Surviving Space: An Introduction to Life Support Systems
Humans are needy. Just to stay alive, we need a breathable atmosphere, water, food, the right temperature, and more. All these human needs are met by — you guessed it! — the Earth. Carl Sagan called it the “Pale blue dot,” and Douglas Adams described it as “an utterly insignificant little blue-green planet.” Irrespective of where you live on that spectrum, our planet’s natural ecological system has provided conditions that humans have evolved to thrive in. Space, on the other hand, is quite the opposite. It lacks all of the elements needed to support human life. Naturally, we copied Earth’s ecosystem through physicochemical processes using mechanical systems to keep humans alive. Physicochemical processes are those that use both physical and chemical processes to complete a reaction. The mechanical systems used to achieve these processes were initially developed for use in submarines. They were then improved to adapt to the space environment and are commonly called Environmental Control and Life Support Systems (ECLSS). Let’s take a look at what these spaceflight life support systems consist of, and eventually, take a deeper dive into how they function.
The very first thing we have to know about these systems is that the right solution depends on mission duration, crew size, re-supply availability, and energy sources. One other important factor that drives all space missions is the launch mass — it is the mass of the rocket/spacecraft including the fuel needed to take-off from Earth, the payload on the rocket, and the structural weight of the rocket itself. All these factors change for every application — on a spacecraft, a space station, or on a base on a celestial body — and are therefore called the mission drivers (and rightfully so!) In case you’re wondering about it, yes! This is the reason why NASA likes sending robotic missions instead of humans. Now that we have to satisfy the needs of live astronauts, we need to keep in mind that these mission drivers can be realized through different combinations of the basic systems — be it for a small duration mission with small crew or a long duration mission with large crew, and every other combination possible. Before we look at these combinations, understanding the basic requirements is essential. These requirements derive from the basic human needs and are as follows:
- Breathing mix — includes breathing oxygen, removing carbon dioxide
- Water
- Food
- Waste disposal
- Temperature and pressure regulation
- Protection from radiation
Now that we know what is required, it’s time to focus on how to achieve the best combination of systems for a given mission (remember how mass is very critical for space missions). So, the simplest solution for a short mission with a small crew is using physicochemical processes. Long-term missions, in addition to using the physicochemical processes, need an entirely different system that is regenerative and mimics nature on Earth. This system is called Controlled Ecological Life Support Systems (CELSS) and Mark Watney partially tries to do this in the movie “The Martian,” where he grows potatoes on Mars to support himself (If you want a quick rundown of the scene where he comes up with a quick-math solution to get a count of his consumables, here it is). We will learn more about how to achieve CELSS at a later time. First, let’s look at the different types of systems that are currently in use — and will stay around.
i. Open-loop Life Support Systems:
Open-loop life support systems are the simplest systems where all the consumables like oxygen, nitrogen, food and water are fed into the system for a single use. These consumables are a part of the cargo, packed to last for the planned duration of the mission (plus some additional stuff for safety). These consumables are renewed through resupply missions in a timely manner. All the waste products generated on-board like carbon dioxide, urine, solid waste, and other things like the dirty laundry are either dumped overboard, or stored separately for return to Earth. Lastly, most systems here have very little closure i.e. not much resources are recycled.
The above mentioned reasons make open-loop systems workable only for short missions with small crews due to its need to carry excess mass without reusing anything. All spacecraft, including the Soyuz and the now-decommissioned Space Shuttle, and even spacesuits use open loop systems. Some other well-known single-use open loop systems were onboard the Mercury, Gemini, Apollo, and Skylab missions. The ones on the Space Shuttle were made to be replaceable during ground maintenance and repair.
ii. Closed-loop Life Support Systems:
Some of the systems above were redesigned to reduce the mass penalties from the open-loop systems and to recover the useful resources by processing the waste products. These systems make up the (partially) closed-loop systems. These systems also require an initial supply of resources but the generated waste products are processed to reuse, reducing the dependence on resupply missions. They can be divided into closed air-loop systems and closed water-loop systems.
