the diameter of

International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...

modules or the diameter of the original ''Skylab''. This large diameter would lead to a module volume of about , allowing for a large amount of space for both storage and habitation. This large interior volume, in turn, would make ''Skylab II'' suitable for its deep-space location, where resupply missions would be rare and astronauts would have to store the food they received from each mission for months at a time. The supplies themselves could be carried in a variety of vehicles, such as the existing

Progress Progress is the movement towards a refined, improved, or otherwise desired state. In the context of progressivism, it refers to the proposition that advancements in technology, science, and social organization have resulted, and by extension w ...

and Dragon, or perhaps a new, SLS-derived logistics module, which could resupply the whole station in one mission.

Potential uses

Once at Earth-Moon L₂, ''Skylab II'' could be a "

stepping stone Stepping stone(s) may refer to: * Stepping stones, stones placed to allow pedestrians to cross a watercourse Places * Stepping Stone, Virginia, US, an unincorporated community * Stepping Stones (islands), Antarctic and sub-Antarctic Buildings ...

" for further human exploration in the Solar System, for example by being a docking site for a crewed lunar lander before the trip to the Moon. The second use would be to add a servicing capability for the astrophysics missions located near Earth-Sun L₂, extending the

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...

mission lifetime of such missions by continually refilling their

liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low t ...

and enabling some astrophysics missions which may otherwise have not been possible or would have been launched in a less-capable state. For

extravehicular activities Extravehicular activity (EVA) is any activity done by an astronaut in outer space outside a spacecraft. In the absence of a breathable atmosphere of Earth, Earthlike atmosphere, the astronaut is completely reliant on a space suit for environmen ...

(EVAs) near the space station, a small, one-person

FlexCraft The Deep Space Habitat (DSH) is a series of concepts by NASA that would be used to support crewed exploration missions to the Moon, asteroids, and eventually Mars.