The Challenges of Building Human Habitats in Space

By 2024, NASA will return astronauts to the Moon for the first time in fifty years. Using the most-powerful launch vehicle ever built - then Space Launch System (SLS) - and the Orion spacecraft, the Artemis III mission will bring the "first woman and next man to the Moon."

Beyond that, NASA, the ESA, and other international and commercial partners plan to set up shop on the Moon. This will entail the creation of the Lunar Gateway (an orbital habitat) and the Artemis Base Camp (a surface habitat) that will allow for a program of "sustained lunar exploration and development."

In the commercial sector, companies like SpaceX are pushing the boundaries to create the world's first entirely-reusable and super-heavy launch system. Known as the Starship, this brainchild of Elon Musk will be making regular trips to Low-Earth Orbit (LEO) and perhaps ferrying people to the Moon and Mars in just a few years' time.

There's simply no denying it, a new age of space exploration is upon us! But whereas the previous space age was all about getting to space, the current age is concerned with staying there. That means developing the technologies for long-duration stays - in other words, space stations.

Space is dangerous

Ask any astronaut, and they will tell you that going to space is not easy. Aside from the chances of being struck by micrometeoroids, increased exposure to radiation, and other dangers associated with floating in a "tin can" (to quote David Bowie), there are also the effects of long periods spent in microgravity on the human body.

Thanks to decades of research aboard the International Space Station (ISS), scientists know that spaceflight takes a toll on the human body and mind. Perhaps the best-known example of this is the NASA Twin Study, where astronaut Scott Kelly spent about a year in space while his twin brother (retired astronaut Mark Kelly) stayed on Earth.

According to the results, which were released in 2019, the human body experiences some significant changes in response to spaceflight. These include loss of muscle and bone density, diminished cardiovascular health and organ function, changes to eyesight and circulation, genetic changes, and psychological effects like insomnia and depression.

All of these hazards cry out for creative solutions. Luckily, human beings have been going to space for over seventy years now and have learned some strategies for keeping our astronauts and spacecraft safe.

A time-honored idea

For over a century, scientists have theorized that one possible solution would be to build habitats in space that rotate to create some type of artificial gravity. The solution is elegant and likely to be very effective.

Fans of science fiction and cinema will no doubt recognize this description for a rotating "pinwheel" station in space. That's because the concept was featured in the classic 1968 film by Stanley Kubrick 2001: A Space Odyssey, which was co-written by famed physicist and mathematician Arthur C. Clarke (and based on his short story "The Sentinel.")

As a scientist and futurist, who believed humanity's future lay in space, Clarke's expertise informed the design of the station (as well as other technical aspects of the film). While this film popularized the idea for millions of people, it was actually a well-dated concept by the time of the film's release.

The concept of a space station that will rotate to simulate the effects of gravity was originally proposed by the Russian/Soviet scientist and "father of astronautics" Konstantin Tsiolkovsky. In 1903, he released a treatise titled "Exploration of Outer Space with Reaction Machines" that spelled out how developments in rocketry could allow for space travel.

In the context of creating space stations, he explained how it would be very easy to provide artificial gravity by simply imparting a rotation into the station:

"The magnitude of artificial gravity will depend on the angular velocity and the radius of rotation. It may be approximately 1000 times less than the Earth's, although nothing hinders us to make it 1000 times more than that of the Earth. For the rotation of the greenhouse (conservatory) or the house, no expenditure of forces is necessary at all. Objects continue to rotate automatically by themselves, by inertia, once they are set in motion. The motion is eternal, as rotation or revolution of the planet."

In 1929, Yugoslav/Slovene aeronautical engineer Herman Potocnik released Problem der Befahrung des Weltraums (The Problem of Space Travel), which contained a detailed illustration of a circular space station in Earth orbit. Potocnik also described how concerns over weightlessness could be mediated by imparting rotation to the station.

