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The Mars Project: Avoiding Decompression Sickness on a Distant Planet

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The Mars Project: Avoiding Decompression Sickness on a Distant Planet NASA/TM--2000-210188 The Mars Project: Avoiding Decompression Sickness on a Distant Planet Johnny Conkin, Ph.D. National Space Biomedical Research Institute Houston, Texas 77030-3498 National Aeronautics and Space Administration Lyndon B. Johnson Space Center Houston, Texas 77058-3696 May 2000 Acknowledgments The following people provided helpful comments and suggestions: Amrapali M. Shah, Hugh D. Van Liew, James M. Waligora, Joseph P. Dervay, R. Srini Srinivasan, Michael R. Powell, Micheal L. Gernhardt, Karin C. Loftin, and Michael N. Rouen. The National Aeronautics and Space Administration supported part of this work through the NASA Cooperative Agreement NCC 9-58 with the National Space Biomedical Research Institute. The views expressed by the author do not represent official views of the National Aeronautics and Space Administration. Available from: NASA Center for AeroSpace Information National Technical Information Service 7121StandardDrive 5285 Port Royal Road Hanover, MD 21076-1320 Springfield, VA 22161 301-621-0390 703-605-6000 This report is also available in electronic form at http://techreports.larc.nasa.gov/cgi-bin/NTRS Contents Page Acronyms and Nomenclature ................................................................................................ vi Abstract ................................................................................................................................. vii Introduction ........................................................................................................................... 1 Martian Resources ................................................................................................................. i Minimum Oxygen Pressure ................................................................................................... 2 Maximum Oxygen Concentration ......................................................................................... 3 The Mars Habitat ................................................................................................................... 4 Inert Gases: Ar or N 2 in a Binary Gas Mixture ..................................................................... 8 Binary Gas Mixtures in Animal Decompressions ................................................................. 10 Inert Gases: Ar and N 2 in a Trinary Gas Mixture ................................................................. I I Trinary Gas Mixtures in Animal Decompressions ................................................................ 13 Metabolic Gases .................................................................................................................... 14 Tissue Ratio and Hypobaric DCS Risk ................................................................................. 15 DCS Risk Associated With Simple Tissue Ratio .................................................................. 16 Simple Inert Gas Mass Balance ............................................................................................. 18 Oxygen Prebreathing ............................................................................................................. 20 Potential Evolved Gas ........................................................................................................... 24 Best "Guess" for DCS Risk ................................................................................................... 25 Bubble Growth Model ........................................................................................................... 26 Conclusions ........................................................................................................................... 31 References ............................................................................................................................. 35 Appendix A: Computing the N 2 to Ar Pressure Ratio ......................................................... A-i Appendix B: Assumptions and Recommendations .............................................................. B-I iii Contents (continued) Page Tables Table I. Three Options Evaluated for a Martian Habitat Atmosphere .................................. 5 Table II. Metabolic and Inert Gases in Option 1 ................................................................... 6 Table III. Physical Characteristics of Gases .......................................................................... 9 Table IV. A Simple Mass Balance at 8.0 psia ...................................................................... 19 Table V. A Simple Mass Balance After a 100-Min Prebreathe at 9.0 psia .......................... 24 Table VI. Variables in Bubble Model for Low Density Case in Lipid Tissue ..................... 27 Table VII. Simulation Data That Do Not Exceed Flammability Limits But Do Not Constrain the N 2 - Ar Ratio in Two Cases .................................................................... 28 Table VIII. Simulation Data That Do Exceed Flammability Limits But Constrain the N 2 - Ar Ratio ................................................................................................................. 30 Figures Figure I. The normal 0 2 dissociation curve (! A), and extraction of O 2 from the blood under normal and exercise conditions (1B) ................................................................... 3 Figure 2. For a specific saturation dive and relative to N 2 in lipid tissue, a bubble containing Ar would grow larger and last longer in both aqueous and lipid tissue in a simulation of low bubble density ..................................................................................................... 10 Figure 3. Peak bubble volume in aqueous and lipid tissues following a 200- to 100-kPa decompression after saturation with a nomloxic N2-He breathing mixture in a simulation of low bubble density ................................................................................... 12 Figure 4. The P (death in rats) as a function of hyperbaric saturation pressure, expressed as feet sea water, and the concentration of Ar in a trinary breathing mixture that includes N 2 .................................................................................................................... 13 Figure 5. The presence of constant metabolic gas pressures (0 2, CO2, and H20 vapor) and the rapid equilibration of pressure across the tissue-bubble interface means that the fraction of any inert gases, only N 2 in this case, decrease as ambient pressure decreases. 14 Figure 6. The P(DCS) for a given TR360 (TR based on a 360-min half-time compartment) is greater at 3.75 psia compared to 4.3 psia in a simulation where exercise is present during a 6-hr exposure ................................................................................................... 17 iv Contents (continued) Page Figure 7. Estimated total inert gas pressure (solid line) after summing the pressure of N 2 in the 360-rain half-time tissue compartment and the Ar pressure in the 720-min 2O half-time tissue compartment ......................................................................................... Figure 8. Decrease in total inert gas pressure (solid line) as a function of time breathing 22 100% 0 2 before EVA .................................................................................................... Figure 9. Decrease in total inert gas pressure (solid line) as a function of time breathing 23 100% 0 2 before EVA from the 10.0-psia habitat .......................................................... Figure IO. Heuristic risk-to-benefit analysis of Ar as part of the breathing environment in 26 an 8.0-psia habitat before an EVA in a 3.75-psia suit .................................................... Figure 11. A bubble grows rapidly and to a large size in lipid tissue with trinary or binary 29 breathing gases in the Mars habitat ................................................................................ 31 Figure 12. A bubble grows large, but not as large as some in Fig. i I ................................. Acronyms and Nomenclature AF argon BGI bubble growth index ¢F4 carbon tetrafluoride CO2 carbon dioxide DCS decompression sickness EVA extravehicular activity fsw feet sea water He helium kPa kilopascals N2 nitrogen N20 nitrous oxide 02 oxygen psia pounds per square inch SF 6 sulfur hexafluoride tigp total inert gas pressure TR tissue ratio vi Abstract A cost-effective approach for Mars exploration is to use the available resources, such as water and atmospheric gases. Nitrogen (N 2) and argon (Ar) in a concentration ratio of 1.68/I .0 are available and could form the inert gas component of a habitat atmosphere at 8.0, 9.0, or 10.0 pounds per square inch (psia). The habitat and space suit are designed as an integrated system: a comfortable living environment about 85% of the time and a safe working environment about 15% of the time. A goal is to provide a system that permits unrestricted exploration of Mars. However the risk of decompression sickness (DCS) during the extravehicular activity in a 3.75- psia suit, after exposure to any of the three habitat conditions may limit unrestricted exploration. This communication is an evaluation of the risk of DCS since a significant proportion, about 25%, of a trinary breathing gas in the habitat might contain Ar. I draw on past experience and published information to extrapolate into untested, multivariable conditions to evaluate risk. A rigorous assessment of risk as a probability of DCS for each habitat condition
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