Why We Should Send Plastic to the Moon

Earth’s nearest neighbor has a carbon problem. Unlike the planet it orbits our moon has surprisingly small quantities of available carbon. If humans hope to turn the moon into a human-compatible outpost sufficient carbon will be a critical part of building that ecosystem.

Carbon can potentially be found in lunar regolith and icy deposits. Kevn M Cannon’s paper, “Accessible Carbon on the Moon” goes into detail on the availability of lunar carbon. Current data indicate that carbon in lunar soil can be found in concentrations of about 100 parts per million. (I wanted to include a relatable reference point you might find on Earth, but at least for soil chemistry, nothing I found quite worked) One atom in ten thousand of lunar soil is carbon, roughly speaking that means if you want to extract 1,000 kg of carbon you need to mine and refine about 5,000 cubic meters of regolith (guesstimated using mental math, but a ballpark. That 5,000 cubic meters is equivalent to 2 Olympic-sized swimming pools. Now at some point, your lunar outposts might become big enough that bulk carbon extraction is worth the time and effort, but what do you do until then?

You ship it, duh, c’mon.

Yes, but not by simply loading up a cargo pod with chunks of graphite/coal/diamond.

Shipping anything to the moon is going to be expensive, escaping the Earth’s gravity followed by safely landing, that’s a lot of fuel/energy. What we need is to balance our costs with our needs. Fortunately for lunar planners, carbon is a very useful material. Carbon serves as the core chemistry of a dizzying array of strong and light materials. Carbon is also one of the core elements of our food. So we know how carbon from Earth is likely going to get to the moon, what will we do with that carbon when we get it there?

This is where things get complicated as there are many economic factors at play.

Carbon usage concerns will really come down to how “self-reliant” engineers and policymakers are trying to make lunar bases*. To that end, how does a “self-reliant” lunar economy connect itself with the rest of humanity? Are we planning on simply growing some percentage of food for our astronauts or are we hoping to be launching rockets fueled using locally made chemistries, and if we are using locally made fuels, what chemistries are acceptable for our rockets? (Look, out for a future post where I share a paper of mine from grad school looking into alternative fuel chemistries for use on the moon)

Going forward I am going to only be talking about carbon in the most abstract of terms, whether it is carbon that makes up methane for fuel, the carbon in food, etc., the rationale is based on the thought that the carbon atoms don’t care where they are. Engineers designing a lunar base are going to be concerned about how much carbon is available for a given task.

At our low end we have carbon to keep people alive. How much carbon do humans cycle through on a given day? On the International Space Station, an astronaut emits less than 0.3 kilograms of carbon dioxide and other carbon-rich substances each day**, about the mass of a standard soda can. Now on the International Space Station, the carbon cycle is an open loop, which means that, at least as of writing in May 2023, effectively all carbon that is brought aboard the International Space Station is disposed of either, by venting out carbon dioxide or sent back to Earth in a cargo return. Currently, NASA budgets about 2.4 kg of food products per person per day getting sent to space, with another 2.8 kg of water required to stay hydrated. Depending on the cost of shipping materials up to a Lunar outpost, being able to reduce the mass of food that needs to be sent from Earth could be considered a worthwhile investment. The question comes down to how expensive is it to make food on the moon. Realistically that is beyond the scope of this post, as the math is far too fuzzy. On the one hand plants can be used to remove carbon dioxide from the air and produce moral improving fresh food. On the flip side, the moon has a month long day night cycle, how much extra power production do your plants need to stay alive during the long night? Even if we assume that for a healthy plant based diet 99+% of our carbon needs to be in plants and soil, a lunar outpost where a dozen crew members are holding onto the remaining 1% of carbon. The local carbon cycle would need less than 1000 kg of carbon to provide for a massive garden.

Where things get more interesting is when we open the carbon loop. What happens when we want to start launching rockets using locally-made carbon-based fuels?

Even with lower gravity and no atmosphere to slow a rocket down, it takes a decent amount of fuel to launch things into lunar orbit and beyond. As an example, if you have a spacecraft that burns methane, with an engine ISP of 363 seconds, and when it can get about 10,000 kg of payload/ship into orbit. You would need to burn 9600 kg of oxygen and methane into orbit, of that combination, 1600 kg would be carbon, aka about 3 Olympic swimming pools of regolith mined and refined for a single launch of this type. As with the discussion on how to use plants as part of the life support system of a lunar habitat, there are far too many topics to give any meaningful analysis of all the very valid combinations of technologies and methodologies that will impact carbon used as a component of fuel. The goal here is to make it clear that it is complicated.