In the 1950s, a similar concept was proposed by German rocket scientist and space architect Werner von Braun. These designs were featured at the time in a series of articles in the national magazine Collier’s titled, “Man Will Conquer Space Soon!”

According to von Braun's detailed description, this "wheel-shaped space station" would measure 250 feet (76 m) in diameter and would orbit Earth around the poles once every two hours. Von Braun also indicated how the station could provide for "synthetic" gravity through rotation:

'To be sure, there are some medical men who are concerned at the prospect of permanent weightlessness — not because of any known danger, but because of the unknown possibilities. Most experts discount these nameless fears. However, there can be no doubt that permanent weightlessness might often prove inconvenient.

What we require, therefore, is a "synthetic" gravity within the space station. And we can produce centrifugal force — which acts as a substitute for gravity — by making the "wheel" slowly spin about its hub (a part of which can be made stationary)."

More recent proposals include the O'Neill Cylinder, named for physicist Gerard K. O'Neill, who came up with the idea after conducting a cooperative study with his students. The concept was publicized in a 1974 article in Physics Today - titled "The Colonization of Space" - and expanded on in O'Neill's 1976 book, The High Frontier: Human Colonies in Space.

Another example is the Stanford Torus, a proposal that resulted from the 1975 NASA Summer Study - hosted by the Ames Research Center and Stanford University. This was essentially a scaled-up version of the Von Braun Wheel that would be capable of housing 10,000 people and would rotate to simulate Earth-normal gravity.

Some concepts that are being considered today include NASA’s Nautilus-X rotating torus concept, which could be integrated into a spacecraft for long-duration missions to deep-space. NASA showed how it could also be attached to the ISS to provide a section with artificial gravity.

There is also the Gateway Foundation‘s proposal for a commercial space station that would consist of inner and outer pinwheel sections, capable of simulating Lunar and Martian gravity (16.5% and 38% of Earth normal), respectively. These rings would incorporate modules that could be used for commercial purposes, tourist accommodations, and research facilities.

In all cases, the concept calls for imparting momentum to the pinwheel in order to get it rotating. Thanks to the conservation of momentum (aka. inertia), the station doesn't require regular acceleration to keep spinning, though added thrust would allow for the residents to modulate the amount of artificial gravity they are exposed to.

Engineering in space

The process of creating structures in space is very similar to creating structures here on Earth: it's a marriage of engineering and architecture. But as Anastasia Prosina, founder and CEO of Stellar Amenities (a design firm specializing in space habitats) explained, the process is inverted when it comes to building in space:

"In architecture, the vision of an architect comes first, and then an engineer helps this vision become a reality. In space architecture, the process starts with a group of engineers who design and assemble the spacecraft, outfitted with the necessary systems. A space architect comes afterwards to help design for the human needs in the confined environment."

As such, the first task in the creation of a space station is to come up with a design that will satisfy all the technical requirements. This means materials that can be pressurized, withstand micrometeoroids, and endure over time. Luckily, some of the best engineers and theorists left detailed plans!

For example, von Braun recommended that his wheel-shaped space station be built from 20 sections of flexible plastic, each composed of nylon and fabric, each of which would be launched from Earth. They would be assembled in orbit, then inflated to provide a breathable atmosphere and ensure the structure remains rigid.

O'Neill's concept specifically called for a station that would allow for the colonization of space by the 21st century, using technology that was readily available. It was also to be built using materials extracted from the Moon and Near-Earth Asteroids (NEAs), the latter of which are thought to be good sources of nickel-iron alloys, platinum, and carbon (which could be fashioned into composites).

In the original paper, he indicated that lunar aluminum and titanium could be used, though he and his students based their design for the cylinder structure on steel cables - aka. "longerons," or load-bearing horizontal structures.

