NASA/TP-2010-216134

Recommendations for Exploration 

Spacecraft Internal Atmospheres:

The Final Report of the NASA Exploration 

Atmospheres Working Group

NASA Exploration Atmospheres Working Group

NASA Johnson Space Center, Houston

October 2010

National Aeronautics and

Space Administration

Johnson Space Center

Houston, TX 77058

STS-114 Mission Specialist Stephen K. 

Robinson is attached to a foot restraint 

on the International Space Station's 

Canadarm2. Image credit: NASA
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NASA/TP-2010-216134

Recommendations for Exploration 

Spacecraft Internal Atmospheres:

The Final Report of the NASA Exploration 

Atmospheres Working Group

NASA Exploration Atmospheres Working Group

NASA Johnson Space Center, Houston

October 2010

National Aeronautics and

Space Administration

Johnson Space Center

Houston, TX 77058
Preface

This report is a result of work conducted during 2005 by the NASA Exploration Atmospheres Working Group (EAWG). It is intended for use by NASA offices and programs that are

developing capabilities for human exploration of the moon, Mars, and the solar system.

The members of the EAWG are gratefully acknowledged for their active participation in the 

working group process and for their insightful contributions to this report. External reviewers 

are acknowledged for the comments and data they provided to the EAWG. The members of the 

EAWG support team, including technical analysts and workshop organizers, are acknowledged 

for their critical roles in the success of the EAWG.

Available from:

NASA Center for AeroSpace Information National Technical Information Service

7115 Standard Drive 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://ston.jsc.nasa.gov/collections/TRS/
iii

Contents

Executive Summary.................................................................................................... 1

1 Exploration Atmospheres Working Group Background and Objectives.................... 2

1.1 Background................................................................................................................. 2

1.2 Charter......................................................................................................................... 3

2 Exploration Atmospheres Working Group Process Overview ................................... 4

2.1 Exploration Atmospheres Working Group Activities................................................. 4

2.2 Exploration Atmospheres Working Group Analysis – Overview .............................. 4

3 Exploration Atmospheres Working Group Analytical Foundation ............................ 5

3.1 History of Spacecraft Atmospheres............................................................................ 5

3.2 Related Work .............................................................................................................. 6

3.3 NASA Requirements and Technical Standards.......................................................... 7

4 Trade Study................................................................................................................. 8

4.1 Identification of Candidate Atmospheres ................................................................... 8

4.2 Study Process.............................................................................................................. 9

5 Recommendations for Mission Systems Design......................................................... 15

5.1 Agency and Constellation Program Level .................................................................. 15

5.2 Crew Exploration Vehicle to International Space Station .......................................... 19

5.3 Crew Exploration Vehicle In-Space Suit.................................................................... 19

5.4 Lunar and Mars Crew Exploration Vehicle ................................................................ 19

5.5 Lunar and Mars Landers............................................................................................. 19

5.6 Lunar and Mars Surface Suits..................................................................................... 20

5.7 Lunar and Mars Habitats............................................................................................. 20

5.8 Mars Transit Vehicle................................................................................................... 21

6 Exploration Atmospheres Working Group Recommendations for Future Work ....... 21

6.1 Human Research Program .......................................................................................... 21

6.2 Technology Development Program............................................................................ 21

6.3 Constellation Program ................................................................................................ 22

6.4 Crew Exploration Vehicle Project .............................................................................. 22

6.5 Advanced Projects ...................................................................................................... 22

7 References................................................................................................................... 23

Appendix A: Exploration Atmospheres Working Group Members ...................................... 25

Appendix B: Exploration Atmospheres Working Group Implementation Plan .................... 27

Appendix C: Exploration Atmospheres Working Group Fault Tree Analysis...................... 37
iv

Appendix D: Using the Analytical Hierarchy Process to Aid in Risk Assessment for 

the Exploration Atmospheres Working Group....................................................... 71

Appendix E-1: Ambulation During Extravehicular Activity on Moon and Mars as 

a Risk Factor for Decompression Sickness............................................................ 85

Appendix E-2: Optimal Total Pressure-Oxygen Concentration Levels for Future 

Spacecraft, Spacesuits, and Habitats..................................................................... 103

Appendix F: Exploration Atmospheres Workshop ............................................................... 123
v

Figures

1 Apollo command module atmosphere on-orbit transition .......................................... 6

2 The EAWG candidate atmosphere design space ........................................................ 9

3 EAWG mission-systems assumptions........................................................................ 10

4 Workshop reference design Point X ........................................................................... 13

5 Integrated atmosphere recommendations for Constellation ....................................... 17

6 Historical spacecraft and recommended Constellation atmospheres.......................... 18

7 Notional control box for vehicle cabin atmospheres .................................................. 18
vi

Tables

1 Disciplines Represented in Exploration Atmospheres Working Group ..................... 3

2 Historical Spacecraft Atmospheres............................................................................. 5

3 The EAWG Candidate Atmosphere Design Points.................................................... 8

4 EAWG Expert Ranking of Candidate Atmospheres................................................... 11

5 EAWG White Paper Topics........................................................................................ 11

6 Human Research Areas............................................................................................... 14

7 Vehicle Research and Technology Development Areas............................................. 14

8 Engineering Challenges.............................................................................................. 14

9 Programmatic Impacts................................................................................................ 15

10 Summary of Recommendations for Constellation Mission Systems.......................... 16
vii

Acronyms

AHP Analytical Hierarchy Procedure

CEV crew exploration vehicle

CM Command Module

CO2 carbon dioxide

COTS commercial off-the-shelf

DCS decompression sickness

EAWG Exploration Atmospheres Working Group

EMI electromagnetic interference

ESAS Exploration Systems Architecture Study

ESMD Exploration Systems Mission Directorate

EVA extravehicular activity

FOM figure of merit

FTA fault tree analysis

HSWG Human System Working Group

ISS International Space Station

LEO low-Earth orbit

LSAM lunar surface access module

OCHMO Office of the Chief Health and Medical Officer

P(DCS) probability of decompression sickness

PFO patent foramen ovale

PLSS Portable Life Support System

psia pounds per square inch, absolute

VSE Vision for Space Exploration
viii
1

Executive Summary

To enable missions to the moon and Mars, it is recommended that NASA’s Constellation 

vehicles, habitats, and spacesuits employ an integrated set of internal atmospheres rather than a 

single design for all elements. These atmospheres must mitigate the risks of fire, decompression 

sickness (DCS), and hypoxia and enable crew excursions on planetary surfaces. The recommendations provided herein are not requirements and reflect technical considerations only.

The initial crew exploration vehicle (CEV) missions in low-Earth orbit, for crew and cargo 

transfer to the International Space Station (ISS), should operate within existing ISS and space

shuttle standard atmosphere designs at 14.7 and 10.2 pounds per square inch, absolute (psia)

nominal total pressures. For lunar sortie missions, the docked CEV and lander (also known as 

lunar surface access module [LSAM)], should operate at a nominal 10.2 psia total pressure, 26.5%

oxygen concentration. These atmospheres ensure compatibility with that of the ISS, do not preclude contingency extravehicular activity (EVA), and do not significantly add to the cost or 

schedule burden of CEV development.

For surface operations, the lunar and Mars landers should also operate at a nominal 8.0 psia, 

32% oxygen. This will require spacecraft interior materials flammability testing at a maximum 

oxygen concentration outside the existing shuttle/ISS database, and will add some cost for such 

testing. This atmosphere allows efficient EVA preparation and egress on the moon, and provides 

a nominal altitude equivalent of 5000 ft. All recommended nominal atmospheres are assumed to 

be at the center of a control box of ±0.2 psia total pressure and ±2.0% oxygen concentration. The

lower end of the lander atmosphere control box may extend to equivalent altitudes over 6000 ft,

which is outside the current NASA standard for hypoxic conditions.

It is especially important to note that these recommendations for the landers must be examined 

more closely prior to development of their requirements. For example, a comprehensive trade

study to evaluate acceptable materials at elevated oxygen levels must be conducted to ensure that 

technical and programmatic considerations are assessed.

In-space and surface EVA suits should operate at a nominal 4.3 psia, 100% oxygen, which is

consistent with current shuttle/ISS suits. The surface suit should also be able to operate at the

lander pressure (8.0 psia) for treatment of DCS, at approximately 6.0 psia for rapid EVA egress, 

and at 3.5 psia in certain contingencies. All of these atmospheres are within current NASA

physiological standards for anticipated suited durations at each condition.

The long-duration lunar and Mars surface habitats should operate at both a nominal 8.0 psia, 

32% oxygen and at 7.6 psia, 32% oxygen (6500 ft equivalent altitude). This will allow the crew

to adapt over an extended period of time to the lower pressure, which will then result in higher 

efficiency for EVA preparation and egress. The 7.6 psia nominal atmosphere is within the current

aviation operational experience base, but would require a modification of the NASA standard on 

hypoxic conditions. It is especially important to note that these recommendations for surface 

habitats must be examined more closely prior to development of requirements for those 

elements.
2

The Mars transit vehicle should operate at nominal values of 14.7 psia, 21% oxygen and 10.2 

psia, 26.5% oxygen. This leverages ISS long-duration physiological data and allows the crew to 

acclimate to a lower pressure prior to Mars arrival.

To support these recommendations, as soon as possible Constellation should institute a modified

standard NASA flammability test to generate ignition threshold data for key spacecraft materials.

This modified test will allow NASA to identify materials at risk from increases in oxygen concentration, minimize potential impacts, and allow for development of sound requirements for 

landers and habitats.

The Human Research, Technology Development, and Constellation Programs should initiate 

activities to enable these atmosphere designs (respectively, human research, technology development, and program-specific studies) to validate these recommendations for the Constellation 

habitable elements.

1 Exploration Atmospheres Working Group Background 

and Objectives

The Exploration Atmospheres Working Group (EAWG) was convened to formulate recommendations on the designs of habitable internal atmospheres for future exploration vehicles. This

report describes the EAWG trade space as well as the outcome of systems analysis of candidate 

atmospheres. The results provided by the EAWG are designed as input to requirements for the 

near- and far-term vehicle development efforts within the Constellation Program, including the

Crew Exploration Vehicle (CEV) Project.

1.1 Background

Over the past 47 years, NASA has developed and operated several types of human spacecraft 

and spacesuits to meet its mission needs. These vehicles have operated with internal habitable 

atmospheres ranging from the low-pressure, pure-oxygen environment of Mercury, Gemini, and 

Apollo to the current space shuttle and International Space Station (ISS) Earth sea-level pressure 

and oxygen content.

The Vision for Space Exploration (VSE) (NASA, 2004) specifies that NASA is to develop and execute human missions to the moon, to Mars, and beyond. It emphasizes a long-term approach to

the human exploration of the solar system by NASA. Interplanetary human spacecraft, planetary

landers, spacesuits, pressurized rovers, and surface habitats may be developed to accomplish this 

goal. Each of these new vehicles will contain a habitable atmosphere to support living and working

activities of its human crew. Each atmosphere will be required to meet agency-level standards for

health and safety, ensuring that risks to the human occupants are controlled to acceptable levels. 

To manage these risks as well as the overall programmatic costs, there is a need for a common 

design and development approach to optimize the systems engineering of multiple spacecraft –

specifically their internal atmospheres.
3

NASA addressed this need through formal systems analysis. In 2004, a small systems analysis team at the Johnson Space Center identified atmosphere-related risks to human physiology, 

spacecraft design and materials, and mission operations. This team then constructed a trade space 

of atmosphere designs based on these risks and produced several candidate design points within 

that trade space that met the NASA design standards (Lange et al., 2005).

In 2005, the EAWG operated specifically within the framework of the VSE to refine the atmosphere trade space into a set of recommendations for exploration programs. This final report 

of EAWG activities and results is consistent with the objectives of the VSE and is designed to 

support Constellation Program needs.

1.2 Charter

The Exploration Systems Mission Directorate (ESMD), as part of the NASA inter-organizational

Human System Working Group (HSWG), chartered the EAWG and tasked it to generate recommendations on the characteristics of internal atmospheres for exploration spacecraft, including 

spacesuits and planetary exploration vehicles. Appendix A provides a complete list of EAWG

members.

The EAWG Implementation Plan, Appendix B, was generated in response to the ESMD request.

This plan laid out EAWG objectives and a general plan for EAWG membership, activities, and 

products. The overall EAWG objective was described as to “refine the atmospheric design space 

to a relatively small domain such that atmospheric requirements for space-based systems can be 

specified.” Table 1 lists the discipline areas represented by the EAWG membership.

Table 1. Disciplines Represented in 

Exploration Atmospheres Working Group

Physiological and Medical

Environmental physiology

Extravehicular activity (EVA) physiology

Space medicine

Toxicology

Mission Operations

EVA operations

EVA systems

Exploration mission-systems architectures

Human factors

Safety and mission assurance

Vehicle and Habitat Systems

Active thermal control systems

Environmental control and life support systems

Food systems

Materials flammability and selection

Space radiation shielding

Spacecraft structures
4

1.3 Specific Objectives

As the EAWG formed and operated, more specific process- and product-oriented objectives were 

generated by its leadership to guide group activities and analytical efforts. These included to

• employ risk-based systems engineering analysis to achieve balanced risk mitigation.

• use a long-range view that considers multiple interacting mission systems.

• apply past NASA program experiences to make use of lessons learned and leverage 

flight-proven technologies and techniques.

• take advantage of insights and novel solutions available outside NASA.

• strive for multidiscipline consensus within the EAWG membership.

• identify the roles of research, technology development, and engineering in meeting the 

challenges presented in the ultimate selection of spacecraft atmospheres.

2 Exploration Atmospheres Working Group Process Overview

This section provides an overview of the process and analytical methods used by EAWG.

2.1 Exploration Atmospheres Working Group Activities

Soon after formation, the EAWG held its first general meeting on June 22, 2005 at the Johnson 

Space Center. EAWG members began their discussion and documentation of major risks related 

to atmosphere design.

EAWG analytical activities, which were initiated after the June meeting, led to the second 

general meeting on August 10-11, 2005 at the Center for Advanced Space Studies in Houston.

EAWG members discussed their evaluations of multiple candidate atmospheres under 

consideration.

Additional analytical activities were conducted during September and October 2005. In lieu of a 

third meeting, a teleconference was held with the EAWG membership on October 4, 2005.

On November 1-3, 2005, the EAWG held a workshop to allow invited reviewers to provide 

inputs to its process. Immediately after the workshop on November 3, 2005, the EAWG held a 

meeting of its members on the results of the workshop and the production of this final report.

The EAWG then held a teleconference on December 9, 2005 to discuss its recommendations.

This concluded the EAWG general meetings.

2.2 Exploration Atmospheres Working Group Analysis – Overview

Three EAWG subgroups were formed to focus on specific risk areas, with each subgroup 

containing members from related disciplines. These subgroups were: Physiological and Medical

Factors, Vehicle and Habitat Systems Factors, and Mission Operations Factors.
5

Several analytical methods were combined to form the systems engineering process supporting 

the EAWGs work. Fault tree analysis (FTA) was used to analyze the major risks identified by the 

EAWG. The Analytical Hierarchy Procedure (AHP) was employed to gather EAWG member

evaluations of candidate atmosphere designs to assess the degree of consensus among members.

White papers were generated by EAWG members and associated NASA personnel to provide 

explanations of specific technical issues and design challenges. Gap analysis was performed to 

identify research, technology development, and engineering challenges as future work to support 

the ultimate selection and validation of exploration spacecraft atmospheres.

3 Exploration Atmospheres Working Group Analytical Foundation

This section provides references to materials that were considered fundamental or controlling for 

the EAWG trade study.

3.1 History of Spacecraft Atmospheres

Lange et al. (2005) summarized previous NASA spacecraft atmosphere designs and operations, 

as shown in Table 2.

Table 2. Historical Spacecraft Atmospheres (Lange et al., 2005)

In addition to these steady-state design conditions, the Apollo Program provides a useful 

example of the transition of a spacecraft atmosphere between two operational conditions. In

Biomedical Results of Apollo, Section II, Chapter 5, Environmental Factors (NASA, 1975), onorbit transition of the Apollo spacecraft atmosphere is described. The document states: “Apollo 

40.0 (5.8) 30 In-suit (2) Salyut, Mir, 101.3 (14.7) 21

ISS/Russian

In-suit (after 36 

hours at 70.3 kPa)

Shuttle 70.3 (10.2) 26.5 29.6 (4.3) 40

240 In-suit (3)

120-140

240(3)

0

0

-

EVA O2 Prebreathe Time,

minutes

References: Carson, et al. (1975), McBarron, et al. (1993), Waligora, et al. (1993), NASA (2002), NASA (2003).

(1) 100% oxygen.

(2) In earlier versions of the Orlan suit, the pressure could be reduced to 26.5 kPa (3.8 psia) for short-duration work regime.

(3) Under emergency conditions, a minimum of 150 minutes of unbroken prebreathe is recommended.

Mask and in-suit; 

staged w/exercise

ISS/US 101.3 (14.7) 21 29.6 (4.3)

101.3 (14.7) 21 29.6 (4.3) In-suit

Skylab 34.5 (5) 70 25.8 (3.75) -

Gemini/Apollo 34.5 (5) 100 25.8 (3.75) -

Mercury 34.5 (5) 100 - -

EVA Prebreathe 

Conditions

EVA Suit 

Pressure,(1)

kPa (psia)

Cabin Oxygen 

Concentration,

volume %

Cabin 

Pressure,

kPa (psia)

Program

40.0 (5.8) 30 In-suit (2) Salyut, Mir, 101.3 (14.7) 21

ISS/Russian

In-suit (after 36 

hours at 70.3 kPa)

Shuttle 70.3 (10.2) 26.5 29.6 (4.3) 40

240 In-suit (3)

120-140

240(3)

0

0

-

EVA O2 Prebreathe Time,

minutes

References: Carson, et al. (1975), McBarron, et al. (1993), Waligora, et al. (1993), NASA (2002), NASA (2003).

(1) 100% oxygen.

(2) In earlier versions of the Orlan suit, the pressure could be reduced to 26.5 kPa (3.8 psia) for short-duration work regime.

(3) Under emergency conditions, a minimum of 150 minutes of unbroken prebreathe is recommended.

Mask and in-suit; 

staged w/exercise

ISS/US 101.3 (14.7) 21 29.6 (4.3)

101.3 (14.7) 21 29.6 (4.3) In-suit

Skylab 34.5 (5) 70 25.8 (3.75) -

Gemini/Apollo 34.5 (5) 100 25.8 (3.75) -

Mercury 34.5 (5) 100 - -

EVA Prebreathe 

Conditions

EVA Suit 

Pressure,(1)

kPa (psia)

Cabin Oxygen 

Concentration,

volume %

Cabin 

Pressure,

kPa (psia)

Program
6

preflight checkout procedures initially encompassed an overpressurization of the Command Module (CM) using 100 percent oxygen. After the Apollo fire, these procedures were modified, and 

a mixture of 60 percent oxygen and 40 percent nitrogen was used to reduce the fire hazard. The 

CM was launched with this gas composition, which eventually was built up to almost 100 percent

oxygen, through leakage makeup with oxygen...”. Figure 1, as excerpted from that document,

illustrates a portion of that transition. The two curves (assumed to be upper/lower limits) show 

how the vehicle cabin atmosphere increased from a launch value of approximately 60% oxygen

to the nominal flight atmosphere of 100% oxygen over an extended period in orbit and during

lunar transit.

Figure 1. Apollo command module atmosphere on-orbit transition

(Biomedical Results of Apollo, NASA, 1975).

3.2 Related Work

Lange et al., in their report “Bounding the Spacecraft Atmosphere Design Space for Future 

Exploration Missions” (2005), provided the major source of information used to initiate EAWG 

activities and analyses.

The Bioastronautics Roadmap (NASA, 2005) was a major source of research and technology 

issues as well as of topics for environmental physiology, EVA, and other areas related to 

spacecraft atmospheres. Some of the recommendations in this report may be used as inputs for 

updating research questions in the Bioastronautics Roadmap.

The NASA Exploration Systems Architecture Study (ESAS) (NASA, 2005) was conducted in

parallel with the EAWG work. The ESAS data released during 2005, used by the EAWG as reference mission-systems information, are consistent with the recommendations in this report. The 

full ESAS report was not available to the entire EAWG during its analytical work, however.
7

The Exploration Extravehicular Activity (EVA) Suit Architecture Study was conducted in 

parallel with the EAWG work. Members of the EAWG also participated in this suit study. The 

recommended suit architecture was not published at the time of release of this report, although it 

is anticipated that the EAWG recommendations will be consistent with the suit architecture study 

results.

3.3 NASA Requirements and Technical Standards

NASA issues agency-level technical standards through the Office of the Chief Engineer and 

the Office of the Chief Health and Medical Officer. The NASA Human-Rating Requirements for 

Space Systems (NASA, 2005) is an integral part of the agency-level requirements process, and

refers to various technical standards that must be applied in the spacecraft human-rating process.

NASA technical standards were used as sources of constraints for EAWG analyses and 

recommendations. NASA-STD-3000, Volume I, Revision B, (NASA, 1995) is invoked by the 

Human-Rating Requirements for Space Systems (NASA, 2005). This document provided the 

design space of all atmospheric pressures and compositions available to meet human constraints related to atmospheric pressure and oxygen content.

At the time of the EAWG work, agency-level human health standards were in preparation but 

were unpublished. An internal NASA document, which is invoked by the Human-Rating 

Requirements for Space Systems (NASA, 2005), was used as the existing source of medical 

standards. It requires hyperbaric treatment capability for crewmembers under certain mission 

scenarios. It also cites NASA-STD-3000 (NASA, 1995) for specifications of habitable 

atmospheres.

It is expected that NASA will baseline new space flight health standards in the near future. The 

effect of these new standards must be taken into account in the ultimate selection of exploration 

atmospheres by the Constellation Program. For example, if the agency were to impose standards

in the areas of hypoxia or decompression sickness (DCS) that differ from those stated in an internal NASA documents, these new standards could affect the EAWG recommendations in this

report.

NASA-STD-6001 (NASA, 1998) was also used as a source of information for the EAWG.

Another internal NASA document is also invoked by the Human-Rating Requirements for 

Space Systems (NASA, 2005). It levies a standard that “spacecraft and habitable modules shall 

be designed and operated so that atmospheric pressure and composition control systems maintain 

a habitable environment under all nominal and contingency operational scenarios.” It requires that 

“provisions shall be made to monitor and control oxygen, nitrogen, carbon monoxide, carbon dioxide, partial and total atmospheric pressure, and credible atmospheric contaminants,” and that 

“crew compartments shall be designed with forced ventilation to prevent stagnant air pockets 

from forming in crew-habitable areas of the compartment.”

Non-NASA technical standards related to human physiology, fire prevention, and other areas 

were sometimes considered, but these standards were not mandated or imposed on the EAWG 

process of deliberation and recommendation.
8

4 Trade Study

This section describes the specific trade study conducted by the EAWG. The intent was to refine

the trade space provided by Lange et al., and to narrow the range of recommended atmospheres 

to be provided to the Constellation Program.

4.1 Identification of Candidate Atmospheres

The EAWG began its deliberations using the six candidate atmosphere design points provided by 

Lange et al. The EAWG then added three more candidate design points, resulting in the set 

shown in Table 3 and figure 2.

Table 3. The EAWG Candidate Atmosphere Design Points

Note: P(DCS) is the EAWG estimate of the probability of decompression sickness. The P(DCS) was computed 

for each design point given a 60-minute in-suit pre-breathe with an ambulatory subject performing a 40hour EVA 

that includes exercise (work) in addition to the ambulation to provide a worst-case prediction for a moon or Mars 

EVA.

Note: Oxygen concentrations in the table are nominal, and the upper control limits will be higher.

Current space suit pressure. This point represents 

shuttle EVA preparation conditions.

7 70.3 (10.2) 18.6(2.7) 26.5 29.6 (4.3) 0.178

Current space suit pressure with assumed 

maximum oxygen concentration from the pressure 

study. O2 concentration control limits are outside of 

existing materials flammability qualification 

envelope.

8 65.5 (9.5) 19.7(2.9) 30 29.6 (4.3) 0.079

Current space suit pressure. Highest oxygen 

concentration point (above that assumed in the 

pressure study) and lowest cabin pressure 

minimize prebreathe time. Outside of existing 

materials flammability qualification envelope.

9 57.2 (8.3) 20.6(3.0) 36 29.6 (4.3) 0.011

Higher space suit pressure. Normoxic cabin 

atmosphere; slightly elevated oxygen 

concentration.

6 91.0 (13.2) 21.0(3.0) 23.1 41.4 (6.0) 0.089

Higher space suit pressure with Earth-normal cabin 

oxygen concentration; well above hypoxic 

boundary. Ground testing may be facilitated.

5 88.5 (12.8) 18.6(2.7) 21 41.4 (6.0) 0.089

Higher space suit pressure with Earth-normal cabin 

oxygen concentration; equivalent to 1829-m (6000-

ft) Earth atmosphere. Lower DCS risk. Ground 

testing may be facilitated.

4 80 (11.6) 16.8(2.4) 21 41.4 (6.0) 0.038

Moderate increase in space suit pressure with 

lower cabin oxygen concentration; at hypoxic 

boundary.

3 73 (10.6) 16.9(2.5) 23.2 34.5 (5.0) 0.089

Lowest cabin oxygen concentration with current 

space suit pressure; at hypoxic boundary.

2 64 (9.3) 17.1(2.5) 26.8 29.6 (4.3) 0.094

Current space suit pressure. Cabin atmosphere 

well above hypoxic boundary, but less than 

normoxic. May allow use of materials certified to 

30% oxygen with tight spacecraft operating control 

bands.

1 65 (9.4) 18.5(2.7) 28.5 29.6 (4.3) 0.089

EVA Suit P(DCS) General Characteristics

Pressure, 

kPa (psia)

Cabin 

Oxygen , 

volume 

%

Cabin 

ppO2, 

kPa(psia)

Cabin 

Pressure, 

kPa (psia)

Pt. No.

Current space suit pressure. This point represents 

shuttle EVA preparation conditions.

7 70.3 (10.2) 18.6(2.7) 26.5 29.6 (4.3) 0.178

Current space suit pressure with assumed 

maximum oxygen concentration from the pressure 

study. O2 concentration control limits are outside of 

existing materials flammability qualification 

envelope.

