Most closed ecosystem development can (and should be) done first on Earth to work out the kinks where there is a back up ecosphere. As I've started working again this week on the ecosystem simulator software my partner and I are commercially developing, I've been considering pressure threshholds and mixes. For structural and economic reasons, any terrestrial proto-CELSS is likely* to be maintained at close to atmospheric air pressure (standard temperature/pressure, STP, is 101 kPa at 23C), with essentially the same air mixture (78N2/21O2). Off Earth, this may not be the case. Below is a chart showing pressures and temperatures covering what is found on both Earth and on Mars, spanning the three phases of water we are most familiar with. The black box around STP covers acceptable limits of temperature and pressure for human habitability.
.
For most of the space age, there have been two schools of thought on cabin pressure.
The 79kPA nitrogen (N2) in air is metabollically inert. There are just as many dioxygen molecules in 21kPa of pure oxygen as there are in 101kPa of regular air. Combustion doesn't care, and neither do the lungs, for the most part. There are of course some subtle differences in heat convection and air movement that in the extreme affect a person's comfort in being able to productively cough, and probably would also affect the way a flame flickers. The thickness of the air also affects your sweat and how it feels when you are wet. Low vapor pressure means moisture evaporation (and its attendant cooling effect) happens more readily. My kids will certainly attest that summer swimming in high altitude (low pressure) Colorado (when it's 95°F) is colder than winter swimming in Florida (at 75°F). This effect will also apply to plant transpiration, but as plants are harder to survey, less research is available. So as far as we know, these human comfort considerations are what drive lower limits of total air pressure. This is why the pure O2 in Apollo was at 30-35kPa and not at 21kPa.
The problem, as Apollo historians might guess, is that flammability increases with the p(O2) level. At above 30kPa, even metal becomes flammable. Again, it's the p(O2) that matters, not the total pressure, or the percent O2. As the joke goes, how do you get an astronaut to bark like a dog? High p(O2), one stray spark, and "woof!"
Talking habitat pressure outside of LEO, it makes sense to use lower total pressure (down to 32kPa), but probably not more than 22kPa of p(O2). So you have to find a filler gas mix to make up about 10kPa. Some of that can be H2O, and some can be CO2. But what are acceptable limits for H2O and CO2?
Most likely, it will need to be some combination of these, depending on what the local resources are. The third option is clearly the most sustainable (i.e., requiring least resupply from Earth). Notice there is no mention of CO2 scrubbers. With plants in your system, you won't need CO2 scrubbers. While there is little research on the topic, metabolic studies suggest humans can tolerate CO2 levels a couple orders of magnitude higher, at least 1 kPa. On the other hand, astronaut Scott Kelly indicated that high levels of CO2 noticeably affected his moods during his record setting long duration stay on ISS (this was at STP and in null g).
One other thing to note with CO2, is that it readily dissolves into liquid water, forming carbonic acid. It is estimated on Earth that about 1/3 of atmospheric CO2 goes into the oceans this way. Since the surface area ratios and volume ratios will likely be different in a CELSS, it's unclear how much CO2 would be absorbed by water, but it would probably not be trivial. As Biosphere 2 learned, Portland cement, too, will take up CO2, sequestering not just the carbon, but the 2 oxygen atoms as well.
While my money is on a bioregenerative system for primary life support, NASA is working on a variety of approaches to manage CO2. It is possible that one of these, or traditional scrubbers will be needed for backup.
These levels are equivalent to p(H2O) of at most 5 kPa. That's for 100%RH at 35°C/95°F. Discussion of atmospheric water is incomplete without involving temperature. Internal habitat temperatures are likely to vary significantly, unless the entire CELSS is located somewhere deep underground (thermal stability is one reason you might do that, but also for radiation shielding). Even so, there is likely to be a nice thermal gradient between your power source and heat sink/radiator. For average air temperatures in the comfort regions (10°C to 35°C), there will necessarily be surfaces within the CELSS that are hotter or colder. Water vapor really likes to condense on colder surfaces. You can expect that, for a Martian hab, the walls separating you from the -70°C outside will probably be colder than the ambient air. On the Moon, that will only be true at night (which lasts 14 Earth days). A smart hab designer will incorporate this into the system, as part of the hydrologic cycle. I'll perhaps go into some of my experiments with this in a later post.
And no, wise guy, methane won't cut it.