Closed air-loop systems are simpler and help in air filtration. Carbon dioxide (CO2) is removed and reduced to breathable oxygen (O2) through the use of lithium hydroxide canisters. These canisters remove CO2 and produce water, which is later used to generate O2. This closes the air loop to a high degree, which means that over 90% of CO2 that is breathed out is recycled to get breathable oxygen. The problem here is that it will also generate solid carbon and methane as residual waste. This new problem is further solved in a closed water-loop cycle.
Water is an essential component needed for the survival of life and NASA, in its assumptions, has each human consuming 3.909 kilograms of potable water and about 26 kilograms of water per day for personal hygiene. Each human also excretes about 4 kilograms of water per day. The idea is to use this excreted water as a resource too. Breathable oxygen can also be generated by using this water through electrolysis. Additionally, these systems can be reliably used even for long duration missions with large crew — astronauts tend to get upset and complain when they don’t get enough oxygen (didn’t I say so? Needy!) Partially or fully closing the water loop reduces the total resupply mass of water, which takes up to 70% of total mass by some estimates. Additionally, by using the Sabatier reaction instead of lithium hydroxide canisters, a significant portion of water and air can be recycled to help the astronauts survive in a sustainable way. Sabatier reaction is the process of generating methane and water from CO2 and hydrogen (H2) at high temperature and pressure. This process is currently being used on the International Space Station (ISS).
Closing the air-loop and water-loop systems will result in reducing the resupply mass to a very high degree assuming there is no leakage into space. Since we don’t live in a perfect world, there will be leaks and losses but until we can achieve a fully closed system, saving over 75% of resupply mass is a great deal. This can be seen in the Fig 4. An essential point we need to consider here is that both the open and closed loop systems use externally supplied energy to operate. This is a big problem because every closed-loop system needs more energy than the former to carry out the recycling processes.
Another need that is essential for human survival in space is maintaining pressure and regulating temperature. The atmosphere monitoring system helps with keeping track of the parameters ranging from pressure and temperature to trace gases, humidity, other particles, etc. Various mechanical devices like transducers (for monitoring pressure), thermistors (for monitoring temperature), electrochemical sensors (for monitoring CO2 and O2 levels), and other such devices are used to provide information to system controllers, which then command the actuators to achieve this regulation. All these systems differ based on if they are used in a space suit or in a space habitat.
Astronauts also insist on being well-fed and having their waste managed (when will these demands stop?!) throughout their mission duration. Since nourishment helps not only maintain the physiological (how the different parts of the body function) but also the psychological (relating to mental state of a person) well-being of the crew, having a well balanced diet is also very important. On the very first missions, the food consisted of a paste that was heated by injecting hot water and from there, we are now at a point where a personalized menu is prepared for each astronaut based on a mixture of frozen, canned and even fresh foods. Eating all the food and constantly keeping the body working generates waste, which has to be disposed of in a hygienic manner.
In the early flights, urine was passed through receptors and transferred into plastic bags or tanks. This was then dumped overboard. The movie Apollo 13 shows an example of this very accurately. With the progress of life support systems, the urine is now processed into fresh water on board the ISS. The fecal matter, on the other hand, was initially collected in plastic bags and sealed and stored with timely ejection into space. Later on, commodes were built on board, and to account for lack of gravity, an artificial air stream is introduced to direct the fecal matter into the storage tanks.
With all the above mentioned information included, we have barely even scratched the surface of life support systems. In the following articles, we will be diving deeper into understanding all the sub-systems like air supply and revitalization, water recovery and management, food and waste management, and workability of these inter-connected systems. We can’t go to Mars and live on it without also considering artificial gravity systems, radiation protection systems, and food production. Significant amount of research is currently being done to address these issues. And if you’re interested in helping solve these issues, definitely look into the programs at University of Colorado — Boulder, University of North Dakota, Baylor College of Medicine, and other such programs in the U.S.A and MELiSSA Foundation in the EU.
References:
- Zabel, Paul. “System Analysis & Evaluation of Greenhouse Modules within Moon/Mars Habitats.” (2012) https://www.researchgate.net/publication/258421865_System_Analysis_Evaluation_of_Greenhouse_Modules_within_MoonMars_Habitats