Broad strokes, if you are using lunar carbon in your rocket fuel, your lunar base should have more than enough carbon to make one hell of a garden and do a certain amount of domestic manufacturing of carbon-enhanced materials.

Without hard numbers that currently do not exist, the emphasis on prioritizing carbon-based materials for payloads sent to the moon will be related to how lunar carbon will be used. For a scenario where the goal is to make food locally, providing sufficient carbon will be relatively simple. As the use of carbon goes from a “closed loop” to one where carbon is used to make consumables that cannot be readily returned to the lunar base’s carbon cycle the ability to refine and extract carbon from local sources will be critical if the goal is to have a moon base be materially self-reliant.

The rubric for deciding to make something that goes on a one-way trip to the moon would likely look something like this.

If the cost of making and shipping something to the moon using non-carbon materials is more than the cost of making/shipping the carbon-based solution, no matter what, use the carbon forward solution.

In cases where the cost of making/shipping something from carbon vs another material is greater than the cost of using alternative materials a more complicated decision tree could still make using the carbon option worthwhile.

If $Cost of mining/refining lunar carbon>$Value/kg of additional Carbon>=$Carbon Cargo-$Non-Carbon alternative, use the carbon based solution

What I am trying to convey above is that if it costs $10,000 to mine a kilogram of carbon on the moon, and the value of carbon to mission planners is $9999/kg, and the difference in the life cycle cost of making and shipping something that uses 1 kg of carbon instead of some other material is less than $9999, then the carbon-based substitute makes more sense. In a more realistic model, we would need to account for the cost of refining the carbon out of the cargo being sent. Like markets here on Earth the tug-of-war of supply and demand would make this function rather dynamic unless of course a space agency/company/national government simply mandated a set mass of carbon being sent to the moon per year.

Author’s Notes/further ramblings

I wish this felt more definitive, but often without additional boundary conditions, it’s difficult to be definitive. While in theory I could have said I have 4 astronauts on 5 month rotations, to allow for 3 overlapping crew rotations per year, and this is how much it would cost to set up a garden on the moon, I would be missing key details like how much does it cost to get to the Moon from Earth, and the answer is not an easy one to lock down. For a crewed mission to the moon, there is an expectation that you do a fuel-intensive Hohmann transfer, which produces a particular price point. That’s fine and dandy for crewed missions, but for re-supply missions where the materials have a longer shelf life, you can take a range of different approaches to the moon that take much longer but use much less fuel and therefore would have a lower cost per kilogram of delivery. Estimating delivery costs also get thrown by the size of the vehicle bringing payloads to and from the surface.

A challenge in modeling the carbon economy on the moon comes down to the mining/refining process. According to a British Columbia Open Textbook on Physical Geology, a gold mine needs densities of about 6 parts per million to be considered economically worthwhile (currently gold is over $60,000/kg). Silver needs rates of about 1000 PPM. Now this assumes you’re only looking for that one material, if you can economically extract multiple useful chemistries from your ore the density requirements of a given substance change dramatically, and I don’t have the personal knowledge base to estimate the economics of lunar mining in the near term.

If you are mining for things like potassium and sodium, which can actually be used to make a kind of rocket fuel the economics could become a bit more palatable,

This gives us a sense of where carbon prioritization will need to occur during mission planning. Food production/life support takes priority. Carbon from food consumption is a middling resource, even if you captured and recycled every gram of carbon from food brought from Earth, you would only add tens of kilograms of carbon per person per year.

Carbon from recycled deliveries is harder to gauge on how much mass would be supplied because it would depend on how much packaging does your delivery require and how often materials are being sent to the base.

 *I am saying bases not colonies as I have a hard time believing that anytime in the next 30+ years we will have habitation where people will spend their full lifetime. Folks going there to do research, maintain science infrastructure, people spending their last days in a unique environment (maybe), but families having kids who grow up there, feels dicey.

**NASA has a handy paper that goes over the inputs and outputs of astronauts on a long term mission

I added up the difference in mass of oxygen used and carbon dioxide emitted, to come up with the breathed carbon, 0.19 kilograms, then I added the mass of all the solids, 0.06 kg urine solid, 0.03 kg feces solid, and misc solids 0.04 kilograms, which fudged, works out to 0.3 kg/person/day