A Primer on Long-Duration Life Support
The practicalities of surviving long enough to take a Martian selfie
Life support is the biggest technical obstacle to the human exploration of Mars.
This fact makes people mad, because there are all kinds of other obstacles that are fun to solve (orbital refueling, landing heavy payloads, making rocket fuel out of Martian air), and life support has a fun factor of zero. It is a thankless world of dodgy sensors, failing bearings, and bacteria trying to grow on absolutely everything.
But if we want to get to Mars alive, we need for this stuff to work.
Requirements
An astronaut in space needs 840 grams of oxygen, 2.8 kilos of water, and 1.8 kilos of dried food a day to stay alive. They also appreciate the little touches, like water to wash with (0.7 kg), fresh clothes (1.5 kg per week), wet wipes (0.2 kg/day) and a toilet (1.4 kg for canisters and wipes).
On the output side, each astronaut exhales around a kilo of carbon dioxide and pees out a liter and a half of urine. They also produce a fairly small quantity of feces and menses (though many women opt to medically induce amenorrhea during space flight).
On shorter missions like Apollo (~12 days) or Shuttle flights (~14 days) it makes sense to pack everything a crew needs with no attempts at recycling. In this paradigm, carbon dioxide can be scrubbed from the cabin air with disposable lithium hydroxide cartridges; everything else is carried along in the space version of a picnic basket.
On longer missions, trying to carry single-use supplies gets unwieldy. A crew of four on a 1,000 day mission to Mars would need 48 tons of consumables, about equal to the mass of the entire spacecraft1. And even if mass were unlimited, there simply wouldn’t be enough room to fit everything on board.
So past a threshold of about 30 days, you have to make some attempt at recycling.
Water
From the diagram above, you can see water is the obvious place to start. Four to five kilos of the stuff flow through each astronaut every day, and you can reclaim most of that amount just by condensing cabin air on a cold surface in the spacecraft.
On the Mir space station, this used to happen organically. Collecting water was a grubby job that involved chasing beach-ball sized spheres of condensate around the colder parts of the spacecraft with trash bags before they could climb into the walls and cause mayhem. Crew members spent three to four hours a day on this dirty and difficult task.
The ISS, a more civilized place, uses a heat exchanger that you can think of as a space-rated dehumidifier. It collects water and reprocesses it for drinking.
Once you’ve squeezed the cabin air dry, the next best source of reclaimed water is urine. Gravity makes it easier to boil astronaut pee on Earth than in space, but a heated centrifuge weighing nearly a ton gets the job done on ISS at about 87% efficiency.
(The limiting factor in urine distillation is actually the high level of calcium from disintegrating astronaut bone, a nice example of how problems in space find ways to compound one another.)
Once the pee is boiled, you’ve reached a 90% level of water recovery. Most of what remains is locked away in an unpleasant substance called urine brine. The ISS has been testing a contraption that dries this out using a system of permeable membranes, but the residue is not easy to handle. To quote a NASA study:
“When wastewater brines are dried, the residual is inevitably a viscous goo, laden with particles of precipitated solids. This brine residual causes several problems for traditional recovery systems, such as clogging pitot tubes, causing bearings to seize, and fouling heat transfer surfaces.”
Drying this enjoyable substance gets you to ~97% closure, at which point you’re left chasing the 90 grams of water trapped each day in astronaut feces.
For NASA, this remains the final frontier. Solutions for unlocking this treasure have ranged from slow roasting, to charring, to simply putting the fecal canisters in a microwave (though presumably not the one in the ship’s break room).
Recovering fecal water puts you near 98% closure, with most of what’s left lost through trash disposal (wet wipes and food packaging).
The water system illustrates the problem of diminishing returns in regenerative life support. Getting near the 98% recovery rate NASA targets for Mars takes four pieces of heavy machinery (the heat exchanger, urine still, brine dryer, and fecal oven) that recover 64%, 26%, 7%, and 1% of cabin water respectively.
Even though half of this machinery recovers only 8% of the water, the crew will be just as dead if any of it stops working.
This trade-off between closure and mechanical complexity comes up repeatedly in life support design, with the added caveat that all the subsystems interact with each other and the bodies of the crew, so they can’t be tested in isolation.
Air
The air system on a spacecraft has a number of jobs to do. It has to replenish oxygen, remove carbon dioxide, and filter out airborne contaminants ranging in size from viruses to hammers. It also has to keep the air in the spacecraft uniformly mixed, detect a variety of toxic gases, and let someone know if the spacecraft is on fire.