8 65.5 (9.5) 19.7(2.9) 30 29.6 (4.3) 0.079

Current space suit pressure. Highest oxygen 

concentration point (above that assumed in the 

pressure study) and lowest cabin pressure 

minimize prebreathe time. Outside of existing 

materials flammability qualification envelope.

9 57.2 (8.3) 20.6(3.0) 36 29.6 (4.3) 0.011

Higher space suit pressure. Normoxic cabin 

atmosphere; slightly elevated oxygen 

concentration.

6 91.0 (13.2) 21.0(3.0) 23.1 41.4 (6.0) 0.089

Higher space suit pressure with Earth-normal cabin 

oxygen concentration; well above hypoxic 

boundary. Ground testing may be facilitated.

5 88.5 (12.8) 18.6(2.7) 21 41.4 (6.0) 0.089

Higher space suit pressure with Earth-normal cabin 

oxygen concentration; equivalent to 1829-m (6000-

ft) Earth atmosphere. Lower DCS risk. Ground 

testing may be facilitated.

4 80 (11.6) 16.8(2.4) 21 41.4 (6.0) 0.038

Moderate increase in space suit pressure with 

lower cabin oxygen concentration; at hypoxic 

boundary.

3 73 (10.6) 16.9(2.5) 23.2 34.5 (5.0) 0.089

Lowest cabin oxygen concentration with current 

space suit pressure; at hypoxic boundary.

2 64 (9.3) 17.1(2.5) 26.8 29.6 (4.3) 0.094

Current space suit pressure. Cabin atmosphere 

well above hypoxic boundary, but less than 

normoxic. May allow use of materials certified to 

30% oxygen with tight spacecraft operating control 

bands.

1 65 (9.4) 18.5(2.7) 28.5 29.6 (4.3) 0.089

EVA Suit P(DCS) General Characteristics

Pressure, 

kPa (psia)

Cabin 

Oxygen , 

volume 

%

Cabin 

ppO2, 

kPa(psia)

Cabin 

Pressure, 

kPa (psia)

Pt. No.
9

Notes: The Normoxic Equivalent and Hypoxic Boundary are taken from NASA-STD-3000 (NASA, 1995).

Lange’s flammability limit is taken from Lange et al. (2005).

Oxygen concentrations in the figure are nominal, and the upper control limits will be higher.

Figure 2. The EAWG candidate atmosphere design space.

4.2 Study Process

A risk-based analytical process was performed by the EAWG that required that the group

examine in detail the following risks as they relate to the design of spacecraft atmospheres:

*

• Hypoxia: human performance degradation due to insufficient oxygen partial pressure

available in the human lung.

• DCS: injury/illness due to the evolution of gas bubbles in human tissues after partial 

reduction of external pressure on the body.

• Fire: rapid, persistent oxidation of a material that releases heat, or heat and light, and is 

generally accompanied by flame.

• Mission impact: reduction in or loss of mission success due to human and/or system

operational factors.

As the first effort in this analytical process, FTA was conducted to fully characterize these risks.

The EAWG, working in its three subgroups, constructed fault tree models. These models were 

then encoded using FTA software by the analytical support team. The models link together multiple human, mission, system, and environmental conditions that form the basis of each risk. The 

models were validated by EAWG subgroup reviews. It was determined that many quantitative 

 *

These risks were used as the key figures of merit (FOMs) for evaluation of the candidate atmospheres.
10

data sets required to perform numerical calculations of risk magnitudes with these FTA models

were not available to the EAWG. Therefore, the FTA analysis was concluded without calculating 

specific risk levels. Nevertheless the models, as constructed, are valuable as a starting point for

future quantitative risk management by exploration programs and vehicle projects. Appendix C 

provides a report on the FTA effort performed by the EAWG.

The full ESAS was not available to the EAWG during the EAWG analysis cycle. A common set

of mission-systems assumptions was generated by the EAWG to provide the necessary context 

for its analyses in the absence of the ESAS. When partial ESAS results became available late in 

the EAWG analysis cycle, the results were reviewed and compared to EAWG common assumptions; small differences were identified that did not cause a significant change in the EAWG

approach or results. Figure 3 illustrates the EAWG mission-systems assumptions, as modified

to match ESAS results that were available to the EAWG as of November 2005.

Figure 3. EAWG mission-systems assumptions.

The AHP was used as a tool to survey EAWG members to gather their expert opinion data on 

the nine candidate atmosphere design points. Each of the four FOMs was weighted, and each of 

the nine candidate atmospheres was ranked against these FOMs. The results were reviewed within

the three EAWG subgroups at the August meeting. The AHP model of the results was used during

this meeting to generate an EAWG cumulative ranking across all three discipline subgroups. This

culminated in the weighted rankings illustrated in Table 4. The numbers in the cells of the table 

are the candidate atmosphere design points described in Table 3. The Phys/Med Ranking column is 

the ranking generated by that subgroup, based on hypoxia and DCS risks. The MissOps Ranking 

column was generated by experts in that subgroup, based on risk to mission operations; and the 

Earth to/from LEO: 

1. Crew: 6

2. Mission Duration: 3 days to 

ISS, 180 days docked to ISS, 

2 days return to Earth

3. Primary Mission Objectives: 

Transfer crew up/down to 

ISS. Provide ISS crew 

emergency return capability.

4. Vehicles: CEV and Space 

Suit I (zero gravity suit)

5. EVAs: contingency only.

Short Lunar/Mars Surface Mission: 

1. Crew: 4 (Lunar), 6 (Mars)

2. Mission Duration: 4 to 89 days 

on surface

3. Primary Mission Objectives: 

Lunar surface exploration. 

Physiological research. 

Learn to live off the land. 

Mission-systems testing for 

growth to later missions.

4. Vehicles: lander, space suit II 

(surface suit)

5. EVAs: 5 to 60. At least 2 crew 

members on each EVA. EVA 

airlock included in lander.

Long Lunar/Mars Surface Mission: 

1. Crew: 4 (Lunar), 6 (Mars)

2. Mission Duration: 90 to 600 days 

on surface

3. Primary Mission Objectives: Mars 

surface exploration. 

Physiological research. 

Learn to live off the land. 

Mission-systems testing for growth 

to later missions.

4. Vehicles: Mars Lander, Habitat, 

space suit II, pressurized rover

5. EVAs: 30 to 300 EVAs. At least 2 

crew members on each EVA and at 

least 1 remaining in the habitat.

Transit to/from Mars: 

1. Crew: 6

2. Mission Duration: 

180 days each way.

3. Primary Mission 

Objective: Transfer 

crew to Mars, Earth.

4. Vehicles: CEV, 

space suit I, Transit 

Habitat

5. EVAs: contingency 

only.

6. Art. Gravity not used.

Notes:

Atmosphere interfaces among vehicles/suits

are indicated by dark lines.

Nitrogen is the assumed diluent for all vehicles.

CEV

Suit I

Lander Trans

Hab

Hab

CEV

Suit II Suit I

Press

Rover

ISS
11

Veh/Hab Ranking was generated by experts in that subgroup, based on fire risk. Appendix D

contains a report on the AHP analysis and results.

Table 4. EAWG Expert Ranking of Candidate Atmospheres

Note: Numbers in cells are the candidate atmosphere design points described in Table 3.

To further analyze and detail significant findings and issues related to atmosphere design, 

EAWG members generated white papers on several topics; these are listed in Table 5. The 

papers, which were made available prior to the workshop to give outside reviewers a deeper 

understanding of the various issues being addressed by the EAWG ,are included in Appendix E..

Table 5. EAWG White Paper Topics

Number Topic

1 Ambulation During EA on Moon and Mars as a Risk Factor for DCS

2 Variable-Pressure EVA Surface Suit

3 Aspects of Oxygen Partial Pressure, Diluents, and Gravity on Atmosphere Selection for Constellation Systems

4 The Effect of Long-Term Partial Pressure Oxygen Exposure

5 Materials Flammability Control for Constellation Program: Impacts for Enriched Oxygen Environments

6 Space Radiation Shielding Materials

Ranking 

Designator

Phys/Med 

Ranking

Miss Ops 

Ranking Veh/Hab Ranking AHP Weighted Ranking

1: Most Preferred 9 9 4 9

2 4 8 5 8

3 8 1 6 4

4 1 2 3 1

5 5 7 7 5

6 6 3 2 3

7 2 4 1 6

8 3 5 8 2

9: Least Preferred 7 6 9 7
12

Analysis of the white papers resulted in the following summary of significant findings:

• Ambulation Effects on DCS

o Based on ground test subject evaluations with otherwise equivalent EVA conditions, 

DCS risk may be significantly higher traversing planetary surfaces than experienced 

on shuttle and ISS.

o Eliminating DCS risk takes on added importance for a multi-EVA surface scenario.

o Significant uncertainty exists concerning how the “lunar loping” gait seen in Apollo 

missions compares with ambulation under ground test conditions.

• Variable Pressure/Multi-Pressure EVA Suit

o A multi-pressure suit appears achievable.

o Such a suit has a higher programmatic risk that requires engineering to minimize 

mass and achieve required reliability for surface EVAs.

o Nevertheless, a multi-pressure suit presents distinct advantages for planetary EVA 

operations and DCS risk mitigation.

• Flammability of Materials

o A 36% oxygen content cabin atmosphere is supportable with current materials 

technologies.

o Engineering development will be needed, however, including additional materials 

testing (additional program cost).

o There may be added operational restrictions related to items such as crew clothing.

Quantitative breakpoints for analysis also were identified as follows:

• 30% oxygen is the maximum flammability test level for ISS/Space Shuttle Programs;

• 70% and 100% oxygen were used operationally by past human programs;

• a 1-hr pre-breathe is a reasonable maximum time for operations and physiology;

• a 3.5-pounds per square inch, absolute (psia) suit pressure is a reasonable minimum based 

on physiology; and

• a 6-psia suit pressure is a reasonable operating maximum based on engineering

capability.

Gap analysis was also performed by the EAWG. Actions taken at the EAWG August meeting 

included providing a definition of research and technology needs related to atmosphere selection 

as well as their impacts on humans, vehicles, and spacesuits. White papers included additional 

information on these areas of needs and their engineering challenges, all of which were then 

captured in the gap analysis.

After the EAWG conducted the analytical processes described above, the group synthesized a 

tenth candidate atmosphere design point to use as a reference design at its workshop to ensure

that outside reviewers could direct their comments toward a specific point in the trade space. It 

was understood that this reference design was not the final recommendation of the EAWG and 

that it was open to change based on workshop results. Figure 4 illustrates this reference design, 

designated Point X, which was designated as a control box centered on 8.5 psia and 32% oxygen.
13

This vehicle atmosphere concept enabled an option of a variable-pressure spacesuit initially 

operated at 6 psia to eliminate DCS risk during nominal EVA depressurization. This suit could 

then be lowered to 4 psia for dexterous EVA tasks.

Figure 4. Workshop reference design Point X.

The Exploration Atmospheres Workshop was held November 1-3, 2005 in Houston, the report is 

of which is included as Appendix F. Inputs from reviewers at the workshop provided new data or 

citations in several areas related to human physiology, including recent data on U.S. Air Force 

studies of hypoxia (Balldin et al., 2005).

Based on the outcome of the workshop and EAWG deliberations, technical gaps relevant to 

atmosphere design were tabulated. Information on these gaps is included here for information 

and consideration but is not considered exhaustive or prioritized. Table 6 lists human research 

areas, Table 7 itemizes technology development areas, Table 8 cites the engineering challenges, 

and Table 9 catalogs expected programmatic impacts and opportunities for Constellation. Note 

that several items in Table 6 are already in the Bioastronautics Roadmap (NASA, 2005) as part 

of the NASA human research agenda for exploration risk mitigation.

After its workshop discussions and findings, the EAWG conducted additional analysis and 

reached consensus on a draft set of recommendations for vehicle and spacesuit atmospheres.

These are documented in Section 5 of this report.
14

Table 6. Human Research Areas

Research Question

What is the probability of DCS as a function of tissue ratio under microgravity, moon partial gravity, and Mars 

partial gravity?

What is the most effective pre-EVA DCS prevention strategy to include pre-breathe with various gases, exercise,

and other medical measures?

What are the best treatment strategies to manage ebulism?

What are the most effective, yet safe and energy- and space-efficient, means of managing DCS in the space flight 

milieu, including the use of hyperbaric oxygen delivery and other promising technologies; and how might they be 

adapted for reduced-G operations?

What is the risk of DCS after an acute environmental insult – eg, leaking module or damaged EMU – and what 

treatment or response options are available under these off-nominal situations?

What are the operational and medical impacts of off-nominal performance of DCS countermeasures?

What are the risk factors that can increase the likelihood of DCS, such as the presence of patent foramen ovale 

(PFO)?

Is it possible to perform interplanetary EVA; and what are the DCS risk mitigation options for interplanetary EVA 

(eg, moon and Mars) given that a tri-gas breathing mixture including argon is present?

What burn treatment (external and inhalation) should be used in reduced gravity?

Table 7. Vehicle Research and Technology Development Areas

Need

Effects of pressure, gravity, oxygen mole fraction, and diluent gas on material flammability.

Acceptable materials for clothing, windows, stowage foam, and radiation protection for oxygen % above 30%.

Detection of fires in alternate environments and atmospheres must be evaluated..

Development of fire suppression systems that are effective for oxygen concentrations > 30% (“No good story for 

putting out fires”).

Methodology for maintaining configurations controls over long duration.

Development of nonflammable coverings and coatings for flammable materials.

Methodology to assess total risk of fire to vehicle/habitat system.

Food processing: Development of an EMI [electromagnetic interference]-acceptable microwave oven.

Develop understanding of ignition mechanisms for high oxygen concentration systems (flow friction and particle 

impact).

Effect of atmospheric noise and voice communications.

Effect of low-atmosphere pressure on food preparation and processing.

Threshold for flammability of hair and skin.

Material flammability during medical operations.

Table 8. Engineering Challenges

Challenge

How do the different oxygen % and pressure combinations change leakage risks in cabins/habitats?

Variable-pressure suit: added complexity, weight, and increased loads certification.

Variable-pressure suit: motorized regulator will most likely be heavier, bulkier, more costly, and susceptible to 

more failure modes.

Variable-pressure suit: the impacts on DCS risk in an emergency situation (suit pressure of 3.3 to 3.9 psia) with an 

initial higher suit pressure (6 to 8 psia) need to be considered; or an emergency pressurization system with 

variable set point could be developed that “follows” the primary regulator set-point changes (see regulator 

challenge).

Variable-pressure suit: relief valve to enable quick reduction of suit pressure to desired operating range function 

would be more difficult to implement in a variable-pressure suit as the final set points are unknown.

Variable-pressure suit: ventilation fan designed to operate at variable pressures will probably not be as efficient as 

a fan finely tuned to work at one specific pressure.

Variable-pressure suit: advanced microphones to work in a variable pressure environment.

Variable-pressure suit: vent loop sensors to operate in a variable-pressure environment (eg, carbon dioxide [CO2]

sensor).
15

Challenge

Variable-pressure suit: swing-bed scrubber to operate in variable pressure (assuming technology becomes 

available).

Variable-pressure suit: power efficiency for technologies designed for a specific pressure range will decrease as 

well as the fidelity of the subsystem.

Variable-pressure suit: increased loads, increased weight, and reduced operating life of the pressure garment.

Variable-pressure suit: soft components subjected to loads greater then 6.0 psi repeatedly may need redesign to 

hard composite components to maintain a longer cycle life. Also, hard goods provide less adjustability (add parts 

mass), are more expensive to design and manufacture, and require more stowage volume.

Variable-pressure suit: glove dexterity will be decreased. Some soft components of the gloves could have to be 

switched to hard components.

Materials flammability: composites and polymers proposed for space radiation control might have to be wrapped 

in nonflammable material.

Materials flammability: polycarbonates (transparent window materials) may be lost at 30% or greater nominal 

oxygen concentrations.

Materials flammability: lack of materials choices affects cost.

Materials flammability: fluoropolymer coatings have been used on flammable commercial off-the-shelf (COTS)

items for shuttle and ISS. Such coatings may not work at higher oxygen concentrations and are fragile.

Radiation shielding: most effective shields are materials rich in hydrogen and carbon, which tend to be flammable. 

Polyethylene will have to be coated with nonflammable material for use as an internal radiation shield or structural 

material.

Table 9. Programmatic Impacts

Impact

Variable-pressure suit: With the design implications required to achieve a variable-pressure suit, there is 

additional program risk (cost, schedule, and technical) over current single/dual-pressure suit design.

Variable-pressure suit: additional formal studies needed within the EVA program to further assess the potential 

issues associated with a variable-pressure suit concept.

Variable-pressure suit (opportunity): substantial long-term benefits over a single- (or dual-) pressure suit to adapt 

to changing requirements or unforeseen operational hurdles.

Partial-pressure oxygen exposure: “Once cabin pressure has been selected, decompression studies with human 

subjects will be necessary to develop pre-breathe protocols that control risk of DCS to acceptable levels.”

Materials flammability: Cost increases due to restricted use of commercial off-the-shelf equipment items at 

enriched oxygen levels.

5 Recommendations for Mission Systems Design

This section captures major technical findings and recommendations resulting from the EAWG 

trade study and closely related efforts. These recommendations are directed toward exploration 

programs within the agency, and specifically toward the Constellation Program. Section 6.0 also 

provides recommendations on future NASA work related to atmosphere design and selection. The

recommendations provided herein are not requirements; they reflect technical considerations 

only.

5.1 Agency and Constellation Program Level

The Constellation Program should plan for a well-integrated design extending across all vehicle 

atmospheres to ensure crew safety, vehicle reliability, mission design flexibility, systems interoperability, and crew operational efficiency.
16

As detailed in Table 10, the initial CEV, designated to provide crew transfers to the ISS, and the 

lunar and Mars CEVs should use atmospheres consistent with current agency technical standards 

and with current program design and operational experience. CEV internal total pressures of 14.7 

and 10.2 psia (nominal) meet these criteria. The CEV must also support a vacuum atmosphere to 

accommodate contingency EVA.

Table 10. Summary of Recommendations for Constellation Mission Systems

Vehicle

Nominal

Total 

Pressure

(psia 

± 0.2 psia )4

Nominal 

Oxygen 

Partial 

Pressure 

(mmHg)4

Nominal 

Oxygen 

Concentration 

(% ± 2.0

percentage 

points)4

Range of Total 

Pressure 

Capability

(psia)1

Tissue Ratio 

(R) After 60 

Minutes Prebreathing3

CEV to ISS 14.7

10.25

160 (0 ft )

140 (3500 ft)

21

26.5 0-14.9

CEV In-Space Suit 4.3 222 100 4.0-4.6

1.55 from 

10.2 psia 

CEV to 4.3-

psia suit

Lunar and Mars CEV 14.7

10.2

160 (0 ft )

140 (3500 ft)

21

26.5 0-14.9

Lunar and Mars Landers 10.2

8.0

140 (3500 ft) 

132 (5000 ft)

26.5

32 0-14.9

Lunar and Mars Surface 

Suits

4.3

6.0

222

310

100

100 3.5-8.0 2

1.13 from 8.0 

psia landers 

to 4.3-psia 

suit; 

1.07 from 7.6 

psia surface 

habitats to 

4.3-psia suit

Lunar and Mars Surface 

Habitats

8.0

7.6

132 (5000 ft)

126 (6500 ft)

32

32 0-14.9

Mars Transit 14.7

10.2

160 (0 ft )

140 (3500 ft)

21

26.5 0-14.9

Note 1: Range of total pressure capability covers Earth launch, Earth entry, and contingencies.

Note 2: Surface suit 3.5-psia capability for suit emergency operations, 8.0 psia for DCS treatment.

Note 3: Sixty minutes in-suit pre-breathe is defined as the time in the suit after purge and leak check until absolute pressure 

on the body reaches 4.3 psia after a nominal depressurization. Nitrogen is the assumed diluent gas.

Note 4: All nominal values are centers of control boxes assumed ±0.2 psia total pressure, ±2 percentage points oxygen.

Note 5: The 10.2-psia recommendation for CEV contingency EVA preparation is based on shuttle experience.

The lunar and Mars landers should provide atmospheres that enable docking with the CEV and 

support-efficient surface EVA preparation and high-EVA frequency for surface sortie missions. 

Atmospheres of 10.2 and 8.0 psia (nominal) internal pressure meet these requirements.
17

The lunar and Mars surface habitat elements should use atmospheres of 8.0 and 7.6 psia (nominal). After extended acclimation of surface habitat crew members, the lower pressure can be 

used, further enhancing efficient and frequent EVA capabilities.

The in-space suit for contingency EVA should provide an internal atmosphere of 4.3 psia 

(nominal). The surface spacesuit used for planned EVA should provide internal atmospheres of 

4.3 and 6.0 psia (nominal). The 6.0-psia atmosphere is useful to enable rapid EVA egress, with 

minimum DCS risk, from the lander and surface habitat. The 4.3-psia atmosphere is useful for 

dexterous EVA task performance.

It is noted that several of these recommended atmospheres may result in conditions slightly 

outside current agency hypoxia standards, as documented in NASA-STD-3000, figure 5.1.2-2

(NASA, 1995). These recommendations are based on the most recent, substantial physiological 

testing evidence, which states that healthy individuals can perform reliably under recommended 

conditions, including equivalent altitudes up to 8000 ft (118 mmHg oxygen pressure) without prior

acclimation and up to 10 000 feet (110 mmHg oxygen pressure) with an extended period of 

acclimation (Balldin et al., 2005; Waligora et al., 1982).

Note also that several of the recommended atmospheres involve oxygen volume concentrations

slightly greater than 30%, which is the maximum nonmetallic materials flammability certification level used by current operational human space flight programs.

Table 10 provides a summary of the recommendations to Constellation. Figure 5 illustrates how 

they relate to and integrate with each other. Figure 6 illustrates the recommended atmospheres in 

the context of historical NASA spacecraft designs.

Figure 5. Integrated atmosphere recommendations for Constellation.

Earth to/from LEO: 

1. Crew: 6

2. Mission Duration: 3 days to 

ISS, 180 days docked to ISS, 

2 days return to Earth

3. Primary Mission Objectives: 

Transfer crew up/down to 

ISS. Provide ISS crew 

emergency return capability.

4. Vehicles: CEV and Space 

Suit I (zero gravity suit)

5. EVAs: contingency only.

Short Lunar/Mars Surface Mission: 

1. Crew: 4 (Lunar), 6 (Mars)

2. Mission Duration: 4 to 89 days 

on surface

3. Primary Mission Objectives: 

Surface exploration. 

Physiological research. 

Learn to live off the land. 

Mission-systems testing for 

growth to later missions.

4. Vehicles: lander, space suit II 

(surface suit)

5. EVAs: 5 to 60. At least 2 crew 

members on each EVA. EVA 

airlock included in lander.

Long Lunar/Mars Surface Mission: 

1. Crew: 4 (Lunar), 6 (Mars)

2. Mission Duration: 90 to 600 days 

on surface

3. Primary Mission Objectives: 

Surface exploration. 

Physiological research. 

Live off the land. 

Mission-systems utilization.

4. Vehicles: Mars Lander, Habitat, 

space suit II, pressurized rover

5. EVAs: 30 to 300 EVAs. At least 2 

crew members on each EVA and at 

least 1 remaining in the habitat.

Transit to/from Mars: 

1. Crew: 6

2. Mission Duration: 

180 days each way.

3. Primary Mission 

Objective: Transfer 

crew to Mars, Earth.

4. Vehicles: CEV, 

space suit I, Transit 

Habitat

5. EVAs: contingency 

only.

Notes:

Pressure values for each vehicle are nominal, in psia.

Atmosphere interfaces among vehicles/suits are indicated by dark lines.

CEV

14.7, 10.2 

Suit I

4.3

Lander

10.2, 8.0

Hab

14.7, 10.2

Hab

8.0, 7.6

CEV

14.7, 10.2

Suit II

4.3, 6.0

Suit I

4.3

Press

Rover

ISS

14.7

CEV

14.7, 10.2

Suit I

4.3
18

Figure 6. Historical spacecraft and recommended Constellation atmospheres.

Each vehicle cabin mixed-gas atmosphere is assumed to be controlled within a range of total 

pressures and oxygen partial pressures, resulting in a notional control box, as depicted in figure 7.

The nominal total pressures, oxygen partial pressures, and oxygen concentrations given in Table 10

are at the center of each control box. It was assumed by the EAWG that the control box for each 

vehicle atmosphere is ±0.2 psia on the total pressure axis and ±percentage points on the oxygen 

concentration axis. When actual control boxes are generated later in each vehicle design cycle, 

they may vary from these assumptions.

Figure 7. Notional control box for vehicle cabin atmospheres.

Oxygen Volume Concentration (%)

Total Pressure (psia)

Curve of Constant

Nominal Oxygen

Partial Pressure

(mmHg)

Notional

Control

Box

Nominal

Oxygen

Concentration

Nominal

Total Pressure

+/- 2

percentage

points

+/- 0.2 psia
19

Note that, using this notional control box, the 10.2 psia, 26% oxygen atmosphere implies a 

maximum oxygen concentration of 28.5%. However, the EAWG assumes that, in this case, 

Constellation will make use of the fact that the Space Shuttle and ISS Programs have certified 

many materials for use at a maximum of 30% oxygen.

5.2 Crew Exploration Vehicle to International Space Station

The initial CEV mission is intended to service the ISS with crew and cargo. Compatibility with 

the nominal ISS atmosphere of 14.7 psia and 21% oxygen as well as the rapid development cycle 

for the initial CEV place heavy emphasis on the use of existing technology and flight operations 

techniques.

Current space shuttle atmosphere designs and attendant capabilities were considered by the 

EAWG to meet the needs of this CEV mission. Per Table 10, the CEV to ISS is recommended to

use a 14.7 psia, 21% oxygen atmosphere for normal operations and a 10.2 psia, 26.5% oxygen

atmosphere for contingency EVA preparation when undocked from ISS. These atmospheres fit 

within both agency health standards and current system design regimes; they also allow the use 

of existing COTS hardware, clothing, stowage containers, etc. that are certified for flammability

to Shuttle/ISS requirements.

