Humanity’s future beyond Earth: Multiplanetary or Islands in Space?

Given all of these advancements, it is clear that humanity's future lies in space. But what will that look like in the long run? Will we live on many planets, moons, and bodies throughout the Solar System? Will we "colonize space" with giant rotating pinwheels and cylinders? Will we live in sealed environments that mimic those on Earth or ecologically engineer other bodies to make them more "Earth-like"?

These options can be sorted into two broad categories that have historically had their share of proponents. The first is the "multiplanetary" vision, the more-conventional approach championed by Buzz Aldrin, James Lovelock, Robert Zubrin, and Elon Musk. The second can be described as the "Islands in Space" vision advocated by individuals ranging from Konstantin Tsiolkovsky, John Desmond Bernal, and Dandridge M. Cole to Gerard K. O'Neill and Jeff Bezos.

Both visions have advantages and drawbacks, and both are supported by a considerable amount of scientific research. In the age of renewed space exploration, such ideas are attracting a growing number of enthusiasts who want to make them happen.

This begs the question: which vision of humanity's future in space will come true? Will one invariably top the other, or are we destined to see both take place in the coming century?

Space is hard!

First, some caveats need to be addressed. Living in space or on other bodies in the Solar System presents numerous challenges. Beyond the warm and comforting embrace of Earth's atmosphere, you have the hard vacuum of space, airless bodies, and planets where the atmosphere is incredibly hostile to life as we know it. Beyond Earth's protective magnetic field, the hazards of solar radiation and cosmic rays become much more acute.

Then there are the effects of exposure to low gravity and microgravity on the human body. As decades of study aboard the International Space Station (ISS) have shown, including NASA's Twins Study, prolonged exposure to microgravity leads to muscle atrophy, decreased bone density, and impacts cardiovascular health, organ function, eyesight, gene expression, and the central nervous system.

Astronaut Scott Kelly (one of the participants in the Twins Study) documented the difficulties of readjusting to Earth's gravity in his book Endurance: A Year in Space, A Lifetime of Discovery. As he described it, the process was an ordeal that involved chronic pain, swelling, fainting spells, and being bedridden for weeks. In short, there's a reason people in the business say "space is hard."

How these problems can be mitigated has been the subject of research studies since the dawn of the Space Age. However, this research has generally been divided between studies that address settlements on the nearest astronomical bodies (like the Moon and Mars) and in space - in Low Earth Orbit (LEO) or at gravitationally-stable Lagrange Points.

Space habitats

The idea of "colonizing space" with large habitats is the more time-honored of the two. In this case, detailed proposals have been made for over a century, beginning with Konstantin Tsiolkovsky - a Russian aeronautical scientist and one of the "forefathers of rocketry." His seminal work, Exploration of the Universe with Reaction Machines (1903), popularized his famous "rocket equation" and was the basis for modern rocket designs.

In addition, this book contained a description of a pinwheel station that would rotate to simulate artificial gravity. This station would have airlocks that would allow spacecraft to dock and closed-loop biological systems to provide food and oxygen for the crew.

The concept was elaborated on by Slovenian astronautical engineer Hermann Noordung (Potočnik) in his book, The Problem of Space Travel (1929). This book contained 100 handmade illustrations of concepts that would allow for a permanent human presence in space. This included designs for a space station which are generally regarded as the first detailed technical drawings of a space station. The station consisted of a rotating habitat wheel powered by a solar collector, an observatory, and a machine room connected by an umbilical.

Irish mathematician and scientist John Desmond Bernal also proposed a concept for a space habitat in 1929. In his book, "The World, the Flesh & the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul," he described how a spherical non-rotating habitat (known as the "Bernal Sphere") could be manufactured using materials harvested in space:

"Imagine a spherical shell ten miles or so in diameter, made of the lightest materials and mostly hollow; for this purpose, the new molecular materials would be admirably suited. Owing to the absence of gravitation its construction would not be an engineering feat of any magnitude. The source of the material out of which this would be made would only be in small part drawn from the Earth; for the great bulk of the structure would be made out of the substance of one or more smaller asteroids, rings of Saturn or other planetary detritus."

Between 1952 and 1954, a series of articles appeared in Collier's magazine titled "Man Will Conquer Space Soon!" This series featured the designs of Werner von Braun, the German rocket scientist who became a chief advisor to NASA in the 1960s. Among his many detailed proposals was the Von Braun Wheel, a 250 ft (76 m) space station that would rotate to provide "synthetic gravity."