On Mars at least, nitrogen (N2) will be in short supply, so you'll probably want to use another inert gas like argon (Ar) or helium (He). Helium on the Moon and on Mars will be a natural byproduct of mining helium 3 for fusion power fuel. Yes, future Martians may indeed sound like animated Disney mice.
'*There is a case for conducting CELSS development on a mountain at very high elevation (>17000ft/5km) for this reason.
.
For most of the space age, there have been two schools of thought on cabin pressure.
International Space Station (ISS)
The ISS is maintained at close to standard pressure and gas mix, due to the fact that the Russians designed Soyuz that way, and Soyuz is the life boat for ISS. Equalizing pressure takes time which may not be available in an emergency situation. This same logic applies when opening the Soyuz door on landing back on Earth at sea level. The US space shuttle was designed the same way for the same reasons. The relatively high pressure differential between the ISS cabin and the outside vacuum results in about a 5% loss per year leak rate. This is not a problem when you are just outside Earth's atmosphere and get resupplies at least 3 times per year.Apollo
The Apollo program utilized a lower pressure of 30 kPa, using pure oxygen. This is possible because what is important (for the most part) is not the total pressure so much as the partial pressure of oxygen (shorthand "p(O2)").The 79kPA nitrogen (N2) in air is metabollically inert. There are just as many dioxygen molecules in 21kPa of pure oxygen as there are in 101kPa of regular air. Combustion doesn't care, and neither do the lungs, for the most part. There are of course some subtle differences in heat convection and air movement that in the extreme affect a person's comfort in being able to productively cough, and probably would also affect the way a flame flickers. The thickness of the air also affects your sweat and how it feels when you are wet. Low vapor pressure means moisture evaporation (and its attendant cooling effect) happens more readily. My kids will certainly attest that summer swimming in high altitude (low pressure) Colorado (when it's 95°F) is colder than winter swimming in Florida (at 75°F). This effect will also apply to plant transpiration, but as plants are harder to survey, less research is available. So as far as we know, these human comfort considerations are what drive lower limits of total air pressure. This is why the pure O2 in Apollo was at 30-35kPa and not at 21kPa.
The problem, as Apollo historians might guess, is that flammability increases with the p(O2) level. At above 30kPa, even metal becomes flammable. Again, it's the p(O2) that matters, not the total pressure, or the percent O2. As the joke goes, how do you get an astronaut to bark like a dog? High p(O2), one stray spark, and "woof!"
| Suits | |
|
| Suit pressures have another consideration. Suits have to articulate joints to enable astronauts to move their limbs. Suits are big balloons. The greater the pressure difference between inside and outside, the harder it is to articulate. So suits are generally kept at lower pressure than spacecraft cabin pressure. Modern EVA suits are at about 30-40kPA (with p(O2) of 21kPA, balance mostly N2). Apollo PLSS suits were at pretty close to 25kPA, pure O2. Discomfort of breathing thin air has to be balanced by discomfort of stiff joints. Mechanical counterpressure is one partial solution, but it is unfortunately not quite as trivial a solution as just wearing spandex. Incidentally, with an oxygen mask, the lower limit of human tolerable pressure is still at least 6kPA, which is still an order of magnitude higher than Martian air pressure. Below this pressure (known as the Armstrong Limit), water boils off the skin. |
Talking habitat pressure outside of LEO, it makes sense to use lower total pressure (down to 32kPa), but probably not more than 22kPa of p(O2). So you have to find a filler gas mix to make up about 10kPa. Some of that can be H2O, and some can be CO2. But what are acceptable limits for H2O and CO2?
Carbon Dioxide (CO2)
CO2 is by far the simpler to consider. CO2 levels in Earth's atmosphere are quite low (albeit, increasing): around 0.04 kPa. The reason for this is that plants are quite efficient at pulling it out of the air and sequestering the carbon. They can easily tolerate higher levels, and indeed grow faster at higher p(CO2), but at the expense of veggie/fruit flavor and micronutrients. However, because they so efficiently convert the CO2 to O2, you can't just crank up the CO2 knob without thinking about your crew barking. Humans by themselves cannot convert O2 to CO2 fast enough to produce sufficient food from just plants. There are three strategies you might use:- On Mars, you could pump in additional CO2 from the weak atmosphere, then vent the excess O2 bearing air, but you'll be losing water and whatever filler gas as well, so your various mechanical ISRU generators will be busy just maintaining your atmosphere. This strategy probably won't work so well on the Moon, where there is no atmosphere, and carbon (the C in CO2) is in short supply.