5.3 Crew Exploration Vehicle In-Space Suit

The CEV in-space suit is assumed to provide only contingency EVA capability in low-Earth 

Orbit (LEO), as well as during transit to and from the moon and Mars. The rapid development 

cycle for this suit places heavy emphasis on the use of existing technology and flight operations 

techniques.

Current EVA Portable Life Support System (PLSS) atmosphere designs and attendant capabilities were considered by the EAWG to meet the needs of this suit. Per Table 10, the in-space 

suit is recommended to use a normal operations atmosphere of 4.3 psia oxygen. This atmosphere

fits within agency health standards and current materials flammability test regimes.

5.4 Lunar and Mars Crew Exploration Vehicle

The CEV is also intended to function in both lunar and Mars crew transit missions. This CEV 

must interface with both lunar and Mars vehicles, including the lunar lander and the Mars transit 

vehicle. Per Table 10, the lunar and Mars CEVs are recommended to use atmospheres like those 

of the ISS CEV.

5.5 Lunar and Mars Landers

The lunar lander is intended to deliver crew and cargo to and from the lunar surface. The Mars

lander is assumed to perform a similar function for access to the martian surface. On the planetary surfaces, these landers support multiple EVA excursions during short-duration surface stays.

These landers must interface with both lunar and Mars vehicles, including the lunar CEV, the 

Mars transit vehicle, and the surface suits.
20

The available development schedule for these vehicles allows the use of new technology and 

flight operations techniques. New atmosphere designs and attendant capabilities were considered

by the EAWG to best meet the needs of these lander missions. Per Table 10, the lunar and Mars 

landers are recommended to use both a 10.2-psia atmosphere for docked operations with the 

CEV and an 8.0-psia atmosphere for planetary surface operations. The 8.0-psia atmosphere 

design extends slightly beyond current agency health standards and current materials flammability test regimes.

Exposed nonmetallic materials for the landers will have to be evaluated for flammability in 

the higher maximum oxygen concentration proposed for their missions. The impact to materials

selection is not expected to be large, given that most exposed surfaces of vehicle hardware will 

be metal or painted metal and standard aerospace electrical wiring, all of which are known to meet

flammability requirements at oxygen concentrations well beyond those proposed for the lunar 

and Mars missions. However, flammability in the increased oxygen concentration could be a 

design driver for composites used in the internal structure.

It is especially important to note that these recommendations for landers must be examined more 

closely prior to development of requirements for the vehicles.

5.6 Lunar and Mars Surface Suits

The surface suit is intended to provide for a multiple, low-overhead EVA capability on the planetary surface. The available development schedule for this suit emphasizes the use of new technology and operations techniques to achieve EVA objectives. The surface suits interface with 

lunar and Mars landers and surface habitats.

The current EVA PLSS nominal operations atmosphere design (4.3 psia) was considered by the

EAWG to meet the dexterous task needs of these surface suits. Per Table 10, the lunar and Mars 

surface suits are recommended to use atmospheres of 4.3 and 6.0 psia, respectively, to provide

flexibility in EVA preparation and operations. The 6.0-psia atmosphere is useful to enable rapid

EVA egress, with minimum DCS risk, from the lander and the surface habitat. The 4.3-psia 

atmosphere is useful for dexterous EVA task performance.

An additional optional atmosphere of 8.0 psia is recommended as an in-suit treatment capability 

for DCS. A 3.5-psia capability is also recommended for certain EVA system contingencies.

5.7 Lunar and Mars Habitats

The lunar habitat is intended to provide for long-duration crew habitation and EVA access on 

the lunar surface. The Mars habitat performs a similar function on the martian surface. While these

habitats interface with the surface suits to support multiple EVA excursions during long-duration 

surface stays, they may interface with pressurized rovers or other similar surface systems pressurized elements.

The available development schedule for these habitats allows the use of new technology and 

flight operations techniques. New atmosphere designs and attendant capabilities were considered

by the EAWG to best meet the needs of these habitat missions. Per Table 10, the lunar and Mars 
21

habitats are recommended to use both 8.0- and 7.6-psia atmospheres for normal operations. After 

an extended period of 8.0-psia operations, the crew can acclimate to the 7.6-psia pressure, which 

provides even greater EVA preparation and egress efficiency. These atmosphere designs extend 

slightly beyond current agency health standards and current materials flammability test regimes.

It is especially important to note that these recommendations for surface habitats must be 

examined more closely prior to developing requirements for those elements.

5.8 Mars Transit Vehicle

The Mars transit vehicle is intended to provide for long-duration crew habitation during transit to

Mars as well as during the return transit to Earth. This vehicle was assumed to interface with the 

in-space suit, the Mars CEV, and the Mars lander.

Per Table 10, the Mars transit vehicle is recommended to initially operate at 14.7- and 10.2-psia 

atmospheres. The total pressure can be decreased to 10.2 psia during transit to Mars, thus achieving a degree of crew acclimation prior to Mars arrival for surface operations at 8.0 and 7.6 psia.

6 Exploration Atmospheres Working Group

Recommendations for Future Work

The EAWG was not intended to provide the ultimate, conclusive point design for all spacecraft

atmospheres because many decision-making factors are inherently programmatic in nature, including cost and schedule issues and impacts. Therefore, recommendations are given below for 

specific NASA programs to perform further work that will leverage the EAWG technical results 

for Constellation mission-systems design and implementation.

6.1 Human Research Program

The EAWG recommends that the Human Research Program consider researching the areas

listed in Table 6. This research would support the Office of the Chief Health and Medical Officer 

(OCHMO) in developing agency-level human health, medical, and environmental standards and 

Constellation in developing lunar and Mars mission capabilities. It is noted that several similar 

research questions are currently in the Bioastronautics Roadmap (NASA, 2005).

It is also recommended that the OCHMO, supported by the exploration medicine team, consider updating the hypoxia standards to allow the Constellation Program to design to the levels 

of atmospheric oxygen associated with the EAWG recommendations in Section 5.0.

6.2 Technology Development Program

The EAWG recommends that the ESMD Technology Development Program consider research 

and technology development in the areas listed in Table 7 to support the Constellation Program

in developing lunar and Mars missions.
22

6.3 Constellation Program

The EAWG recommends that the Constellation Program perform program-specific trade studies

to evaluate the atmosphere recommendations in this report to generate atmosphere requirements

for Constellation vehicles and suits. These studies should account for program factors that the

EAWG did not analyze. These may include many program advantages and disadvantages, such 

as cost impacts per year for gas leakage, EVA pre-breathe time, and the nonmetallic materials 

development/test/certification/usage control process.

The EAWG recommends that the Constellation Program perform systems engineering to address

the areas listed in Table 8. The EAWG also recommends that the Constellation Program study

the issues and potential programmatic impacts (eg, cost, schedule, definition of acceptable risk) 

listed in Table 9.

Because of the uncertainties in the final maximum oxygen percentage concentration to be used,

Constellation should modify the standard NASA flammability test as soon as possible to generate ignition threshold and material flammability data for key spacecraft materials over the range 

of conditions identified in this report. The modified test will allow NASA to identify materials at 

risk from possible increases in oxygen concentration and minimize potential impacts.

It is recommended that, in future testing, the NASA groups involved in materials flammability 

testing begin to plan for certification testing at increased oxygen concentration levels.

6.4 Crew Exploration Vehicle Project

The EAWG recommends that the CEV Project support the atmosphere trade study described 

in Section 6.3. CEV should also incorporate findings from the Human Research Program, the 

Technology Development Program, Constellation, and the Exploration EVA team into its vehicle 

atmosphere design to support lunar and Mars missions.

6.5 Advanced Projects

The EAWG recommends that Constellation Advanced Projects support the trade study described

in Section 6.3 from an EVA and future habitable elements standpoint. For example, a comprehensive

trade study to evaluate acceptable materials at the elevated oxygen levels recommended for both 

landers and surface habitats must be conducted to ensure that technical and programmatic considerations are assessed.

The Exploration EVA team should also incorporate findings from the Human Research Program, 

the Technology Development Program, Constellation systems engineering, and the CEV Project

into its suit atmosphere design to support lunar and Mars missions.
23

7 References

Balldin U, Webb J, Tutt R, Wheeler D, Fischer J, Thompson W, et al. Effect of Low Grade 

Hypoxia at Night in Special Operations Forces Aircraft Operations. AFRL-HE-BR-TR-2005-

0163, Biosciences and Protection Division, Brooks City-Base, San Antonio, Texas, November 

2005.

Lange KE, Perka AT, Duffield BE, Jeng FF. Bounding the Spacecraft Atmosphere Design Space 

for Future Exploration Missions, NASA/CR-2005-213689, Johnson Space Center, Houston, 

2005.

NASA, Bioastronautics Roadmap, NASA/SP-2005-6113, Johnson Space Center, Houston, 2005.

NASA, Biomedical Results of Apollo, NASA/SP-368, Johnson Space Center, Houston, 1975.

NASA, Exploration Systems Architecture Study (ESAS) Final Report, NASA/TM-2005-214062, 

NASA Headquarters, Washington, D.C., 2005.

NASA, Flammability, Odor, Offgassing and Compatibility Requirements and Test Procedures 

for Materials in Environments that Support Combustion, NASA-STD-6001, NASA 

Headquarters, Washington, D.C., 1998.

NASA, Human-Rating Requirements for Space Systems, NPR 8705.2A, NASA Headquarters, 

Washington, D.C., 2005.

NASA, Human Systems Integration Standards, NASA-STD-3000, Volume I, Revision B, NASA 

Headquarters, Washington, D.C., 1995.

NASA, The Vision for Space Exploration, NASA Headquarters, Washington, D.C., 2004.

Waligora JM, Horrigan DJ Jr, Bungo MW, Conkin J. Investigation of the combined effects of 

bedrest and mild hypoxia. Aviat Space Environ Med. 1982;53(7):643-646.
24
25

Appendix A:

Exploration Atmospheres Working Group Members
26

Barghouty, Nasser Code XD41, NASA MSFC

Buderer, Melvin Code SK, NASA JSC

Campbell, Paul Lockheed Martin Space Operations

Chambliss, Joe Code EC1, NASA JSC

Conkin, Johnny Code SK, NASA JSC

Drake, Bret Code EX, NASA JSC

Ewert, Michael Code EC2, NASA JSC

Fitzpatrick, Daniel Code SD37, NASA JSC

Galbreath, Gregory Code ES2, NASA JSC

Gernhardt, Michael Code CB, NASA JSC

Henninger, Donald Code EC1, NASA JSC

Jones, Jeff Code SD, NASA JSC

Khan-Mayberry, Noreen Code SF23, NASA JSC

Lange, Kevin Jacobs Sverdrup Technology

Lawson, Michael Code EC1, NASA JSC

Lin, Chin Code EC2, NASA JSC

McCandless, Jeff NASA HQ, NASA ARC

Morin, Lee Code CB, NASA JSC

Patrick, Jeff Code XA, NASA JSC

Pedley, Michael Code ES4, NASA JSC

Perchonok, Michele Code SF3, NASA JSC

Powell, Michael Code SK, NASA JSC

Ruff, Gary Code PTO0, NASA GRC

Rust, Randy Code NX, NASA JSC

Severance, Barbara Code DF, NASA JSC

Trevino, Robert Code EC5, NASA JSC

Urban, David Code RUC0, NASA GRC

Waligora, Jim USRA
27

Appendix B:

Exploration Atmospheres Working Group

Implementation Plan
28

INTRODUCTION

This is a product of the Crew and Thermal Systems Division (CTSD), Engineering 

Directorate, Johnson Space Center. The chair or lead for this activity is Don 

Henninger, Chief Scientist for CTSD and Paul Campbell of Space Life Sciences is 

Deputy. Support for the development of this document was provided by the Human 

Systems Research and Technology Theme Systems.

1.0 BACKGROUND

As NASA plans human exploration missions and develops new spacecraft and 

habitats, it must specify environmental requirements for the human-occupied systems. 

One such requirement is the definition of the human habitation atmosphere – pressure 

and constituent gases. Human-occupied spacecraft and habitat environments must 

provide a safe atmosphere. Total atmospheric pressure and constituent gas 

composition must be consistent with human physiology, the space environment, the 

spacecraft or habitat design, and with space exploration mission objectives. 

NASA’s currently operating human spacecraft, the Shuttle and the International Space 

Station (ISS), nominally operate at 101 kPa (14.7 psi). An exception is that prior to an 

Extravehicular Activity (EVA), Shuttle atmospheric pressure is lowered to 70 kPa 

(10.15 psi) to reduce pre-breathe times for the EVA in suits where the pressure is 30 

kPa (4.3 psi) (see Figure 1). In the case of the ISS, an airlock is used for egress and 

ingress during EVA events. Now that NASA is preparing to design and build new 

spacecraft and habitats, it is prudent to assess the optimum design space parameters 

for the human habitable atmospheres (spacecraft cabins, planetary surface habitats, 

airlocks, spacesuits, and pressurized rovers). Preliminary design bounds have been 

added to facilitate the discussion (see Figure 1).
29

Figure 1. Assumed design bounds based on respiration requirements. Historical 

spacecraft cabin atmosphere conditions are shown for comparison.

2.0 OBJECTIVES AND ANTICIPATED RESULTS

The objective of this task is to refine the atmospheric design space to a relatively 

small domain such that atmospheric requirements for space-based systems can be 

specified.

3.0 APPROACH AND PRODUCTS

An Exploration Atmospheres Working Group (EAWG) will be formed consisting of 

technical and programmatic stakeholders to evaluate current knowledge and define 

the viable design space domain. A general process is shown in Figure 1. The 

EAWG will conduct additional analyses to further refine the design space domain. 

After reaching a consensus on the design space domain, a workshop will be 

convened to review these results and solicit additional inputs. The EAWG will then 

make recommendations for additional research and technology development to 

enable NASA to specify exploration atmosphere requirements. The EAWG will 

support development of requirements and integration into the Exploration Systems 

Mission Directorate (ESMD) Constellation Systems documentation.
30

Select Candidate Design Values Select Candidate Design Values Optimize

Define Requirements Define Requirements

Define Missions Define Missions

Define Bounds on Cabin and Space Suit 

Atmosphere Design Space

Define Bounds on Cabin and Space Suit 

Atmosphere Design Space

Evaluate Detailed Impacts

(Risk, Performance, Mass, Cost, etc.)

Structures

Life Support

Space Suits

Safety 

Radiation Shielding

Materials

Human Factors Operations

Research (Answer 

Enabling Questions)

Research (Answer 

Enabling Questions)

Define Risks Define Risks

Refine

Figure 2. Envisioned atmosphere selection process.

The EAWG will determine the technical analysis approach and process(s) to be used 

to arrive at its recommendations on atmospheric pressure and composition. A straw 

man technical approach to be considered is quantitative risk analysis (see Figure 2), 

to ensure that the recommended atmosphere(s) control all risks to exploration crews 

and missions.
31

Figure 3. Notional risk-based process for definition of habitat and suit 

atmosphere requirements.

Form EAWG: May 2005

Identified technical and programmatic stakeholders or designees will be invited by the 

EAWG chair to review and discuss the topic. Competing positions relative to future 

human exploration missions will be identified and documented. The EAWG will 

develop a requirements framework to validate the design space in such a way that 

requirements for future spacecraft, space suits, and habitats can be specified. 

Risk Risk Analysis
32

The following are the tentative stakeholders or designees invited to participate as 

members of the EAWG: 

Area of Expertise Name Center

1 Radiation Barghouty, Nasser MSFC

2 Medical Barratt, Michael R. MD JSC

3 Science Buderer, Melvin C. JSC

4 Decompression Sickness Conkin, Johnny JSC - NSBRI

5 Human Standards Connolly, Janis JSC

6 Radiation Cucinotta, Francis A. JSC

7 Medical Duncan, James M. JSC

8 Medical - EVA Fitzpatrick, Daniel T. JSC

9 EVA, Decompression Sickness, Crew Gernhardt, Michael L. JSC

10 Mission Architecture Gruener, John E. JSC

11

Technology Development, Integrated Testing, & 

Plant Growth Henninger, Donald L. JSC

12 EVA Kearney, Lara E. JSC

13 ECLSS - Advanced Life Support Lawson, B.M. (Mike) JSC

14 Thermal Lin, Chin H. JSC

15 Headquarters - Requirements Division McCandless, Jeff Hqs

16 Analytical Systems Packham, Nigel JSC

17 Structures Pedley, Michael D. (Mike) JSC

18 Food Systems Perchonok, Michele H. JSC - NSBRI

19 Materials Flammability Ruff, Gary GRC

20 Safety & Mission Assurance Rust, Randolph S. (Randy) JSC

21 EVA Trevino, Robert JSC

22 Materials Flammability Urban, David L. GRC

23 Mission Operations Webb, Dennis J. JSC

24 Human Factors Woolford, Barbara J. JSC

Confirmed as of 5-17-05

Chair - Don Henninger/JSC/281-483-5034

Deputy Chair - Paul Campbell/JSC/281-483-0079

Table 1. Exploration Atmosphere Working Group (EAWG) roster as of 17 May 2005.

Evaluate current knowledge and define viable design space: July 2005

The EAWG will be comprised of experts with distinct areas of responsibility pertinent 

to exploration atmospheric environments. It is likely that many participants may be 

unaware of important requirements drivers that must be considered when undertaking 

such an activity. Members of the EAWG will be asked to provide a background 

briefing outlining their areas of responsibilities relevant to the EAWG.

Members shall describe their areas of responsibility, parametric limits under which 

their responsible technologies will operate, and associated risks to proper operation of 

their technology or responsible areas.

A compiled presentation package will be made available as a reference. Design 

requirements and risks will be categorized and reported as preliminary findings. 

A proposed technical approach will be presented to the EAWG for general comment 

and consideration. This approach will be used to narrow the design space by 

minimizing risk factors governed by atmospheric parameters. 

The EAWG will also address the following:
33

• Determine the “needs” vs. “wants” of each participant or stakeholder. 

• Compare & contrast critical needs and risks

• Recognize overlapping areas of O2 percent/pressure where shared 

requirements can be developed. 

• Determine areas that have no connection but have influence on other systems, 

and establish connections with Interface Control Document’s (ICD’s) to outline 

areas of technical responsibility.

• Reference the Bounding the Spacecraft Atmosphere Design Space for Future 

Exploration Missions document.

Workshop: August 2005

The EAWG will conduct the Exploration Atmospheres Workshop. This workshop will 

serve as a mechanism to validate and formulate recommendations for humanoccupied atmospheres and recommend additional research and technology 

development to enable NASA to specify exploration atmosphere requirements. 

Workshop activities may include the following:

• Solicit additional inputs from a NASA internal community,

• Identify additional constraints,

• Validate the proposed technical approach, and

• Concurrence of design domain.
34

Additional Analyses: September 2005

The EAWG shall conduct additional analyses to further refine the design space 

parameters. Identify and include additional information within the risk analysis. 

Recommendations: September 2005

The EAWG shall make recommendations for additional research and technology 

development to enable NASA to specify exploration atmosphere requirements.

Final Recommendations: January 2006

The EAWG shall make final recommendations for Exploration Atmosphere Level 2 

requirements. These recommendations will be reviewed with Human Systems 

Working Group (HSWG).

Support to Constellation Program: April –TBD 2006

The EAWG shall support the integration of these recommended requirements into the 

Constellation Systems requirements documentation for the Constellation Spiral 1 

System Requirements Review (SRR).

4.0 Management Plan

As a strategic investment in research and technology development The Human 

Systems Working Group (HSWG), in support of the Human Support Research & 

Technology (HSRT) Theme, has authorized the formulation of this plan.

Under sanction of HSWG, Donald Henninger (NASA/EC) is the appointed lead of this 

study with Paul Campbell (SA2) as the deputy. 
35

SCHEDULE

The following timeline illustrates the approach and tentative dates that the EAWG will 

follow to deliver requirement recommendations.

Figure 4. Schedule of activities.

S O

FY 05 FY06

M J J A N D J F M A M J J

Form the Exploration Atmospheres Working Group (EAWG).

EAWG to evaluate current knowledge and define the viable design 

space parameters.

EAWG to convene an Exploration Atmospheres Workshop to confirm 

work to date and solicit additional inputs.

EAWG to conduct additional analyses to further refine the design 

space domain.

EAWG shall make recommendations for additional research and 

technology development to enable NASA to specify exploration 

atmosphere requirements.

EAWG shall make recommendations for Exploration Atmosphere 

Level 2 Requirements.

EAWG shall support integration of these requirements into 

Constellation Systems requirements documentation for the Crew 

Exploration Vehicle (CEV) System Requirements Review (SRR).

CEV 

SRR
36

5.0 BUDGET

The following details the Exploration Atmospheres budget estimate. Existing fund 

sources and charge codes will be utilized.

FY 05* FY 06**

FTE $K FTE $K

NASA FTE ($150K / FTE) 1.6 240 1.6 240

 Analysis & Coordination 0.4 0.4

 Medical 0.1 0.1

 EVA 0.1 0.1

 Materials 0.1 0.1

 Structures 0.05 0.05

 Safety & Mission Assurance 0.1 0.1

 Fire Prevention, Detection & Suppression 0.1 0.1

 Advanced Life Support 0.05 0.05

 Thermal Control 0.1 0.1

 Human Factors 0.05 0.05

 Science 0.05 0.05

 Technology & Systems Development 0.05 0.05

 Mission Operations 0.05 0.05

 Programmatic & Management 0.3 0.3

NASA Travel 15.5 15.5

Support Contractor FTE ($150K / FTE) 1.4 210 1.4 210

Support Contractor Travel & Materials 9.5 9.5

Materials - workshop 10

Total 3 485 3 475

* 3rd & 4th Quarters of FY 05

** 1st & 2nd Quarters of FY 06

6.0 SUPPLEMENTAL DOCUMENTATION:

Constellation Spiral 1 Systems Requirements Review [ESMD Draft/ May 3rd, 2005]

Bounding the Spacecraft Atmosphere Design Space for Future Exploration Missions 

[NASA/CR-2005-DRAFT]
37

Appendix C:

Exploration Atmospheres Working Group

Fault Tree Analysis
38

Exploration Atmospheres Working Group

Fault Tree Analysis

I. Introduction

a. The Exploration Atmospheres Working Group (EAWG) is charged with refining the atmospheric 

design space to a relatively small domain such that atmospheric requirements for space-based 

systems can be specified. The EAWG will support development of requirements and integration 

into the Exploration Systems Mission Directorate (ESMD) Constellation Systems 

documentation. The working group consisted of various technical and programmatic 

stakeholders from across the agency including the Crew Office, S&MA, EVA, M&P, and Food 

Systems. 

b. Fault Tree Analysis is a deductive, failure-based approach. The analysis process begins by 

identifying an undesirable event called a “top event “. Then using a system familiarization 

approach, determine the causes of the undesirable top event by using a systematic, backwardstepping process. This type of analysis can be either qualitative or quantitative in nature. Once 

the fault trees are created they can be updated and quantified as details change and mature.

c. The EAWG decided on two logic approaches: Fault Tree analysis and Analytical Hierarchical 

Process (AHP). It was decided to do both in parallel and compare the results prior to making any 

recommendations.

II. Basic Approach

a. The EAWG decided to examine nine specific atmospheric combinations that were derived in a 

white paper entitled “Bounding the Spacecraft Atmosphere Design Space for Future Exploration 

Missions” written by Kevin Lange, Alan Perka, Bruce Duffield, and Frank Jeng. The nine points 

are listed in Table 1 below. 

Table 1

The points are described as follows: the first number is the PSI, the second number is the PPO2 

and the third number is the PSI of the EVA suit. The EAWG was divided into three teams: 

Mission Operations, Flammability, and Medical. Each team created fault trees within their area 

of expertise. The Mission Operations team initially created six fault trees, each representing a 

different Constellation element. The Medical team initially created two fault trees; the first 

dealing with Decompression Sickness (DCS) and the second dealing with Hypoxia. The 

Flammability team created a single fault tree. As the fault trees developed, the Mission 

Operations team decided that the sixth fault tree, CREW-MISSION-OPS-FTA, contained events 

that were being included in other fault trees and so could be deleted. The Medical team decided 

that the Hypoxia fault tree was not going to give any insight into the differences between the 

nine atmosphere points being discussed. 

III. Conclusions

a. The Medical team and the Flammability team were able to rank the nine points. The Mission 

Operations team was unable to get the rankings completed in the time allowed during the second 

EAWG meeting. The Mission Operations’ fault trees were not able to be completed. The results 

for the other two teams are shown in Table 2.

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

9.4/28.5/4.3 9.3/26.8/4.3 10.6/23.2/5.0 11.6/21/6.0 12.8/21/6.0 13.2/23.1/6.0 10.2/26.5/4.3 9.5/30/4.3 8.3/36/4.3
39

Table 2

These rankings were determined by simple tabulation of events ranked 1, 2, 3, etc. The point with 

the most events rank 1 was ranked highest risk overall and so on. This is a very basic ranking 

approach and can vary depending on the analyst. This method ranked the atmospheres similarly to 

those of the AHP and in the case of the DCS fault tree, matched rather well the calculated risk 

probabilities.

IV. Future Work

a. The current fault trees for all three teams are at an appropriate level of detail given the current 

program parameters. As the program matures providing more detailed parameters, the fault 

trees can be updated to reflect the new level of knowledge. Events can be added or subtracted as 

appropriate and even fault trees can be added or deleted. In the event other atmosphere points 

that are identified the fault trees could be used to show the relative ranking of the new points and 

how they relate to the original nine points. As more detail becomes available the events in the 

fault trees may be able to be quantified and an interval of probabilities could be determined for 

the risk of each point.

b. Development of the fault trees is an iterative process tool and can be used in part or in whole, by 

the Constellation Program PRA or by the PRAs of various projects under Constellation.

