The titular space station featured in the classic television space opera Babylon 5 is a five-mile-long ‘tin can,’ essentially a giant, hollow cylinder in space, home to a quarter of a million humans (and aliens). “It’s a port of call,” the opening narration intones. “Home away from home for diplomats, hustlers, entrepreneurs and wanderers … two million, five hundred thousand tonnes of spinning metal, all alone in the night.”

The design of the Babylon 5 space station still remains one of the best examples of an O’Neill space habitat in fiction. Popularized by Princeton University’s Gerard O’Neill in the 1970s and early 1980s, his idea was to build giant cities in space, each home to up to several million people. It would be a new ‘high frontier,’ which O’Neill envisaged we could inhabit by the dawn of the twenty-first century. Working with 1970s technology, the scale of O’Neill’s ideas were far too ambitious. But just with a new Babylon 5 on the horizon, maybe it’s also time for us to revisit O’Neill’s concepts and ask the question: could cities in space still be humanity’s future?

O’Neill promoted his ideas on talk shows, in public presentations, in his 1976 book The High Frontier, and even in testimony before Congress. He imagined three scales of space station. Island One and Two would be spherical, measuring about 500 meters and 1,600 meters (one mile) in diameter, respectively. A popular variation on Island Two was the Stanford Torus, which was a ring rather than a sphere, about 1.8 kilometers (1.1 miles) across, with the space inside the habitat ring 130 meters in diameter.

Island Three would have been the big tamale: two counter-rotating cylinders 6.4 kilometers (four miles) wide and up to 32 kilometers (20 miles) long. The various habitats would be built using material mined from the Moon and near-Earth asteroids (asteroids may be easier to reach because their lower gravity means less rocket fuel is required to enter into orbit, to land, and then to launch from.) This material would then be catapulted to the deep-space habitat-assembly site by an electromagnetic slingshot known as a ‘mass driver’.

Impressive, yes? But O’Neill’s plans depended upon an industrial and engineering infrastructure in space that we just didn’t have in the 1970s, and we frankly still aren’t anywhere close.

“O’Neill’s habitats were conceived to be assembled in a single extended operation on an epic scale,” says Anthony Longman, a Cambridge University-trained architect who is one of the key players in a recent NASA Institute for Advanced Concepts (NIAC) study into space habitats.

Much was contingent on the space shuttle, which back in 1976 hadn’t even flown any test flights. The original plan for the shuttle fleet was to launch hundreds of missions every year and allow humans to fully develop low-Earth orbit. Yet throughout its entire lifetime, the space shuttle flew only 135 missions.

“The shuttle simply was not up to it,” says Jerry Stone, who leads a study for the British Interplanetary Society (BIS) updating the original Island One design. “That was a major reason why O’Neill’s ideas couldn’t go ahead at the time.”

With the shuttle unable to perform as the workhorse needed to bring launch costs down, and with the cost of building O’Neill’s space habitats vaguely estimated to be anywhere between $4 billion and $200 billion, turning concept into reality was out of reach.

“It required sustained investment on an enormous scale over a long time frame,” says Longman. “The level of technological and financial risk involved made his proposals very easy to shoot down.”

For much of the four decades since, the concept of building large space habitats has remained impractical, but things are beginning to change. Like it or loathe it, it’s undeniable that the billionaire space race is helping to drastically reduce launch costs, while providing an impetus to conquer the final frontier not seen since the heady days of Apollo.

“We need the appropriate infrastructure to achieve something like [an O’Neill cylinder],” says Stone.

Stone met O’Neill in 1977 when the Princeton professor came to London to give a talk at the BIS headquarters. “A few of us took him for a meal after the meeting, and we had a fantastic conversation with him. That’s what started off my interest in the subject.”

This interest has culminated in the SPACE (Study Project Advancing Colony Engineering) initiative, which is a long-term study being conducted by the BIS that is exploring how we could build O’Neill’s habitats today, and how they would be different compared to the plans from the 1970s. The initiative has two deliberate restrictions to help it stay grounded in reality. One is that its projected cost cannot exceed that spent on the Apollo program (accounting for inflation) and two, it would all have to be done with current technology.

The SPACE team realized that the construction of a space habitat wouldn’t begin with building Island One. Instead, they came up with the idea for an ‘Island Zero’ — a small station, designed as a place where the workforce could live as they receive material catapulted from the Moon or asteroids with which to build larger habitats. Island Zero would be constructed from eight individual units, not dissimilar to the inflatable modules designed by Bigelow Aerospace, and launched by vehicles such as SpaceX’s Falcon 9, or even Starship if its development proves successful. One of the key concepts behind O’Neill’s original habitats was the generation of simulated gravity through rotation. So to be a true O’Neill habitat, Island Zero would also have to generate the effect of gravity via centrifugal force through rotation about a central axis.

Studies have shown that the faster the rotation, the more the human body is unable to adapt to that rotation. Most humans struggle with rotations faster than 4rpm, while almost everybody seems happy with 1rpm or less, but in a small habitat that would be too slow to generate a simulated gravity of 1g. The sweet spot seems to be about 2rpm, and Stone’s SPACE team have therefore designed the eight units of Island Zero to be distributed radially outwards from a central node, like spokes on a wheel, with a ring-shaped cylinder connecting them to produce an outer wheel with a radius of 250 meters, where the crew can experience 1g of gravity from a rotation rate of about 2rpm.

The Island Zero concept would be highly adaptable, and suitable not only for low Earth orbit, but for a base at L5 or the Moon. “We could use an Island Zero as the proposed Lunar Gateway,” suggests Stone. An Island Zero could even be placed into Martian orbit to support ground operations. Plus, an Island Zero could be made larger by adding more units and filling in the gaps in the spokes, or extending them.

