The Gas Nobody Budgets For

There is no nitrogen recycling on the International Space Station. None. Twenty-five years of continuous habitation, the most expensive machine ever built, and the nitrogen in that air is tank nitrogen. It gets used, it gets vented out the airlocks, and more comes up on the next resupply.¹

Nobody thinks about that, because nobody thinks about nitrogen. Ask a man on the street what you need to breathe and he says oxygen. Oxygen gets the headlines. Oxygen is what we’re going to bake out of the lunar soil with mirrors, what MOXIE pulled out of the Martian air, what the plants make for us in the greenhouse. Oxygen is the one everybody has a plan for.

But look at what you’re actually breathing. Better than three-quarters of every breath you take is nitrogen, and it does nothing. It doesn’t react. Your lungs pass it in and pass it right back out. It is, chemically speaking, the most useless gas in the room.

And you will die without it.

Why the useless gas matters

I’ve spent forty-one years around industrial construction, most of it on the North Slope, and a fair piece of that in Class I Division 2 areas where the whole design philosophy is built around one question: what happens if this thing finds an ignition source. You learn to respect an atmosphere. You learn that the difference between a safe room and a bomb is sometimes just the mixture.

NASA learned it the hard way on January 27, 1967.

Apollo 1 was sitting on the pad running a plugs-out test, cabin pressurized with pure oxygen at 16.7 psi — about two pounds above sea-level pressure, to keep the hull pushed out the way it would be in vacuum. That is more than five times the partial pressure of oxygen you’re breathing right now. In an atmosphere like that, materials nobody would ever call flammable become flammable.² Aluminum burns. Velcro burns like a fuse. Three men died in under a minute.

The fix wasn’t better wiring, though they did that too. The fix was nitrogen. In March 1968 NASA changed the launch atmosphere to sixty percent oxygen and forty percent nitrogen, and only switched over to low-pressure pure oxygen once the ship was in orbit, where a fire is merely catastrophic instead of instantly fatal.³

So that’s the first thing nitrogen does. It’s the fire break. It’s the inert filler that sits between your oxygen molecules and keeps your habitat from being a bomb with people in it.

Can you run pure oxygen? Yes, at low pressure, and we did it for years. Mercury and Gemini flew that way. But you’re accepting a fire risk that never goes away, in a sealed can you can’t walk out of, and you’re running your crew on a hair trigger for the entire mission. That’s not a design. That’s a gamble you keep making.

The second thing nitrogen does is feed you. Every protein in your body, every strand of DNA, every enzyme, is built on nitrogen atoms. You can’t get them from the air — your body has no way to touch N₂ — so you get them from your food, and your food gets them from the soil, and the soil gets them from bacteria. I’ll come back to that.

The problem with the Moon

Here’s where it gets ugly.

The Moon is a fine place to get oxygen. Lunar regolith runs a little over forty percent oxygen by weight — it’s all locked up in oxides, in the silicon and the aluminum and the iron and the titanium, and prying it loose takes energy, but the energy is right there overhead for two weeks at a stretch.⁴ Heat the dirt, run the reaction, collect your oxygen. Every serious lunar architecture has some version of this.

Now go looking for nitrogen in that same shovel of dirt.

The Apollo soil samples come in anywhere from a couple dozen to a bit over a hundred parts per million — call it eighty for a round number.⁵ Eighty. Parts per million. The very richest samples anybody has measured top out at about 120 parts per million.⁶ There is more carbon in lunar soil than nitrogen, and there isn’t much carbon.

Put those two numbers next to each other, because that’s the whole article in one line: the same shovel of dirt is something like five thousand times richer in oxygen than in nitrogen.

And it isn’t even lunar nitrogen, most likely. What’s in there got blown in on the solar wind and implanted a few nanometers deep in the outside of the grains — which means it lives on the surface skin of the soil, and it means the mature, well-weathered soil has more of it than fresh material.⁶ Though I’ll tell you what I found interesting when I went digging: the scientists don’t actually agree on where it came from. The solar wind explanation has trouble accounting for how much of it is there and for the isotope ratios, and the honest papers say the origin of the bulk of the trapped lunar nitrogen is still unknown.⁷

That’s worth saying out loud in a magazine like this one. Somebody reading this is going to go settle that question someday.