 PSI/PPO2/EVA (Atmosphere Number)

Risk Level Overall Ranking 

Number Flammability DCS

Highest Risk 1 8.3/36/4.3 (9) 10.2/26.5/4.3 (7)

 :

 : 2 9.5/30/4.3 (8) 9.5/30/4.3 (8)

 :

 : 3 9.4/28.5/4.3 (1) 9.4/28.5/4.3 (1)

 :

 : 4 9.3/26.8/4.3 (2) 9.3/26.8/4.3 (2)

Middle Risk 5 10.2/26.5/4.3 (7) 10.6/23.2/5.0 (3)

 :

 : 6 10.6/23.2/5.0 (3) 13.2/23.1/6.0 (6)

 :

 : 7 13.2/23.1/6.0 (6) 12.8/21/6.0 (5)

 :

 : 8 12.8/21/6.0 (5) 11.6/21/6.0 (4)

Lowest Risk 9 11.6/21/6.0 (4) 8.3/36/4.3 (9)
40

V. Appendix

a. Medical Team DCS Fault Tree

DCS

MULTI-DCS-OCCURS TYPE-II-DCS-OCCURS

INAD-PREBREATH-MDCS

0.0E+0

FREQ-EVA-PER-CREW-MDCS

0.0E+0

CREW-PROC-FAILURE-MDCS MECH-FAILURE-MDCS

1.0E+0

PROTOCOL-RISK-MDCS PROCEDURAL-ERROR-MDCS

0.0E+0

FAILURE-OF-TESTBED-MDCS

0.0E+0

RESEARCH-FAILURE-MDCS

0.0E+0

SUIT-FAILURE-MDCS

0.0E+0

VEHICLE-FAILURE-MDCS

LIFE-THREAT-DCS-OCCURS MEDI-SIGN-RESI-DCS

INADEQUATE-PREBR-LT-DCS INADEQUATE-TREAT-LT-DCS MECH-FAILURE-LT-DCS

1.0E+0

TIME-TO-TREAT-LT-DCS

1.0E+0

VEHICLE-FAILURE-LT-DCS

1.0E+0

SUIT-FAILURE-LT-DCS

1.0E+0

PRESSURE-TREAT-LT-DCS

1.0E+0

SUIT-DESIGN-FAIL-LT-DCS

1.0E+0

FREQ-EVA-PER-CREW-LT-DC

1.0E+0

CREW-PROC-FAIL-LT-DCS

1.0E+0

PROTOCOL-RISK-LT-DCS

1.0E+0

FAILURE-TESTBED-LT-DCS

1.0E+0

RESEARCH-FAIL-LT-DCS

PROCEDURAL-ERROR-LT-DCS

INADEQUATE-PREBR-MSR-DCS INADEQ-TREAT-MSR-DCS MECH-FAIL-MSR-DCS

PROCEDURAL-ERROR-MSR-DCS

1.0E+0

PROTOCOL-RISK-MSR-DCS

1.0E+0

FAILURE-TESTBED-MSR-DCS

1.0E+0

RESEARCH-FAIL-MSR-DCS

1.0E+0

CREW-PROC-FAIL-MSR-DCS

1.0E+0

FREQ-EVA-CREW-MSR-DCS

1.0E+0

PRESSURE-TREAT-MSR-DCS

1.0E+0

TIME-T0-TREAT-MSR-DCS

1.0E+0

SUIT-DESIGN-FAIL-MSR-DCS

1.0E+0

SUIT-FAIL-MSR-DCS

1.0E+0

VEHICLE-FAIL-MSR-DCS

Loss of Mission

due to DCS

Crew member

has multiple

DCS occurrence

Inadequate

PreBreathe occurs

Failure to compensate

for frequency of EVAs

per crew member

Failure of Crew to

follow PreBreathe

Procedures

Mechanical failure

leads to DCS

occurrance

Failure to provide the

proper PreBreathe

Procedures

Inherent Risk for

individual

Wrong Test Bed used

to develope

Procedures

Inadequate Research

Data

Suit failure causes

DCS Vehicle System failurecauses DCS

Life Threatening DCS event occurs

Medically significant

residual from DCS

event occurs

Inadequate prebreath

causes Life Threatening

DCS event

Failure to compensate

for frequency of EVA

per crew

Failure of crew to

follow prebreath

procedures

Failure to provide

adequate treatment Mechanical failure causinglife threatening DCS event

Suit failure causes life

threatening DCS event

Vehicle system failure

causes life threatening DCS event

Failure to treat DCS in

timely manner

Suit inadequate for

treatment use

Failure to have

adequate pressure

for treatment

Failure to have

adequate research

data

Failure to use

adequate testbed for

research

Failure to have correct

procedures in place

Inherent risk for

individual

Type II DCS

Occurs

Failure of prebreath to

prevent medically

significant residual DCS

Failure of crew to

follow prebreath

procedures

Failure to compensate

for frequency of EVA

per crew

Failure to provide

adequate treatment

Mechanical failure occurs

causing DCS with

medically significant

residual

Vehicle system failure

occurs

Suit failure causes

MSR DCS

Failure to treat event

in timely manner

Suit not able to be

used for treatment

Failure to use

adequate pressure to

treat event

Failure to provide

correct prebreath

procedures

Failure to provide

adequate research

data

Failure to use correct

testbed for research

Inherent risk for

individual

 DCS - Loss of Mission Due to DCS 2005/07/14 Page 2
41

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

Description 9.4/28.5/4.3 9.3/26.8/4.3 10.6/23.2/5.0 11.6/21/6.0 12.8/21/6.0 13.2/23.1/6.0 10.2/26.5/4.3 9.5/30/4.3 8.3/36/4.3 Comments and 

Rationale

Loss of Mission due 

to DCS

Crew member has 

multiple DCS 

occurrence

Failure of Crew to follow 

Prebreathe Procedures 5 5 5 8 5 5 1 5 9

Atmospheres with 

zero prebreathe 

have no risk of 

incorrectly performing prebreathe. Shuttle 

prebreathe is more 

complex than for 

the 60 min 

prebreathe 

atmospheres 

proposed

Failure to compensate 

for frequency of EVAs 

per crew member

5 5 5 8 5 5 1 5 9

Atmospheres with 

a smaller physiological adjustment 

to be made 

between vehicle 

atmosphere and 

suit atmosphere 

will have lower risk 

of accumulated 

problems.

Inadequate

Prebreathe occurs

Inherent Risk for 

individual 5 5 5 8 5 5 1 5 9

Assuming 

individual has 

successfully 

adapted to vehicle 

atmosphere, the 

scenarios that 

require a larger 

change between 

vehicle and suit 

atmosphere pose 

more risk for 

individuals.
42

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

Failure to provide the 

proper Prebreathe 

Procedures

Wrong Test Bed used 

to develop Procedures 4 4 4 8 7 4 1 4 9

If no prebreathe is 

required, procedures can't be 

incorrect. Higher 

pressure atmospheres will be 

easier to test and 

develop, and while

shuttle procedures 

are complex, 

experience with 

them mitigates the 

risk somewhat

Inadequate Research 

Data 3 4 5 6 8 9 7 2 1

Higher pressure 

atmospheres will 

be easier to test. 

Shuttle atmosphere 

risk is mitigated by 

experience.

Mechanical failure 

leads to DCS 

occurrence

 (0.000E+000) Suit 

failure causes DCS 7 7 4 2 2 2 7 7 7

Variable pressure 

suits will be more 

complex, and thus 

have a higher risk. 

Shuttle design is 

not optimum 

design, but risk is 

mitigated 

somewhat by 

experience.

(0.000E+000) Vehicle 

System failure causes 

DCS

-- -- -- -- -- -- -- -- --

Depends on 

vehicle, not 

atmosphere

Type II DCS Occurs

Life Threatening DCS 

event occurs
43

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

Failure of crew to 

follow prebreathe

procedures

5 5 5 8 5 5 1 5 9

Atmospheres with 

zero prebreathe 

have no risk of 

incorrectly 

performing 

prebreathe. 

Shuttle prebreathe 

is more complex 

than for the 60 min 

prebreathe 

atmospheres 

proposed

Failure to compensate 

for frequency of EVA 

per crew

5 5 5 8 5 5 1 5 9

Atmospheres with 

a smaller 

physiological 

adjustment to be 

made between 

vehicle atmosphere 

and suit 

atmosphere will 

have lower risk of 

accumulated 

problems.

Inadequate prebreathe

causes Life 

Threatening DCS 

event

Inherent risk for 

individual 5 5 5 8 5 5 1 5 9

Assuming 

individual has 

successfully 

adapted to vehicle 

atmosphere, the 

scenarios that 

require a larger 

change between 

vehicle and suit 

atmosphere pose 

more risk for 

individuals.
44

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

Failure to have correct 

procedures in place

 (1.000E+000) 

Failure to use adequate 

testbed for research

4 4 4 8 7 4 1 4 9

If no prebreathe is 

required, the 

procedures can't 

be incorrect. 

Higher pressure 

atmospheres will 

be easier to test

and develop, and 

while shuttle 

procedures are 

complex, 

experience with 

them mitigates the 

risk somewhat

 (1.000E+000) 

Failure to have 

adequate research data

3 4 5 6 8 9 7 2 1

Higher pressure 

atmospheres will 

be easier to test. 

Shuttle atmosphere 

risk is mitigated by 

experience.

Failure to provide 

adequate treatment

Failure to have 

adequate pressure for 

treatment

-- -- -- -- -- -- -- -- --

Depends on 

architecture and 

treatment system 

selected, not 

atmosphere 

selected

Suit inadequate for 

treatment use 3 3 6 8 8 8 3 3 3

Variable pressure 

suits will be more 

likely to have a 

treatment pressure 

capability

Failure to treat DCS in 

timely manner -- -- -- -- -- -- -- -- --

Depends on 

architecture and 

operations, not 

atmosphere design
45

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

Mechanical failure 

causing life 

threatening DCS event

Suit failure causes life 

threatening DCS event 7 7 4 2 2 2 7 7 7

Variable pressure 

suits will be more 

complex, and thus 

have a higher risk. 

Shuttle design is 

not optimum 

design, but risk is 

mitigated 

somewhat by 

experience.

 (1.000E+000) 

Vehicle system failure 

causes life threatening 

DCS event

Medically significant 

residual from DCS 

event occurs

Failure of crew to follow 

prebreathe procedures 5 5 5 8 5 5 1 5 9

Atmospheres with 

zero prebreathe 

have no risk of 

incorrectly 

performing 

prebreathe. 

Shuttle prebreathe 

is more complex 

than for the 60 min 

prebreathe 

atmospheres 

proposed

Failure to compensate 

for frequency of EVA 

per crew

5 5 5 8 5 5 1 5 9

Atmospheres with 

a smaller 

physiological 

adjustment to be 

made between 

vehicle atmosphere 

and suit 

atmosphere will 

have lower risk of 

accumulated 

problems.
46

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

Failure to provide 

adequate treatment

Failure to use adequate

pressure to treat event -- -- -- -- -- -- -- -- --

Depends on 

architecture and 

treatment system 

selected, not 

atmosphere 

selected

Suit not able to be used 

for treatment

336888333

Variable pressure 

suits will be more 

likely to have a 

treatment pressure 

capability

Failure to treat event in 

timely manner -- -- -- -- -- -- -- -- --

Depends on 

architecture and 

operations, not 

atmosphere design

Failure of prebreath to 

prevent medically 

significant residual 

DCS

Inherent risk for 

individual

555855159

Assuming 

individual has 

successfully 

adapted to vehicle 

atmosphere, the 

scenarios that 

require a larger 

change between 

vehicle and suit 

atmosphere pose 

more risk for 

individuals.

Failure to provide 

correct prebreath 

procedures

Failure to use correct 

testbed for research

444874149

If no prebreathe is 

required, the 

procedures can't 

be incorrect. 

Higher pressure 

atmospheres will 

be easier to test 

and develop, and 
47

Atmosphere Points (PSI / PPO2 % / Suit P)

Point #1 Point #2 Point #3 Point #4 Point #5 Point #6 Point #7 Point #8 Point #9

while shuttle procedures are complex, experience 

with them mitigates 

the risk somewhat

Failure to provide 

adequate research data 3 4 5 6 8 9 7 2 1

Higher pressure 

atmospheres will 

be easier to test. 

Shuttle atmosphere 

risk is mitigated by 

experience.

Mechanical failure 

occurs causing DCS 

with medically 

significant residual

Suit failure causes MSR 

DCS -- -- -- -- -- -- -- -- --

New suits will be 

developed for exploration, so all 

atmospheres have 

risks of suit failure 

from new hardware

Vehicle system failure 

occurs -- -- -- -- -- -- -- -- --

Depends on 

vehicle, not 

atmosphere

1 2 3 4 5 6 7 8 9

1 0 0 0 0 0 0 12 0 3

2 0 0 0 2 2 2 0 3 0

3 5 2 0 0 0 0 2 2 2

4 3 6 5 0 0 3 0 3 0

5 9 9 12 0 9 9 0 9 0

6 0 0 2 3 0 0 0 0 0

7 2 2 0 0 3 0 5 2 2

8 0 0 0 14 5 2 0 0 0

9 0 0 0 0 0 3 0 0 12

3 4 5 8 7 6 1 2 9
48

b. Flammability Team

FLAMMABILITY-TREE

FIRE

IGNITION-SOURCE

OVERHEAT

SHORTS

STATIC

FUEL-OR-O2-FAIL

FLAMMABLE-LEAKS

OXYGEN

ATM-REG-FAILS

FAIL-TO-RECOVER-FIRE

DETECTOR-FAILS INCORRECT-ALARM-SETPT

CREW-FAILURE IMPROPER-MATERIAL

CONFIGURATION-FAILS

FUEL

O2-LEAKS

PPE-FAILS

AUTO-DETECT-FAIL

FIRE-SUPPRESS-FTA

EXTINGUISH-INEFFECTIVE

FIRE-SUPPRESS-FAIL

RECOVERY-FAILS

Fuel or O2 failure

Oxygen

Ignition source

Fire occurs

Atmosphere regulation fails

Static charge

Electrical shorts

Equipment overheats

Leaks of flammable fluids

Detector fails Incorrect alarm

setpoint

Improper material selection

Fuel

Configuration

control fails

Fail to recover from

ignition/fuel/O2

failures

Crew fails to

detect/respond to

ignition/fuel/O2

failures

Death/Permanent

Injury due to Fire

Leaks of O2

Automated

system fails to

detect fire

Extinguishing

not effective

Suppression

system fails

to activate

Personal protective

equipment failure

Fire suppression

unsuccessful

Fail to recover

from fire

 FLAMMABILITY-TREE - Death/Permanent Injury due to Fire 2005/08/09 Page 4
49

Fault Tree Logic 

for Flammability Type Description

Atmosphere Points (PSI / PPO2 % / Suit P) Rationale/Comments

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0 11.6/21/6.0 12.8/21/6.0

13.2/23.1/

6.0

10.2/26.5/

4.3

9.5/30/

4.3

8.3/36/

4.3

Overall rankings 3 4 6 9 8 7 5 2 1

In general, the basic events 

for each atmosphere point 

were in agreement, so this 

ranking was assumed for 

the overall top level 

rankings of the atmosphere 

points as shown to the left. 

Based on the rankings, it 

appears that failure of the 

fuel or O2 is the driver most 

dependent on the chosen 

atmosphere point, 

combined with overheating 

of equipment and 

ineffective extinguishing 

also most dependent on the 

atmosphere point to result 

in overall risk of 

flammability as a function of 

atmosphere point. 

FLAMMABILITYTREE

Top 

AND 

Gate

Death/Perm

anent Injury 

Due to Fire

FIRE AND 

Gate

Fire occurs 

due to 

combination 

of ignition 

source and 

fuel or O2

failure

FAIL-TORECOVER-FIREOR 

Gate

Failure to 

recover 

from 

ignition, fuel 

and/or O2

failures

DETECTORFAILSBasic 

Event

Detector 

fails to 

detect 

potential 

fire

No effect of atmosphere 

selection.The only potential 

effect of atmosphere might 

be on the 

concentration/composition 

of species produced by an 

incipient fire. 

INCORRECTALARM-SETPTBasic 

Event

Incorrect 

alarm 

setpoint

No effect of atmosphere 

selection.The only potential 

effect of atmosphere might 

be on the 

concentration/composition 
50

Fault Tree Logic 

for Flammability Type Description

Atmosphere Points (PSI / PPO2 % / Suit P) Rationale/Comments

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0 11.6/21/6.0 12.8/21/6.0

13.2/23.1/

6.0

10.2/26.5/

4.3

9.5/30/

4.3

8.3/36/

4.3

of species produced by an 

incipient fire. 

CREW-FAILURE Basic 

Event

Crew fails 

to 

detect/resp

ond to 

ignition/fuel/

O2 failures

No effect of atmosphere 

selection.The only potential 

effect of atmosphere might 

be on the 

concentration/composition 

of species produced by an 

incipient fire. 

IGNITIONSOURCEOR 

Gate

Ignition 

source is 

present

OVERHEAT Basic 

Event

Ignition 

source due 

to 

equipment 

overheats

No effect of atmosphere 

selection

STATIC Basic 

Event

Ignition 

source due 

to static 

charge

Check on atmosphere 

dependency

SHORTS Basic 

Event

Ignition 

source due 

to electrical 

shorts

No effect of atmosphere 

selection

FUEL-OR-O2-FAIL OR 

Gate

Fuel or O2

failure 

occurs

FUEL OR 

Gate

Fuel failure 

occurs

IMPROPERMATERIALBasic 

Event

Improper 

material 

selection

3 4 6 9 8 7 5 2 1

Higher O2 restricts material 

selection. Potential 

decrease in margin of 

safety.

FLAMMABLELEAKSBasic 

Event

Leaks of 

flammable 

fluids

No effect of atmosphere 

selection

CONFIGURATION

-FAILS

Basic 

Event

Configuratio

n control 

failure

3 4 6 9 8 7 5 2 1

Higher O2 could impose 

more stringent configuration 

control. Chance for error 

increases.

OXYGEN OR 

Gate

Oxygen 

failure 

occurs
51

Fault Tree Logic 

for Flammability Type Description

Atmosphere Points (PSI / PPO2 % / Suit P) Rationale/Comments

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0 11.6/21/6.0 12.8/21/6.0

13.2/23.1/

6.0

10.2/26.5/

4.3

9.5/30/

4.3

8.3/36/

4.3

ATM-REG-FAILS Basic 

Event

Atmosphere 

regulation 

fails

3 4 6 9 8 7 5 2 1

Lower O2 atmospheres 

would allow more time for 

detection before O2

concentration became high

O2-LEAKS Basic 

Event Leaks of O2 3 4 6 8 9 7 5 2 1

Lower O2 atmospheres 

would allow more time for 

detection before O2

concentration became high; 

points 4 and 5 reversed

RECOVERYFAILSOR 

Gate

Failure to 

recover 

from fire

PPE-FAILS Basic 

Event

Personal 

protective 

equipment 

(PPE) fails

Ask medical if 36 to 21% O2

makes a difference

FIRE-SUPPRESSFAILOR 

Gate

Fire 

suppression 

unsuccessf

ul

FIRE-SUPPRESSFTABasic 

Event

Fire 

suppression 

system fails 

to activate

No effect of atmosphere

AUTO-DETECTFAILBasic 

Event

Automated 

system fails 

to detect 

fire

No effect of atmosphere

EXTINGUISHINEFFECTIVEBasic 

Event

Extinguishin

g not 

effective

3 4 6 9 8 7 5 2 1

Higher concentrations of 

suppressant required to 

extinguish fire in high O2
52

c. Mission Operations Team

CEV-MISSION-OP'S-FAILURE

1.0E+0

CEV-RADIATION

1.0E+0

CEV-SYSTEM-FAILURE CEV-CABIN-FLAMMABILITY

1.0E+0

CEV-CABIN-FALM-THERMAL

1.0E+0

CEV-CABIN-FLAM-SHORT

CEV-CABIN-LEAK

CEV-CABIN-LEAK-MAJOR

1.0E+0

CEV-CABIN-LEAK-MMOD

1.0E+0

CEV-CABIN-LEAK-RUPTURE

CEV-CABIN-LEAK-MINOR

1.0E+0

CEV-CABIN-LEAK-HW

1.0E+0

CEV-CABIN-LEAK-SEALS

CEV-CREW-MEDICAL

1.0E+0

CEV-CABIN-MED-DCS

1.0E+0

CEV-CABIN-MED-HYPOXIA

1.0E+0

CEV-CABIN-MED-OTHER

CEV-DOCKING-FAILURE

1.0E+0

CEV-DOCK-HW

1.0E+0

CEV-DOCK-LEAKAGE

1.0E+0

CEV-DOCK-PROX-OPS

CEV-TRANS-PRESS

1.0E+0

CEV-TRANS-DOCK

1.0E+0

CEV-TRANS-EQUIP

1.0E+0

CEV-TRANS-LEAK

CEV Pressure

Transistion Failure CEV DockingFailure CEV Crew MedicalCondition

Severe cain

leak in CEV

Minor cabin

leak in CEV

Leakage of CEV

Cabin Volume Flammable conditionin CEV

CEV fails to

meet one or more

mission objectives

CEV crew exhbits

other medical

conditions

CEV pressure

transistion failure

during docking

Failure of CEV

pressure transistion

hardware systems

CEV cabin volume

leakage

Failure of CEV

docking hardware

Failure of CEV

docking seals

CEV Prox Op's

system malfunction

CEV crew exhibits

symptoms of Hypoxia

CEV crew exhibits

DCS symptoms

Rupture of CEV

cabin volume

Leakage of CEV

cabin seals

Severe leak

caused from MMOD

Leakage of CEV

cabin hardware

TPS Failure Electrical Short

Excessive CEV

Radiation

CEV System Failure

CEV-MISSION-OP'S-FAILURE - CEV fails to meet one or more mission objectives 2005/08/16 Page 2
53

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

CEV Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

CEV-MISSIONOPS-FAILURETop OR 

Gate

CEV fails to 

meet one or 

more mission 

objectives

CEV-CABINFLAMMABILITYOr 

Gate

Flammable 

condition in 

CEV

CEV-CABINFLAM-THERMALBasic 

Event TPS Failure

CEV-CABINFLAM-SHORTBasic 

Event 

Electrical 

short

CEV-CABIN-LEAK Or 

Gate

Leakage of 

CEV cabin 

volume

CEV-CABIN-LEAKMAJOROr 

Gate

Severe cabin 

leak in CEV

CEV-CABINLEAK-MMODBasic 

Event 

Leak caused 

by MMOD

CEV-CABINLEAK-RUPTUREBasic 

Event 

Leak caused 

by rupture

CEV-CABIN-LEAKMINOROr 

Gate

Minor cabin 

leak in CEV

CEV-CABIN 

LEAK-HW

Basic 

Event 

Leakage of 

CEV cabin 

hardware

CEV-CABINLEAK-SEALSBasic 

Event 

Leakage of 

CEV cabin 

seals

CEV-CREWMEDICALOr 

Gate

CEV crew 

medical 

condition

CEV-CABINMED-DCSBasic 

Event 

CEV crew 

exhibits DCS 

symptoms

CEV-CABINMED-HYPOXIABasic 

Event 

CEV crew 

exhibits 

Hypoxia 

symptoms
54

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

CEV Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

CEV-CABINMEDS-OTHERBasic 

Event 

CEV crew 

exhibits other 

medical 

conditions

CEV-DOCKINGFAILUREOr 

Gate

CEV docking 

failure

CEV-DOCK-HW Basic 

Event 

Failure of 

CEV docking 

hardware

CEV-DOCKLEAKAGEBasic 

Event 

Failure of 

CEV docking 

seals

CEV-DOCKPROX-OPSBasic 

Event 

CEV Prox 

op's 

malfunction

CEV-RADIATION Basic 

Event 

Excessive 

CEV radiation

CEV-SYSTEMFAILUREBasic 

Event 

CEV system 

failure

CEV-TRANSPRESSOr 

Gate

CEV pressure 

transition 

failure

CEV-TRANSDOCKBasic 

Event 

CEV pressure 

transition 

failure during 

docking

CEV-TRANSEQUIPBasic 

Event 

Failure of 

CEV pressure 

transition 

systems

CEV-TRANSLEAKBasic 

Event 

CEV cabin 

volume 

leakage

1 Highest Risk 0 0 0 0 0 0 0 0 0

2 0 0 0 0 0 0 0 0 0

3 0 0 0 0 0 0 0 0 0

4 0 0 0 0 0 0 0 0 0

5 Medium Risk 0 0 0 0 0 0 0 0 0
55

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

CEV Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

6 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0

8 0 0 0 0 0 0 0 0 0

9 Lowest Risk 0 0 0 0 0 0 0 0 0
56

LSAM-MISSION-OP'S-FAIL

1.0E+0

LSAM-RADIATION-FAILURE

1.0E+0

LSAM-SYSTEMS-FAILURE LSAM-CABIN-FLAMMABILITY

1.0E+0

LSAM-FALM-SHORT

1.0E+0

LSAM-FLAM-THERMAL

LSAM-CABIN-LEAK

LSAM-LEAK-MAJOR

1.0E+0

LSAM-LEAK-MMOD

1.0E+0

LSAM-LEAK-RUPTURE

LSAM-LEAK-MINOR

1.0E+0

LSAM-LEAK-HW

1.0E+0

LSAM-LEAK-SEALS

LSAM-CREW-MEDICAL

1.0E+0

LSAM-CABIN-MEDS-DCS

1.0E+0

LSAM-CABIN-MEDS-HYPOXIA

1.0E+0

LSAM-CABIN-MEDS-LUNAR

1.0E+0

LSAM-CABIN-MEDS-OTHER

LSAM-DOCKING-FAILURE

1.0E+0

LSAM-DOCK-HW

1.0E+0

LSAM-DOCK-LEAKAGE

1.0E+0

LSAM-DOCK-PROX-OPS

LSAM-EVA-FAILURE

1.0E+0

LSAM-AIRLOCK-EVA

1.0E+0

LSAM-EVA-CREW

1.0E+0

LSAM-EVA-SUIT

1.0E+0

LSAM-EVA-SUIT-PRES

Failure of LSAM EVA Failure in LSAM

docking systems LSAM crew medical event

Minor leakage

in LSAM cabin Severe leakagein LSAM cabin

Leak in LSAM Cabin Flammable condition in LSAM

LSAM Fails to

Achieve One or

More Objectives

LSAM crew exhibits other medical

condtions (trips,

falls physological ailments)

Crew illness

prevents EVA Failure of LSAM EVA suit

Failure of LSAM

airlock to support EVA

EVA suit pressure

failure during

LSAM EVA

LSAM prox op's

system malfunction Failure LSAMdocking seals Failure of LSAMdocking hardware

LSAM crew exposure to contaminants

dust, propellants, etc

LSAM crew exhibits

Hypoxia symptoms LSAM crew exhibitsDCS symptoms

Leakage of LSAM cabin seals Leak of LSAM

cabin hardware Rupture of LSAMcabin volume LSAM MMOD damage

Electrical short in LSAM Failure of LSAM TPS system

LSAM exceeds

Radiation limits Failure of LSAM systems

 LSAM-MISSION-OP'S-FAIL - LSAM fails to meet one or more mission objectives 2005/08/16 Page 3
57

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

LSAM Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

LSAM-MISSIONOPS-FAILURE

Top 

OR 

Gate

LSAM fails 

to meet 

one or 

more 

mission 

objectives

LSAM-CABINFLAMMABILITYOr 

Gate

Flammable 

condition in 

LSAM

LSAM-CABINFLAM-THERMALBasic 

Event 

TPS 

Failure

LSAM-CABINFLAM-SHORTBasic 

Event 

Electrical 

short

LSAM-CABIN-LEAK Or 

Gate

Leakage of 

LSAM 

cabin 

volume

LSAM-CABINLEAK-MAJOROr 

Gate

Severe 

cabin leak 

in LSAM

LSAM-CABINLEAK-MMODBasic 

Event 

Leak 

caused by 

MMOD

LSAM-CABINLEAK-RUPTUREBasic 

Event 

Leak 

caused by 

rupture

LSAM-CABINLEAK-MINOROr 

Gate

Minor cabin 

leak in 

LSAM

LSAM-CABIN 

LEAK-HW

Basic 

Event 

Leakage of 

LSAM 

cabin 

hardware

LSAM-CABINLEAK-SEALSBasic 

Event 

Leakage of 

LSAM 

cabin seals

LSAM-CREWMEDICALOr 

Gate

LSAM crew 

medical 

condition

LSAM-CABINMED-DCSBasic 

Event 

LSAM crew 

exhibits 

DCS 

symptoms
58

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

LSAM Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

LSAM-CABINMED-HYPOXIABasic 

Event 

LSAM crew 

exhibits 

Hypoxia 

symptoms

LSAM-CABINMED-LUNARBasic 

Event 

LSAM crew 

exhibits 

illness due 

to 

Contamina

nts (Dust, 

Propellants

, ETC.)

