In science fiction, we often deal with enormous space ships that likely contain a huge amount of breathable air. Just as often, life support systems dramatically seize to function and crew members begin to suffocate immediately in defiance of common sense and, possibly, basic chemistry.
The otherwise cool movie Sunshine is a particularly egregious example of this, but also the otherwise admirably science-compatible Stargate Universe suffers from the same misconception. Seems like every second episode the ship suddenly runs out of air. So, what would the breathing situation on board the Destiny really look like? Time for the Obsessive Compulsive Nerd Squad to take action and do some calculations!
Total air volume
Let’s assume (for the sake of argument), Destiny is basically a very flat pyramid shape of about 300 x 500 x 40 meters, that would result in a volume of roughly 2,000,000 cubic meters for the ship in total. Let’s assume that half of that is taken up by machines and structures, which leaves us with 1 million cubic meters of air inside the ship.
| 2000000 | cubic meters ship volume |
| 1000000 | cubic meters of air |
| 200000 | cubic meters of air if 20% of ship is pressurized |
Lung throughput
The people on the Destiny lead quite an active lifestyle, so they will breathe about 10 liters of air in one minute, bringing us to 14.4 cubic meters per person per day. Air on earth typically has about 20% oxygen content, and because only a fraction of this is absorbed by the lungs with each breath, it is possible to breathe the same air multiple times. However, discharged carbon dioxide is poisonous if it’s not removed from the atmosphere. Let’s assume that a given volume of air can only be breathed once, just to be on the safe side. This leaves us with a theoretical 70,000 person-days of air inside the Destiny.
There are about a hundred people on board, resulting about 700 days of fresh air for everyone, almost two years! At this point in the story, only 20% of the ship is pressurized due to large breaches of the hull. If the remaining parts of the ship are occupied by the crew, they will “only” have about 140 days of fresh air within the habitable zones:
| 10 | liters of air per minute for one person |
| 14400 | liters breathed per day |
| 14.4 | cubic meters breathed per person day |
| 69444.44 | Person-days of air |
| 100 | people on board |
| => 694.44 | days of air for entire ship |
| => 138.89 | days if only 20% of the ship is pressurized |
Total oxygen supply
Now we all know an estimate that assumes the entire volume of air can only be breathed once is wildly pessimistic. How much oxygen is actually in Destiny’s atmosphere? Again, taking Earth-like air as a reference, Destiny’s habitable zones contain a total of 40,000 cubic meters of O2. Assuming a need for 600 liters of oxygen per person-day, the crew of destiny consumes 60 cubic meters per day, taking us to a total oxygen supply of about 660 days:
| 25 | liters of actual oxygen consumption per person-hour |
| 600 | liters of oxygen per person-day |
| 60 | cubic meters per crew-day |
| 40000 | cubic meters of O2 inside the habitable areas |
| => 666.67 | days of oxygen supply total |
CO2 buildup
However, this is calculation is also not the entire truth, since we would not be able to use every last ounce of O2 present in the air, and more importantly: CO2 poisoning will become an issue if it is not scrubbed from the atmosphere. For a more realistic calculation on what the situation without any kind of air treatment capability looks like, let’s have a look at the CO2 buildup over time:
Even though mild symptoms of CO2 poisoning set in at about 1%, only a carbon dioxide content of 5% or more will seriously begin to incapacitate the crew. If the average human puts out 500 liters of CO2 per day, putting the output of the entire crew at 50 cubic meters per day, it will take them about 40 days to achieve 1% saturation and 200 days to generate enough CO2 until it becomes hazardous given the total air volume of their habitat.
| 20 | liters of CO2 produced per hour |
| 480 | liters CO2 output per person per day |
| 48 | cubic meters CO2 output for the crew per day |
| 2000 | cubic meters of CO2 equal 1% saturation in the habitable areas |
| => 41.67 | days until 1% saturation |
| => 208.33 | days until 5% saturation is reached |
So where does this leave us?
In theory, there is enough oxygen for almost two years in the accessible compartments, but we now know that without CO2 scrubbers, the air inside a ship like the Destiny would last for about half a year assuming a crew size of 100 people. Mild CO2 poisoning symptoms would set in after one month. However, these time spans are not even close to the instant Hypercapnia and suffocation symptoms found in most SciFi scripts.
Becoming even more obsessive
For proper life support to work, Destiny would have to have liquid air storage units in order to re-pressurize its compartments. We are not told how large they are, but this reserve will likely add a sizable portion to the air that makes up the ship’s atmosphere.
Since stars contain pretty much every element, and considering the enormous lifespan of the spacecraft, we can assume that Destiny replenishes its chemical resources whenever it passes through a star. This would most likely encompass the uptake of gaseous components for life support as well, including new oxygen.
So once again, we’re left with a show that could have used better science advisors. Or any science advisors for that matter.