In his book, O'Neill expanded on the idea by claiming an "industrial seed" on the Moon - consisting of self-replicating robots and a mass-driver could harvest lunar ore and launch it to where the space station (dubbed "Island One") was being assembled. However, he also recommended the use of in-situ resources to reduce costs:

"[I]t appears that the establishment of space manufacturing would give a much greater payoff: a productive factory in space, with a self-supporting workforce of 10,000 people... The reasons for that greater payoff are post-Apollo advances in-vehicle systems, and all the 'bootstrap process' - using the material and energy resources of space to build manufacturing capacity."

In 1977, NASA released "Space Settlements: A Design Study," a detailed plan for the creation of a Stanford Torus. In terms of construction facilities, the study recommended establishing a machine shop in space, with rolling mills, extrusion presses, casting beds, and other equipment.

Construction materials would be sourced from the Moon, such as lunar silica for the manufacture of windows and solar panels and elemental oxygen to create water, rocket fuel, and oxygen gas. Metals would also be sourced from the Moon, such as aluminum, titanium, magnesium, and iron.

The construction process would rely on a space-based metal forming process that would take advantage of abundant solar energy, heat dissipation, and low-gravity. The station's frame would be constructed from aluminum plates and ribs, which would then be covered with metal plates to provide shielding (more on that below).

Putting things in motion

Perhaps the single-greatest advantage for a rotating space station is that very little force is needed to generate the sensation of gravity. This was identified by Tsiolkovsky and later theorists who recognized how the physics of space would actually be accomodating in this regard. As Tsiolkovsky noted in Exploration of Outer Space with Reaction Machines:

"For the rotation of the greenhouse (conservatory) or the house, no expenditure of forces is necessary at all. Objects continue to rotate automatically by themselves, by inertia, once they are set in motion. The motion is eternal, as rotation or revolution of the planet."

Von Braun explained how a simple rocket mounted to the exterior of the station could get things moving and would never have to be used again (unless further acceleration was called for):

"To the space station proper, we attach a tiny rocket motor which can produce enough power to rotate the satellite. Since there is no resistance that would slow the "wheel" down, the rocket motor does not need to function continuously... If our 250-foot ring performed one full revolution every 12.3 seconds, we would get a synthetic gravity equal to that which we normally experience on the ground."

By the 1970s, research into the effects of spending time in a rotating inertial frame had progressed and possible negative effects were identified. For example, NASA's design study indicated that when inhabitants move around inside the space station, they would experience the Coriolis force, as well as "pseudo gravity":

"At low velocities or low rotation rates the effects of the Coriolis force are negligible, as on Earth, but in a habitat rotating at several rpm, there can be disconcerting effects. Simple movements become complex and the eyes play tricks: turning the head can make stationary objects appear to gyrate and continue to move once the head has stopped turning. 

"This is because Coriolis forces not only influence locomotion but also create cross-coupled angular accelerations in the semicircular canals of the ear when the head is turned out of the plane of rotation. Consequently, motion sickness can result even at low rotation rates..."

As a result, the study recommended that rotation rates with a Stanford Torus be kept to 3 rotations per minute (rpm) or less. O'Neill also addressed research into the phenomenon in The High Frontier and recommended that the rotation rate be kept low (1 to 3 rpm). He also indicated that as settlements became larger, this would be less of an issue:

"In the case of habitats in space, the range of interest is between one and three rotations per minute - high enough to be of concern, but low enough that most of the subjects so far tested have been able to adapt to it, usually within a day or two. For the larger habitats, which will almost surely follow the first small "models" the rotation rates can be kept below on rotation per minute without compromising the efficiency of design."

Protection from the elements

In space, it is necessary to shield against the elements at all times. Since being in space means being surrounded by vacuum (or near-vacuum) conditions, stations need to be fully-pressurized and heated, and environmental conditions need to be constantly monitored. For this reason, impacts from micrometeoroids or orbital debris are considered a major hazard.

To protect against these threats, NASA and other space agencies have developed multiple types of HyperVelocity Impact Technology. These include simple (monolithic) shields, whipple shields (multi-layered), honeycomb, foam, or hybrid shielding. Another means of protection is to ensure that space stations have curved surfaces, which increases their "effective thickness."