LSAM-CABINMEDS-OTHERBasic 

Event 

CEV crew 

exhibits 

other 

medical 

conditions 

(Trips & 

Falls, 

Physiologic

al 

Ailments)

LSAM-DOCKINGFAILUREOr 

Gate

LSAM 

docking 

failure

LSAM-DOCK-HW Basic 

Event 

Failure of

LSAM 

docking 

hardware

LSAM-DOCKLEAKAGEBasic 

Event 

Failure of 

LSAM 

docking 

seals

LSAM-DOCKPROX-OPSBasic 

Event 

LSAM Prox 

op's 

malfunction

LSAM-RADIATION Basic 

Event 

Excessive 

LSAM 

radiation

LSAM-SYSTEMFAILUREBasic 

Event 

LSAM 

system 

failure

LSAM-EVAFAILUREOr 

Gate

Failure of 

LSAM EVA
59

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

LSAM Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

LSAM-AIRLOCKEVABasic 

Event 

Failure of 

LSAM 

airlock

LSAM-EVACREWBasic 

Event 

Crew 

illness 

prevents 

EVA

LSAM-EVA-SUIT Basic 

Event 

Failure of 

LSAM EVA 

suit

LSAM-EVA-SUITPRESBasic 

Event 

LSAM suit 

pressure 

failure

1 Highest 

Risk 0 0 0 0 0 0 0 0 0

2 0 0 0 0 0 0 0 0 0

3 0 0 0 0 0 0 0 0 0

4 0 0 0 0 0 0 0 0 0

5 Medium 

Risk 0 0 0 0 0 0 0 0 0

6 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0

8 0 0 0 0 0 0 0 0 0

9 Lowest 

Risk 0 0 0 0 0 0 0 0 0
60

SUIT-MISSION-OP'S-FAIL

1.0E+0

SUIT-CONSUMABLE-FAILURE

1.0E+0

SUIT-RADIATION SUIT-AVAILABILITY

1.0E+0

SUIT-AVAIL-MAINT

1.0E+0

SUIT-AVAIL-SPARE

SUIT-LEAK

1.0E+0

SUIT-LEAK-HELMET

1.0E+0

SUIT-LEAK-SEALS

1.0E+0

SUIT-LEAK-TEAR

SUIT-MOBILITY-FAILURE

1.0E+0

SUIT-MOBIL-FATIGUE

1.0E+0

SUIT-MOBIL-FIT

1.0E+0

SUIT-MOBIL-JOINT

1.0E+0

SUIT-MOBIL-PRESS

SUIT-TASK-FAILURE

1.0E+0

SUIT-TASK-FIT

1.0E+0

SUIT-TASK-MOBIL

1.0E+0

SUIT-TASK-TOOLS

EVA suit task failure Suit mobility failure EVA suit leak failure EVA suit availability failure

Suit fails to meet one or more

mission objectives

Joint/Bearing

Contamination Suit fit preventstask completion

Suit mobility

prevents task

completion

Lack of adequate

tools to complete task

Suit mobility

limited by fit

Suit mobility

limited by crew fatigue

High pressure

limits suit mobility

Suit exhbits

O2 valve seal leak

EVA suit exhbits

tear or puncture Suit Repair parts not available Suit undergoingmaintenance

EVA suit radiation

monitor system fails

Failure to maitain

EVA suit consumables

Helmet/Sheild

Crack/Puncture

 SUIT-MISSION-OP'S-FAIL - EVA suit fails to meet one or more mission objectives 2005/08/16 Page 4
61

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

EVA Suit Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

SUIT-MISSIONOPS-FAILURETop OR 

Gate

LSAM fails to 

meet one or 

more mission 

objectives

SUITAVAILABILITY Or Gate

EVA suit 

Availability 

Failure

SUIT-AVAILMAINTBasic 

Event 

Suit 

undergoing 

maintenance

SUIT-AVAILSPAREBasic 

Event 

Suit spares 

unavailable

SUITCONSUMABLEFAILURE

Basic 

Event 

Failure to 

maintain EVA 

suit 

consumables

SUIT-LEAK Or Gate EVA suit leak 

failure

SUIT-LEAKSEALSBasic 

Event 

Suit exhibits 

leak at seal or 

valve

SUIT-LEAKHELMETBasic 

Event 

Suit Helmet 

crack or Shield 

puncture

SUIT-LEAK-TEAR Basic 

Event 

suit exhibits 

tear or 

puncture

SUIT-MOBILITYFAILURE Or Gate Suit mobility 

failure

SUIT-MOBILFATIGUEBasic 

Event 

Suit mobility 

limited by crew 

fatigue

SUIT-MOBILJOINT-CONTBasic 

Event 

Suit joint or 

bearing 

contamination 

limits mobility

SUIT-MOBIL-FIT Basic 

Event 

Suit mobility

limited by fit
62

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

EVA Suit Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

SUIT-MOBILPRESSBasic 

Event 

High pressure 

limits suit 

mobility

SUIT-RADIATION Basic 

Event 

EVA Suit 

radiation 

monitor system 

fails

SUIT-TASKFAILURE Or Gate EVA suit task 

failure

SUIT-TASK-FIT Basic 

Event 

Suit fit 

prevents task 

completion

SUIT-TASK-MOBIL Basic 

Event 

Suit 

mobility/Transl

ation prevents 

task 

completion

SUIT-TASK-TOOLS Basic 

Event 

Lack of 

adequate tools 

to complete 

task

1 Highest Risk 0 0 0 0 0 0 0 0 0

2 0 0 0 0 0 0 0 0 0

3 0 0 0 0 0 0 0 0 0

4 0 0 0 0 0 0 0 0 0

5 Medium Risk 0 0 0 0 0 0 0 0 0

6 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0

8 0 0 0 0 0 0 0 0 0

9 Lowest Risk 0 0 0 0 0 0 0 0 0
63

P-ROVER-OP'S-FAIL

1.0E+0

P-ROVER-DOCK-FAIL

1.0E+0

P-ROVER-RADIATION P-ROVER-EVA-FAIL

1.0E+0

P-ROVER-EVA-HW

1.0E+0

P-ROVER-EVA-LEAKAGE

1.0E+0

P-ROVER-EVA-OBSTUCT

P-ROVER-FLAMMABLE

1.0E+0

P-ROVER-FALM-SHORT

1.0E+0

P-ROVER-FLAM-THERMAL

P-ROVER-LEAK

1.0E+0

P-ROVER-LEAK-DAMAGE

1.0E+0

P-ROVER-LEAK-SEALS

P-ROVER-SYSTEM-FAIL

1.0E+0

P-ROVER-LIFE-SUPPORT

1.0E+0

P-ROVER-MECH-SYST

P-ROVER-TRANSPORT

1.0E+0

P-ROVER-TRANS-CREW-INCAP

1.0E+0

P-ROVER-TRANS-TERRAIN

Failure of Rover

Transport capability

Leakage failure

of the Pressruized Rover

Flammable environment

in Pressurized Rover

Press Rover

fails to support

EVA capabilities

P Rover fails

to meet one or

more mission

objectives

Failure of Pressurized Rover systems

Mechanical system

failure Life supportfailure P. Rover crewincapacitated

Failure of P.

Rover to navigate

terrain

Leakage of P.

Rover seals

Damage to P.

Rover causes

leakage

P. Rover Electrical

short P. Rover TPSFailure

Obstruction

prevents P. Rover

EVA

Failure of P.

Rover EVA hardware Leakage of PRover EVA Airlock

Failure of Pressurized

Rover Radiation

Shielding

Failure of Press

Rover to provide

docking function

 P-ROVER-OP'S-FAIL - Pressurized Rover falis to meet one or more mission objectives 2005/08/16 Page 5
64

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

P. Rover Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

P-ROVERMISSION-OPSFAILURE

Top 

OR 

Gate

P. Rover fails to 

meet one or 

more mission 

objectives

P-ROVERDOCKINGFAILURE

Basic 

Event 

P. Rover 

docking failure

P-ROVER-EVAFAILOr 

Gate

P. Rover fails to 

support EVA 

capability

P-ROVER-EVAHWBasic 

Event 

Failure of P. 

Rover EVA 

hardware

P-ROVER-EVALEAKAGEBasic 

Event 

Leakage at 

P.Rover 

LSAM/Hab I/F 

(Airlock)

P-ROVER-EVAOBSTRUCTBasic 

Event 

Obstruction 

prevents 

P.Rover EVA

P-ROVER-CABINFLAMMABILITYOr 

Gate

Flammable 

condition in P. 

Rover

P-ROVER-FLAMSHORTBasic 

Event 

P. Rover 

electrical short

P-ROVER-FLAMTHERMALBasic 

Event 

P. Rover TPS 

failure

P-ROVER-CABINLEAKOr 

Gate

Leakage of P. 

Rover cabin 

volume

P-ROVER-LEAKDAMAGEBasic 

Event 

Damage to P. 

Rover causes 

leak
65

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree Logic for 

P. Rover Op's Type Description 9.5/30/4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

P-ROVER-LEAKSEALSBasic 

Event 

Leakage of P. 

Rover seals

P-ROVERRADIATIONBasic 

Event 

Excessive P. 

Rover radiation

P-ROVERSYSTEM-FAILUREOr 

Gate

P. Rover system 

failure

 P-ROVERECLSS-FAILBasic 

Event 

P. Rover 

ECLSS failure

 P-ROVERMECH-SYS-FAILSBasic 

Event 

P. Rover 

Mechanical 

System failure

P-ROVERTRANSPORTOr 

Gate

P. Rover 

transport failure

P-ROVERTRANS-CREWINCAP

Basic 

Event 

P. Rover crew 

incapacitated

P-ROVERTRANS-TERRAINBasic 

Event 

Failure of P. 

Rover to 

translate terrain

1 Highest Risk 0 0 0 0 0 0 0 0 0

2 0 0 0 0 0 0 0 0 0

3 0 0 0 0 0 0 0 0 0

4 0 0 0 0 0 0 0 0 0

5 Medium Risk 0 0 0 0 0 0 0 0 0

6 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0

8 0 0 0 0 0 0 0 0 0

9 Lowest Risk 0 0 0 0 0 0 0 0 0
66

HAB-MISSION-OP'S-FAILURE

1.0E+0

HAB-RADIATION

1.0E+0

HAB-STOWAGE-FAILURE HAB-CABIN-LEAK

1.0E+0

HAB-CABIN-DOCK-LEAK

1.0E+0

HAB-CABIN-LEAK-HW

HAB-CONSUM

1.0E+0

HAB-CONSUM-CONTAM

1.0E+0

HAB-CONSUM-USE

1.0E+0

HAB-EXP-CONSUM

1.0E+0

HAB-O2-SYSTEMS-FAIL

HAB-EVA-FAIL

1.0E+0

HAB-EVA-AIRLOCK

1.0E+0

HAB-EVA-CREW

1.0E+0

HAB-EVA-SUIT

HAB-FLAMM-FAILURE

1.0E+0

HAB-FLAM-SHORT

1.0E+0

HAB-FLAM-THERMAL

HAB-RECYCLE-SYSTEM-FAIL

1.0E+0

HAB-RECYCLE-CONTAM

1.0E+0

HAB-RECYCLE-HW

Failure of Hab

recycle systems

Flammable environment

in Habitat

Failure of Habitat

EVA capability

Habitat consumable

not sufficient

Habitat cabin

leak

Hab fails to

meet one or more

mission objectives

Expired consumables Hab recycle

systems contamination

Failure of Hab

recycle hardware

Hab electrical

short

Hab crew medical Hab TPS failure

event

Failure of Hab

EVA suit

Failure of Hab

airlock

Mechanical systems

failure

Under-estimated

consum. or spares

usage

Contamination

of Hab consumbales

Failure during

docking

Hab hardware

system failure

Failure in Habitat Inadequate stowage Radiation shielding

HAB-MISSION-OP'S-FAILURE - Habitat fails to meet one or more mission objectives 2005/08/16 Page 6
67

Atmosphere Points (PSI / PPO2 % / Suit P)

Fault Tree 

Logic for HAB 

Op's

Type Description 9.5/30/

4.3

8.3/36/

4.3

9.4/28.5/

4.3

9.3/26.8/

4.3

10.6/23.2/

5.0

11.6/21/

6.0

12.8/21/

6.0

13.2/23.1/

6.0

10.2/26.5/

4.3 Rationale

HABMISSIONOPS-FAILURE

Top 

OR 

Gate

HAB fails to meet one 

or more mission 

objectives

HAB-CABINLEAKOr 

Gate Habitat cabin leak

HAB-CABINDOCK-LEAKBasic 

Event Failure docking

HAB-CABINLEAK-HWBasic 

Event Hab hardware failure

HABCONSUMOr 

Gate

Habitat consumable 

not sufficient

HABCONSUMCONTAM

Basic 

Event 

Contamination of Hab 

consumables

HABCONSUMUSE

Basic 

Event 

Underestimated 

consumable or 

spares usage
68

Atmosphere Points (PSI / PPO2 % / Suit P)

HABCONSUM-EXPBasic 

Event 

Hab consumables 

expired

HAB-O2-

SYSTEMSFAIL

Basic 

Event 

Mechanical Systems 

failure

HAB-EVAFAILOr 

Gate

Failure of Habitat 

EVA capability

HAB-EVAAIRLOCKBasic 

Event Failure of Hab airlock

HAB-EVACREWBasic

Event 

Hab crew Medical 

Event occurs

HAB-EVASUITBasic 

Event 

Failure of Hab EVA 

suit

HAB-FLAMMFAILUREOr 

Gate

Flammable 

environment in 

Habitat

HAB-FLAMSHORTBasic 

Event Hab electrical short
69

Atmosphere Points (PSI / PPO2 % / Suit P)

HAB-FLAMTHERMALBasic 

Event Hab TPS failure

HABRADIATIONBasic 

Event 

Failure in Habitat 

radiation shielding

HABRECYCLESYSTEM-FAIL

Or 

Gate

Failure of Habitat 

recycle systems

HABRECYCLECONTAM

Basic 

Event 

Hab recycle system 

contamination

HABRECYCLE-HWBasic 

Event 

Failure of Hab 

recycle hardware

HABSTOWAGEFAIL

Basic 

Event Inadequate storage

1 Highest Risk 0 0 0 0 0 0 0 0 0

2 0 0 0 0 0 0 0 0 0

3 0 0 0 0 0 0 0 0 0

4 0 0 0 0 0 0 0 0 0

5 Medium Risk 0 0 0 0 0 0 0 0 0

6 0 0 0 0 0 0 0 0 0

7 0 0 0 0 0 0 0 0 0
70

Atmosphere Points (PSI / PPO2 % / Suit P)

8 0 0 0 0 0 0 0 0 0

9 Lowest Risk 0 0 0 0 0 0 0 0 0
Engineering and Science Contract Group

2224 Bay Area Boulevard

Houston, Texas 77058

71

Appendix D:

Using the Analytical Hierarchy Process to Aid in Risk Assessment 

for the Exploration Atmospheres Working Group
72

Introduction

This report explores the avenues taken by the Advanced Integration Matrix (AIM) on behalf 

of the Exploratory Atmospheres Working Group (EAWG) to gain EAWG consensus on 

possible modification of atmospheric pressure and oxygen content in order to facilitate safe 

and rapid Extravehicular Activity (EVA) for future long term missions. A brief questionnaire, 

using the Analytical Hierarchy Process (AHP) was developed to assess the qualitative 

preference of the group for certain proposed design points with each point having somewhat 

different perceived risks and rewards. A fault tree analysis was done in conjunction with this 

study but since there is little probabilistic data available currently comparing the key 

elements of the process, a qualitative ranking of design points similar to the AHP study part 

A was done. AHP has the capability to integrate the qualitative and quantitative data from 

both the simpler AHP studies and the fault tree analysis, but will rely on the opinion of 

experts as to the relative importance of key points such as fire risk and decompressions 

sickness risk to the overall risk. Previous efforts cited in the literature have shown the 

validity and usefulness of using AHP to combine quantitative and qualitative results in a joint 

effort.

Background

In order to prepare for eventual trips back to the Moon and Mars, a working group being 

called the EAWG was formed to study possible compromise on a vehicle/or living quarters 

atmosphere that might streamline and increase EVA efficiency while maintaining an 

atmosphere that does not significantly increase fire risk. While there are other risks which 

will be discussed as they are related to this subject, the efficiencies necessary to do 

repeated and frequent EVA is the driving force for such change. While the implications are 

not necessarily obvious, the ability to do the required EVA effort involves being able to 

rapidly and safely deploy for EVA without risk of Decompression Sickness (DCS). DCS is 

brought on by nitrogen or another inert gas being pulled from body tissues by a pressure 

gradient which occurs as a result of moving from a relatively high pressure vehicle or habitat 

to a relatively low pressure EVA suit. The proposed reduction in cabin/vehicle pressure 

would be done in order to bring the pressure closer to that of the EVA suit and thus shorten 

the time necessary to equilibrate the body with the space suit atmosphere. The time period 

required for equilibration is the prebreathe time. DCS can be prevented by a preparation via 

the prebreathe procedure that takes place at equal pressure to that of the vehicle/habitat, 

but at 100% oxygen. The oxygen gradually displaces the nitrogen in the tissues thus 

preventing bubbles of nitrogen from forming when pressure is reduced. If cabin pressure is 

high, this process can take hours and depending on the prebreathe protocol could also put 

the astronaut at considerable risk for DCS.

Flammability of materials increases as the volume percent of oxygen increases. As the 

pressure in the vehicle or habitat is decreased, the volume percentage of oxygen must be 

increased to keep the partial pressure of oxygen the same and allow normal lung function. 

According to Dalton’s Law the total pressure of a gas mixture is equal to the sum of the 

partial pressures. If the total pressure is dropped and the partial pressure of oxygen needs 

to be the same for adequate lung function, then the percentage of oxygen by mass or by 
73

volume must be increased. This proposed increase in oxygen poses potential concerns. 

The approach taken by the fire safety community is to test all materials in well defined 

experimental settings and to eliminate those materials that do not pass specific tests based 

on flammability and off-gassing characteristics. In doing the testing up to this juncture, 30% 

oxygen has been considered the point where most testing stops. Therefore, few materials 

have been tested above the 30% oxygen limit. Though data is lacking, it is felt that many 

currently available materials would be lost if the proposed atmosphere is over 30% oxygen. 

It is believed that the electrical wire coating Nomex and the proposed radiation shielding 

Polyethylene could be lost if oxygen goes above 30% [1]. Possibly the flammable materials 

could be jacketed in metal but that could create other problems possibly related to scattering 

of high energy radiation particles. 

These are but a few of the issues involved for the EAWG to consider. Data is both available 

and uncertain in some areas and in other areas totally unavailable. If risk probabilities were 

available for all possible events, one could use classical statistics to project the probability or 

likelihood of future events. However some of the activities proposed have never been done 

before, although data is available from similar activities for some of the proposed risk 

studies. There is enough difference in the areas where data was collected and analyzed as 

compared to the projected activity, to leave a great deal of uncertainty in the evaluation. 

The Analytical Hierarchy Process

The Analytical Hierarchy Process (AHP) allows one in the absence of quantitative data or 

only partially available quantitative data, to combine that data with qualitative judgments to 

aide in evaluation of complex decisions. How much risk one proposes to take is one such 

complex decision. If all the necessary data was available from past experience to do a 

probability projection into the future, it would still tell us what designs would have failed or 

succeeded in the past instead of proposing ways of designing future in-system-excellence 

[2].

AHP has been used for risk assessment for environmental issues, national security issues, 

in the information technology sector and other areas and is being used to evaluate 

responses from the EAWG working group [3].

AHP uses pairwise comparisons in matrix format allowing participants to judge the 

importance of one item in relation to another and to make a quantitative judgment. If the 

participants have knowledge of the subject being considered, the AHP process captures that 

knowledge and assesses its consistency. An example is given in Figure 1, where each 

branch of a tree is evaluated against every other using a scale with magnitude 2-9 in favor 

of one item or 2-9 in favor of the other item. The negative signs indicate only direction 

toward one alternative and so for example -8 is equal to +8 in magnitude only the signs 

indicate preference for different alternatives. The number one means the assessment is the 

same for both items. The pairwise comparisons form a matrix which uses a ratio scale of

paired comparisons so if a participant judges “a” more relevant than “b”, “a” is selected 

along with a multiplier of 1-9 and the effect of “b” on “a” is recorded as the reciprocal. A 

square matrix is formed with the comparisons needed to make the overall evaluation Table 

1, with the comparison of each entity to itself along the diagonal and equal to 1. 
74

Table 3-A Simple Example of an AHP Matrix for Pairwise Comparison

a b

a 1 8

b 1/8 1

The matrix can then be multiplied or added using standard techniques to look at how effects 

on a small scale affect the larger picture. The maximum eigenvalue is used to evaluate a 

respondent’s consistency of response. An example of consistency is that if: “A” > “B”; “B” > 

“C” then “A” > “C”.

Figure 1- Slider bar used for pairwise comparison in the AHP Assessment

Mission Operations Risk Flammability Risk

no 

entry

-

9

-

8

-

7

-

6

-

5

-

4

-

3

-

2 1 2 3 4 5 6 7 8 9

A hierarchy is a representation of a complex problem in a multilevel structure whose first 

level is the goal followed successively by levels of factors, criteria, sub criteria, and so on 

down to a bottom level of alternatives [4]. The main purpose of arranging goals, attributes, 

issues and stakeholders in a hierarchy is that we don’t have enough detail to identify a 

cause and effect relationship. The elements being compared should be clustered into 

homogenous groups so they can be meaningfully compared with respect to elements in the 

next highest group. Functional hierarchies decompose complex relationships into their 

constituent parts according to their essential relationships. A valuable observation about the 

hierarchical approach is that the functional representation of a system may differ from 

person to person but people tend to agree on the bottom level of alternative actions to be 

taken and the level above it. 

The AHP for EAWG

The choice made for this study was to have the EAWG participants rate or rank nine design 

points in preferential order based on the criteria from the next tier above in the analysis, 
75

shown in Figure 8. The nine design points are outlined in Figure 22

Table 2

. The points are 

bounded by a red line representing 30% oxygen which is the point to which most testing of 

materials for flammability has been done. The points are bounded on the bottom by the 

hypoxic boundary, the blue line that cuts across the page from upper left to lower right. 

Point nine is outside the proposed 30% boundary. The points were originally published as 

six design points [1], but as discussion with the EAWG progressed additional design points 

were added. explains the pressure and partial pressure relationships of oxygen and 

the major inert gas that is present in the proposed cabin atmosphere at the proposed design 

points. Design points on the upper left of the design space would require prebreathe times 

greater than 1 hour but are assumptions based on the idea that variable pressure suits are 

available at 35-40 kPa and some nitrogen elimination could occur while in transit to the EVA 

work site. 

 

2 Courtesy of Kevin Lange [1]
76

Figure 2-Showing the Nine Design Points Being Considered by the EAWG

Figure 3 and Figure 4 are graphical examples of ranked design points for both Flammability 

and Decompression Sickness, done by EAWG members. Early in the study all participants 

were asked to rank the design points but after reconsideration, a decision was made to have 

the design points ranked by experts in four major areas: Hypoxia risk, DCS risk, 

Flammability risk, and Mission Operations risk. After the nine design points were ranked, all 

participants were asked to do pairwise comparisons on the four aforementioned areas. The 

comparisons serve as weighting factors on the ranking of design points and allow the design 

alternatives to be compared based on higher level criteria. 
77

Table 4-Candidate Design Point Descriptions

Candidate Spacecraft and Space Suit Atmosphere Designs

Pt. 

No.

Cabin 

Pressure, kPa 

(psia)

Cabin ppO2, 

kPa(psia)

Cabin 

Oxygen, 

volume%

EVA Suit 

Pressure, 

kPa (psia)

General Characteristics

1 65 (9.4) 18.5(2.7) 28.5 29.6 (4.3) Current space suit pressure. 