But settled or not, the engineering answer is the same. At eighty parts per million you would have to process something on the order of twelve thousand tons of regolith to recover a single ton of nitrogen, and that’s at a hundred percent capture, which you will not get. Meanwhile the same twelve thousand tons hands you thousands of tons of oxygen almost as a byproduct.

So the Moon gives you air you can’t breathe. It gives you the dangerous fifth and withholds the safe four-fifths.

Mars has something the Moon doesn’t

Mars flips it.

Curiosity’s own instrument sampled the air at the surface and measured it: 95 percent carbon dioxide, 2.6 percent molecular nitrogen, 1.9 percent argon, a sixth of a percent oxygen.⁸ The atmosphere is thin — surface pressure around 610 pascals, roughly six-tenths of one percent of ours — so 2.6 percent of very little is still very little in absolute terms.⁹

But it’s there. It’s in gas form. And it’s free.

You don’t mine it, you don’t cook it out of rock, you don’t smelt anything. You run a compressor. Suck in Martian air, freeze out the carbon dioxide, separate the argon, and you’ve got nitrogen. It’s a refrigeration problem, and we’ve been solving refrigeration problems industrially for over a century. Every air separation plant on Earth does the same trick.

I don’t think people weigh this properly when they argue Moon versus Mars. The Moon is closer, and closer is worth a lot — three days from rescue instead of eight months. But the Moon has no nitrogen and Mars does, and if you’re talking about a settlement that grows its own food and keeps its own air for decades, that’s not a small footnote. That’s a resource that decides whether you’re a colony or a very expensive campsite.

Further out, the numbers get better still. Ammonia ice is common in the outer solar system, past the frost line — comets, the icy moons, the bodies nobody has gone to yet. Ammonia is one nitrogen atom with three hydrogens attached, and it doesn’t take exotic chemistry to crack it. That’s where the bulk inventory is, if we ever get serious about hauling it. That’s a long game. But it’s the only place with enough of it to matter at scale.

The barrel

Now the hard case. The one I care most about.

Take the rotating habitat — our barrel — and ask the same question. Where does its nitrogen come from?

It doesn’t. There’s no regolith under the floor to bake, no atmosphere outside to compress. Whatever nitrogen you seal inside that hull on day one is every nitrogen atom that structure will ever have, unless a ship brings more.

That changes what the problem even is. On the Moon it’s a mining problem. On Mars it’s a chemical engineering problem. In the barrel it’s a plumbing problem.

Every time somebody cycles an airlock, a volume of air goes outside and doesn’t come back. Every seal weeps a little. Every hull penetration is a slow leak somebody hasn’t found yet. Those are nitrogen atoms leaving the system permanently — not degraded, not used up, just gone, out where you can’t chase them.

And here’s the part that should keep a designer up at night: you can recover oxygen from carbon dioxide. You can recover water. We have machines for both and they fly today. But a nitrogen atom that goes out an airlock is a nitrogen atom you must replace from outside the system, forever.

So in the barrel, the winning design isn’t the one with the cleverest biology. It’s the one that loses the least. Airlock volume recovery — pumping the lock down into a holding tank instead of venting it to space. Leak detection that finds a slow one before it’s a big one. Choosing the number of airlock cycles per day like it’s a budget item, because it is one. That is unglamorous work. Nobody makes a documentary about the pump-down tank. But it decides how long a habitat can live without a supply ship.

Fixing isn’t making

Now, when I started thinking this through, my first instinct was that we’d grow the nitrogen. Algae. Something like what the oceans do. And I want to walk through why that’s half right, because I think a lot of people land in the same place I did.

It’s true that bacteria are the reason nitrogen is available to life at all. That’s what the nitrogen-fixing organisms do — rhizobia in the nodules on legume roots, free-living Azotobacter in soil, cyanobacteria in water. They carry an enzyme called nitrogenase that pries apart the triple bond in N₂ and hands you ammonia, which plants can actually use.¹⁰ It’s a beautiful piece of machinery and it’s the reason there’s protein in the world.