In 1974, physicist Gerard K. O'Neill conducted a cooperative study with his physics students at Princeton University. The topic was how a space station could be constructed using raw materials from the Moon and near-Earth asteroids (NEAs). The habitats would rotate to simulate gravity and be illuminated and powered by solar power. Their study was publicized in a 1974 article in Physics Today and explored more extensively in O'Neill's book, The High Frontier: Human Colonies in Space (1976).

The architecture O'Neill described consisted of three main habitats ("Islands") supported by many smaller cylinders (used for growing crops) and zero-gravity industrial modules. They consisted of the following":

• Island One: a rotating sphere measuring one mile (1.6 km) in circumference and 1,681 feet (512 m) in diameter, with people living in the equatorial region (similar to a Bernal sphere).
• Island Two: another spherical station measuring 5,200 feet (1,600 m) in diameter
• Island Three: two counter-rotating cylinders measuring 20 miles (32 km) long and 5 miles (8 km) in diameter, with an outer ring supporting farming modules. In the center, where gravity is lowest, is the manufacturing block.

Island Three (aka. an O'Neill Cylinder) is where the largest population would live. It would be aimed at the Sun and rely on solar collectors to provide power. It would also have a system of adjustable mirrors in its interior to simulate Earth-like conditions. As O'Neill wrote:

"The cylinder circumference is divided into six regions, three 'valleys' alternating with three arrays of windows. By locating three large, light planar mirrors above the widows and pointing the cylinder axes always towards the Sun, we can arrange that the valleys will receive natural sunshine, and that the Sun will appear motionless in the sky even though the cylinder is rotating.

"Varying the mirror angle will give dawn, the slow passage of the Sun across the sky during the day, and sunset. The day-length, weather, seasonal cycle and heat balance of the colony can be regulated by the same schedule of mirror-angle variation. A large paraboloidal mirror at the end of each cylinder can be collecting solar energy twenty-four hours per day, to run the community's power plant."

In 1975, another concept was proposed as part of a NASA Summer Study, a cooperative study hosted by NASA's Ames Research Center and Standford University. This led to the Stanford Torus, a scaled-up version of the Von Braun Wheel that would be capable of housing up to 10,000 people and simulated Earth-like 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 percent and 38 percent of Earth's normal), respectively. These rings would incorporate modules that could be used for commercial purposes, tourist accommodations, and research facilities.

Terraforming 101

With the beginning of the Space Age in 1958 (the launch of the Soviet satellite Sputnik I), space agencies and astronautical engineers began contemplating long-term missions beyond Earth and the Earth-Moon system. In addition to exploration missions, there were many proposals for how permanent human outposts could be created on neighboring planets.

Lastly, the study and modeling of planetary climates led to revolutionary ideas about how extraterrestrial environments could be altered to make them more amenable to humans. This is popularly known as "terraforming" (or ecological engineering), where a planetary environment is gradually transformed to be more "Earth-like" - complete with a denser, breathable atmosphere, stable temperatures, a water cycle, and a self-sustaining planetwide biosphere.

In 1961, Carla Sagan wrote one of the first proposal papers for terraforming, titled "The Planet Venus," where he argued that photosynthetic bacteria could be introduced into Venus' atmosphere to convert the carbon dioxide, reducing the greenhouse effect and lowering the density of the atmosphere. This proposal proved unfeasible with the subsequent discovery of sulfuric acid clouds and that the atmosphere was too dense for such organisms to survive.

However, Sagan's paper got the ball rolling.

In 1973, Sagan followed up on his previous paper with an article titled "Planetary Engineering on Mars," where he proposed introducing dark plants or low albedo materials to the Martian polar ice caps. This would allow them to absorb more solar radiation, melt, and thicken Mars' atmosphere.

In his book, Islands in Space: The Challenge of Planetoids (1964), NASA engineer Dandridge M. Cole recommended importing ammonia ices from the outer Solar System to Mars. These would melt in the Martian environment and thicken the atmosphere, triggering a greenhouse effect that would warm the environment and cause the surface ice to melt. Since ammonia is mostly nitrogen by content, this process would also introduce a buffer gas into the atmosphere that could be mixed with oxygen to become breathable.

In 1976, NASA officially addressed the issue of planetary engineering in a study titled "On the Habitability of Mars: An Approach to Planetary Ecosynthesis." The study examined the various challenges and found that "[n]o fundamental, insuperable limitation of the ability of Mars to support a terrestrial ecology is identified." It further indicated that introducing greenhouse gases and photosynthetic organisms could someday create a livable environment for humans.

In 1984, NASA scientist and environmentalist James Lovelock and ecologist Michael Allaby wrote The Greening of Mars (1984), a science fiction book set in the year 2245 that tells the story of how humans colonized and terraformed Mars. The process began with surplus nuclear missiles being repurposed to introduce chlorofluorocarbons (CFCs) and microbes to Mars. The CFCs, a super greenhouse gas, would warm the Martian atmosphere and melt the polar ice caps, thickening the atmosphere.