Carbon dioxide is what kills you first if the air system fails. A CO2 concentration much above 5% (50,000 ppm) will incapacitate a crew, meaning that whatever removes the gas from cabin air has to be pretty reliable and not break for too long. (Astronauts have given themselves bad headaches just by falling asleep in a corner of the spacecraft that had inadequate ventilation).
The early space program through the Shuttle era scrubbed carbon dioxide with disposable canisters of lithium hydroxide, a powder that chemically binds the gas. On the ISS, CO2 is removed by a machine about the size of a car engine that pumps dried cabin air over a bed of reusable zeolite beads. These beads adsorb CO2 on their surface but do not react with it chemically. Once a bed is full, it is shunted to vacuum and heated until the trapped gas vents off into space.2
Scrubbing CO2 out of cabin air poses a special challenge because there is no distinct waste stream to process. The crew exhales into the same ambient air they breathe in, and it’s thermodynamically hard to get all the gas out of a dilute solution. The scrubber on the space station struggles to bring CO2 levels below 3500 ppm, about nine times the baseline level on Earth, and three times more than you might encounter in a stuffy office. Whether this affects astronaut performance depends on whom you ask (the Americans say definitely; the Russians, whose paycheck gets docked for whining, say it’s no problem).
In comparison to CO2, replenishing oxygen is straightforward. You can make all the oxygen you want by running an electric current through water. But the equipment for doing this is heavy, and leaves gaseous hydrogen as a byproduct. The newly separated H2 and O2 want desperately to reunite back into a molecule of water, and thwarting this desire takes a fair amount of mass and engineering. So in many mission designs, it can make sense to just forget recycling and treat oxygen as a consumable.
Food
Food may be my favorite technical barrier to Mars travel, because everyone assumes it has been solved, or that it is easy to solve, while the people working on it mop the sweat from their brow during the day and try keep the shaking in their hands from rattling the ice cubes in their whisky glass at night.
Astronauts have hated space food ever since the first meat cubes came back uneaten from Project Gemini. Even on the ISS, where fresh foods are often available, getting crews to eat adequately is a struggle. Whether it’s because the stomach senses satiety differently in zero gravity, or because the space station smells like a toilet, crews have historically consumed only 80% of their rations.
On a multiyear mission, such a calorie level would lead to malnutrition and embarrassing deficiency diseases like space scurvy. So we need to come up with ready-to-eat meals that are nutritious, storable for five years without refrigeration, and appetizing enough that a crew can eat them for a thousand days without wanting to murder each other.
These kinds of meals don’t exist. Their closest equivalent is the military meal ready-to-eat (MRE). But as any soldier or prepper will tell you, an MRE is not something you can subsist on. The meals are not nutritionally complete, and soldiers’ own backronym for the combat ration (meals refusing to exit) sheds light on a notorious shortcomings. Defense department guidelines stress that soldiers should not be fed MREs for more than 21 days at a time.
The absence of cold storage on a spaceship might surprise you. Surely the one thing space doesn’t lack is coldness. Why not just hang six tons of frozen burritos out the airlock and be done with it?
But a spacecraft is more like a thermos than a freezer. Everything in the main pressure vessel will tend to come the same temperature, so a special cold storage area would need to be outside crew capsule. Not only does this mean regularly using the airlock (an incredibly risky process that wastes air) but it would deprive the crew of the considerable radiation protection afforded by sleeping behind a wall of chicken tetrazzini.
So a space root cellar is out. What we’re left with on the spacecraft are the cooking facilities of an office break room: a microwave, hot water tap, and a mini fridge.
Growing your own
Of course on Earth, this problem has been solved for ten thousand years or more . Plants make tasty food on demand, they recycle a number of astronaut waste products, and they can really spruce up a spacecraft. So why not take some with us?
But for all the appeal of subsistence farming as a lifestyle, it comes with problems. We know next to nothing about zero-g agronomy, and we’ve already lost some space crops to mold. Making the crew dependent on plants puts astronauts one bad harvest away from starving to death.
Plants also need a huge amount of room to grow, and their inedible parts have to be composted. Doing this has proven challenging even in closed-cycle farming experiments on Earth, so NASA quite rightly doesn’t want to add it to all their other problems in space.
So while a Mars crew will likely try growing plants to supplement their menu, they can’t rely on them for core nutrition. One way or another, we need to find a way to store room-temperature food for up to five years without it turning into an unappetizing brown mush.
