O’Neill Cylinders: Inside the forgotten plan to build spinning worlds for humanity beyond Earth

Long before fantasies of terraforming Mars captured the public imagination, a different, perhaps grander, vision for humanity’s future in space took shape. In the 1970s, a Princeton physicist named Gerard K. O’Neill asked a simple question: is a planetary surface the best place for an expanding technological civilization? His answer was a resounding no. Instead, he proposed something straight out of science fiction yet grounded in rigorous physics: colossal, spinning artificial worlds built in the vacuum of space. These “islands in the void,” known as O’Neill Cylinders, weren’t just space stations; they were meticulously designed habitats capable of supporting millions of people in an Earth-like environment, complete with blue skies, rolling hills, and pleasant weather.

The vision of Gerard K. O’Neill

The concept of the O’Neill Cylinder was born in the fervent technological optimism of the post-Apollo era. Gerard K. O’Neill, an accomplished physicist, was not a science fiction writer but an educator trying to engage his students. He challenged them to design a large-scale space habitat using only 1970s technology. The results of these studies, refined over years and presented at NASA conferences, were surprisingly practical. O’Neill argued that moving to space wasn’t about escaping a dying Earth, but about tapping into the limitless resources of the solar system.

His core argument was elegantly simple: planets are inefficient. They have deep gravity wells that make launching materials incredibly expensive, limited surface area, and fixed day-night cycles. Why not build our worlds where we need them? He proposed constructing these habitats at Lagrange points, stable gravitational pockets in space, primarily between the Earth and the Moon. From here, humanity could access:

• Constant solar energy: Uninterrupted, 24/7 sunlight to power a thriving civilization.
• Zero-gravity construction: Assembling massive structures without the constraints of gravity would be far easier.
• Extraterrestrial resources: Sourcing raw materials like aluminum, silicon, and titanium from the Moon or asteroids would be vastly more energy-efficient than lifting them from Earth.

This was a fundamental shift in thinking. The goal was no longer to conquer other worlds, but to create new ones perfectly tailored to human needs.

Designing a world from scratch

The most famous of O’Neill’s designs was the “Island Three” habitat, now universally known as the O’Neill Cylinder. It was a breathtaking feat of engineering designed to house millions. The structure consisted of a pair of counter-rotating cylinders, each about 5 miles in diameter and 20 miles long. The counter-rotation was crucial, as it would cancel out any gyroscopic effect, allowing the habitat to keep its solar panels perpetually aimed at the sun.

The magic of the design was its ability to create an Earth-like environment. By rotating roughly once every two minutes, the centrifugal force on the inner surface would simulate a full 1G of gravity, allowing people to walk, plants to grow, and water to flow just as it does on Earth. The interior surface area of a single pair of cylinders would be over 500 square miles, comparable to a large city and its surrounding countryside.

The interior landscape was divided into six longitudinal strips: three land areas and three enormous windows. Giant external mirrors would reflect sunlight through these windows, and by adjusting their angles, residents could create a familiar 24-hour day-night cycle, seasonal variations, and even control the weather. Looking up from a field or a town inside the cylinder, one wouldn’t see a ceiling but a blue “sky” arcing overhead to the opposite side of the habitat, with other towns and forests visible miles away. It was a plan to build paradise, piece by piece.

Building the islands in the void

O’Neill and his supporters at NASA knew that the biggest hurdle was not the physics but the logistics. How could humanity possibly construct something so massive? The answer was to live off the land, but in this case, the “land” was the Moon. Launching millions of tons of material from Earth would be prohibitively expensive. Instead, O’Neill proposed establishing a small lunar base to mine the moon’s soil, which is rich in metals and silicon.

The key piece of technology to make this work was the mass driver, an electromagnetic catapult. This device would use powerful magnetic fields to accelerate buckets of lunar dust to escape velocity, flinging them into a precise orbit where a space factory would catch them. Without the need to overcome a thick atmosphere or deep gravity, this was a far cheaper way to get building materials into space.

Once in orbit, these materials would be processed in solar-powered smelters and fabricated into the components for the cylinders. Construction would take place in the weightlessness of space, with automated machinery and a dedicated workforce assembling the habitat piece by piece. The initial habitat would then serve as the industrial base to build more, leading to an exponential expansion of humanity into the solar system, a self-sustaining breakout into the cosmos.

The legacy and future of space habitats

So why are we not living in spinning space worlds today? The grand vision of the 1970s faded as political winds shifted. The focus moved from large-scale colonization to the more pragmatic, Earth-orbit-focused Space Shuttle and later the International Space Station. The sheer upfront cost and complexity of O’Neill’s plan, while theoretically sound, were too immense for a world grappling with economic and political challenges. The dream was shelved, relegated to the pages of science fiction and the hearts of space enthusiasts.

However, O’Neill’s legacy is far from forgotten. His ideas have profoundly influenced a generation of scientists, engineers, and thinkers. They have become a staple of science fiction, appearing in everything from the Gundam anime series to films like Elysium. More importantly, the core principles of his vision are experiencing a powerful resurgence. Entrepreneurs like Jeff Bezos have explicitly cited Gerard K. O’Neill as their primary inspiration, with Blue Origin’s long-term goal being “millions of people living and working in space.” The renewed push for lunar missions and the burgeoning field of asteroid mining are direct steps toward acquiring the off-world resources O’Neill knew were essential. The dream is being reawakened.

In conclusion, the O’Neill Cylinder was more than a fantastical concept; it was a detailed, science-based roadmap for humanity’s future. Gerard K. O’Neill proposed a path that sidestepped the challenges of planets, opting instead to create custom-built worlds using the vast material and energy resources of space. His plan involved mining the Moon, using mass drivers for transport, and building spinning habitats that could replicate Earth’s gravity and environment. While the political will to build them faded in the 20th century, the logic behind them has never been more relevant. As we look back to the Moon and beyond, the forgotten plan for islands in the void offers a compelling, inspirational, and perhaps even necessary vision for our future among the stars.

Image by: Maria Orlova
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