But read that carefully. Fixing converts nitrogen. It doesn’t create it.

Those bacteria take N₂ that’s already in the air and turn it into a form life can grab. Every nitrogen atom in that ammonia was already in your habitat. The biology is a conversion step, not a source. You cannot grow your way out of an empty tank.

That’s the distinction this article turns on, and it took me a minute to get straight in my own head: usable nitrogen and total nitrogen are two different inventory problems. Fixation solves the first one. Only a ship or a planet solves the second.

Which doesn’t make the biology optional — it makes it essential in a different way. In a closed habitat, nitrogen is constantly walking out of the air and into the crops, through the crops into the crew, and out of the crew into the waste stream. If that waste stream doesn’t come back around, your soil starves while your air stays fine. The European work on this maps out the whole loop: urine and solid waste broken down to ammonium, then Nitrosomonas bacteria oxidizing ammonium to nitrite, then Nitrobacter taking nitrite to nitrate — which is the form plants and cyanobacteria actually want. Then the plants feed the crew and it goes around again.¹¹

Two things about that loop are worth flagging. Ammonia is dangerous in a sealed can if it gets loose, and it’s toxic to the plants themselves above a certain concentration — so the same compound that’s your fertilizer is your hazard.¹¹ And nitrogenase is poisoned by oxygen; the fixing organisms have to wall it off to work at all, which is why Anabaena grows special cells called heterocysts just for the job.¹² You’re asking one system to do two chemically opposed things at once. That’s a real design constraint, not a detail.

And this is where it comes back around to the worms. Everything I’ve written about soil and worms and bees in these pages is the same argument at a different scale: the invisible workforce nobody puts in the budget. Nobody lists Nitrosomonas on the equipment schedule. But if it doesn’t come along, your closed loop isn’t closed.

What I’d actually do

Four things, and none of them are exotic.

Bring the seed inventory, and size it honestly. Nitrogen is heavy and boring and you’re paying freight on it, but there is no substitute for having the atoms on site. Budget it like steel.

Treat every leak path as a permanent loss. Pump down the airlocks. Instrument the hull. Count the cycles. A habitat that loses two percent of its nitrogen a year is on a twenty-year clock whether anybody’s tracking it or not.

Close the biological loop from day one, not as a phase two. The waste stream is a nitrogen mine you already own.

And when you’re picking where to go, put nitrogen on the list next to water and sunlight. We ask whether there’s ice. We ask about the regolith. Ask where the nitrogen comes from, and ask it out loud, before the design is frozen.

Somebody is going to build the first real closed habitat in my lifetime or shortly after. When it fails — and the first one will fail at something, that’s how commissioning works — I’d bet a paycheck it won’t be the glamorous system that gets it. It’ll be something like this. The gas nobody budgeted for, walking quietly out the airlock, a few pounds at a time.

Does this sound like we’ve got it all figured out? I don’t think so either. If you see it differently, or you think there’s a better way to manage the nitrogen problem in the barrel, or you’ve got a piece of this I haven’t considered, I want to hear it. Write in. Tell us where we’re wrong, or where we’re onto something. This magazine only gets better with more people pulling on the same rope.

Ideas in Star Bound are published open — take them, build them better. Our open proposal.

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A note on where this comes from. I am not a metallurgist, a spacecraft engineer, or a degreed engineer of any kind. But I am a practical one, with more than forty years in heavy industrial construction: welding, millwright work, hydraulics, mining, and thirty-three years of electrical construction and commissioning on the North Slope of Alaska, in a place that punishes anything built wrong. Much of that time I have worked as a field engineer, which means I am the one standing in front of the equipment when the drawing turns out to be wrong, and the fix has to be made on the spot, with what is on hand, and has to work the first time. Anyone who has done that long enough learns that a lot of engineering gets corrected in the field by people with more experience than the people who drew it. That is the eye I bring to these subjects: not what should work on paper, but what actually gets built, and what breaks. The facts I take from other people are footnoted so you can check them and read further. The ideas built on top of those facts are my own, and I would rather see them knocked down by somebody who knows better than left standing because nobody looked.