Cabin atmosphere well above 

hypoxic boundary, but less than 

normoxic. May allow use of 

materials certified to 30% 

oxygen with tight spacecraft 

operating control bands.

2 64 (9.3) 17.1(2.5) 26.8 29.6 (4.3) Lowest cabin oxygen 

concentration with current 

space suit pressure; at hypoxic 

boundary.

3 73 (10.6) 16.9(2.5) 23.2 34.5 (5.0) Moderate increase in space suit 

pressure with lower cabin 

oxygen concentration; at 

hypoxic boundary.

4 80 (11.6) 16.8(2.4) 21 41.4 (6.0) Higher space suit pressure with 

Earth-normal cabin oxygen 

concentration; equivalent to 

1829-m (6000-ft) Earth 

atmosphere. Lower DCS risk. 

Ground testing may be 

facilitated.

5 88.5 (12.8) 18.6(2.7) 21 41.4 (6.0) Higher space suit pressure with 

Earth-normal cabin oxygen 

concentration; well above 

hypoxic boundary. Ground 

testing may be facilitated.

6 91.0 (13.2) 21.0(3.0) 23.1 41.4 (6.0) Higher space suit pressure. 

Normoxic cabin atmosphere; 

slightly elevated oxygen 

concentration.

7 70.3 (10.2) 18.6(2.7) 26.5 29.6 (4.3) Current space suit pressure. 

This point represents shuttle 

EVA preparation conditions.

8 65.5 (9.5) 19.7(2.9) 30 29.6 (4.3) Current space suit pressure 

with assumed maximum oxygen 

concentration from the pressure 

study. O2 concentration control 

limits may be outside of 

existing materials flammability 

qualification envelope.

9 57.2 (8.3) 20.6(3.0) 36 29.6 (4.3) Current space suit pressure. 

Highest oxygen concentration 

point (above that assumed in 

the pressure study) and lowest 

cabin pressure minimize 

prebreathe time. Outside of 

existing materials flammability 

qualification envelope.

Note: Points 1 – 6 are from Table 4.1 in Lange et. al., “Bounding the Spacecraft Atmosphere Design Space for 

Future Exploration Missions,” NASA/CR-2005-213689, referred to in this table as the “pressure study”. Points 7-

9 were added by the EAWG after the first meeting of the group.
78

Figure 3-An Example of Un-weighted Ranking of Nine Design Points based on 

Flammability

Figure 4-Figure 2-An Example of Un-weighted Ranking of Nine Design Points based 

on DCS
79

The following charts which show results of AHP analysis done by the using the software 

package Expert Choice. Figure 5 shows the results of the initial AHP process that made no 

division of the respondents based on expertise. All respondents were asked to complete all 

parts of the questionnaire (attached as Appendix A). A number of respondents felt 

uncomfortable responding to areas of the questionnaire that were out of their areas of 

expertise. Because of this, the survey was reanalyzed (no new questionnaire) dividing the 

respondents into areas of expertise and having the ranking of alternatives done by persons 

with expertise in the given area. Figure 7 shows the results of “experts” performing the 

ranking of alternatives and with the weighting factors determined by the overall group. At 

meeting number two, a third ranking was done with ranking of alternatives done by the 

experts in attendance and the weighting of objectives done by all respondents (Figure 6). 

Figure 6 and Figure 7 have graphical demonstrations of the contributions made in the 

alternative areas i.e. the nine ranked points. Looking at Figure 6, if there’s a substantial 

reduction in risk to design point 9 ranking due to low risk for DCS, it shows a shorter green 

section of the bar for DCS as compared to point 7 which has a high risk of DCS.

Figure 5-Chart that Demonstrates the Weighting Factors on the Left and the Results 

of Ranking the Nine Alternatives as influenced by the Weighting Factors; Study done 

prior to the second EAWG meeting with no Division based on Expertise
80

Figure 6-Chart that Demonstrates the Weighting Factors on the Left and the Results 

of Ranking the Nine Alternatives as influenced by the Weighting Factors; Study done 

during the second EAWG meeting with Division based on Expertise for Ranking the 

data points
81

Figure 7-Chart that Demonstrates the Weighting Factors on the Left and the Results of 

Ranking the Nine Alternatives as influenced by the Weighting Factors; Study done prior 

to the second EAWG meeting with Division based on Expertise for Ranking the data 

points

Figure 8 shows the structure of the AHP structured process used with the nine design point 

alternatives at the bottom of the structure and the four risk alternatives: Flammability, DCS, 

Mission Operations, and Hypoxia shown on the next tier above. Persons in attendance at 

the second meeting of the EAWG seemed most comfortable with the approach using expert 

opinion to perform the ranking, rather than using the entire group to rank the points.
82

Figure 8-Diagram of the AHP Structure as Designed for the EAWG to consider two 

Atmospheres: Atm. A and Atm. B.

Mission Segment Goal Risk

CEV/Lander Atmosphere A

Flammability Risk DCS Risk Mission Operations 

Risk Hypoxia Risk

Design Pt 

Alternatives ranked 

1-9

related to 

Flammability for 

ATM A 

Design Pt 

Alternatives ranked 

1-9

related to Mis_Ops 

for ATM A 

Design Pt 

Alternatives ranked 

1-9

related to Hypoxia 

for ATM A 

 Design Pt 

Alternatives ranked 

1-9

related to DCS for 

ATM A 

Mission Segment Goal Risk

HAB/Rover Atmosphere B

Flammability Risk DCS Risk Mission Operations 

Risk Hypoxia Risk

Design Pt 

Alternatives ranked 

1-9

related to 

Flammability for 

ATM B 

 Design Pt 

Alternatives ranked 

1-9

related to Mis_Ops 

for ATM B 

 Design Pt 

Alternatives ranked 

1-9

related to Hypoxia 

for ATM B 

Design Pt 

Alternatives ranked 

1-9 related to DCS 

for ATM B

Please Consider the Following when Rating the Design Points 1-9:

Risks Associated During Meeting 2 with Flammability:

• Level of Risk due to % O2 

• Loss of Possible Materials due to Failure to Meet Fire Standards 

Risks Associated During Meeting 2 with Mission Operations:

• Failure to reach EVA productivity goals 

• Failure to contribute to EVA/IVA safety 

• Time Required for Preparation of Safe EVA 

• Glove non-flexability and Finger Injury 

Risks Associated During Meeting 2 with Hypoxia

• Risk of Operation Below the Hypoxic Limit 

• Risk of Failure of the O2 Support System 

Risks Associated During Meeting 2 with DCS

• Lack of Timely Response to Symptoms and Treatment

• Risk Exacerbated at design point due to Strenuous EVA 

• Complexity of the Pre-breath Protocol 

A Parallel Fault Tree Analysis and Comparison with AHP

A fault tree analysis (FTA) is a deductive, top-down method of analyzing system design and 

performance. It involves specifying a top event to analyze (such as a fire), followed by 

identifying all of the associated elements in the system that could cause that top event to 

occur. Then using a system familiarization approach, it determines the causes of the 

undesirable top event by using a systematic, backward-stepping process. This type of 

analysis can be either qualitative or quantitative in nature. Once the fault trees are created 

they can be updated and quantified as details change and mature [5].
83

A parallel effort using FTA was done by the EAWG. Since there was probability data 

available only in the DCS category and the available data has a degree of uncertainty,

having been obtained from studies which were not identical to planetary EVA, the design 

point alternatives were ranked in a similar 1-9 point scale, just as was done in the AHP 

study. Table 3 gives the rankings for the FTA process. These results are very close in the 

Flammability and DCS areas to AHP results. 

Table 5-Ranking of the Nine Data Points According to Fault Tree Analysis

FTA is being used to capture details of the possible cabin pressure and oxygen 

percentages. The rankings portion of the FTA process, and Part A of the AHP questionnaire 

are in general agreement, but the comparison in part B of the AHP questionnaire cross over 

the boundaries of many individuals expertise. In preparation for the third meeting a strategic 

approach was taken that assumes certain spacecraft will have equivalent atmospheres, 

allowing them to dock directly with each other and therefore it is assumed that: 

Group A: consists of CEV, Lander, and Mars Transit Habitat/Vehicle – with the common 

atmosphere A.

Group B: consists of a planetary Surface Habitat, and a Pressurized Rover – and have a 

common atmosphere B. Atmosphere A and B may or may not be the same. Another round 

of AHP questionnaires were sent to the working group with exactly the same points to 

evaluate but for two different atmospheres. Results of that analysis will be available during 

the September 27-29, 2005 workshop.

The final step in the evaluation is still to make an informed assessment of the overall 

contribution of major risk factors. That informed assessment and recommendation, could be 

done with AHP, by the EAWG team after looking at all the factors involved.

Bruce E Duffield

ESCG Life Support Analyst

Thermal and Environmental Analysis Section

FTA Ranking of the Nine Data Points

Flammability 

(Based on Expert 

Judgment)

4 5 6 3 7 2 1 8 9

DCS (Based on 

Probability)

9 4 5 6 3 2 1 8 7

Going from left to right the most desirable point is on the left; least desirable on the right
84

Bibliography:

1) Lange KE, AT Perka, BE Duffield, FF Jeng, Bounding the Spacecraft Atmosphere 

Design Space for Future Exploration Missions, NASA/CR—2005–213689, June 2005.

2) Davidson GG, AW Labib, Learning From Failures: Design Improvements Using a 

Multiple Criteria Decision Making Process, IMECHE 2003, pg 207.

3) See TK, L Kemper, Multiattribute Decision Making Using Hypothetical Equivalents, 

Proceedings of DETC 2002 of ASME Design Engineering Technical Conferences, Sept 

29-Oct 2, Montreal Canada.

4) Saaty TL, Fundamentals of Decision Making and Priority Theory Vol. VI, University of 

Pittsburgh, RWS Publications, 1994.

5) Deckard G. IV, Report to the Advanced Integration Matrix on the Fault Tree Analysis 

Effort for Exploratory Atmospheres Working Group, September, 2005.
85

Appendix E-1:

Ambulation During Extravehicular Activity on Moon

and Mars as a Risk Factor for Decompression Sickness
86

White Paper

Authors: Johnny Conkin, Ph.D., and Michael R. Powell, Ph.D.

PURPOSE:

This White Paper documents ambulation as a risk factor for decompression sickness 

(DCS) during extravehicular activity (EVA) on Moon and Mars, and how to mitigate this risk 

through efficient prebreathe protocols. 

BACKGROUND:

Standing, walking, and even stepping are such ubiquitous activities in our daily 

experience that healthy people do not consider these as exercise. In reality, they represent 

substantial lower body exercise from the standpoint of kinematics. The muscles, joints, and 

bones in the lower body evolved to efficiently transport a substantial mass (our body) over a long 

distance without difficulty. Ambulation on the Moon and Mars in a space suit will be the 

primary means to explore the surface. Even though the net forces transferred to the lower body 

will be lower in a reduced gravity field and the range and rate of joint motion will be reduced 

due to the suit exoskeleton, ambulation still represents the greatest expenditure of energy during 

the EVA(12).

It is well documented (1,3,5,6,10) that exercise of the lower body increases the risk and 

severity of Type I “pain-only” decompression sickness (DCS) in the feet, ankles, knees, and hips. 

It is beyond the scope of this White Paper to review the mechanisms to account for increased risk 

due to ambulation (see Appendix for some details). Suffice it to say that there is no 

disagreement that lower body exercise increases the risk of DCS and the number of venous gas 

emboli (VGE). But there is disagreement on the magnitude of the effect (3,7,8,9,11). Much of 

the disagreement is because several factors determine the magnitude of the effect, such as the 

magnitude of decompression stress at the start of the exercise, the type and intensity of lower 

body exercise, and even the time spent walking at reduced pressure. An understanding of the 

risk with each of these factors in isolation has not been achieved, so opinions vary. 

Ambulation as a factor to increase the risk of DCS is not a concern if nitrogen (N2) 

pressure in the tissues is less than total suit pressure. Under this condition, there is no N2

supersaturation in the tissues during EVA, and bubble formation and growth is not possible. 

This was the case during the Apollo program where astronauts lived at 5.0 psia in 100% oxygen 

(O2) for several days prior to EVA on the Moon in 3.7 psia suits. However, future exploration 

missions to Moon and Mars will be conducted from habitats that have an inert diluent gas. The 

presence of this gas, most likely N2, and the possibility of operating at a suit pressure lower than 

the habitat diluent pressure makes ambulation a risk factor for DCS and VGE. The risk can be 

mitigated through a combination of habitat atmosphere selection, additional in-suit prebreathe, or 

a higher suit pressure. 
87

FUNDAMENTAL ASSUMPTION:

The assumption here is that any prebreathe compensation to reduce the risk of DCS and 

VGE due to ambulation on Earth would be as effective applied to ambulation on Moon and 

Mars. Note that the acceptable level of DCS risk for Moon and Mars missions has not been 

determined.

Example of ambulation as a risk factor for DCS and VGE, and how to mitigate this risk 

with additional in-suit prebreathe:

A detailed example, based on analysis, is now provided to demonstrate the magnitude of 

the ambulation effect on DCS and VGE risk and the magnitude of the prebreathe compensation 

needed to mitigate the ambulation effect. 

The successful 10.2 psia staged decompression protocol currently used on the Shuttle is 

one option for the Crew Exploration Vehicle (CEV) and Moon habitat. But EVA on the Moon 

or Mars will involve significant ambulation, possibly quite different than on Earth (Moon Loping 

(12)), and it is possible that the current Shuttle staged decompression protocol would need to be 

more conservative for use on the Moon (see Figure 1).
88

Figure 1. Because of musculoskeletal stress, these are not equivalent activities when assessing 

the risk of DCS.
89

Details about the Space Shuttle Staged Protocol:

The Shuttle cabin initially operates at 14.7 psia, at about 21% oxygen. There is an initial 

60 min 100% O2 mask prebreathe at 14.7 psia if subsequent time at 10.2 psia is less than 36 hrs, 

or else the initial prebreathe is omitted. Before EVA, the EVA crewmembers don space suits. 

The final in-suit prebreathe depends on the time spent at 10.2 psia breathing 26.5% O2; it is 75 

min if equilibrated for 12 hrs and 40 min if equilibrated more than 24 hrs. Finally, the EVA 

crewmembers decompress to 4.3 psia in their suits.

Details about computed Tissue Ratio (TR) as an Index of Decompression Stress:

TR is P1N2 / P2 where P1N2 is calculated N2 pressure in a compartment with a 360 min 

half-time just prior to EVA, and P2 is 4.3 psia for the current EVA suit. The staged protocol 

provides denitrogenation through a combination of 100% O2 prebreathe with a mask and in the 

suit, and several hrs of breathing a reduced ambient ppN2 of 7.5 psia. Equilibration at a ppN2 of 

7.5 psia plus 70 min of additional in-suit prebreathe results in a computed TR of 1.524.

We computed TR from 143 staged Shuttle prebreathe protocols performed in flight. The 

results from 80 of the 143 total EVAs done first from the Shuttle are compared to 245 prebreathe 

tests performed in altitude chambers at the Johnson Space Center (JSC).

The presence of Doppler-detected gas bubbles (VGE) in the pulmonary artery (venous 

blood) was determined with an ultrasound Doppler bubble detector in the tests performed in 

altitude chambers.

Refer to Figs. 2 through 4 for a summary of our analysis.
90

RESULTS

0.50 0.84 1.18 1.52 1.86 2.20

computed tissue ratio

0

25

50

75

Count

0.0

0.1

0.2

0.3

Proportion per Bar

0.50 0.84 1.18 1.52 1.86 2.20

computed tissue ratio

0

25

50

75

Count

0.50 0.84 1.18 1.52 1.86 2.20

computed tissue ratio

0

25

50

75

Count

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Proportion per Bar

0.50 0.84 1.18 1.52 1.86 2.20

computed tissue ratio

0

25

50

75

Count

Figure 2. Histograms showing the distribution of computed tissue ratio in the 245 records from 

subjects in altitude chambers at JSC (upper) and 80 records of astronauts that performed their 

first 10.2 psia staged EVA from the Shuttle (lower). A normal density function is imposed on 

each histogram to provide a visual reference to each mean computed tissue ratio (peak of curves) 

and the variability about each mean (spread of curves). The means are very similar, about 1.52, 

but the standard deviation is five times smaller in the EVA data (0.046) compared to the chamber 

data (0.26). 

 
91

Figure 3. The probability of DCS [P(DCS)] and probability of VGE [P(VGE)] decreases on 

subsequent EVAs after the first EVAs from the Shuttle (from Conkin, 2001). All 143 EVAs 

were performed after > 12 hrs of denitrogenation at 10.2 psia while breathing 26.5% O2. Eighty 

of the 143 EVAs were the first EVAs of the mission. The predicted risk of DCS and VGE are 

greater than zero in both cases, but no DCS has been reported during EVA, and VGE have never 

been monitored during EVA to confirm the predicted risk. Note the impressive decrease in DCS 

and VGE risk when the nonambulatory condition (no use of weight bearing leg muscles) is 

present, which we consider is present during EVAs from the Shuttle. 
92

1.301.351.401.451.501.551.601.651.70

computer tissue ratio

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

probability of decompression sickness

Figure 4. At a computed tissue ratio of 1.52, the best estimate of DCS risk after 80 Shuttle 

staged denitrogenations given nonambulatory EVA is about 4% compared to 15% if the same 

EVA is done under ambulatory conditions (from Conkin, 1996). The contribution of ambulation 

toward the risk of DCS is a significant unknown if EVAs are done on the Moon and if the 

Shuttle staged denitrogenation protocol were adopted for the Moon habitat.
93

Table I is a summary of the observed and predicted risk of DCS from EVA and research 

experience. Notice that the mean TR of 1.52 from 80 of 143 total EVAs done first from the 

Shuttle is comparable to 245 research tests in altitude chambers. But there was 18.3% (13.7 –

23.8%, 95% confidence interval (CI)) DCS observed in ambulatory subjects in altitude chambers 

compared to no DCS (0 – 4.5% CI) reported from astronauts during EVA. The risk of DCS for a 

1.52 TR is predicted higher for ambulatory EVAs on the Moon [15.5% (0 – 32% CI)] compared 

to nonambulatory EVAs from the CEV [4.0% (2 – 8% CI)]. Also notice that the predicted risk 

of 15.5% for ambulatory EVAs on the Moon is comparable to the 18.3% risk for ambulatory 

subjects in altitude chambers. 

TABLE I: Observed and Predicted Risk of DCS

Test Condition TR ± SD, n Observed

%DCS (CI)

Predicted

%DCS (CI)

μg EVA (3)

Predicted

%DCS (CI)

Moon EVA (2)

Flight μg 1.527 ± 0.046, 80 0% (0 – 4.5) 4.1% (2 – 8) 15.9% (0 – 34)

Research 1g 1.520 ± 0.26, 245 18.3% (13.7 – 23.8) 4.0% (2 – 8) 15.5% (0 – 32)

CI = 95% confidence interval

CONCLUSIONS

• DCS has not been reported during EVAs at 4.3 psia from the Shuttle, so a similar staged 

protocol for the CEV will likely result in similar success. 

• Inactivity of the lower body in μg before and during EVA likely reduces the risk of Type 

I DCS in the lower body, but ambulation on the Moon and Mars given the same TR is 

predicted to quadruple the risk of DCS and triple the risk of VGE to a level observed 

during ambulatory tests in JSC ground-based altitude chambers. 

• A long final in-suit prebreathe time of 2.5 hrs may be required if the current Shuttle 

staged protocol were adopted for a Moon habitat to compensate for ambulation on the 

Moon (see Fig. 5), and even longer if suit pressure were less than 4.3 psia.

• At present, there is no good estimate of what the type of walking referred to as “Moon 

Loping” will have on bubble formation in tissues.
94

0 30 60 90 120 150 180

in-suit prebreathe time (min)

5.0

5.5

6.0

6.5

7.0

7.5

8.0

computed P1N2 (psia)

In-Suit Prebreathe needed for Ambulatory EVAs on Moon

in 4.3 psia Suit if 10.2 psia Staged Protocol is Selected

0 30 60 90 120 150 180

in-suit prebreathe time (min)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

P(DCS)

Figure 5. To compensate for the increased risk of DCS and VGE at 4.3 psia due to ambulation 

on the Moon, additional in-suit prebreathe time is required to reduce the risk if the current 10.2 

psia staged conditions are selected for a Moon habitat. The acceptable level of DCS risk for 

Moon EVAs has not yet been defined. 
95

• Therefore a different staged protocol is also a consideration to reduce the total prebreathe 

time to 60 min. For example a 9 psia habitat with 30% O2 would result in about 5% risk 

of DCS at 4.3 psia, or a 8.3 psia habitat with 36% O2 would result in about 1% risk of 

DCS at 4.3 psia (see Fig. 6).

Moon Habitat Options for EVA at 4.3 psia

Skylab Mercury/Gemini/Apollo

Early Apollo Design

Shuttle EVA Preparation

Shuttle/ISS

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Cabin Volume Percent Oxygen

Cabin Total Pressure, kPa

0

2

4

6

8

10

12

14

Cabin Total Pressure, psia

Normoxic Equivalent

Hypoxic Boundary

Assumed Flammability Limit

Historical Designs

P(DCS)

0

0.0005

0.005

0.02

0.05

0.089

0.15

0.20

0.25

0.30

R

0.64

0.86

1.02

1.17

1.30

1.40

1.51

1.58

1.65

1.71

0

0.02

0.089

0.0005

0.30

P(DCS) (see Table)

Figure 6. Curves for constant DCS probability (isorisk) from Conkin, 1996 for a 29.6 kPa (4.3 

psia) space suit with a 60 min prebreathe (6.89 kPa = 1.0 psi). Calculations based on a four hr 

EVA on the Moon that includes exercise plus ambulation equivalent to walking in 1-g. If 

prebreathe time is limited to 60 min, and acceptable DCS risk is < 10%, then the resulting 

narrow design space for the Moon habitat is restricted to the shaded region. Figure from Lange, 

2005.
96

REFERENCES

1. Cook SF. Part II. Role of exercise, temperature, drugs, and water balance in decompression 

sickness. In: Fulton JF, editor. Decompression sickness. Philadelphia: WB Saunders; 

1951:223-35.

2. Conkin J, KV Kumar, MR Powell, PP Foster, JM Waligora. A probability model of 

hypobaric decompression sickness based on 66 chamber tests. Aviat Space Environ Med 

1996; 67:176-83.

3. Conkin J, Powell MR. Lower body adynamia as a factor to reduce the risk of hypobaric 

decompression sickness. Aviat Space Environ Med 2001; 72:202-14.

4. Lange KE, Perka AT, Duffield BE, Jeng FF. Bounding the spacecraft atmosphere design 

space for future exploration missions. NASA Contractor report 2005-213689 Houston: 

Johnson Space Center, pp. 24 as Fig. 3-6, June 2005.

5. Henry FM. Effects of exercise and altitude on the growth and decay of aviator’s bends. J 

Aviat Med, 1956; 27:250-59.

6. Pilmanis AA, Olson RM, Fischer MD, et al. Exercise-induced altitude decompression 

sickness. Aviat Space Environ Med 1999; 70:22-9.

7. Balldin UI, Pilmanis AA, Webb JT. The effect of simulated weightlessness on hypobaric 

decompression sickness. Aviat Space Environ Med 2002; 73:773-8.

8. Powell MR, Waligora JW, Norfleet WT. Decompression in simulated microgravity; bedrest 

and its influence on stress-assisted nucleation. Undersea Biomed Res 1992; 19(S):54.

9. Vann RD, Gerth WA. Is the risk of DCS in microgravity less than on earth? ASMA, 68th 

Annual Scientific Meeting, Chicago IL, May 11-15, 1997, Abstract # 45.

10. Ferris EB, Jr., Engel GL. The clinical nature of high altitude decompression sickness (Chap. 

2). In: Fulton JF, editor. Decompression sickness. Philadelphia: WB Saunders; 1951:27-29.

11. Webb JT, Beckstrand DP, Pilmanis AA, Balldin UI. Decompression sickness during 

simulated extravehicular activity: ambulation vs. non-ambulation. Aviat Space Environ Med 

2005; 76:778-81.

12. Goldberg JH, Alred JW. Prediction of physical workload in reduced gravity. Aviat Space 

Environ Med 1988; 59:1150-57.
97

APPENDIX

Discussion about Lower Body Adynamia 

“Space walk” during an EVA in low Earth orbit is a misnomer. Astronauts do not walk 

in the conventional sense but only anchor their legs to a stable structure so that the upper body 

can affect some task. We characterize the lack of musculoskeletal activity and therefore the lack 

of dynamic forces in the lower body over several days of adaptation to μg and during EVAs as 

Lower Body Adymania (LBA). We define LBA as restricted lower body movement, particularly 

walking or even a standing posture through contraction of antigravity musceles, during both the 

denitrogenation phase (prebreathing) at site pressure and during the exercise phase while at 

altitude. In simpler terms, if you do not ambulate (walk) in a gravity field, then you are

considered adynamic.

LBA is a condition of sufficient magnitude to reduce the risk of DCS and VGE. A 

person who is inactive, more specifically neither standing erect nor walking, before and during a 

prebreathe and while at altitude is less likely to have DCS or VGE than the same person who is 

inactive at altitude but walks both before and during the exposure. The mean latency times of 

the first symptom and VGE are extended in adynamic subjects compared to ambulatory subjects. 

We feel there is enough data to recommend LBA as a technique to use before and during an 

altitude exposure if the goal is to reduce the risk of DCS and VGE, or reduce the variability in 

the DCS and VGE outcome for some people. We cannot say that adynamia before or during the 

exposure is more important since these data are not yet available. Our working hypothesis is that 

any benefit derived from adynamia before an exposure is lost if the adynamia is not maintained 

during the exposure because of the rapid regeneration of tissue micronuclei. 

The fundamental question about mechanisms to account for the observations about LBA 

cannot be answered here. We observe macroscopic outcomes and then infer mechanisms on a 

microscopic scale. A comprehensive review and discussion of micronuclei is beyond the scope 

of this report, but information is available (1,2,3). The fundamental untested premise of 

adynamia is about the control of nucleation processes within tissues and fluids. In the absence of 

supersaturation (the difference between dissolved gas partial pressure and the absolute pressure), 

the spontaneous rate of nucleation is inconsequential when micronuclei on the order of microns 

in radius are considered. This is not to say, however, that the number or distribution of 

micronuclei sizes cannot be influenced before a supersaturation exists when mechanical energy 

is added to the system. A case in point is the observation that vigorous exercise during a 90 min 

prebreathe reduces, not increases, the incidence of DCS and VGE (4,5). The enhanced removal 

of N2 during the dual-cycle exercise appears to dominate the DCS and VGE outcomes, 

regardless of how the number or distribution of micronuclei was changed.

