The advantages of a normal Earthian atmosphere apply equally well to the Hab as they do to the ISS and other spacecraft. However, there are several compelling reasons for a reduced atmospheric pressure for the Hab:
- Lower Hab mass.
- Less air to be manufactured, which therefore reduces the mass and energy requirements of the ISAP system.
- Potential for a zero prebreathe protocol for EHA/EVA.
The higher the atmospheric pressure of the Hab, the stronger and therefore heavier it will need to be. This is contrary to our requirement of reducing the mass of the Hab as much as possible because of the significant challenge of landing such a heavy object on Mars.
Atmospheres in early spacecraft had low total pressure, low oxygen pressure, or both. However, not all of these atmospheres would be suitable for an 1.5 year stay on Mars. Research conducted in recent years has more clearly defined the limits for artificial atmospheres suitable for extended exposure.
The minimum partial pressure of oxygen required to support human physiology is considered to be 16kPa. However, for long-duration space missions, a minimum partial pressure of oxygen of 18kPa is recommended (Duffield, 2003). This is based on a previous study about planetary surface habitats (Campbell, 1991), which reviewed 33 different considerations related to atmospheric pressure and composition.
From a physiological perspective, an O2 pressure of 18kPa is perfectly safe. This is equivalent to about 1370m altitude (approximately the altitude of Kathmandu, Nepal), which does not even qualify as “high altitude” in mountain medicine (1500 – 3500m). Acclimatisation to reduced O2 pressure at altitude is characterised by an increase in pulse and breathing rate. Most people can ascend to 2400m (where O2 pressure is about 16kPa) without difficulty, however, altitude sickness may occur above this level. Astronauts can be conditioned for an O2 pressure of 18kPa by training in a hypobaric chamber, or at a moderate altitude (e.g. Black Mesa, US). In a microgravity environment there would already be increased strain on the cardiovascular system , and it would be preferable not to cause any further strain; however, the habitat is in a gravity environment on the surface of Mars, and although this is still a reduced gravity environment compared with Earth, the increased load on the heart will be mitigated.
The next design question is how much buffer gas to include. A pure oxygen atmosphere introduces an unacceptably high risk of fire, such as the one that occurred in the Apollo 1 Command Module. The upper limit of oxygen concentration with regard to fire safety has not clearly defined, but 30% is considered a reasonable upper limit (Campbell, 1991). This gives us a total atmospheric pressure of 60kPa, about 60% of Earth.
Buffer gas refers to the component of the atmosphere comprised of metabolically inert gases, which usually means nitrogen (N2), plus the noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe). The buffer gas portion of the atmosphere of Earth is almost entirely N2 (99%), with about 1% Ar and trace amounts of He, Ne and Kr. As described in the section on In Situ Air Production, because we’re making buffer gas in an economical way by simply using the Martian atmosphere with dust, CO2 and contaminants removed, our buffer gas on Mars will be about half-half N2 and Ar, possibly with trace amounts of Ne, Kr and Xe.
Nominal atmospheric concentrations of CO2 and H2O must also be determined. According to JSC 20584 (Spacecraft Maximum Allowable Concentrations for Airborne Contaminants), the maximum CO2 concentration is 0.7%. A CO2 concentration of 1% can cause drowsiness, with more serious symptoms occurring at higher concentrations. A typical concentration in normal spacecraft operations is 0.5%, which is a reasonable design goal. This gives us a CO2 partial pressure of about 0.3kPa.
With regard to water vapour, NASA specifies a RH (Relative Humidity) of 30-70%, i.e. an average of about 50%. Our target temperature is 295K (about 72°F or 22°C, which is optimal for human comfort and productivity), and the saturated water vapour pressure at this temperature and pressure is about 2.6kPa. Our average water vapour partial pressure will be 50% of this, or about 1.3kPa.
Any other gases present in the atmosphere should be present in trace amounts only.
Proposed design for Mars habitat atmosphere.
|Gas||Partial pressure (kPa)|
|Carbon dioxide (CO2)||0.3|
|Water vapour (H2O)||1.3|
|Buffer gas (N2/Ar)||40.4|
This atmosphere will produce differences in sound quality that the crew will be required to adapt to. The higher density of Ar compared with N2 will have the effect of lowering audio frequencies, including astronaut voices. Sound will also not be as loud or travel as far due to the reduced atmospheric pressure.