Carbon Dioxide Scrubbers

[This is an article in progress, and is currently being written]

Why Remove C0₂?

A steady supply of oxygen alone is insufficient to keep astronauts breathing. While the intake of oxygen is essential for respiration, the by-product of this respiration is the exhalation of approximately one kilogram of carbon-dioxide per day. The concentration of this gas in Earth's atmosphere is roughly 0.04%, but in the close confines endured by astronauts, accumulations of CO₂ can quickly reach toxic levels.

Symptoms Experienced According to CO₂ Level

CO2 Concentration	Symptoms
1%	Drowsiness
3%	Impaired hearing, increased heart rate and blood pressure, stupor
5%	Shortness of breath, headache, dizziness, confusion
8%	Unconsciousness, muscle tremors, sweating
>8%	Death

NASA has set strict limits of acceptable CO₂ concentration on spacecraft. The longer the duration of the flight, the lower the permissible maximum: for example, while allowing a maximum of 2% for a one-hour period, NASA recommends that the concentration not exceed 0.5% over a 1000-day stay (an as-yet hypothetical duration, as the record for longest consecutive stay in space at the time of writing is held by Russian cosmonaut Valery Polyakov at 438 days). The proportionate decrease in concentration acceptability in accordance with duration accounts for the fact that the longer the period of time, the higher the likelihood of CO₂-induced behavioral changes negatively affecting missions requiring close personal contact in confined spaces.

Moderating CO₂ levels over such a long period of time naturally implies weight and power demands, yet NASA has adopted more stringent regulations than the Navy, which allows up to 2.5% concentrations for submarine personnel over a 24-hour period. While both types of crews operate in a confined area with an artificially-regulated atmosphere, repairing a CO₂ removal system in space constitutes a potentially more complex task where one cannot simply surface in the event of an unsuccessful attempt brought on by concentration difficulties. Hypothetically, a well-developed Mars colony could relax such limitations slightly once it is large enough to contain multiple fail-safes and back-ups, thereby reducing the energy demands on the colony as a whole.

Biological C0₂ Scrubbers

On Earth,plant life removes carbon dioxide from the air and replaces it with oxygen. This elegant solution has yet to feature as the primary form of CO₂ scrubbing for spacecraft, principally because of the number of plants required in conjunction with the costs of providing optimized lighting and air conditioning. The International Space Station, for example, has plants aboard, but not in the quantities necessary to have any appreciable impact on CO₂ concentrations. Yet, such a system could potentially fill the air-recycling needs of future Mars habitats or even spacecraft.