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 mi