Meanwhile, microbes (some of which would be genetically engineered) would expand to cover the surface. Slowly, other species of plants, trees, and animals would be introduced to create a water cycle and a regenerative biosphere.

Other proposals include using orbital mirrors to direct sunlight onto the surface to melt the polar ice caps or redirecting asteroids to impact the surface and kick up dust to warm the atmosphere. Both ideas were suggested in a 1993 paper titled "Technological Requirement for Terraforming Mars," co-written by Mars Society founder Dr. Robert Zubrin and Christopher McKay of the NASA Ames Researcher Center.

Beyond Mars, there have been multiple proposals for terraforming Venus, though it would be thirty years after Sagan's initial proposal paper that new research would be conducted. In 1991, British scientist Paul Birch wrote "Terraforming Venus Quickly," where he introduced the idea of flooding Venus' atmosphere with hydrogen gas to convert atmospheric carbon dioxide into water and graphite. While the latter would need to be sequestered, enough water would be created to cover up to 80 percent of the planet in oceans.

Birch also proposed how the atmosphere could be cooled using solar shades stationed at the Sun-Venus L1 Lagrange point. Since Venus receives about twice as much solar radiation as Earth, this would reduce the greenhouse effect and could even freeze atmospheric carbon dioxide, producing dry ice that would fall to the surface.

Yet another proposal was made by Birch in a 1993 paper titled "How to Spin a Planet," where he suggested speeding up Venus' rotation. This could be done in numerous ways, like directing asteroids to strike one of Venus' hemispheres, with the idea of creating an Earth-like diurnal cycle, cooling the planet, and reducing atmospheric density.

Then came a paper in 1996 titled "The stability of climate on Venus," authored by planetary physicist Mark Bullock and astrobiologist David H. Grinspoon. They demonstrated how the atmosphere could be reduced by introducing refined magnesium and calcium to the atmosphere. These elements would bond with carbon dioxide to create carbonates that could be harvested and used as building materials.

In 2003, NASA scientist Geoffrey A. Landis released a study titled "Colonization of Venus," indicating how cities could be built above Venus' upper cloud layer. At this altitude, temperatures would be tolerable for humans, and the atmospheric density would allow the cities to remain afloat. These cities could act as solar shields and processing stations where carbon dioxide could be converted into super-materials (like graphene).

Which is better?

Both visions present their share of benefits and challenges. For instance, the cost of either vision would be astronomical and require an incremental approach. They would both require extensive labor in hostile environments (which could be handled by robots initially) and would entail generations of adjustment. But given the right strategies, neither vision presents any insurmountable challenges. But which one is arguably the more feasible of the two?

When it comes to habitats in space, the benefits are clear. By creating closed-loop ecosystems in space that simulate artificial gravity, humans could settle anywhere in the Solar System. Compared to settling on other planets, O'Neill Cylinders and other rotating space stations are the more economical option. They can be built in near-Earth space, tailored to meet human needs, and spun up to simulate Earth-normal gravity.

As he explained in the High Frontier, O'Neill and his students considered what the most promising places to settle in the Solar System would be. They concluded that planets were unsuitable for multiple reasons:

"The prospects for colonization of other planetary surfaces are unappealing. First, the total areas involved are too small: the Moon and Mars total only about the land area of Earth; neither has an atmosphere. Both have the wrong gravity for maintenance of our bodies in good health, and the Moon has a fourteen-day night which would require any colonists there to do without natural sunshine for weeks at a time."

In 1979, Gerard K. O'Neill described the motivations for his space settlement studies in an interview with Omni Magazine. As he indicated, the benefits of the "High Frontier" include relieving population pressures (and the social problems this entails). In addition, these habitats would not be reliant on Earth since they could be built using space resources and provide their own food, fuel, and energy. As he explained:

"First of all, there would be fewer people living on Earth and an increasing fraction living in space, where there's unimaginable room. Those in space colonies would of course find the situation much more open and free. They'd be living in relatively small-scale structures, in habitats that would be community-size rather than nation-size. With a few thousand to perhaps fifty thousand people in each space colony, government could be as simple and intimate as a New England town meeting.

"Yet each colony could be quite self-sufficient, using pure solar energy to generate power for travel, agriculture, environmental control, and so on. Since the colony would be growing its own food, there would be no reason for it to tie into a large-scale governmental structure."