Sources

1. Clauwaert P. et al. (2017). “Nitrogen cycling in Bioregenerative Life Support Systems: Challenges for waste refinery and food production processes.” Progress in Aerospace Sciences 91 — the review notes there is currently no reuse of nitrogen on the International Space Station — https://www.sciencedirect.com/science/article/abs/pii/S0376042117300234 ↩
2. Wikipedia, “Apollo 1” — cabin at 16.7 psi pure oxygen during the plugs-out test, more than five times the normal partial pressure of oxygen — https://en.wikipedia.org/wiki/Apollo_1 ↩
3. NASA History, “50th anniversary of NASA deciding on a mixed-gas atmosphere for Apollo, a direct result of the Apollo fire” (March 2018) — https://www.nasa.gov/history/50th-anniversary-of-nasa-deciding-on-a-mixed-gas-atmosphere-for-apollo-a-direct-result-of-the-apollo-fire/ ↩
4. Wikipedia, “Lunar resources” — regolith composition, oxygen in the low-to-mid forties percent by mass — https://en.wikipedia.org/wiki/Lunar_resources ↩
5. NASA Lunar Sample Compendium (C. Meyer), soil 12001/12003 — nitrogen determinations of 54 to 110 ppm across several labs, with other Apollo soils in the compendium ranging from about 23 to 130 ppm — https://curator.jsc.nasa.gov/lunar/lsc/12001.pdf ↩
6. Kim J. S. et al. (1995). “Nitrogen isotope abundances in the recent solar wind.” Nature 375, 383–385 — regolith nitrogen as high as 0.012 percent by mass, correlated with solar-wind-implanted noble gases — https://www.nature.com/articles/375383a0 ↩
7. Wieler R. et al. (1999). “Evidence for a predominantly non-solar origin of nitrogen in the lunar regolith revealed by single grain analyses.” Earth and Planetary Science Letters 167 — https://www.sciencedirect.com/science/article/abs/pii/S0012821X99000126; see also Hashizume K. et al. (2000), Science 290, 1142 — https://www.science.org/doi/10.1126/science.290.5494.1142 ↩
8. NASA, “With Mars Methane Mystery Unsolved, Curiosity Serves Scientists a New One: Oxygen” (2019) — SAM instrument surface composition: 95% CO₂, 2.6% N₂, 1.9% Ar, 0.16% O₂, 0.06% CO — https://www.nasa.gov/missions/with-mars-methane-mystery-unsolved-curiosity-serves-scientists-a-new-one-oxygen/ ↩
9. Wikipedia, “Atmosphere of Mars” — average surface pressure about 610 Pa, 0.6 percent of Earth’s — https://en.wikipedia.org/wiki/Atmosphere_of_Mars ↩
10. Hendrickx L. et al. (2021). “Optimizing Nitrogen Fixation and Recycling for Food Production in Regenerative Life Support Systems.” Frontiers in Astronomy and Space Sciences — rhizobia, Azotobacter, cyanobacteria; nitrogenase efficiency highest under anaerobic conditions — https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2021.699688/full ↩
11. Yang J. et al. (2021). “Development of Nitrogen Recycling Strategies for Bioregenerative Life Support Systems in Space.” Frontiers in Microbiology 12 — the MELiSSA loop: ammonium to nitrite (Nitrosomonas europaea) to nitrate (Nitrobacter winogradskyi); ammonium hazardous if leaked and harmful to plants at high concentration — https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.700810/full ↩
12. Verseux C. et al. (2021). “A Low-Pressure, N₂/CO₂ Atmosphere Is Suitable for Cyanobacterium-Based Life-Support Systems on Mars.” Frontiers in Microbiology 12 — nitrogenase inactivated by oxygen; Anabaena fixes nitrogen in heterocysts — https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2021.611798/full ↩