Body Waste
The space toilet has been a reliable source of fascination for every rising generation of space nerds. Years of engineering effort have reduced the humiliation level of the device (Shuttle astronauts had to practice centering their anus in the cross-hairs of a video camera) but releasing the hounds on the ISS remains a much less joyful experience than, say, using a Japanese shower toilet for the first time.
On the space station, feces gets vacuumed into a bag full of antimicrobial powder. This bag is then sealed and placed in a cylindrical canister for long term storage, until it can be sent down to Earth.
Only the most daring life support engineers want to crack these cylinders open and try processing the stuff. But storing it on a Mars mission is a challenge because the containment cylinders take up a lot of space. Long-term storage is a special worry on the Martian surface, where the bacteria in human waste pose a serious potential contamination hazard, and have to be prevented from leaking into the environment over timescales of many decades.
Medicine
Early space medicine was a zero-gravity version of “walk it off”. Apollo doctors filled the spacecraft’s glove box with amphetamines and tranquilizers and wished the crew a good flight.
As missions grew longer, the medical kit expanded. Mir had a medical case of surgical instruments with a bilingual instruction manual, and often flew a doctor as one of the crew. The ISS is practically an orbiting Walgreens, offering residents the use of an ultrasound machine, defibrilator, pregnancy test, and a full suite of pharmaceuticals from aspirin to Zoloft.
Being long and remote, a Mars mission will require a medical bay that can handle health risks inherent to the mission (bone fractures, vision loss, toxic burns, cancer) as well as unexpected illness. While a spacecraft might seem like the last place to worry about infectious disease, the immune system gets weird in space and old viruses can reactivate. At the same time, the crew is sharing their living space with a YOLO experiment in fungal and microbial evolution in a highly mutagenic environment.
Even though ISS astronauts pop pills like candy, our knowledge of how pharmaceuticals behave in space is minimal (there have been just two experiments). We’re also in the dark about how long a shelf life medication has in space, which is a particular issue for things like antibiotics.
When it comes to administering anesthesia or doing surgery, knowledge is purely theoretical, and limited to common sense advise like not using anesthetic gases in a confined space.
A problem unique to spaceflight is figuring out the downstream effects of drugs and their metabolites on the life support system, which is full of sensitive components like catalyst beds that can be poisoned by small quantities of material. There’s little point in figuring out how to administer chemotherapy in space if it’s going to brick the water purifier and kill the whole crew.
Fire
Fire doesn’t usually get lumped in with life support, but since not being on fire is essential to the crew’s survival, I like to throw it in.
The worst space fire to date happened in 2003, when a defective oxygen canister on Mir ignited and burned for fourteen minutes, spraying droplets of molten metal clear across the spacecraft. The crew emptied three fire extinguishers and sustained second-degree burns in a futile attempt to put it out, but ultimately they just had to wait for it to finish burning on its own.
Though NASA and Roscosmos downplayed the incident, this was a very close call. Had the fire started a few days earlier, before a departing Shuttle had emptied the module of trash bags, it would likely have spread throughout the station. As it was, the limited fire filled Mir with smoke and nearly forced the crew to evacuate.
Fire scares on the ISS have so far been more benign, but any long-duration spacecraft is by its nature a fire hazard. A Mars-bound spacecraft will be a crowded environment with miles of wiring and significant heat sources (the zeolite beds in the carbon dioxide scrubber need to be heated to 400°C, for example). And because there’s no way to cut a mission short, the crew will not just need to be able to extinguish a fire, but live in its ashes for up to two years.
This means that the life support system has to be able to absorb the combustion products of a major fire and keep working, or at least be fixable afterwards. That’s a heavy design constraint to adhere to, especially when you can’t simulate an actual microgravity fire on Earth.
Exercise
Exercise is an odd duck. NASA requires space station crews to work out for multiple hours a day to combat both bone loss and muscle atrophy. This seriously eats into available crew time while stressing the life support system.
There is a class of drugs called biphosphonates that inhibits bone loss, but NASA screwed up the relevant experiment and forgot to set up a control group who would take these drug in space without exercising. So we still don’t know if drugs alone provide adequate protection against bone loss in space, and ISS crews still spend hours each day working out like gerbils on a treadmill.
Culturally, it doesn’t help that the astronaut corps is a bunch of health nuts who just love to work out. There is a pronounced tendency in the literature to assume that exercise is always good, and horror at the idea of turning our astronauts into a bunch of zero-gravity layabouts.
