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.
Space suits typically use pure oxygen as breathing gas, which simplifies the PLSS (Personal Life Support System) and thus reduces the suit mass. Spacesuit pressures have ranged from 26kPa to 57kPa, although usually a lower pressure is preferred as this improves mobility. Space suits with relatively high pressure are sometimes referred to as “zero prebreathe”, as they permit an astronaut to transition from an Earth-like atmospheric pressure (101.3kPa) to the suit without needing to spend time prebreathing. Prebreathing involves breathing pure oxygen for a period of time (30 minutes to several hours) at atmospheric or mid-level pressures in order to purge nitrogen from the blood and other tissues, which is necessary with low-pressure suits to prevent DCS (decompression sickness, also known as “the bends”).
Zero prebreathe suits for Mars are highly desirable (Rouen, 1996), as they would result in considerable saved time and much greater convenience for the crew, who will wish to conduct EVA/EHA most sols. It is easier to achieve this with MCP suits, as these can support a higher pressure without sacrificing mobility. Gas pressurised suits require as low as possible air pressure in order to facilitate effective mobility and reduce crew fatigue.
The amount of prebreathing is based on the decompression ratio (also known as the R value), which is the ratio between the partial pressure of buffer gases in the spacecraft or habitat atmosphere, and the total pressure in the space suit. From Campbell, 1991:
An R value of more than 1.0 may have a risk of decompression sickness. This risk increases with increased R value. The risk of decompression sickness during a mission is statistically cumulative over a number of decompressions during that mission; therefore, the mission duration and frequency of EVA must be considered when determining an appropriate value for R for a given mission. A statistical analysis of cumulative risk shows that for R=1.22, the risk of decompression sickness after ten decompressions is 7 percent, while for R=1.4 the risk is 37 percent. Therefore, cumulative risk is an important criterion for long-term planet surface missions and it increases rapidly as R is increased above 1.0. If the decompressions are not separated by enough time to eliminate previously formed gas bubble nuclei from body tissues, the risk of decompression sickness on subsequent decompressions is also greatly increased.
Our crew will be spending approximately 540 days on the surface of Mars, with an estimated 3-4 EHA’s per week, for a total maximum of around 200-300 EHA’s each. Considering this very large number of EHA’s, a zero prebreathe protocol would only be an option for R=1.0, which, if we accept our above habitat atmosphere design, would necessitate a marssuit pressure of 40.4kPa.
As also reported by Campbell (1991), oxygen toxicity can occur when exposed to a high partial pressure for an extended period, i.e. above 40.5kPa for more than approximately 24 hours. The same report also states:
For EVA’s of six to eight hours, 100 percent oxygen can be used at pressures up to 41.4 kPa (6.0 psia) with absolutely no risk.
It’s unlikely that any EHA/EVA on Mars will need to exceed 6-8 hours; or, alternately, this duration could be set as an upper limit. The proposed suit pressure of 40.4kPa fits within these limits, and supports the proposed Hab atmosphere design.
Current MCP suits support a pressure of approximately 30kPa. As this is an active area of research, it seems entirely plausible that MCP suits with an operating pressure of 40.4kPa can be developed in time for a human mission to Mars. From private communications with Dava Newman, Professor of Aeronautics and Astronautics and Engineering Systems at MIT and a leading MCP suit researcher:
We’ve made some outstanding progress this year on our active materials and believe we can now achieve the 30 kPa pressure level. It shouldn’t be asking too much for us to achieve the mid-30 kPa, especially by 2030!
Aspects of atmosphere design relevant for the Hab apply equally to the Rover. We wish to provide a safe and healthful atmosphere while keeping the mass of the Rover as low as practical. There are several advantages if the Rover atmosphere has the same composition and pressure as the Hab:
- No adaptation or prebreathing required when transitioning between the Hab and the Rover.
- The same equipment (such as laptops, scientific instruments and sensors) can be used in both environments. This reduces cost and complexity of design, and part counts.
- The same zero prebreathe protocol can be used for both the Hab and the Rover.
With the Hab and Rover having the same atmospheres it may also be possible to connect the Rover directly to the Hab: perhaps with a person-sized hatch to make it easy to transfer between them without needing to suit up or go through the airlock, or perhaps simply with a pipe to freshen the Rover’s air.
P. D. Campbell, “Internal Atmospheric Pressure and Composition for Planet Surface Habitats and Extravehicular Mobility Units,” Lockheed Engineering and Sciences Company, Contract NAS9-17900, Job Order K1-ETB, Report No. JSC-25003, for NASA Man-Systems Division, 1991.
B. E. Duffield, “Advanced Life Support Requirements Document,” JSC-38571, Revision C, National Aeronautics and Space Administration, Lyndon B. Johnson Space Center, Houston, Texas, 2003.
JSC 20584, “Spacecraft Maximum Allowable Concentrations for Airborne Contaminants,” Toxicology Group, Medical Operations Branch, Medical Sciences Division, Space and Life Sciences Directorate, NASA, Johnson Space Center, June 1999.
M. Rouen, “EVA Advanced Research and Development Road Map,” SAE Technical Paper 972460, 1997, doi:10.4271/972460.