If an object impacts a 2.54-cm (1 inch) surface at a 90° angle (i.e., perpendicular to its face) the effective thickness will be equal to the thickness of the surface itself - 1 inch. But if the impact comes in at a tilted angle (ex. 30°, 45°, 60°), the effective thickness would be 2.93, 3.59, and 5.08 cm (1.15, 1.414, and 2 inches), respectively.

In short, angling a surface relative to the path of an impacting object can effectively double the amount of protection. This knowledge dates back to the Middle Ages, where engineers found that if they designed towers that were rounded, rather than square, they would be able to endure more punishment from siege engines.

In World War II, engineers found that if their tanks had angled or sloped surfaces, enemy tanks or anti-tank guns would have a much harder time penetrating them. Unless the guns had a particularly high muzzle velocity, the shells were more likely to ricochet and then explode.

In space, this would amount to building stations that take advantage of tubular or cylindrical sections. The walls of this structure would not only be more resilient to micrometeoroid impacts, but they would also hold their shape better over time. This is due to something known as a "pressure differential," which gets rather significant in space.

In space, conditions are that of a vacuum (or near-vacuum), which means space stations need to be pressurized at all times. This creates a significant difference in pressure between the inside and exterior of the station, which causes stress to its surfaces. Since curved surfaces naturally reinforce themselves against pressure, they are less likely to deform over time.

Another major concern is radiation, which can take the form of cosmic rays or solar radiation. Beyond Earth's protective magnetic field and its atmosphere, humans are vulnerable to a particular type of cosmic ray known as "heavy primaries" - nuclei of helium, carbon, iron, and other elements that have been stripped of their electrons.

There is also the sudden bursts of radiation periodically emitted from our Sun (aka. solar flares) that greatly increase the amount of charged particles astronauts are exposed to. Every few decades, a particularly powerful burst is emitted that interferes with radio transmissions and power grids here on Earth and would be fatal to astronauts directly exposed to it.

One potential means of protecting humans from radiation in space is to use electromagnetic fields, which curve the path of charged particles that pass through them. NASA explored this very idea with the Standford Torus and concluded that a charged plasma field (of 10 to 15 gigavolts/nucleon) that sustains high electrical potential in the vicinity of the habitat would be highly-effective:

"A shield of this capability would also protect against the effects of the strongest solar flares, and no shelter would be needed. The difficulty is that the structural mass required to resist the magnetic forces between superconducting coils precludes this design even for the most favorable geometry, namely, a torus."

There is also the option for passive protection, which amounts to using dense concentrations of matter to provide natural shielding. Once again, O'Neill explained how this could be done easily enough by using lunar resources or leftover slag to create shielding. He also showed how certain depths of soil inside the station, as well as its atmosphere, would shield against radiation.

"The later space communities," he wrote, "will have atmospheric depths, and thicknesses of structure below the ground, so great that they too will afford to their inhabitants' protections from cosmic rays comparable to that of Earth."

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Building rotating habitats in space entails all kinds of challenges, not the least of which is the massive commitment in time, resources, and energy it would require. According to the 1975 Summer Study that resulted in the design for the Stanford Torus, the creation of all the industry needed to produce a city-sized habitat in space would cost the equivalent of two Apollo Programs.

Adjusted for inflation, that works out to over $300 billion today, which would likely be spread over a period of about a decade. Nevertheless, from an engineering and physics standpoint, the concept is sound. And assuming that humanity's presence in space continues to grow at its present rate, the construction of habitats will become a viable possibility before too long.

Since the creation of space stations at various points in our Solar System will open up surrounding areas of space for commercial, science, and exploration operations, they might even become a necessity. To ensure that these habitats meet the needs of their living occupants (not just humans, but plants, animals, and other creatures), artificial gravity is a must.