More pertinently, Island Zero would be a base of operations from which to construct Islands One, Two, and Three. “It’s basically a bootstrap,” says Stone. With advances in materials science, such as the development of new alloys and new materials such as carbon nanotubes, as well as the giant leaps made in robotics and engineering that we can see in everyday life, we are in a far better position for the engineering challenges of building a city in space.

Anthony Longman has also recognized the need to start small. In 2019, as part of a team led by Robert Skelton, who is a Professor of Aerospace Engineering at Texas A&M University, he contributed to a new report on space habitats for NASA’s NIAC program. This report contained a new and ingenious design of space habitat, one that starts off small and economical, but which can then expand from the inside out.

“My approach from the first has been to ask what impact the requirement for expandability, for closed ecosystem life support, and for landscape for recreation, would have on the design of a space habitat,” says architect Longman.

It begins with a small, cylindrical, inflatable unit designed to have the capacity for repeated expansion. Around this core, an initial, half-meter-thick radiation shield built from asteroidal material is assembled. This shield doesn’t just protect from solar radiation but acts as a cocoon inside which the habitat can grow. Doors in the radiation shield allow fresh construction materials to be brought inside the habitat, while the shield itself is made bigger and thicker. Along the rotational axis, a microgravity industrial area is set up, the products of which could pay for the continued development of the habitat. Eventually, the habitat would grow to a radius of 224 meters, with four rotating segments — the cylindrical habitat enclosed by a membrane, the radiation shield, and two mirrors that reflect sunlight into the habitat — all spinning at 2rpm to generate 1g of simulated gravity. The central region is hollow, the interior surface covered with 2.4 million square meters of agricultural space, plus a central park of 10,000 trees for the habitat’s population of 8,000 people to enjoy. Beneath the interior surface are 52 levels for homes, office space, and so forth.

Key to Skelton and Longman’s proposal is something called tensegrity, which balances opposing forces of compression and tension using structures linked by cables. You may have seen tensegrity, which is a word coined by Buckminster Fuller, in action in something as simple as desk toys, but Skelton was the first to develop the principles of tensegrity for large-scale aerospace engineering.

Within Skelton and Longman’s habitat, “the atmospheric pressure provides the compression force and is balanced by the tension forces in the enclosure membranes resulting from atmospheric pressure,” says Longman. This reduces the amount of mass needed to build a stable structure. And the upshot of this is the habitat wouldn’t just be a solid and expensive ‘tin can,’ but a more cost- and mass-efficient structure.

An O’Neill habitat is not intended to be just a space station or even just an orbiting city. It’s designed to be a living, breathing environment with a delicately poised, closed-loop biosphere. The park and agricultural space are vital to making the habitat reasonably self-sufficient.

“When people talk about life support technology, they are generally talking about hydroponics, which is a great solution for many situations, but not for really long-term applications like space habitats,” says Longman. “That’s because on a large scale a hydroponic system has an unacceptable number of failure points.”

Instead, he sees hydroponics supporting a skeleton crew for a few early years, while they work on developing a permanent, soil-based life-support system.

The soil won’t be shuttled up from Earth — too much mass, too much cost. Instead, lunar regolith can be turned into soil using the same species of fungi that we use to clean up toxic waste on Earth. In particular, the fungi is able to metabolize harmful, carcinogenic molecules called polycyclic aromatic hydrocarbons (PAHs), and after a few years, the fungi makes the resulting soil suitable for growing plants and food.

At this point, the habitat could support its maximum population, which raises a question: who would such habitats be for?

“It is not my expectation that the early examples will be settlements, so much as commercial enterprises, analogous to today’s cruise ships, a few of which now host permanent resident populations,” says Longman. He suggests that the first rotating habitat would be similar to an Antarctic Research Station, perhaps orbiting Mars. But O’Neill’s vision was for millions of people to eventually be living in orbiting cylinders.

“Millions of people living without fear of volcanoes, earthquakes, tsunamis, hurricanes, and simply without the weather extremes that we have in most places here on Earth. Life on a habitat could be much more comfortable,” says Stone.

However, the countries on Earth that are often most affected by natural disasters frequently tend to be poorer. Are their populations really likely to be those who migrate to space? Or is it more likely that an orbiting habitat would become the preserve of the wealthy? Stone doesn’t agree.

“There’s going to be all kinds of jobs [on a habitat], from the manufacturing side of things. And police, hospitals, schools — if you’re an engineer, a carpenter, you will be wanted up there,” he says.

Avoiding the transferral of the social inequalities that we have on Earth to a space habitat is possibly going to be the biggest challenge of all when building an O’Neill cylinder.

There are larger issues at stake, however. With all the potential existential risks that we face right now, the idea of having a lifeboat for humanity is appealing. “We would be looking at a situation whereby if something happened to the Earth, it may not necessarily mean the end of the human race,” says Stone.

The advent of the billionaire space race means that the founding of at least an early stage habitat — perhaps an Island Zero, or the beginning of Longman’s expanding, cocooned habitat — is no longer beyond us. Microgravity industries, and solar power satellites generating energy to beam down to the people of Earth, could help fund further development of these habitats. And as the infrastructure grows, so does their commercial viability. Furthermore, we could move much of our polluting industries and energy-consuming data centers off Earth and into space, helping to pave the way for a cleaner planet. In the words of Babylon 5, the development of a space habitat would be a “dream given form,” and a solid foothold in space from which we could really begin spreading farther into the Solar System.

It’s a dream that began with O’Neill’s vision, but it was a vision that was ahead of its time. Perhaps its time is now, and Stone sums up the mood that reflects this growing optimism.

To learn about Longman and Skelton’s concept for a space habitat, visit Longman’s website at Sky Frame Research