The Air We Breathe: Lessons in Atmosphere from the ISS and Biosphere 2
(08/01/2025)
Welcome back. In our last article, we introduced our two guiding stars for grounding our closed-loop city in reality: the ISS, our pragmatic engineer, and Biosphere 2, our ambitious gardener. Today, we put them to the test on the most critical and immediate challenge of any sealed habitat: creating and maintaining a stable, breathable atmosphere.
The Engineer: The ISS's Mechanical Lung
For over two decades, the International Space Station has kept astronauts alive in a vacuum. Its approach to air is a masterpiece of reliable, mechanical life support.
Making Oxygen: The ISS doesn't carry all its oxygen; it makes it. The Oxygen Generation System uses electricity from the station's solar panels to split water (H₂O) into hydrogen and breathable oxygen (O₂). The oxygen is released into the cabin, and the hydrogen is either vented or, more recently, used in other systems.
Removing Carbon Dioxide: Every breath we exhale releases carbon dioxide (CO₂), which is toxic in high concentrations. The ISS's Carbon Dioxide Removal Assembly continuously scrubs the air, capturing CO₂ molecules in beds of a porous mineral called zeolite. In a crucial step toward closing the loop, the Sabatier system then reacts this captured CO₂ with the waste hydrogen from the oxygen generator. This reaction produces water—which is recycled back into the system—and methane, which is vented into space.
The lesson from the ISS is one of absolute reliability. Through redundant, well-understood mechanical and chemical processes, it provides a safe atmosphere, day in and day out. It is a perfect mechanical lung, but one that still exhales valuable carbon (as methane) into the void.
The Gardener: Biosphere 2's Planetary Ambition
Biosphere 2 took a radically different approach. It was designed to breathe like a planet. The vast array of plants within its rainforest, farm, and other biomes were meant to produce all the oxygen and consume all the CO₂ generated by the crew and the soil, creating a self-regulating, living atmosphere.
The Humbling Lesson: The experiment provided one of the most important lessons in the history of closed systems. The project's scientists, in an effort to create incredibly fertile soil for the plants, inadvertently created the perfect conditions for a population explosion of soil microbes. These trillions of invisible organisms consumed oxygen and released CO₂ at a rate that the plants simply could not keep up with.
The Result: Over 16 months, atmospheric oxygen plummeted from 21% to a dangerously low 14%. The biospherians grew fatigued, and to ensure their safety, pure oxygen had to be injected from the outside, breaking the seal.
The lesson from Biosphere 2 was one of profound humility. A living atmosphere is not a simple equation of plants versus animals; it is an infinitely complex web of seen and unseen life. A purely biological system is powerful, but not yet predictable or stable enough to be trusted with human lives.
Synthesis: The Hybrid Atmosphere of Our City
Our closed-loop city learns from both the engineer and the gardener. It cannot afford the resource loss of the ISS nor the instability of Biosphere 2. Therefore, it employs a hybrid, dual-system approach.
The Mechanical Core: For guaranteed safety, the city has an engineered life support system based on the ISS model. Electrolyzers produce a constant baseline of oxygen, and advanced scrubbers are always on standby to remove CO₂ and other contaminants. This is the non-negotiable safety net.
The Biological Lungs: The primary work of atmospheric regulation is done by the city's vast biological systems: its vertical farms, parks, algae bioreactors, and green walls. This massive plant and algae biomass generates the bulk of the oxygen and absorbs most of the CO₂, creating a vibrant, healthy, and naturally balanced environment.
Crucially, we fuse the two systems to create a truly closed loop. The CO₂ captured by the mechanical scrubbers is not vented into space. It is treated as a valuable resource and piped directly to the vertical farms and algae tanks, providing a rich source of carbon to boost their growth and oxygen production. The engineer thus feeds the gardener, creating a single, robust, and regenerative system.
With a stable atmosphere secured, we next turn to the liquid of life. Join us next time as we explore the challenges of water recycling, learning from the ISS's advanced purifiers and Biosphere 2's artificial rain.
