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Curriculum · Tutorial

Estimating atmospheric requirements for space settlements

Every settlement has to make its own air. In this tutorial you’ll size the atmosphere for a settlement — balancing what keeps people healthy, what keeps fires in check, and what it costs to ship and maintain.

⏱ 45–60 min🎓 High school (Grades 9–12)NGSS-aligned
Start the tutorial Teacher version

What you’ll be able to do

By the end, you can size an atmosphere and defend your choices.

  • 1Convert between total pressure, partial pressure of oxygen, and percent oxygen.
  • 2Explain why a lower total pressure forces a higher percent oxygen to keep people healthy.
  • 3Describe how oxygen concentration — not just partial pressure — drives flammability.
  • 4Weigh the competing demands of human comfort, fire risk, cost and structure when choosing an atmosphere.
  • 5Estimate how much of each gas a settlement needs using Dalton’s Law, Amagat’s Law and expansion ratios.

Part 1 · The core relationship

How total pressure sets the oxygen fraction

Air is a mixture, and each gas contributes its own “partial pressure”. Oxygen’s share of the total is what your body — and any fire — actually responds to.

Partial pressure of O₂ = Total pressure × Fraction of O₂Fraction of O₂ = Partial pressure of O₂ ÷ Total pressure

Example 1. A container has a total pressure of 700 mmHg and a partial pressure of oxygen of 150 mmHg. What percent oxygen do you have?

Example 2. You must have 20% oxygen in a system that runs at only 0.65 Earth atmospheres of pressure. What is the partial pressure of oxygen?

Try it yourself

Move the sliders and watch the oxygen fraction — and the comfort and fire read-outs — respond.

0.70 atm

Earth at sea level ≈ 1.0 atm

0.160 atm

Earth at sea level ≈ 0.16 atm

Resulting oxygen fraction

23%

Human comfort
Comfortable for people
Fire risk
Near Earth-like fire behaviour

Notice the tension: drop the total pressure to save on structure and gas, and the same life-supporting partial pressure of oxygen becomes a much larger fraction of the air — which pushes fire risk up. That trade-off is the heart of the design problem.

Part 2 · Keeping people healthy

How much oxygen do people actually need?

As a rule, the lower the total pressure, the higher the percent oxygen you need to keep people healthy. People survive at altitude, but not without consequences.

NASA’s foundational study Space Settlements: A Design Study (SP-413) recommends keeping the partial pressure of oxygen close to what the lungs need for good respiration — roughly 0.13 atm (~100 mmHg) — with a tolerance band on either side. Go too low and you risk hypoxia; too high and you risk other effects on the blood. We’ll use that recommended value in the sizing problem later.

Part 3 · Keeping fires in check

Why the oxygen fraction — not just the pressure — drives fire

A fire needs three things at once: fuel, heat, and oxygen (an oxidiser). Remove any one and it can’t burn — that’s the fire triangle.

The Apollo 1 tragedy showed the stakes: a spark in a pure-oxygen cabin caused a fire that killed three astronauts during a ground test. NASA later switched ground tests to a nitrogen–oxygen mix.

A key NASA finding: oxygen concentration matters more than partial pressure for flammability. A higher percent oxygen raises fire risk faster than a higher partial pressure does. So a designer wants to keep the oxygen fraction as low as human health allows.

The fire triangle
  • 🔥 Heat — an ignition source
  • 🪵 Fuel — anything that burns
  • 💨 Oxygen — the oxidiser

What does the trade-off look like?

Hold the oxygen partial pressure steady and lower the total pressure, and the oxygen fraction climbs fast. At a low total pressure, each extra bit of oxygen pushes the percentage up much more quickly.

Total pressure (atm)pO₂ (atm)% Oxygen
0.250.140%
0.250.1560%
0.250.280%
0.50.120%
0.50.1530%
0.50.240%
10.110%
10.1515%
10.220%

There’s no single percentage where things “suddenly” catch fire — the designer has to decide how far to deviate from well-understood Earth-like conditions, weighing fire risk, human comfort, the cost of shipping and replenishing gas, material availability, and the structure needed to hold the pressure.

Part 4 · The rest of the air

What the non-oxygen components do for you

Air is far more than oxygen. The three biggest other components each earn their place.

Nitrogen

  • An inert filler that dilutes the oxygen — a built-in safety margin against fire and sudden pressure loss.
  • All life needs nitrogen to build cells. Plants can’t use N₂ directly; they rely on nitrogen-fixing bacteria or fertiliser, and animals get it by eating plants.

Carbon dioxide

  • People breathe it out. Too much is dangerous (hypercapnia); too little is also harmful (hypocapnia).
  • Plants need it to photosynthesise, and higher CO₂ can boost plant growth. Scrubbers pull the excess back out of the air.

Water vapour

  • People are comfortable with some moisture; too little dries eyes, skin and airways, too much stops sweat from cooling us.
  • It also drives plant transpiration, electronics condensation and static, metal corrosion, slippery surfaces — and, at 100%, rain.

Beyond air, remember that other gases in industrial systems have flammable and explosive limits (LEL/LFL and UEL/UFL) — ranges of concentration where they can ignite. Good practice is to keep those in mind whenever a system holds a gas other than air.

Part 5 · Putting it together

How much air does a settlement need?

A settlement of 1,000,000 m³ runs at 0.75 atm and 20 °C. Using the NASA-recommended oxygen partial pressure and nitrogen for the rest, how many 100 m³ cargo containers of each gas does it take to fill it?

  1. 1

    Split the air into its parts (Dalton’s Law)

    Total pressure = pO₂ + pN₂. With pO₂ = 0.132 atm, nitrogen makes up the rest: pN₂ = 0.618 atm.
  2. 2

    Find each gas’s partial volume (Amagat’s Law)

    Partial volume = (partial pressure ÷ total pressure) × total volume.
    O₂: 176,000 m³N₂: 824,000 m³
  3. 3

    Adjust the expansion ratio for settlement pressure

    Liquefied gas expands more at lower pressure, so scale the 1-atm ratio by the total pressure.
    O₂: 0.000872 (liquid per m³ gas)N₂: 0.001078
  4. 4

    Convert to litres of liquid, then to containers

    Liquid volume = partial volume × adjusted expansion ratio, divided into 100 m³ containers.

Oxygen containers

1.5

153.5 m³ liquid O₂

Nitrogen containers

8.9

887.9 m³ liquid N₂

With the example inputs (1,000,000 m³ at 0.75 atm, pO₂ = 0.132 atm) you get about 1.5 containers of oxygen and 8.9 of nitrogen. Each of these assumes air behaves as an ideal gas — a good first estimate, but a design team can refine it with the van der Waals equation.

Check your understanding

Three quick questions

Answers check instantly and the reasoning reveals as you go. Nothing is recorded — this is just for you.

Q1. A habitat runs at a total pressure of 0.60 atm. The designer wants a partial pressure of oxygen of 0.18 atm. What percent oxygen is that?

%

Q2. You need 21% oxygen and the cabin runs at 0.50 atm total. What partial pressure of oxygen does that give? (atm)

atm

Q3. Two cabins hold the same partial pressure of oxygen, but cabin A is at 1.0 atm total and cabin B at 0.5 atm total. Which is more prone to fire?

Go deeper

Sources and further reading

Oxygen & the human body

Flammability

Air components & non-ideal gases

Ready to design the whole settlement?

This tutorial is one piece of Stellr’s Space Design Challenge. Get the full material and run it with your class.

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