- You could also use combustion (of dried plant waste hydrocarbons) to help turn excess O2 back into CO2 and H2O. As long as you keep the p(O2) below 23 kPA (and take standard fire safety precautions), this is not as dangerous as it might sound. However, you will be releasing particulates and other volatiles into the atmosphere which will burden your filter system and/or lungs. On frigid Mars and beyond, combustion would have the advantage of producing heat, for both warmth and for cooking.
- You could balance the plant mass with animal mass. The animals will offset plant O2 production by converting it back into O2 they'll also provide an additional food source (for crew), and method of recycling inedible plant wastes. You will have to take basic hygiene measures to prevent these animals from becoming disease vectors, and from destroying plants needed for human consumption.
Most likely, it will need to be some combination of these, depending on what the local resources are. The third option is clearly the most sustainable (i.e., requiring least resupply from Earth). Notice there is no mention of CO2 scrubbers. With plants in your system, you won't need CO2 scrubbers. While there is little research on the topic, metabolic studies suggest humans can tolerate CO2 levels a couple orders of magnitude higher, at least 1 kPa. On the other hand, astronaut Scott Kelly indicated that high levels of CO2 noticeably affected his moods during his record setting long duration stay on ISS (this was at STP and in null g).
One other thing to note with CO2, is that it readily dissolves into liquid water, forming carbonic acid. It is estimated on Earth that about 1/3 of atmospheric CO2 goes into the oceans this way. Since the surface area ratios and volume ratios will likely be different in a CELSS, it's unclear how much CO2 would be absorbed by water, but it would probably not be trivial. As Biosphere 2 learned, Portland cement, too, will take up CO2, sequestering not just the carbon, but the 2 oxygen atoms as well.
While my money is on a bioregenerative system for primary life support, NASA is working on a variety of approaches to manage CO2. It is possible that one of these, or traditional scrubbers will be needed for backup.
Water (H2O)
Obviously, water in liquid phase is required for both plant and animal life. Because of our sensitivity to it, water's unique physics, and the fact that Earth's temperature range spans the entire liquid phase of water, H2O in the atmosphere is its own branch of science and engineering called psychrometrics. Atmospheric water vapor is commonly measured as relative humidity (%RH), which is a percentage of the condensation p(H2O) for a given air temperature and pressure. Humans are most comfortable (at STP) in 40-60% RH. Most plants like a higher level: 80-90%RH. Electronics like a lower level: 40-50%RH.These levels are equivalent to p(H2O) of at most 5 kPa. That's for 100%RH at 35°C/95°F. Discussion of atmospheric water is incomplete without involving temperature. Internal habitat temperatures are likely to vary significantly, unless the entire CELSS is located somewhere deep underground (thermal stability is one reason you might do that, but also for radiation shielding). Even so, there is likely to be a nice thermal gradient between your power source and heat sink/radiator. For average air temperatures in the comfort regions (10°C to 35°C), there will necessarily be surfaces within the CELSS that are hotter or colder. Water vapor really likes to condense on colder surfaces. You can expect that, for a Martian hab, the walls separating you from the -70°C outside will probably be colder than the ambient air. On the Moon, that will only be true at night (which lasts 14 Earth days). A smart hab designer will incorporate this into the system, as part of the hydrologic cycle. I'll perhaps go into some of my experiments with this in a later post.
Filler gas (Ar, He, N2)
If you were keeping track of the numbers, you'll have noted that humidity can account for no more than 5 kPa and CO2 at best 1 kPa of the 10 kPa adddional pressure you'll need for habitat comfort. You'll need at least another 4 kPa of filler gas, probably a bit more so you won't be at 100%RH everywhere in the hab. The filler gas mix you use will depend on what is available/economical. In some cases, it may actually be the most expensive component of your atmosphere.And no, wise guy, methane won't cut it.
On Mars at least, nitrogen (N2) will be in short supply, so you'll probably want to use another inert gas like argon (Ar) or helium (He). Helium on the Moon and on Mars will be a natural byproduct of mining helium 3 for fusion power fuel. Yes, future Martians may indeed sound like animated Disney mice.
'*There is a case for conducting CELSS development on a mountain at very high elevation (>17000ft/5km) for this reason.
Permalink: https://celss.net/tiki-view_blog_post.php?postId=3992