What Is An O’Neill Cylinder?

An O’Neill cylinder is a space-colony design made of two counter-rotating cylinders, each about 20 mi (32 km) long and 5 mi (8 km) wide, proposed by physicist Gerard K. O’Neill in the 1970s. Spinning roughly 28 times an hour, the cylinders use centrifugal force to mimic Earth-like gravity for the people living on their inner surface.

Many people believe that the Earth will soon be in danger and the sprawling nature of humanity is the undeniable cause. With the rapid technological progress and advancement of the past few centuries, we’re quickly exhausting the resources from planet Earth in order to power our industrial needs and global commerce. Many futurists feel that we will be left with no option but to explore and colonize space if we intend to survive into a future when resources on Earth can no longer meet our requirements.

Overpopulation is an imminent challenge that makes the need for space travel and colonization even more urgent. That being said, building a space habitat is no easy pursuit and is loaded with daunting challenges, such as the need for construction facilities in space, the recreation of livable communities in space, the recycling and processing of waste, the simulation of artificial gravity, and (most importantly) convincing governments and global organizations that this venture is worth pursuing.

The prospect of space colonization paves the way for devising methods to extract energy from resources on other planets. On Earth, harnessing energy from the Sun using solar panels isn’t particularly efficient, and faces inevitable barriers caused by the atmosphere and the daily occurrence of darkness (e.g., nighttime).

However, in space, solar constructs can perpetually harness energy from the Sun without interruption. Utilizing this copious amount of energy would permit us to travel throughout our solar system without worrying about energy expenditure. Moreover, chemical resources would be in great supply in our solar system. To begin with, NASA has recently embarked on a project to generate fuel, water, and oxygen from resources present on the Moon.

Given these foundations for why organizations should foray into developing a space habitat, allow me to introduce the O’Neill cylinder, a space settlement design consisting of two counter-rotating cylinders proposed by renowned physicist Gerard K. O’Neill in the 1970s.

Origin Of The Idea For The O’Neill Cylinder

While teaching physics to his students at Princeton University, O’Neill assigned them the task of designing a megastructure in space in order to demonstrate that living and surviving in space is actually a possibility.

His students came up with numerous designs to accommodate human habitation in space. After a long session of brainstorming, O’Neill boiled their theories down to the idea of a cylinder-like space settlement design. He laid out the concept in a 1974 Physics Today article titled The Colonization of Space, then fleshed it out in his 1976 book The High Frontier: Human Colonies in Space. The largest of his proposed habitats, the one most people now picture, was labeled Island Three, which is the design we have come to call the O’Neill cylinder (or an O’Neill colony).

Design Of The O’Neill Cylinder

The O’Neill cylinder design consists of two cylinders rotating in opposite directions on a bearing to mitigate the gyroscopic effect. Each cylinder was proposed to be about 20 miles (32 km) long and 5 miles (8 km) in diameter, with 6 broad stripes running along its length (3 habitable land surfaces and 3 windows). O’Neill envisioned industrial processes and recreational facilities to be located on the central axis in a virtually zero-gravity environment.

Gravity Simulation

One key difference between living on Earth and living in space (or on any other astronomical body) is the difference in gravity. Artificial gravity is needed for stability, and the O’Neill cylinder has a provision to achieve exactly that. As the two giant cylinders spin on their axis, anything resting on the inner surface is pushed outward against it, and that centrifugal effect feels just like gravity. We can pin down the spin rate from the acceleration equation a=v²/r. Plugging in Earth’s gravitational acceleration (9.81 m/s², or 32.2 ft/s²) and a radius of about 4 km (the 5-mile diameter cut in half), the cylinder needs to rotate roughly 28 times per hour, slow enough that residents would not feel dizzy, to reproduce a comfortable 1g.

Earthly Environment Simulation

Maintaining an atmosphere with a constitution similar to that of Earth is the next challenge when building a space habitation. The O’Neill cylinder is prudently designed with a ratio of gases similar to what is found on Earth. However, there is a caveat: the total pressure is only about half of that at sea level (with oxygen kept close to its share in ordinary air, topped up with nitrogen). This would not impact our breathing substantially, yet this minor trade-off pays off in two ways, cutting down the amount of gas needed to fill the habitat and reducing how thick and heavy the cylinder walls have to be. The proposed O’Neill cylinder also has provisions wherein the habitat would be able to control its own micro-climate using an arrangement of mirrors and by altering the ratio of gases in the cylinder.

