There are two people 400km apart. Both are floating, waiting for nitrogen to pass from their bodies’ tissue into their blood to eventually be exhaled through their lungs. One is underwater at 6m completing a decompression stop and the other is preparing for an Extra Vehicular Activity on the International Space Station. There’s a neat symmetry to it.
The only real association I had between scuba diving and space exploration was photos of astronauts training underwater in NASA’s Neutral Buoyancy Lab in Houston. The giant pool is used to train astronauts ahead of missions with the best practical simulation of zero gravity available on earth.
In fact the closer you look many of the key diving considerations around gas composition and minimising the risk of DCS exist in the design and safe operation of spacecraft. The key difference in space is that the risk surrounds movement of the human body to an environment below the standard atmospheric pressures of earth. It’s hypobaric rather than hyperbaric.
Spacecraft Atmospheric Composition
There are basic similarities in attempting to keep humans alive in space and underwater. Humans need a safe breathable gas to survive in both. The medium is different (vacuum vs. water) with each obviously posing different challenges.
In 1959 NASA was choosing its first class of astronauts and didn’t know what skillset would be required for this new form of exploration. In the end military test pilots were chosen to have the core skills necessary but submariners and even divers were considered.
NASA examined multiple options for the atmospheric composition inside spacecraft with detailed studies on 100% oxygen, nitrox (nitrogen and oxygen) and heliox (helium and oxygen) combinations. Interestingly, Heliox was proven to have poor thermal performance and reduced crew comfort in testing – an endorsement for those preferring to use a denser exotic gas like argon for suit inflation.
On its journey to the moon the Apollo spacecraft used an atmosphere of 100% oxygen at approximately 0.3 atm, significantly lower than the 1 atm of sea level pressure on earth. This decision was largely driven by simplicity – much like a pure oxygen rebreather it’s the easiest to implement.
However, on the launchpad the cabin was supplied with a 60/40 nitrox blend at just over ambient pressure (to allow leak checks on the craft) and reduce the risk of fire associated with a pure oxygen atmosphere. Exhaled CO2 was removed using scrubbers that would be familiar to rebreather divers.
During launch, Apollo would then reduce the cabin pressure and increase the oxygen content up to the target of 100% oxygen with any nitrogen being slowly lost over time to the minute natural leakage of the craft. This meant the astronauts would be exposed to an ascent from a saturated state at 1 atm to 0.3 atm with the associated risk of DCS.
It took Apollo approximately 10 minutes to reach orbit and unlike diving there is clearly no option to slow the ascent – rockets don’t have brakes.
To further help prevent DCS, astronauts would pre-breathe oxygen to off-gas nitrogen ahead of the ascent, effectively performing the role of a decompression stop. This protocol was successful with no documented cases of DCS. However, the risks are evidenced by anecdotal reports from astronauts complaining of minor joint pain on reaching orbit – later believed to be low level DCS.
Any diver who has done a nitrox course might question how the astronauts planned to safely breathe 100% O2 for possible mission lengths that could last days? Breathing enriched oxygen mixes underwater can quickly lead to oxygen toxicity.
In space, the key is the reduced atmospheric pressure within the capsule – resulting in a far lower partial pressure of oxygen (PPO2) of approximately 0.3 atm. By comparison, a 35 minute dive to 25m using EAN35 results in a PP02 of 1.23 atm and a 20% use of the daily allowable exposure according to PADI DSAT tables. The reduced PPO2 is key.
The Soviet/Russian Soyuz spacecraft and the International Space Station (ISS) both use air at standard atmospheric pressure. This removes the risk of DCS on ascent but does increase it in an emergency decompression scenario and raises complications when needing to perform a spacewalk.
Decompression in Space
The most obviously diving-like activity done by astronauts is the spacewalk or more accurately called an EVA (Extra Vehicular Activity). The spacesuits worn are effectively small spacecraft; providing power, thermal regulation and a safe independent atmosphere. The astronauts can then safely work outside the confines of their parent spacecraft or station. They function similarly to a pure oxygen rebreather- scrubbing the waste CO2 while replacing used oxygen.
To maintain any dexterity in the joints of the suit they have to be operated at as low a pressure as possible. The higher the pressure differential between the inside and outside the more effort is required to move.
The current NASA spacesuits maintain an atmosphere of approximately 0.3 atm of pure oxygen. On Apollo this made EVAs straightforward as they had already performed their decompression and off-gassing on the ground with the cabin maintaining a similarly reduced pressure. On the ISS, with its standard earth pressure and atmosphere, a significant course of decompression is required before making any spacewalk.
In this scenario the astronaut’s tissues will be fully saturated with nitrogen at the sea level pressure of the station so the decompression obligations are significant. The most common process used is camping out in the ISS airlock where the pressure can be reduced and the O2 percentage increased – effectively mirroring staged decompression underwater.
The airlock is large enough that the astronauts can relax, sleep and prevents them from having to spend extended periods decompressing wearing their spacesuits. NASA has other tailored, and suitably complex, procedures using exercise to increase off-gassing that reduce the overall time spent decompressing. These are carefully monitored and can’t be applied to diving. Strenuous exercise on decompression stops is not recommended for divers.
Final Thoughts
The physiological challenges are the same: how to effectively remove nitrogen from your body while moving from a state of higher to lower pressure. The fundamental difference is that in space the decompression price is paid before the activity starts rather than at the end as it is underwater. Unfortunately, I’m unlikely to be able to afford to test the efficacy of the processes on the next tourist flight to the ISS.
References
Preventing Decompression Sickness Over Three Decades of Extravehicular Activity
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