But it’s very important to find out whether we can get away without exercising during a Mars surface stay, since the amount of mass we can land on the planet is a serious constraint on habitat size. Having to include exercise equipment means limiting available exploration time, packing more food and oxygen, and upsizing the life support system.
Laundry
Laundry on the ISS is handled with disposable clothing, which the astronauts try to stretch for as long as possible. Astronaut Daniel Petit explains the drill:
We were stretching [changing clothes] out to about every 8 to 10 days, and the indicator that it was time to change your underwear would be when you started to get a rash around your waist, and then it was time to change that pair of underwear. Then you would down-mode it. You’d wear your underwear for prime daytime underwear, then you’d down-mode it to exercise underwear. We’d exercise for two, two and a half hours a day, and you’d use a different set of clothes for exercising. So, your day clothes would be down-moded to exercise clothes, and then from there you’d down-mode them to rags for cleaning up messes.
Sending disposable clothing to Mars means figuring out where to put it all without making astronauts feel like they live inside an old closet. New clothes also gunk up the air filters with lint.
Washing clothes, on the other hand, creates a new wastewater stream for the life support system to handle. When you remember that substances like hair conditioner and lotion have historically crippled the water system on the ISS, you can understand the apprehension that a couple of Tide pods inspire in the engineering department.
The laundry problem is of course exacerbated by exercise. And on a Mars surface mission, it may be further exacerbated by the need to handle dust.
NASA’s strategy for the time being is to search for fabrics that don’t soil easily and try to develop a kind of superFebreeze for space. But like the food problem, the laundry issue is both unglamorous and a fairly serious capability gap.
Stowage
The final problem I’ll mention in connection with life support is stowage—where to put things. We tend to look at life support systems in terms of mass balance, but in many cases the more severe constraint on a spacecraft is volume.
While the ISS is enormous by space standards (it has about the volume of an airliner), its inside is still a warren of cabling and equipment, much of which folds into the walls and requires hours of prep work to get access to.
Cargo bags sometimes leave entire areas of the space station off limits, and planning and managing how to Tetris all this stuff together and move it around is a full-time job for logistics personnel on the ground.
Just keeping track of what is on the space station, and where it’s hiding, is a challenge. Though items are tracked almost obsessively, down to telling the crew which trash bag to put a discarded item in, mission control routinely sends up WANTED posters of missing items. There’s an informal contest among the astronaut corps to see how long an object can stay lost on the space station (the record so far is eight years).
Because of size constraints, stowage on a Mars spacecraft will be much harder. The main pressure vessel has to fit inside a rocket fairing, putting strict limits on available volume.
Inside that space, the crew will need to have ready access to all equipment that might break, whatever spare parts they need to fix it, and still find room for three years’ worth of supplies, sleeping quarters, space suits, a toilet, and a gym.
Further Reading
For more on wrangling water globules aboard Mir, I highly recommend David Wolf’s entertaining oral history
To read about the challenges of removing carbon dioxide in ISS, here’s a 2024 status report on the latest tech being tested there.
For a deep dive on complexity vs. recovery tradeoffs in water reclamation, see Water Recovery Trades for Long-Duration Space Missions
On the question of space food, see Progresses [sic] in processing technologies for special foods with ultra-long shelf life DOI: 10.1080/10408398.2020.1853034
A 2025 paper on the latest efforts in creating a space laundromat.
For a real deep dive into the issues I raise in this post, see Logistics Needs for Future Human Exploration Beyond Low Earth Orbit (2017). DOI 10.2514/6.2017-5122
Here and elsewhere I use ballpark figures from NASA’s several studies on what a Mars mission architecture would look like. They tend to posit a transit vehicle that weighs around 100 tons. A Starship derivative would be larger, but has similar constraints on habitable volume.
Similar to the water case, you add machinery to process the captured CO2, with diminishing returns. The next step in this journey for CO2 would be a Sabatier reactor, which would recover oxygen, leaving methane as a byproduct. You can vent this methane or process it further in an oven to recover hydrogen and leave carbon black (soot) as a final waste product.
What is the advantage of making oxygen from water on a spacecraft? Isn't the mass of water you would need to bring with you greater than the mass of oxygen it produces?
Is there any data on collecting moisture from the male astronauts’ ejaculate? Surely another 0.1% available there! (Are male astronauts allowed to ejaculate while in space?)