There are at least four areas that we contemplate when considering micronuclei: the rate 

of micronuclei formation, the number of micronuclei formed, the distribution of micronuclei 

size, and the rate of micronuclei dissolution, which includes mechanisms to stabilize 

micronuclei. Each of these processes is complex to characterize in a physical system and more 
98

so when energy is added too or subtracted from a biological system. For example, the 

mechanical energy transferred through tendons and ligaments from muscle contractions. 

Supersaturation could be tolerated indefinitely if the rate of nucleation or rate of bubble growth 

were very slow. Therefore, the full potential for evolved gas as bubbles would not be realized if 

the supersaturation is reduced by the blood’s removal of dissolved gas faster than either the rate 

of nucleation or the rate of bubble growth. The rate of nucleation for a given supersaturation 

may not be a rate-limiting step in bubble formation because there may be stabilized or easily 

formed micronuclei that exist in the body. Thus, the appearance of bubbles after decompression 

may depend simply on the growth of readily available micronuclei, and well-controlled 

experiments can help to resolve the mechanism(s) of adynamia. Stephan (viscous) adhesion or 

tribonucleation (6,7,8,9), either before or after a decompression, is a process that could increase 

the rate of nucleation in living tissue and fluids, so experiments about micronuclei before a 

decompression should eliminate tribonucleation as a variable after the decompression. To 

complete the discussion, there is evidence that preformed and stabilized micronuclei do not exist 

before decompression, at least in a species of translucent crab (2,10).

We postulate that with an adynamic period of several hrs, a new distribution of 

micronuclei is established. The larger micronuclei are unstable and therefore short-lived and, in 

part due to surface tension (16), fewer stabilized smaller micronuclei transform into growing 

bubbles for a given supersaturation (15). The application of a high pressure spike, either 

hydraulic or pneumatic, filtration, or ultracentrifugation of a sample, are all accepted means to 

reduce the number and size of micronuclei (change the distribution) as evident by fewer bubbles 

or cases of DCS after a subsequent decompression (6,8,11). The idea of “using up” micronuclei 

faster than they are generated as a means to understand increased resistance to DCS on repeated 

exposures has also been discussed (1).

Our approach to use LBA to reduce or otherwise control hypothetical micronuclei 

populations appears to be the first use of this concept, especially as it applies to decompressions 

in μg. Violent muscular contractions in bullfrogs prior to hypobaric decompressions (12) were 

associated with bubble formation in the resting animals while at altitude, and has been known 

since the early 1940s. The number of bubbles was reduced if the frogs were allowed to recover 

as long as one hr after electrical stimulations. The authors offered two explanations: a shortlived local increase in carbon dioxide that facilitated bubble growth at altitude, or the inception 

of micronuclei or some other short-lived entity that would later facilitate the growth of bubbles at 

altitude. This same concept was recently tested in humans (13) where twenty subjects were 

exposed to 6.2 psia on three separate and random occasions without the confounding of 

prebreathe or any exercise at altitude during a two hr exposure. Each subject did 150 deep knee 

bends in a period of ten mins either two hrs, one hr, or just prior to ascent with the remaining 

time spent adynamic in a chair. As with the frogs, it was hypothesized that exercise before 

decompression would generate a population of some entity (micronuclei, macronuclei, vaporfilled cavities trapped on vascular endothelium, or increase the concentration of carbon dioxide) 

that would diminish in size or concentration given enough time prior to ascent. They used 

subsequent VGE information to indirectly test the hypothesis. They observed that intense lower 

body activity just prior to the altitude exposure did cause more VGE to appear, and to appear 

earlier compared to when exercise was done earlier. The critical observation was that the 

predisposing factor(s) diminished with time while sitting quietly in a chair prior to the ascent. 
99

The predisposing factors are assumed absent or reduced for EVAs in μg, but are assumed present 

for EVAs on the Moon or Mars.

LBA to reduce the risk of DCS, possibly by reducing the number and size of micronuclei, 

may have a limited utility in many situations but is important when relating DCS and VGE 

results obtained in altitude chambers to astronauts during EVA in μg. Astronauts do not move 

the lower body the same in space as they do on Earth. They are generally adynamic for several 

days prior to and during a space EVA. Indirect evidence suggests that astronauts and 

cosmonauts may have benefited from LBA since there has never been a report of DCS during an 

EVA. We acknowledge that underreporting can be attributed to additional factors that mask a 

minor pain-only symptom while in a space suit. Also, a bias, though denied, not to report any 

problems in an operational setting should be acknowledged, and is a constant concern to Flight 

Surgeons (14).

It is not possible to logically characterize an astronaut on the surface of the Moon or Mars 

as being adynamic even if the gravity is just 1/6th Earth-equivalent on the Moon and 3/8th on 

Mars. An astronaut in a Moon or Mars habitat would experience significant physical activity in 

the legs and would not unload all the forces in the lower body while ambulating on the surface. 

The additional risk of DCS and VGE from “Moon Loping” as compared to normal walking is 

unknown. 
100

Figure 1. The rate of DCS as a result of exercise after an ascent to 3.0 psia without prebreathing. 

Standard exercise was 10 step-ups on a nine inch stool in 30 sec, repeated at 5 min intervals. 

Figure modified from Henry, 1956.
101

Figure 2. The probability of any Grade of VGE versus the PB time on 100% O2 under four simulated

conditions:  with a 50 yo adynamic subject,  with a 50 yo ambulatory subject,  with a 20 yo 

adynamic subject, and  with a 20 yo ambulatory subject, all with ascent to 4.3 psia and with light 

upper and lower body exercise as part of the exposure. Figure from Conkin et al, 2003.

REFERENCES FOR APPENDIX

1. Hills BA. Decompression sickness, Vol. 1. The biophysical basis of prevention and treatment. 

New York: John Wiley and Sons; 1977. p. 76-90.

2. Hemmingsen EA. Nucleation of bubbles in vitro and in vivo. In: Brubakk AO, Hemmingsen 

BB, Sundnes G, editors. Supersaturation and bubble formation in fluid and organisms. 

Trondheim, Norway: Tapir Publishers; 1989: 43-68.

3. Powell MR, Waligora JM, Norfleet WT, Kumar KV. Project ARGO - Gas phase formation in 

simulated microgravity. NASA Technical Memorandum 104762. Houston: Johnson Space 

Center; 1993.

4. Loftin KC, Freeman-Perez S, Beene D, Hnatt L. Metabolic rate measurements comparing supine with 

upright upper-body exercises. NASA Contractor Report 4549. Houston: Johnson Space Center; 1993.

5. Webb JT, Fisher MD, Heaps CL, Pilmanis AA. Prebreathe enhancement with dual-cycle ergometry 

may increase decompression sickness protection. Aviat Space Environ Med 1996; 67-618-24.
102

6. Evans A, Walder DN. Significance of gas micronuclei in the aetiology of decompression sickness. 

Nature 1969; 222:251-52.

7. Hayward ATJ. Tribonucleation of bubbles. Brit J Appl Phys 1967; 18:641-44.

8. Ikles KG. Production of gas bubbles in fluid by tribonucleation. J Appl Physiol 1970; 28:524-

27.

9. McDonough PM, Hemmingsen EA. Bubble formation in crabs induced by limb motion after 

decompression. J Appl Physiol 1984; 57:117-22.

10. McDonough PM, Hemmingsen EA. A direct test for the survival of gaseous nuclei in vivo. 

Aviat Space Environ Med 1985; 56:54-6.

11. Vann RD, Grimstad J, Nielsen CH. Evidence for gas nuclei in decompressed rats. Undersea 

Biomed Res 1980; 7:107-12.

12. Whitaker DM, Blinks LR, Berg WE, Twitty VC, Harris M. Muscular activity and bubble 

formation in animals decompressed to simulated altitudes. J Gen Physiol 1945; 28:213-23.

13. Dervay JP, Powell MR, Fife CE. Effective lifetime of tissue micronuclei generated by 

musculo-skeletal stress. ASMA, 68th Annual Scientific Meeting, Chicago IL, May 11-15, 1997, 

Abstract # 68.

14. Bendrick GA, Ainscough MJ, Pilmanis AA, Bisson RU. Prevalence of decompression 

sickness among U-2 pilots. Aviat Space Environ Med 1996; 67:199-206.

15. Van Liew HD, Raychaudhuri S. Stabilized bubbles in the body: pressure-radius relationships 

and the limits to stabilization.

16. Walder DN. Serum surface tension and its relation to the decompression sickness of 

aviators. J Physiol (London) 1948; 548:48-49.

17. Conkin J, Powell MR, Gernhardt ML. Age affects severity of venous gas emboli on 

decompression from 14.7 to 4.3 psia. Aviat Space Environ Med 2003; 74:1142-50.
103

Appendix E-2:

Optimal Total Pressure-Oxygen Concentration Levels for Future 

Spacecraft, Spacesuits, and Habitats
104

Authors: Richard A. Scheuring, DO, MS, UTMB/Wyle Laboratories,

Johnny Conkin, PhD, NASA/JSC, Jeff Jones, MD, NASA/JSC, Mike Gernhardt, PhD, NASA/JSC

Introduction

This paper describes proposed environmental atmospheres for future long duration spacecraft, 

spacesuits, lunar and Mars habitats. Several atmospheric design points for the Constellation 

missions have been developed by the Environmental Atmosphere Working Group (EAWG) 

ranging from normoxic to moderately hypoxic while simultaneously hypobaric. These 

environments were analyzed to achieve a balance among the risk of decompression sickness, the 

overhead required to perform an exploration extravehicular activity (EVA), short and long term 

human performance at less than normoxic levels of partial pressure of oxygen (ppO2), and the 

fire hazard. Atmospheres in future vehicles for exploration missions will likely be less than 

standard atmospheric pressure, with an ambient inspired ppO2 less than a Earth sea level 

equivalent of 3.07 pound per square inch absolute (psia), or 159 mmHg. A hypobaric and 

reduced oxygen environment will be the integrated solution to safety, engineering, operational, 

and medical concerns that have as their goal routine and safe exploration of the Lunar or Martian 

surface. Indeed, the Constellation program goals and proposed mission architecture emphasize 

EVA with exploration of planetary surfaces as the central driving operation. However, there are 

important limits posed by human physiology, materials, and equipment factors (e.g. 

hematological and circulatory changes, flammability of materials, thermal performance of 

equipment within the vehicle and habitat) that must be considered when choosing these 

atmospheric parameters. 

Assumptions

Crewmembers will have been pre-screened for blunted respiratory response to 

hypoxia.13,16,20,25,34 Additionally, efficient and frequent EVA’s drive the exploration initiative; 

low pressure EVA suit operations are preferred to high pressure suit ops to improve capability; 

there is an operational value to a short in-suit prebreathe; vehicle atmosphere composition may 

not mitigate the risk of decompression sickness (DCS) during an EVA; dedicated hyperbaric 

treatment capability may not be present on the vehicle or habitat,9 and the vehicle and habitat 

materials have been certified for in-flight use in reduced oxygen partial pressure in microgravity. 

Finally, the alveolar partial pressure of oxygen (PAO2) is not directly measured in crewmembers. 

It is therefore assumed the ppO2 correlates with the PAO2 based on established respiratory gas 

pressures and gas exchange ratio tables in the diving and aviation literature.11

Unknowns

There are several unknowns that exist when considering exposing humans to a chronic mildly 

hypobaric/hypoxic environment. There may be a synergistic effect between mild hypoxia and 

chronic microgravity exposure that alters blood rheology.15,26,27,29 Will the proposed transition 

rate from launch to en route cabin atmosphere avoid acute physiologic effects? Likewise, will the 

transition to Mars atmospheric parameters be tolerated without adverse events? Immune function 

is known to be altered in microgravity and in non-spaceflight stressors such as hypoxia.31,32 What

are the implications for wound healing, infection resistance and reactivation of latent virus in the 

proposed hypobaric/mildly hypoxic environment? The proposed mission profile will demand 

frequent EVAs; how will the repetitive exposure to alterations in cabin and EVA suit 

environmental atmospheres (pressure and oxygen tension changes) impact cellular physiology? 
105

Lastly, inherent environmental atmospheric sensor errors exist. Determining the actual 

environmental atmosphere, as opposed to the indicated pressure and oxygen concentration, may 

be difficult. How will these unknowns influence the final choice of environmental atmospheric 

parameters?

Background

The EAWG initially offered nine environmental atmosphere design candidates for consideration 

for the crew exploration vehicle (CEV) and lunar habitat (Table 1), with design point 9 ranking 

as the most preferred.5 Operating at a cabin pressure of 8.3 psia and ppO2 of 3.0 psia (155 

mmHg), the ppO2 altitude equivalent was approximately 1,600 feet above sea level. However, 

this design point approaches the existing materials flammability qualification envelope.24 The 

authors of this flammability study stated that it is possible to operate safely in a cabin O2 

concentration of 36%, especially if helium is added as a diluent. Historically, much higher 

oxygen concentrations were used for the Apollo missions (cabin pressure 5.0 psia @ 100% O2) 

and Skylab (cabin pressure 5.0 psia @ 70% O2) (Figure 1). Interestingly, the available materials 

that were nonflammable in this environment were severely restricted, including crew clothing 

and packaging foams.4 Usable materials were generally metallic in nature. In the present day, 

modification of available off-the-shelf hardware for safe use in the 70-100% O2 environment is 

difficult and costly unless it is provided in a metallic case.24 

The materials used on earlier missions were flame resistant but heavy. However, a key driver in 

developing the exploration vehicles will be to reduce mass and stowage volume. In addition to 

materials flammability and mass concerns, there are physiologic consequences of long term 

exposure to elevated oxygen levels (greater than 400 mmHg or 7.72 mmHg), or hyperoxic 

conditions.11 Limits on exposure to hyperoxic environments is aimed at preventing oxidative 

stress on the organ systems of the crew. With higher levels of oxygen concentration, there is 

potential for progressive oxygen toxicity sustained mainly in the pulmonary system, but also 

possible in the nervous system, if under hyperbaric conditions (greater than 14 days at fraction of 

inspired oxygen concentrations (FiO2) above 178 mmHg (3.44 psia).23,33 The current 

Constellation environmental atmosphere design points do not approach these higher oxygen 

exposure limits.

Pt. 

No.

Cabin 

Pressure, kPa 

(psia)

Cabin 

ppO2, 

kPa(psia)

Cabin 

Oxygen, 

volume%

EVA Suit 

Pressure, 

kPa (psia)

P(DCS) General Characteristics

1 65 (9.4) 18.5(2.7) 28.5 29.6 (4.3) 0.089 Current space suit pressure. Cabin atmosphere 

well above hypoxic boundary, but less than 

normoxic. May allow use of materials certified 

to 30% oxygen with tight spacecraft operating 

control bands.

2 64 (9.3) 17.1(2.5) 26.8 29.6 (4.3) 0.094 Lowest cabin oxygen concentration with current 

space suit pressure; at hypoxic boundary.

3 73 (10.6) 16.9(2.5) 23.2 34.5 (5.0) 0.089 Moderate increase in space suit pressure with 

lower cabin oxygen concentration; at hypoxic 

boundary.

4 80 (11.6) 16.8(2.4) 21 41.4 (6.0) 0.038 Higher space suit pressure with Earth-normal 

cabin oxygen concentration; equivalent to 1829-

m (6000-ft) Earth atmosphere. Lower DCS 

risk. Ground testing may be facilitated.
106

Pt. 

No.

Cabin 

Pressure, kPa 

(psia)

Cabin 

ppO2, 

kPa(psia)

Cabin 

Oxygen, 

volume%

EVA Suit 

Pressure, 

kPa (psia)

P(DCS) General Characteristics

5 88.5 (12.8) 18.6(2.7) 21 41.4 (6.0) 0.089 Higher space suit pressure with Earth-normal 

cabin oxygen concentration; well above hypoxic 

boundary. Ground testing may be facilitated.

6 91.0 (13.2) 21.0(3.0) 23.1 41.4 (6.0) 0.089 Higher space suit pressure. Normoxic cabin 

atmosphere; slightly elevated oxygen 

concentration.

7 70.3 (10.2) 18.6(2.7) 26.5 29.6 (4.3) 0.178 Current space suit pressure. This point 

represents shuttle EVA preparation conditions.

8 65.5 (9.5) 19.7(2.9) 30 29.6 (4.3) 0.079 Current space suit pressure with assumed 

maximum oxygen concentration from the 

pressure study. O2 concentration control limits 

may be outside of existing materials 

flammability qualification envelope.

9 57.2 (8.3) 20.6(3.0) 36 29.6 (4.3) 0.011 Current space suit pressure. Highest oxygen 

concentration point (above that assumed in the 

pressure study) and lowest cabin pressure 

minimize prebreathe time. Outside of existing 

materials flammability qualification envelope.

Table 1. EAWG Design Candidates. (Campbell PD, Henninger D. EAWG Results to Date. 

EAWG Atmospheres Workshop, November 2005)

Figure 1. Historical Spacecraft and Current Shuttle/ISS Cabin and Suit Atmospheres

(Adapted from Henninger D, Campbell PD. Briefing to SLSD on EAWG Recommendations, 

December, 2005.NASA/JSC Bioastronautics Exploration Research and Technology Office)

In contrast to the normoxic or hyperoxic environmental atmosphere proposals, mild to 

moderately hypoxic and hypobaric scenarios, equivalent to 6,000 to 8,000 feet above sea level, 

Skylab Mercury/Gemini/Apollo

Early Apollo Design

Shuttle EVA Preparation

Shuttle/Mir/ISS

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Cabin Volume Percent Oxygen

Cabin Total Pressure, kPa

0

2

4

6

8

10

12

14

Cabin Total Pressure, psia

Normoxic Equivalent

Hypoxic Boundary

Historical Designs

Region of

Unimpaired

Performance Hypoxic

Region

Region of

Oxygen Toxicity
107

have been suggested for consideration.4,5,7 The advantage of this hypoxic/hypobaric environment 

has been identified as decreased DCS risk, reduced oxygen pre-breathe time and lower suit 

pressure for improved mobility and dexterity.5,9,14 More recently, the EAWG has proposed an

operational environmental altitude equivalent to 3,500 feet above sea level.6 The spaceflight and 

surface operations EVA suits recommendation is consistent with current Shuttle/ISS suit 

atmosphere of 4.3 psia @ 100% O2. Other recommendations for long-duration outpost habitats 

on planetary surfaces expose the crews to 8.0 psia @ 32% O2 and 7.6 psia, 32% O2 (altitude 

equivalents between 5,000 and 6,500 feet, respectively) (Table 2).

Vehicle

Nominal

Total Pressure

(psia +/- 0.2 

psia)4

Nominal 

Oxygen 

Partial 

Pressure 

(mmHg) 4

Nominal 

Oxygen 

Concentration 

(% +/- 2.0 

percentage 

points)4

Range of 

Total 

Pressure 

Capability

(psia) 1

Tissue Ratio (R) 

After 60 

Minutes 

Prebreathing3

CEV to ISS 14.7

10.25

160 ( 0 ft )

140 (3500 ft)

21

26.5 0-14.9

CEV In-Space Suit 4.3 222 100 4.0-4.6

1.55 from 10.2 

psia CEV to 4.3 

psia suit

Lunar and Mars CEV 14.7

10.2

160 ( 0 ft )

140 (3500 ft)

21

26.5 0-14.9

Lunar and Mars 

Landers

10.2

8.0

140 (3500 ft)

132 (5000 ft)

26.5

32 0-14.9

Lunar and Mars 

Surface Suits

4.3

6.0

222

310

100

100 3.5-8.0 2

1.13 from 8.0 

psia Landers to 

4.3 psia suit; 

1.07 from 7.6 

psia Surface 

Habitats to 4.3 

psia suit

Lunar and Mars 

Surface Habitats

8.0

7.6

132 (5000 ft)

126 (6500 ft)

32

32 0-14.9

Mars Transit 14.7

10.2

160 ( 0 ft )

140 (3500 ft) 

21

26.5 0-14.9

Table 2. Summary of Recommendations for Constellation Mission Systems 

Note 1: Range of total pressure capability covers Earth launch, Earth entry, and contingencies. 

Note 2: Surface suit 3.5 psia capability for suit emergency operations, 8.0 psia for DCS treatment.

Note 3: 60 minute in-suit prebreathe is defined as the time in the suit after purge and leak check until 

 absolute pressure on the body reaches 4.3 psia after a nominal depressurization.

Note 4: All nominal values are centers of control boxes assumed +/-0.2 psia total pressure, +/-2 percentage 

 points oxygen.

Note 5: 10.2 psia recommendation is based on Shuttle experience, for CEV contingency EVA preparation.
108

Methods

An extensive review of the current database of altitude physiology, alterations in physiology 

occurring with exposure to microgravity, characteristics of materials exposed to various 

environmental atmosphere conditions, and prior spaceflight mission technical reports was 

conducted. Appropriate charts, graphs and observations were extracted from studies and tests 

relevant to the operational environments anticipated for exploration missions, and many are 

included in this paper.

Results

The proposed spacecraft and habitat environmental atmosphere take into the account the 

following:

Crewmember Physiology

Under standard atmospheric conditions of 14.7 psia pressure and ppO2 3.0 psia (159 mmHg), 

approximately 98% of the hemoglobin will be saturated with oxygen during passage through 

pulmonary capillaries. This is reflected in the oxygen-hemoglobin dissociation curve when the 

PAO2 is 100 mmHg (Figure 2). The PAO2 takes into consideration the dilutional effects of water 

vapor and carbon dioxide at the level of the alveoli, hence the lower value. 

 Figure 2. The normal oxygen-hemoglobin dissociation curve. (Guyton AC, Hall JE. 

 Textbook of Medical Physiology, 10th Ed. WB Saunders: Philadelphia, 2000, p. 466-

 467.)

The oxyhemoglobin dissociation curve describes the relationship between the PAO2 (x-axis) and 

the oxygen saturation (SaO2) (y-axis). Hemoglobin’s affinity for oxygen increases as successive 
109

molecules of O2 bind, with a greater number of O2 binding as the PAO2 increases until the 

maximum amount that can be bound is reached. At a PAO2 above 60 mmHg, the standard 

dissociation curve is relatively flat. Conversely, below a PAO2 of 60mmHg, the hemoglobin’s 

affinity of O2 diminishes, allowing O2 to be unloaded to the peripheral tissues.17 This accounts 

for the sigmoidal shape of the curve. Technically, as the PAO2 falls below 100 mmHg (1.93 psia) 

the hemoglobin begins to desaturate, resulting in a relative “hypoxic” zone. Clinically, symptoms 

of hypoxia are not observed in healthy individuals until the PAO2 enters the steep portion of the 

curve, generally below 60 mmHg (1.16 psia) corresponding to a hemoglobin saturation of less 

than 90%. This corresponds to an equivalent altitude in non-acclimatized individuals of greater 

than 10,000 ft above sea level11,17 Interestingly, the amount of alveolar carbon dioxide, PACO2, 

and water vapor pressure change little at this altitude, adding to the dilutional effects. Reduced

atmospheric pressure with concomitant reduction in PAO2 below 60 mmHg has several acute 

effects (Figure 3), including decreased mental proficiency, visual acuity, muscle fatigue, nausea, 

headache, and impaired discrete motor movements. 11,15,17

 Figure 3. Relationship of Alveolar O2 and CO2 Composition to Performance. (NASA 

 STD-3000 58)

Prolonged human exposure to reduced oxygen tension usually results in the induction of adaptive 

changes. The most dramatic evidence of adaptation to hypoxic conditions is observed in the 

changes in blood viscosity. Prolonged non-lethal hypoxia results in increases in the total blood 

volume and oxygen carrying capacity of the blood. The hematocrit may rise from a normal value 
110

of 40 to 45 in an average male up to 60, over the period of 4 weeks, with a concomitant increase 

of 20 to 30 % in the blood volume.11,17,28 

Adaptation to microgravity has hematologic effects that interact negatively with the adaptations 

to hypoxia. Reductions in total blood volume (TBV) have been noted since the early Gemini and 

Apollo while working in a hyperoxic (ppO2 5.0 psia/259 mmHg) environment.19 These findings 

were observed in crewmembers during the Skylab missions (ppO2 2.54 psia/127 mmHg) as well, 

averaging reductions in TBV by 13% on average.

19 Skylab crewmembers also experienced a 7%-

14% decrease in their red blood cell volume (RBV). Although red blood cell count and TBV 

both decrease in chronic microgravity, the hematocrit (RBV/TBV) remains near preflight levels. 

The change in RBV is believed to be linked to changes in erythropoietin (EPO) secretion by the 

kidneys. EPO is typically secreted to stimulate RBC production in response to reduced arterial 

PO2.

1,4,11,17 However, with the loss of plasma volume (up to 27% less/kg body mass (ml/kg))29

encountered initially in microgravity, the kidneys may suppress EPO production as the blood 

viscosity increases. Also, the need for oxygen carrying capacity is reduced as lower extremity 

muscles become inactive and lose mass. Additionally, a small amount of circulating RBC mass is 

lost initially in microgravity due to neocytolysis.1 Neocytolysis refers to the observed decrease in 

RBC mass due to destruction of RBCs either newly released or scheduled to be released from the 

bone marrow. 

