ECLSS Primer: Atmosphere Management

Introduction

How do we maintain a breathable atmosphere in space? Here on Earth, we take a lot of things for granted, including the air we breathe. In space, many environmental factors are actively working against—or are openly hostile—towards life. The vacuum of space combined with the microgravity environment results in a whole host of challenges in sustaining a breathable atmosphere. Today I’ll be discussing some of those challenges, along with the state-of-the-art designs for atmosphere management systems in space habitats like the International Space Station (ISS).

This post will be part of a longer series of deep-dives into different subtopics in bioastronautics. Full disclaimer: I am not an ECLSS expert, and will be working only with publicly-available data, such as those published by NASA or at industry conferences. Many systems (such as some Russian systems, those used by SpaceX’s Crew Dragon, and most of NASA’s Lunar Gateway) do not have a lot of public information on their ECLSS system development. As always, if you catch any errors or facts that need correcting, feel free to contact me.

Let’s start with some basics. At the highest level, the environmental control and life support system is responsible for keeping the crew alive and healthy. Generally, there are five primary elements of ECLSS:

Atmosphere Management (CO2, O2, N2, trace contaminants, particulates, and microorganisms, pressure)
Water Management (potable, hygiene, and recovery)
Waste Processing (collect, store, process from trash, water, and food waste)
Food Supply (provision and/or production)
Crew Safety (fire detection and suppression, radiation shielding)

Before diving into the intricacies of atmosphere management, I want to first discuss loop closure and options for ECLSS technology. This will lay a foundation for the types of ECLSS technology that are available. Generally, ECLSS systems can either be open- or closed-loop. This status can be determined by whether or not mass is crossing over the spacecraft boundary. For example, the Apollo missions relied on a mostly open-loop ECLSS system, meaning that the waste outputs were not recycled back into the system. Instead, the waste—including carbon dioxide, waste water, and fecal matter—was either vented overboard or stored until the return to Earth. Alternatively, modern day systems on the ISS try to process as much waste as possible, through systems like the Water Recovery System (WRS).

Additionally, ECLSS systems can either be regenerable or non-regenerable, which corresponds to whether or not the systems are composed of reusable or one-time use technologies. For example, the Skylab space station used a molecular sieve to remove carbon dioxide from the cabin atmosphere. Unlike previous uses of LiOH canisters, this technology was not one-time use; it could be desorbed to vacuum and used again to continue to revitalize the atmosphere. Although this technology was regenerable (meaning it could be reused over and over again), the carbon dioxide itself was vented to the vacuum of space, and so the system was ultimately open-loop. A true closed-loop system (something like the Earth’s biome) would recycle and reuse all of its elements. A true regenerable, closed-loop system could use the same components over and over again to recycle matter through the system without replacement. As missions increase in duration, the aim is to use as many closed-loop systems and regenerable technologies as possible in order to conserve mass and energy.

Viewed at the system level, there are four primary options for providing ECLSS consumables ins pace:

Launch all consumables needed from the start (Mercury, Gemini, Apollo…)
Periodically resupply consumables (Mir, ISS)
Recycle waste mass in space (ISS)
Utilize in-situ resources (Future Deep Space Missions)

These broad categories are applicable to every aspect of the ECLSS systems, not just atmosphere management. So now that we’ve established a baseline of our options, I want to discuss the matter at hand: air, how to provide it, and how to revitalize it.

Background: The Human Body is Messy

In order to start thinking about the complexity of the air revitalization part of the ECLSS system, it’s helpful to visualize all of the human metabolic inputs and outputs:

Our little stick figure astronaut, based on nominal values for a 70 kg male crewmember-day (NASA BVAD)

As you can see, there’s a lot of stuff that goes into and passes out of the human body each day. There are the obvious things, like food, water, and waste; but there are also the more invisible processes, such as how the human body takes in an air mixture (usually an ~80/20 mixture of nitrogen/oxygen), extracts the oxygen, and exhales carbon dioxide. There is also the matter of humidity, which humans generate through both respiration (breathing) and perspiration (sweating). On top of it all are trace contaminants: things like sweat solids (every gotten so sweaty that you feel salty? Me too), skin particles, hair particles, and other matter. These all need to be accounted for in space, as a build up of contaminants like sweat solids and hair can be detrimental to onboard systems and human health.

But before you can think about all the systems need to handle metabolic inputs/outputs, there is something even more basic that the human body can’t survive without: pressure.