Blue Origin founder and CEO Jeff Bezos also lauded the benefits of space habitats during a presentation he made in May 2019. Speaking to Blue Origin employees, Bezos articulated his vision for a civilization of a trillion humans in the solar system, which he posited would also mean a civilization with "1000 Mozarts and 1000 Einsteins," which would only be possible through the creation of "O'Neill colonies":

"These are very ideal climates. They're shirtsleeve environments. This is Maui on his best day, all year long. No rain, no storms, no earthquakes. What does the architecture even look like when it no longer has its primary purpose of shelter? We'll find out. But these are beautiful, people are going to want to live here. And they can be close to Earth so that you can return. This is important because people are going to want to return to Earth. They're not going to want to leave Earth forever.

"They'll also be really easy to go between. The amount of energy required to go between these O'Neill colonies, from one to another, to visit friends, to visit family, to visit one that is a recreational area. Very, very low energy needs to transport, and quickly. It's a day trip."

Many of the same benefits are cited when making a case for terraforming. These include accessing the resources of the Solar System, having new places to settle, and opportunities for scientific research, innovation, and social experimentation. Moreover, establishing infrastructure and a human presence on other planets has the added benefit of opening up yet more places in the Solar System for human exploration and settlement.

In his book, Entering Space: Creating a Spacefaring Civilization (2000), in the chapter titled "Extraordinary Engineering," Robert Zubrin explained how terraforming is essential to humanity's long-term expansion and settlement of space:

"A major challenge that humans will face as we become a Type II and then Type III civilization is that of transforming the environments found on other planets to more Earth-like conditions. This must be done because environments friendly to life are a product of the activity of life. Thus, as humans move out into space it is unlikely that we will find environments that perfectly suit our needs. Instead, as life and humanity have done historically on Earth, we will have t improve the natural environments we find to create the worlds we want. Applied to other planets, this process of planetary engineering is termed 'terraforming.'"

This tracks with what O'Neill and his students concluded in their original study. As they explained, the resources of the Moon and asteroids are needed to create their proposed habitats. While they emphasized how these habitats could be self-sustaining, there's no guarantee that they won't need to be replenished with resources (such as water and minerals) over the long haul.

Zubrin and other proponents have also noted that the lower gravity on the Moon and Mars means that less energy is needed to launch payloads into space. This makes them more suitable for resource extraction and could facilitate the migration of heavy industry (like mining and manufacturing) away from Earth. Zubrin also noted in Entering Space how a settlement on the Moon is preferable to ones in near-Earth space:

"As a destination for space colonization, the Moon has the undeniable advantage that it is the closest of any major or minor planetary bodies, reachable with existing chemical propulsion in a three-day flight. It is also clear that we have the capability to establish permanent bases on the Moon...

"The lunar surface contains vast amounts of oxygen, silicon, iron, titanium, magnesium, calcium, and aluminum, tightly bound into rocks as oxides, but there nevertheless... These resources give the Moon an enormous advantage as a destination for colonization over geocentric orbital space, where there is nothing at all to work with."

Why not do both?

To break it down, building human settlements on other planets means having to contend with long-distance travel, hostile environments, higher radiation, and lower gravity. Building habitats in space addresses many of these problems but also comes at a tremendous cost. The resources required to build ones large enough to house billions (or a trillion) of humans would mean mining near-Earth asteroids, the Moon, Mars, the Asteroid Belt, and perhaps beyond.

When comparing the two approaches, one can't help but notice that they present what might be called "complementary problems." In short, the drawbacks of one can be addressed by the benefits of the other. For example, settlements on the Moon, Mars, Venus, and beyond could provide the necessary building materials to create habitats in space. Once complete, these rotating space stations could be visited by planet-bound humans who prefer to live on rocky bodies but require periodic exposure to Earth-normal gravity (call it "gravity therapy").

What's more, space habitats could serve as stepping stones to other planets in the Solar System. Within the Earth-Moon system, rotating stations could be constructed using materials harvested from NEAs and the Moon. These could then facilitate settlements on the Moon and transits to Mars, where more habitats could be built using materials from the Red Planet. Once operational, these habitats could help ferry settlers to and from the Martian surface and resources destined for Earth and elsewhere.

You can't build massive space habitats without harvesting resources from nearby asteroids, planets, and satellites. Similarly, you can't have healthy settlers on other planets where the gravity is low, not without periodic exposure to Earth-normal gravity. And if you're planning on spreading throughout the Solar System and leveraging the resources to terraform other planets, you need all the infrastructure you can handle!

In short, to ensure our continued survival and development as a species, humanity's best option is to "go multiplanetary" and establish "Islands in Space." This will ensure that we have all the space, resources, and energy we need, plus endless opportunities for growth and social experimentation. It's a bold vision and would require tremendous work. But in exchange, it promises tremendous returns!