Imagine a sprawling megastructure floating endlessly through space. One day, humans
may occupy an orbital settlement several hundred kilometers from Earth. Perhaps, humans will settle
closer to the moon, or in the main asteroid belt beyond Mars. If humans ever migrate into space,
what would our settlement look like, and how would we navigate life in the final frontier?
A space colony must withstand the harsh conditions of space,
but it must also create a safe and comfortable environment for generations of human colonists.
Any habitat must have replenishable sources of food, water, breathable air, and medical supplies;
and it must spin to produce artificial gravity, though scientists are unsure how much gravity we
need. Space colonists also require protection from solar radiation. The Sun releases a continuous
stream of charged particles, called solar wind, and intermittently strikes in deadly
bursts known as solar particle events. Further radiation from galactic cosmic rays and charged
particles trapped in the Van Allen Belts may endanger the occupants of any space colony.
Scientists have overcome some of these hurdles while constructing the International Space Station
or ISS. In 1998, the first modular piece of the ISS launched into Earth’s orbit. Over the next
13 years, international astronauts and engineers pieced together a microgravity research laboratory
roughly 400 kilometers above the Earth. The ISS orbits our planet every 90 minutes, flying about
28,000 kilometers per hour. For more than 20 years, the ISS has safely accommodated
teams of astronauts and remains the longest habitable space station ever constructed.
The ISS is not a permanent, self-sustaining habitat, nor does it provide a comfortable
environment for scalable colonization. Astronauts occupy cramped corridors and
survive on vacuum-packed or freeze-dried meals. In this challenging, isolated environment,
astronauts are susceptible to social and psychological problems, like anxiety and
depression. Prolonged exposure to weightlessness, or microgravity, also causes changes to the
body’s organ systems, including muscle atrophy, visual impairment, and a loss of bone density.
Though the ISS is not a permanent settlement, it has yielded valuable insight into life in free
space. Were humans to construct a larger structure — say, for 100 or 1,000 colonists — we may build
upon the technologies used aboard the ISS. For instance, any space settlement must be
equipped with a closed-loop life support system. The ISS life support system is not a closed-loop
system, but it is generations beyond the earliest days of spaceflight when astronauts carried all
their oxygen and water in tanks. Over the years, the ISS has served as a test platform
for an Environmental Control and Life Support System, or ECLSS, which recycles water, maintains
temperature, and ventilates habitable cabins. Two devices generate oxygen by electrolyzing water,
while carbon dioxide and other volatile gases are absorbed and vented into space.
For the past few years, the European Space Agency has been testing a different life support system,
which processes roughly 50% of CO2 on the ISS to create oxygen. Newer still, a photobioreactor,
built by German scientists, uses photosynthetic algae to consume
CO2 and release oxygen in the presence of light. This system also produces biomass,
demonstrating how future life support systems could supply astronauts with replenishable food.
Still, some oxygen must arrive from Earth in tanks,
not only for emergency storage, but also for use in spacesuits.
Currently, the ISS cannot pressurize generated oxygen to support astronauts on spacewalks.
The station also relies on regular shipments of nitrogen, the most abundant gas in the cabin air.
As future spaceflights, like NASA’s project Artemis, take astronauts farther into space, life
support systems will advance incrementally. New systems may generate an Earth-like cycle, which
provides breathable air and replenishable food for astronauts, and eventually space colonists.
If we design an efficient closed-loop system, how might scientists construct a habitat capable of
supporting a large colony in free space? Scientists have attempted to answer that
question for over 100 years, but many of the most influential ideas come from the mid-1970s.
At the time, an influential physicist named Gerard O’Neill believed humans had the
technology to construct orbital cities, potentially capable of housing millions
of people. O’Neill’s first conceptual space colony features huge cylinders,
about 32 kilometers long and 6 kilometers in diameter. Each cylinder encases a long
strip of artificial terrain, yielding 1,300 square kilometers of living space.
The O’Neill Cylinder, as it is known today, creates artificial gravity by rotating on its
axis. The outer shell has thick radiation shields, and its walls are covered in solar windows,
providing colonists with warmth and light. Incredibly, the O’Neill Cylinder was designed
to support a thriving natural ecosystem. Inside each cylinder, O’Neill imagined rivers, forests,
and parks, alongside cities and neighborhoods, populated by over a million colonists.
Another innovative megastructure was proposed in 1975 by a NASA Summer Study at Stanford
University. Their conceptual model, known as the Stanford Torus, is a donut-shaped habitat with a
hollow residential tube. A population of 10,000 colonists could occupy the tube’s inner ring,
where cars and homes are held in place by the torus’s rotational gravity.
Shortly after the Stanford Torus, O’Neill proposed another habitat called the Bernal Sphere. In this
design, O’Neill downsized his massive cylinders to a globe less than 2 kilometers in diameter.
On account of its smaller size, the Bernal Sphere rotates more quickly than a larger-scale colony,
completing just over one rotation per minute to produce earth-normal gravity.
These inventive habitats inspired many more concepts in the 21st century. In 2006,
the Kalpana One Orbital Settlement was proposed as an improvement upon
the megastructures from the 1970s. Kalpana One uses rotating cylinders,
like the O’Neill Cylinder, to create Earth-normal gravity. Caps at the end
of each cylinder provide a continuous source of light and solar power, while shields in the
hull structure protect colonists from radiation. More recently, two scientists proposed a small
rotating habitat that can launch directly from the surface of the Earth. One of those scientists also
proposed an orbital settlement, without radiation shields, in Equatorial Low Earth Orbit, thereby
reducing the settlement’s total mass by over 95%. Unfortunately, few organizations possess the
resources to build or transport materials for a space colony, even a light or compact one. Thus,
scientists have proposed innovative solutions to lower the cost of construction and transportation.
If humans construct a colony near the Moon,
we could cheaply build structures out of lunar regolith. Raw materials, launched
from the Moon to a construction site in free space, could be assembled into walls, windows,
and machines using 3D printing technology. For colonies constructed in the main asteroid belt,
humans could harvest resources from nearby asteroids and the dwarf planet Ceres.
We may possess the technology to put a structure in space, but it’s unlikely humans
will create an affordable and replenishable habitat in the coming century. However, new,
hyper-efficient designs may accelerate humanity’s push toward space colonization. Today, our society
is largely bound to the Earth, but tomorrow, our descendants may be living amongst the stars.