Cabin Pressure

The first step toward creating a space habitat is to provide what space cannot: a pressurized volume. This is generally done through the construction of a tin can-like structure that is able to be filled with air to some predetermined pressure. I find it helpful to think through ECLSS in terms of the aspects of space that will kill you first: if exposed to the vacuum of space, you have several seconds of life remaining. After you establish a box with some pressure, your astronaut will be able to survive for some time until the saturation of carbon dioxide or lack of oxygen begins to present a problem.

But pressure is slightly more complex than filling a tin can with air. Here on Earth (at a standard sea level reference) we have a total pressure of 29.92 in. Hg., or 14.7 psi. The mix of gases that make up this standard atmosphere is roughly 78% nitrogen, 21% oxygen, 0.97% argon, 0.04% carbon dioxide, and some traces of other gases. The ISS keeps a normoxic, dual-gas environment at the standard atmosphere, which means it maintains roughly 21% oxygen with the rest of the atmosphere consisting of nitrogen to maintain sea level pressure.

So why is this a big deal? Turns out it can sometimes be detrimental to stick to sea level pressure. There are a few reasons for this: at higher pressure, the spacecraft is more prone to leaks through all the different joints it has, and can lose more consumables over time as a result. Spacecraft must be designed to “feed the leak,” meaning that designers must provision enough consumables such that the atmosphere is maintained even with losses.

However, it also turns out that we can’t hold extravehicular activity (EVA) suits at 1 atm, because astronauts wouldn’t be able to move the joints of the suit. (Imagine trying to fix a car in an inflatable suit; the joints won’t be very forgiving, and every motion would be exhausting) Therefore, if astronauts plan on performing EVA, they will have to undergo prebreathing in order to go from the standard 14.7 psia inside the the Station to the lower total pressure of 4.5 psia in the EVA suits in order to avoid decompression sickness. Like SCUBA divers ascending from higher pressures back to sea level, astronauts would risk the nitrogen in their blood expanding, causing a host of health issues that could be potentially fatal. The prebreathe process requires astronauts to breathe 100% oxygen for several hours to purge the nitrogen from their blood stream. If designers want to reduce this downtime, they could keep a higher oxygen percentage and a lower total pressure in the primary habitat, meaning the astronauts would have less nitrogen in their blood to begin with.

Another catch: if you decrease the total pressure, you need to increase the partial pressure of oxygen such that it provides enough partial pressure for human lungs to extract the oxygen they need from the air mixture. (Side note: this is why your body has to work harder at higher altitudes; the percentage of oxygen remains the same, but the total pressure is lower, meaning that your lungs get less oxygen from each breath) The trade of cabin pressure is linked to the concern for habitat leaks and the time dedicated to EVA. However, it should be noted that a higher oxygen percentage results in a more flammable environment. It’s up to the designers to decide what risk they’re will to take and whether or not they’re will to have astronauts go through a prebreathe protocol, as well as whether or not they’re willing to lose consumables overboard through leaks.

Carbon Dioxide Removal

What system, if any, take priority in space? Contrary to popular belief, if you were locked in an airtight room, the problem you would have wouldn’t be a lack of oxygen—it would actually be the concentration of carbon dioxide that kills you first.

Carbon dioxide removal on the ISS is reliant on two primary systems: the Carbon Dioxide Removal Assembly (CDRA) in the US system, and the Russian Vozdukh system. CDRA relies on a regenerable 4-bed CO2 removal system that includes silica gel to dehydrate the air and zeolite to adsorb CO2. Alternatively, Vozdukh relies on a 3-bed regenerable amine adsorbant system rather than the zeolite. The US CAMRAS system also relied on amine; however, amine can produce potentially toxic off-gases, including ammonia.

Initially, the CO2 from CDRA was vented overboard. However, improvements to the ISS have resulted in further loop closure, and eventually the CO2 was fed back into the Sabatier reactor to recover up to 50% of the oxygen, with a methane byproduct that was vented overboard. Unfortunately, the Sabatier reaction had a set of issues that resulted in it being decommissioned in 2017. The CDRA’s on-orbit performance has been consistent in maintaining < 6 mmHg CO2 for up to 9 crew, although it was only designed for a crew of 4. That being said, CDRA has several failure modes, resulting in significant on-orbit maintenance requirements. In certain periods of its lifetime, it never reached 90 days of continuous operation without problems. These failures include valve issues, shorts, and some issues with the sorbents.