Day And Night Simulation

With the human habitat situated in a vacuum (space), the cylinder essentially turns into a huge thermos! The theoretical O’Neill cylinder tried to overcome this issue by using a series of mirrors hinged on each of the three windows. This way, direct sunlight could be directed into the cylinder to simulate day time. Similarly, by turning the mirror away, a night-like ambience could be created. This simulated ‘night’ would also permit the heat produced biologically to radiate out of the cylinder.

What Are Island One, Island Two And Island Three?

O’Neill did not sketch a single blueprint. He laid out a graded series of habitats, numbering them by ambition. Island One is the modest first step: a slowly rotating sphere, known as a Bernal sphere, roomy enough for on the order of 10,000 residents living around its equator. Island Two is a larger sphere, an intermediate stage. Island Three is the giant most people picture, the pair of counter-rotating cylinders we have been describing here. Around the same time, a NASA summer study in 1975 worked out a parallel wheel-shaped design, the Stanford torus, sized for a similar population of roughly 10,000 people.

The point of starting small was practical. A Bernal sphere or a Stanford torus could plausibly be built first, prove that people can live and work in spin gravity, and pay its way before anyone attempted a structure 20 miles long. In that sense the O’Neill cylinder was always meant to be the destination rather than the opening move, an alternative to the far harder task of trying to terraform Mars.

How Big Can A Space Cylinder Get? The McKendree Cylinder

The size of a spinning habitat is limited less by imagination than by the strength of whatever its walls are made from. A cylinder spun fast enough to fake gravity is constantly trying to fling itself apart, much like an overspun flywheel, and only the tension in its hull holds it together. Build that hull from a stronger material and you can build a bigger drum before it tears. O’Neill sized his cylinders for the steel and aluminum available in the 1970s.

In 1996, engineer Thomas McKendree worked through what stronger, future materials might allow in the journal Nanotechnology, and in 2000 he presented a scaled-up version at NASA’s Turning Goals Into Reality conference. This design, now called the McKendree cylinder, imagines walls of carbon nanotubes or diamond-like carbon, both far stronger than steel. On paper that pushes the radius from O’Neill’s few kilometers out to hundreds of kilometers, and the length to several thousand, enough interior surface to rival the land area of a large country. It remains an intriguing thought experiment rather than a plan, since we cannot yet manufacture flawless carbon nanotubes at anything like that scale. Lofting the raw materials cheaply would also lean on ideas such as the space elevator.

Why Does Spin Gravity Have A Catch? The Coriolis Effect

Spinning to fake gravity works, but it is not a flawless copy of standing on Earth, and the differences are worth knowing. The first is the gravity gradient. Because the pull comes from rotation, and that pull grows with distance from the spin axis, your feet near the outer rim feel a little more gravity than your head, which sits closer to the center. The effect is small, and the bigger the cylinder the smaller it becomes, which is one more reason to build large.

The second is the Coriolis effect, the same sideways nudge that steers hurricanes on a spinning Earth. Inside a rotating habitat it pushes on anything that moves, so climbing a ladder, throwing a ball, or simply turning your head can feel subtly off and, when the spin is fast, can bring on dizziness and nausea. NASA’s review of artificial gravity notes that the remedy is a slow spin paired with a large radius, keeping the rotation down to only a few turns a minute at most. That is precisely why O’Neill’s cylinder is so enormous. At a radius of roughly 4 km it needs to turn only about 28 times an hour, well under one revolution per minute, slow enough that residents would barely notice either effect.

Despite the design of the O’Neill cylinder being technically sound, the idea is still too sophisticated to be built with our present technology, and would demand cheap, frequent launches and large-scale mining of materials in space first. Thus far, its implementation has been confined to the realm of science fiction. The concept does have a high-profile modern champion, though: Blue Origin founder Jeff Bezos, who studied at Princeton while O’Neill taught there, has said he envisions a future of giant O’Neill colonies housing a trillion people. Between Bezos’ vision and the broader push from companies like SpaceX, perhaps some day O’Neill cylinders will actually help humanity settle in the great vastness of space!