The spiritual successor to the CDRA is the 4BCO2 system. This system includes a sorbent upgrade to Zeolite 13x adsorbent, which is more effective than the previous 5A zeolite sorbent and less prone to dusting. Other improvements include a shorter cycle time (meaning the beds can suck up CO2 and desorb it out of the system to be processed faster, potentially resulting in a lower baseline level of CO2 in the cabin), and reduced power requirements. You can read more about the design of the 4BCO2 system here and here, along with some of its performance data.

Additionally, there are several other demonstrations onboard the ISS, including the European Advanced Closed Loop System (ACLS). This system was a demo of an integrated CO2 removal and water processing system, and results from its flight are still pending. Although some of the water systems on ISS are linked to the air revitalization system, I’ll be saving a discussion of those technologies for another primer.

Oxygen Generation

The ISS relies on several different systems in order to produce oxygen. The primary oxygen generation system on the ISS is the Oxygen Generation Assembly (OGA), which resides in the Tranquility module. This system uses electrolysis on water from the Water Recovery System (WRS) in order to produce breathable oxygen. Previous systems, like the Space Shuttle, relied on cryogenic oxygen. However, the advances on the ISS have allowed for greater loop closure by relying on water reclaimed from condensate and other waste streams.

Schematic of anode feed electrolysis; Mitlitsky et al., “Applications and Development of High Pressure PEM Systems,” Portable Fuel Cells Conference, Lucerne, Switzerland June 21-24, 1999

The Russian segment of the ISS relies on the Elektron and Vika systems. The Vika solid oxygen generator uses potassium chlorate to produce ~600 liters of oxygen a day (roughly equivalent to one crewmember’s worth of oxygen). Like the OGA, Elektron uses hydrolysis to turn water into oxygen.

As you can see, there are a whole host of options for each stage of the atmosphere management. Each option has advantages and drawbacks, and needs to be considered within the context of the mission. Some systems are improvements over others; some are simply different ways of accomplishing the same task. The general trend in ECLSS is to increase loop closure and use of regenerable technology, as can be seen from advances in different US systems, below:

Summary of spacecraft functions and solutions over time, modified from Rollock, 2023

Trace Contaminants, Temperature, and Humidity

The US Trace Contaminant Control Subassembly (TCCS) removes products from off-gassing, including alcohol, aldehydes, aromatics, esters, hydrocarbons, keytones, sulfides, and inorganics. This system relies on an activated charcoal bed, catalyitc oxidizer, and a LiOH bed for removal. Other systems, like the Trace gas Analyzer for Air Quality Monitoring, or ANITA, and the Airborne Particulate Monitor, or APM, monitor the cabin air for other contaminants such as aerosols, skin flakes, lint, sweat, and particles from personal care products. The Russian segment relies on the Russian Harmful Impurities Removal System, which is similar to the TCCS.

Although there is some humidity removal from the CO2 systems, the ISS primarily relies on the Common Cabin Air Assembly (CCAA), the Condensing Heat Exchanger (CHX), and the Water Separator Air Duct to pull water from the air, rehydrate the air, and control temperatures. In the microgravity environment, there is no convection (no gravity to pull heavier air molecules down), and so these systems are all heavily reliant on fans and other forced circulation.

Additional Considerations

In microgravity environments, the convection forces responsible for most of our terrestrial air-mixing are no longer a factor. This means that as astronauts breathe, the carbon dioxide has the tricky tendency to gather around wherever they happen to be, while more oxygen-rich air remains elsewhere. This is also a problem for heat management, where the heat put out by electronics and human bodies alike will simply remain around the object unless the air is mixed around by force. This is most commonly achieved through the use of fans.

However, fans are loud; there are entire studies dedicated to the tradeoff between acoustic noise and air ventilation in crew quarters. And even if the fans aren’t that loud, they create a constant noise environment for the crew. Many astronauts report some amount of hearing loss despite our best efforts to mitigate noise.

Additionally, it isn’t always worth it to close every ECLSS loop. For shorter mission durations < ~30 days, the increased mass of the closed-loop systems generally outweighs the cost of consumable replenishment. Therefore for systems like the Space Shuttle or Crew Dragon, it doesn’t make sense to rely on full loop closure. Instead, consumables launched with the crew for the mission duration and replenished upon return.

These are just some examples of how interconnected the ECLSS systems are to everything from structural design to human factors; in order to think about space habitats, you have to think about the big picture and how all of these systems affect each other, sometimes in unexpected ways. Hopefully this provided a quick overview of the systems—as always, if you have any questions, feel free to contact me on Twitter or via email!