So there may be two conflicting processes going on simultaneously during exploration transit 

phases: a normoxic or mildly hypoxic atmosphere causing the synergistic effects of reduced 

ppO2 and subsequent increased oxygen carrying capacity in the blood by increasing RBC 

production vs. the commonly observed effects of plasma loss and RBV loss in response to 

microgravity. Which process predominates? In an attempt to understand the possible 

physiological interaction of hypoxia and microgravity, researchers at NASA Glenn Research

Center analyzed existing data assuming a long duration mission in mildly hypoxic conditions, 

equivalent to 5,000-8,000 ft.15 Observations made from the data (Table 2) reveal that relatively 

little change occurs in blood viscosity between 0-5,000 ft. However, the blood viscosity 

increases between 15-50% when crew members have been exposed to microgravity at altitudes 

of 8,000 ft. The authors concluded that the combination of hemoconcentration from PV loss and 

increased RBV result in increased blood viscosity. The clinical concern is that increased blood 

viscosity, in the setting of reduced circulation in the lower extremities and overall reduced 

venous system tone, may increase the risk of cardiovascular events, such as thrombi 

formation.15,27,29

Atm 

Equivalent 

Altitude 

(ft)

Change 

in PV 

(ml)

Total 

PV 

(ml)

Change 

in RBV 

(ml)

Total 

RBV 

(ml)

TBV 

(ml)

Hematocrit 

(%)

*Relative 

Viscosity

0-5000 0 2764 0 1824 4588 40 4

6000 -91 2673 170 1994 4667 43 4.6

7000 -183 2581 340 2164 4745 46 5.3

8000 -274 2490 509 2333 4823 48 6

*Relative Viscosity is defined as the blood’s viscosity relative to that of water at 37oC.
111

Table 2. Estimated changes in blood volume components and hematocrit based on the 

case of long duration exposure (chronic conditions) to both microgravity and mildly hypoxic 

atmospheric conditions. Note that an average body mass of 68.2 kg was assumed to conform to 

Skylab 4 astronaut conditions on return (R+0). (Courtesy of Griffin DW, Meyers JG. Biomedical 

Effects of Proposed CEV Atmospheres. NASA Glenn Research Center, 2005)

However, there have been no observed or reported episodes of thrombus or embolus in either 

animals or humans during short or long duration spaceflight, so this concern may be only 

theoretical. This observation is consistent with periodic health assessment information from ISS: 

after the initial period of hemoconcentration that occurs during early microgravity exposure 

(adapting to fluid shifts and subsequent diuresis), the hematocrit normalizes (ISS crew at ppO2 of 

145-170 mmHg) and stays in the low normal range throughout the remainder of the mission. One 

means to reduce the perceived risk associated with this uncertainty is to operate the vehicle 

atmosphere within our existing microgravity experience base during the microgravity transit 

phase, then transition toward the lower habitat pressures and ppO2 levels gradually. Gravity in 

lunar and Mars missions and its effects on pulmonary and cardiovascular physiology is another 

consideration, although it is not likely to have physiological significance. In summary, the lunar 

missions will serve to build our experience base prior to conducting the longer duration Mars 

missions. 

Launch to En Route Cabin Pressure and Oxygen Concentration Changes

The proposed atmospheric transition after launch is displayed graphically in figure 4. The 

transition from sea level atmospheric pressure and oxygen concentration (14.7 psia and 21% O2) 

to an en route cabin pressure and oxygen concentration range of 10.2 psia and 26.5% O2 (3,500 

ft) to 8.0 psia and 32% O2 (5,000 ft) would likely use procedures similar to those currently 

practiced on shuttle and ISS missions. The launch cabin pressure would be staged down initially 

to ~10.2 psia and the O2 enriched to ~26.5% consistent with existing shuttle and station flight 

rules.23 If it was desired to reduce the ppO2 to lower levels to provide for a slow acclimatization 

to the Lander and Habitat ppO2 (~ 2.56 psi), then it should be relatively straight forward to 

breath down the oxygen gradually over time consistent with some TBD acclimatization protocol. 

During the lander/CEV docked operations the cabin pressure and FiO2 would be consistent with 

the CEV limitations on O2 concentration (< 30%). Once the crew had transferred into the Lander 

and undocked from the CEV, the cabin pressure could be further reduced and the O2

concentration would be elevated to 32%. There is no physiological time constraints to when this 

depress could occur. Consequently, there is little risk of DCS associated with this pressure 

transition as it is below the threshold for tissue supersaturation.
112

Figure 

4. Concept for Lunar-Mars CEV Atmosphere Transition on Earth Ascent. (Adapted from 

Henninger D, Campbell PD. Briefing to SLSD on EAWG Recommendations, January, 

2006.NASA/JSC Bioastronautics Exploration Research and Technology Office)

This approach is consistent with the observation that the hypoxic ventilatory response (HVR), 

which is the increase in ventilation brought about by hypoxia, takes place over a period of days.34 

This is shown by an increase in ventilation and a decrease in PACO2 (Figure 5). Why is this 

important to consider with respect to determining the transition rate from sea level to mildly 

hypoxic/hypobaric cabin atmosphere? The terrestrial analog to the clinical changes that occur in 

response to acute exposure to an increase in cabin altitude is acute mountain sickness (AMS), a 

condition affecting otherwise fit people on ascending rapidly to altitude. Symptoms of AMS 

include breathlessness, headache, nausea, vomiting, disturbed sleep and poor physical 

performance.26,34 It is important to point out that this increase in ventilation varies greatly among 

individuals, and does not usually begin until the inspired PO2 is reduced to approximately 90 

mmHg (1.71 psia), an altitude equivalent of 10,163 ft above sea level, but can occur at lower 

levels/altitude equivalents in susceptible individuals. This limit is also imposed on pilots 

operating in commercial airlines and military flight operations.7,12 In summary, ranges of time 

must be considered in altitude transition effects, and both acute and chronic acclimatization. The 

Figure below shows the difficulty of giving a specific answer to the question of “how long does 

it take to acclimate to a reduced-oxygen condition?”

Skylab Mercury/Gemini/Apollo

Early Apollo Design

Shuttle EVA Preparation

Shuttle/Mir/ISS

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Cabin Volume Percent Oxygen

Cabin Total Pressure, kPa

0

2

4

6

8

10

12

14

Cabin Total Pressure, psia

Normoxic Equivalent

Hypoxic Boundary

Historical Designs

8

Pre- launch Transition

14.7 psi, 21% O2 

Ascent 

Transition

10.2 psi,

26.5% O2 Lunar Sortie CEV
113

 

Figure 5. Time courses of a number of acclimatization and adaptive changes plotted on a 

long time-scale, the curve of each response denoting the rate of change, fast at first then 

tailing off. Included are: heart rate, hyper and hypoventilation, the CO2 ventilatory response 

(CO2VR), hemoglobin concentration ([Hb]), changes in capillary density (Cap. Dens), 

hypoxic ventilatory response (HVR) and the pulmonary hypoxic pressor response (PHPR). 

(Adapted from Ward MP, Milledge JS, West JB. High Altitude Medicine and Physiology. 

University of Pennsylvania Press, Philadelphia, 1989, pgs. 67-96)

Risk of Decompression Sickness

Decompression sickness is a potentially debilitating and life-threatening condition that occurs 

when inert gas, typically N2, evolves out of the blood and body tissues. The evolved gas can 

compress nocioceptive tissues causing pain (“the bends”), or interrupt venous or arterial blood 

flow, or other vascular and neurological disorders.3,11 An individual is at risk of developing DCS 

whenever exposed to an ambient pressure lower than the tissue nitrogen tension. In order to 

estimate the severity of DCS, the tissue ratio, or R-value, R, was developed. The R-value is 

defined as the ratio of the tissue nitrogen tension in 360 minute half-time tissue before 

depressurization to the ambient pressure after depressurization:

 R = PN2-Tissue

 PSuit
114

In general, the higher the R-value above 1.0, the greater the likelihood of DCS.4,21 However, a 

number of other variables influence the likelihood of developing DCS, including the time the 

individual is exposed to reduced pressure, the degree of physical activity, the ambulation contact 

forces at reduced pressure, the pressure profile, repeated exposure to hypobaric pressures etc.9 To 

reduce the risk of DCS when transitioning from the cabin atmosphere to the EVA suit 

environment, crewmembers are exposed to 100% O2 for varying periods of time in an attempt to 

“washout” N2 from the body tissue. These procedures reduce but do not entirely eliminate N2

from the tissues, but do reduce the EVA crewmembers’ R-value at the time of decompression to 

the lower EVA suit pressure. The current shuttle and ISS EVA suit operates at a suit pressure of 

4.3 psia for maximum mobility and reduction of crewmember fatigue. It is assumed that 

planetary EVA suits will operate at pressures near 4.3 psia.

For the proposed habitat cabin atmospheric pressure of 8.0 psia (414.5 mmHg) at 32% ppO2, the

ppN2 is 5.43 psia (281.9 mmHg). The R-value without additional in-suit prebreathe at a suit 

pressure of 4.3 psia is:

 R = 5.43 = 1.23

4.3

The R-value, after a proposed maximum acceptable 60 minute in-suit 100% oxygen prebreathe, 

is calculated as:

 R = 4.86 = 1.13

4.3

Reducing In-suit Prebreathe Time by Living in a Hypobaric and Mild Hypoxic Environment

At this time an acceptable R-value for exploration EVA’s has not been determined. That 

determination will be made as part of an integrated approach that would first define the 

acceptable decompression risk for different phases of the mission (The Exploration DCS Risk 

Definition and Contingency Plan). The prebreathe verification tests would be conducted using an 

EVA simulation that is appropriate with respect to metabolic rates, time, and ambulation contact 

forces. In general, for a given suit pressure, the amount of prebreathe time required for a given 

R-value will be reduced by reducing the nitrogen partial pressure in the habitat or lander. 

Prebreathe time could be completely eliminated if the habitat atmosphere was 100% O2. 

However, a balance must be achieved between the increased risk of fire at high O2 concentration 

and the decreased risk of DCS as N2 pressure in the habitat is reduced. The concentration of O2

and therefore risk of fire for a given total pressure can be slightly reduced if mild hypoxia is 

accepted. The degree of hypoxia anticipated is equivalent to living in Denver Colorado, or 

Albuquerque New Mexico, at about 5,280-6000 feet altitude.

Even small reductions in the nitrogen partial pressure of the habitat can result in significant 

reduction in prebreathe time. To illustrate this point we compare the prebreathe times required to 

achieve different R-values from different CEV and habitat atmosphere options: 
115

I. 10.2 psia @ 26.5% O2 with a 60 minute in-suit prebreathe to achieve an R-value of 1.55 for 

contingency EVAs from the Crew Exploration Vehicle (CEV). This is a hypobaric and mildly 

reduced-oxygen environment, equivalent to breathing air at 3,500 feet altitude. To achieve the 

same R-value as the lower habitat pressures in option II and III would require 224 and 252 

minutes, respectively. 

II. 8.0 psia @ 32.0% O2 with a 60 minute in-suit prebreathe to achieve an R-value of 1.13 for 

lunar EVAs. This is a hypobaric and mildly reduced-oxygen environment, equivalent to 

breathing air at 5,000 feet altitude. 

III. 7.6 psia @ 32.0% O2 with a 60 minute in-suit prebreathe to achieve an R-value of 1.07 for 

Mars EVAs. This is a hypobaric and mildly reduced-oxygen environment, equivalent to 

breathing air at 6,500 feet altitude. 

Whereas the specific acceptable R-value for exploration EVAs has yet to be determined, it is 

clear that reduction in habitat nitrogen partial pressure will result in a significant reduction in 

prebreathe time.

Environmental Atmosphere Sensor Error

The environmental control and life support system (ECLSS) group has determined that 

environmental sensor error exists in measuring both total cabin pressure and oxygen 

concentration. The ECLSS community estimates that a +/- 0.2 psia error exists in measuring the 

total atmospheric pressure and +/- 2% error in measuring O2 concentration (Figure 6). However, 

we would anticipate improved sensor capability in future vehicles.

Oxygen Volume Concentration (%)

Total Pressure (psia)

Curve of Constant

Nominal Oxygen

Partial Pressure

(mmHg)

Notional

Control

Box

Nominal

Oxygen

Concentration

Nominal

Total Pressure

+/- 2 %

+/- 0.2 psia

 Figure 6. Notional Control Box for Vehicle Cabin Atmospheres. (Adapted from

 Henninger D, Campbell PD. Briefing to SLSD on EAWG Recommendations,

 December, 2005.NASA/JSC Bioastronautics Exploration Research and 

Technology Office)
116

Discussion

Humans adapt to hypoxic exposure over a period of days to weeks (45-60 days) by increasing 

minute ventilation, splenic contraction, redistribution and increase in circulating blood volume, 

augmenting the oxygen carrying capacity of the blood.26,27,34 The proposed mission transitions 

the vehicle atmospheric profile from a launch atmosphere of 14.7 psia and 21% O2 to a CEV 

atmosphere of 10.2 @ 26.5% O2 over a period of a couple days. The en route to surface cabin 

pressure would be further reduced to 8.0psia @ 32% O2. The corresponding launch PAO2 (103 

mmHg or 1.98 psia) to CEV (86 mmHg or 1.65 psia) and surface PAO2 (81 mmHg or 1.56 psia) 

represents an altitude equivalent of 3,500 ft to 5,000 ft., well within the acceptable physiological 

range. This statement is supported by work done with United States Air Force (USAF) pilots 

studied in mildly hypoxic environments (5,000 ft to 8,000 ft above sea level), with some 

limitations.34 The authors measured cognitive performance (continuous performance tasks, 

grammatical reasoning, math processing, and spatial orientation tasks. Although the pilots 

reported subjective symptoms associated with acute hypoxic exposure there were no statistically 

significant changes in their cognitive performance below 8,000 ft. However, the hypoxia 

exposure was of short duration (hours) and limited to few objective physiological variables 

(SaO2 and heart rate). 

Figure 7 graphically represents the Constellation mission environmental atmospheres that have 

been proposed in this whitepaper contrasted with historic spacecraft atmospheres. 

Figure 7. Historical Spacecraft and Recommended Constellation Atmospheres.

(Adapted from Henninger D, Campbell PD. Briefing to SLSD on EAWG Recommendations, 

December, 2005.NASA/JSC Bioastronautics Exploration Research and Technology Office)

Skylab Mercury/Gemini/Apollo

Early Apollo Design

Shuttle EVA Preparation

Shuttle/Mir/ISS

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Cabin Volume Percent Oxygen

Cabin Total Pressure, kPa

0

2

4

6

8

10

12

14

Cabin Total Pressure, psia

Normoxic Equivalent

Hypoxic Boundary

Historical Designs

 CEV to ISS

 CEV to ISS

 (EVA Prep) Lunar Sorties CEV

Lunar/Mars Long Duration
117

One concern raised by a staged depressurization is that it will take approximately 36 hours to 

equilibrate at the LSAM habitat pressures, so the first EVAs from the LSAM (~12-24 hours after 

establishing the LSAM atmosphere) would require a slightly longer prebreathe as the crew will 

not be fully equilibrated at the reduced ppN2. NASA’s current prebreathe protocol testing would 

need to account for the details of this procedure. Interestingly, the historic atmospheric standard, 

contained in the NASA Human Integration Standards 3000 (HSIS) document for cabin 

atmospheric pressure is the range of 190 to 760 mmHg (3.6-14.7 psia). At pressures below 190 

mmHg crews are required to don pressure suits and to breathe 100% oxygen. Crewmembers are 

not directed to don an O2 mask in the current standard, as long as the vehicle cabin ppO2

provides a PAO2 of at least 103mmHg (1.98 psia) which is sea level equivalent.22 The current ISS 

flight rule defines the lower limit requiring supplemental oxygen as an inspired ppO2 of 111 

mmHg23, [which results in a PAO2 of 62 mmHg (at e.g. 6.35 psia (329 mmHg)/34% O2

ambient)], clearly below the current HSIS standard. The 10,000 ft. altitude equivalent (111 

mmHg ppO2) also represents the maximal altitude that DOD and commercial FAA pilots may fly 

without supplemental oxygen (accepted masking level).7,12

Although technically the crew member is initially mildly hypoxic, the SaO2 is maintained over 

94% at rest. The worst case scenario taking into consideration the environmental atmosphere 

sensor error of -0.2 psia and -2% O2 concentration for the CEV and lunar habitat exposes the 

crewmember to a PAO2 of 73 mmHg (1.40 psia) and 65.3 mmHg (1.26 psia), respectively. As 

was stated earlier, desaturation of Hb occurs at a PAO2 <60 mmHg. Even in the worst case 

scenario for sensor error taken into consideration, the crew will not approach this level of 

hypoxia. After acclimatization, less impairment at rest would be expected.

Little consideration has been given to the effects of exercise in hypoxic environments to this 

point where high metabolic workloads are expected. While resting pulmonary blood flow may 

allow adequate time for equilibration of oxygenation of Hb during pulmonary transit time, 

exercise causes an attendant increase in cardiac output. In the hypoxic environment, this time is 

shortened, resulting in precipitous falls in SaO2 secondary to the reduced capillary transit time. 

Although this is a valid concern, oxygen delivery to the tissues should not be compromised as 

exercise in the habitat will not exceed 30 minutes. However, studies will need to be performed to 

account for these hypoxic effects when developing exercise countermeasures for protecting 

aerobic fitness and bone loss mitigation. In regards to the EVA environment, it is important to 

point out that EVA with suit pressures above 3.7 psia with 100% O2 are slightly hyperoxic. 

Therefore, performance limitations during EVA attributable to reduced O2 availability would be 

not expected.

A similar concern for operating in the hypoxic range is raised in regards to reducing the margin 

of oxygen delivery to the crew during a contingency, such as fire or exposure to toxic chemicals 

(such as propellants and combustion products). Material flammability testing will need to 

confirm ignition characteristics in this environment. Also, regardless of the environmental 

oxygen concentration, a crew member will have supplemental oxygen supplied via the portable 

breathing apparatus (PBA) in the event of a toxic environmental exposure.

In reviewing the various proposed design points generated by the EAWG and others in the space 

engineering community it is tempting to push the physiological envelope to accept lower cabin 

pressures and ppO2 levels. Indeed, some authors suggest that crewmembers exposed to 
118

moderately hypoxic conditions during long-duration missions should exhibit greater resistance to 

severe, acute hypoxia and DCS after a period of acclimatization.30 However, it is important to 

note that although the 3,500 ft - 5,000 ft altitude equivalent is well within the operating envelope 

of physiological function, further compromise of the proposed cabin and suit atmosphere 

parameters may exceed the adaptive responses the body to hypoxia and microgravity (Figure 

5).9,11,15 

Figure 5. Total pressure versus percentage of oxygen and defined regions of hypoxia,

normoxia, and hyperoxia. (NASA-STD-3000 56)

It is for this reason that we recommend conducting the microgravity portions of these short 

duration CEV to ISS and lunar transit missions within our existing space flight experience base. 

This would not result in any compromise to the capability of the CEV and LSAM to support safe 

and efficient EVA, and eliminates the need to conduct expensive and likely inconclusive ground 

studies on the combination of mild hypoxia and micro-gravity. The current NASA prebreathe 

protocols for the Space Shuttle and ISS are based on a final R-value of 1.65-1.68 after the O2

prebreathe period.22 Current model predictions suggest that having an R-value between 1.22 and 

1.40 will significantly reduce the risk of DCS in crewmembers.4,8

The proposed R-value of 1.23 without an additional O2 prebreathe period fits well within these 

recommendations. Reducing the partial pressure of N2 in the habitat through a combination of 

reduced total pressure and increased O2 concentration without incurring a significant hypoxic or 

fire risk results is a significant reduction in final in-suit prebreathe time to achieve a satisfactory 

R-value. The benefit of increased joint mobility while operating in the EVA suit will increase 

crewmember productivity and decrease fatigue. However, it is important to recognize that 

appropriate human ground trials will be required to validate any prebreathe protocols. 

Additionally, the long term microgravity exposures (Lunar surface operations, Mars transit, and

Mars habitat) will require that data be collected to develop an operationally-focused research 

program consisting of: 1) validation of the proposed prebreathe protocols for exploration EVA 
119

operations, 2) extending the testing and certification of spaceflight materials to higher O2

concentrations that envelope these habitat atmosphere recommendations, 3) validate food 

preparation strategies at the proposed pressures, and 4) reduce the uncertainties of the proposed 

atmospheric effects on blood rheology, immune function, and cellular physiology in response to 

repetitive atmospheric changes associated with EVA operations.

Conclusion

Recommendations

We have provided an evidence-based approach for selecting the optimal total pressure-oxygen 

concentration levels for future spacecraft and habitats. Careful consideration of the current 

evidence reveal crewmembers will have minimal detrimental physiological effects of mildly 

reduced oxygen partial pressure equivalent to 3,500 ft to 5,000 feet above Earth sea level.

Mission efficiency can be significantly improved under these atmospheric parameters by 

reducing or eliminating the dedicated oxygen prebreathe by the EVA crew. Depending on the 

hypobaric and mild hypoxic conditions, there is a two to eight-fold reduction in the in-suit 

prebreathe time to achieve the stated R-values. 

These recommendations are consistent with existing NASA Shuttle and ISS standards and flight 

rules for breathable atmosphere and oxygen concentration, so that the CEV and habitat can be 

designed with no new materials limitations. The short term CEV to ISS and lunar transit 

missions will stay within the known operational experience base and should not require any new 

Earth-based physiological testing for the combined effects of microgravity and hypoxia. 

However, the proposed lander and habitat recommendations will require that the current NASA 

Standard 3000 (HSIS) total pressure and oxygen concentration limits be amended to 

accommodate the new environmental atmosphere ranges. These recommendations will also 

require materials ignition and flammability testing and certification to 34% oxygen 

concentration. Data collected during lunar missions (with increasing duration) will be used to 

formulate the plan for Mars exploration, with the assumption that the physiological interactions

of reduced gravity and lower oxygen tension will be diminished as the gravity level increases on 

the Martian surface relative to the Moon. Implementing these recommendations, in addition to 

bringing some new challenges, will provide significant improvements in operational productivity 

for planetary surface exploration.

Acknowledgements

The authors would like to thank David Gillis, MD, PhD at UTMB/Wyle laboratories for 

reviewing the physiological considerations of this whitepaper. Dr. Gillis provided valuable input 

from a clinical perspective based on his years of experience as a Naval flight surgeon and 

practicing anesthesiologist. 
120

References

1. Alfrey CP, Udden MM, Leach-Huntoon C, Driscoll T, Pickett MH. Control of red blood 

cell mass in spaceflight. J Appl Physiol 1996; 81: 98-104.

2. Balldin U, et al. Effect of Low Grade Hypoxia at Night in Special Operations Aircraft 

Operations. AFRL-HE-BR-TR-2005-0163. November, 2005.

3. Bove AA. Diving Medicine, 3rd Ed. WB Saunders Co. Philadelphia, PA. 1997, pgs. 159-

171.

4. Campbell PD, Harris JD. Internal Atmospheric Pressure and Composition for Planet 

Surface Habitats and Extravehicular Mobility Units. March, 1991. LESC-29278.

5. Campbell PD, Henninger D. EAWG Results to Date. EAWG Atmospheres Workshop, 

November 2005.

6. Campbell PD, Henninger D. Recommendations for Exploration Spacecraft Internal 

Atmospheres: The Final Report of the NASA Exploration Atmospheres Working Group. 

JSC-63309. January, 2006.

7. Code of Federal Regulations, Title 49, Part 121.331-.337. Washington, DC: U.S. 

Government Printing Office, 2003.

8. Conkin J. The Mars Project: Avoiding Decompression Sickness on a Distant Planet. 

NASA/TM-2000-210188.

9. Conkin J. EAWG Subgroup Overview: Physiological and Medical. Exploration 

Atmosphere Workshop, November, 2005.

10. Conkin J. et. al. A probabilistic model of hypobaric decompression sickness based on 66 

chamber tests. Aviat Space Environ Med, 1996:67, 176-183.

11. Dehart RL, Davis JR. Fundamentals of Aerospace Medicine 3rd Ed. Lippincott Williams 

& Wilkins, Baltimore, MD. 2002, p. 19-38.

12. Fisher, PW. USAF School of Aerospace Medicine Flight Surgeon's Guide, Chapter 2-

"High Altitude Respiratory Physiology"

13. Georgopoulos D, Walker S, Anthonisen NR. Increased chemoreceptor output and 

ventilatory response to sustained hypoxia. J Appl Physiol. 1989 Sep;67(3):1157-63.

14. Gernhardt M, Conkin J. The Effect of Long-Term Partial Pressure Oxygen Exposure. 

NASA Johnson Space Center Environmental Physiology Laboratory, 2005.

15. Griffin DW, Meyers JG. Biomedical Effects of Proposed CEV Atmospheres. NASA 

Glenn Research Center, November, 2005
121

16. Guillemin K, Krasnow MA. The hypoxic response: huffing and HIFing. Cell. 1997 Apr 

4;89(1):9-12.

17. Guyton AC, Hall JE. Textbook of Medical Physiology, 10th Ed. WB Saunders: 

Philadelphia, 2000, p. 466-467.

18. Henninger D, Campbell PD. Briefing to SLSD on EAWG Recommendations, December, 

2005. NASA/JSC Bioastronautics Exploration Research and Technology Office.

19. Johnson PC, Driscoll TB, LeBlanc AD. Biomedical Results from Skylab. NASA-SP-377, 

1998.

20. Kline DD, et. al. Blunted respiratory responses to hypoxia in mutant mice deficient in 

nitric oxide synthase-3. J Appl Physiol 2000; 88: 1496-1508.

21. Lange KE, et. al. Bounding the Spacecraft Atmosphere Design Space for Future 

Exploration Missions. NASA/CR-2005-213689

22. NASA-STD-3000 Vol. 1, Sect. 5.1

23. NASA-Johnson Space Center Flight Rules, Volume B: B13-54.

24. Pedley MD, Ruff GA. Materials Flammability Control for Constellation Program: 

Impacts for Enriched Oxygen Environments. NASA Glenn Research Center, August, 

2005. 

25. Prisk GK, Elliot AR, West JB. Sustained microgravity reduces the human ventilatory 

response to hypoxia but not to hypercapnia. J Appl Physiol 2000; 88: 1421-1430.

26. Roach RC, Loeppky JA, Icenogle MV. Acute mountain sickness: increased severity 

during simulated altitude compared with normobaric hypoxia. J Appl Physiol. 

1996;81(5): 1908-1910.

27. Sagesaka T. Influence of red blood cell concentration on the initiation time of blood 

coagulation: risk of thrombus formation by hemoconcentration. Clin Hemorheol 

Microcirc., 2004; 31(4):243-9

28. Sanchez C, Merino C, Figallo M. Simultaneous measurement of plasma volume and cell 

mass in polycythemia of high altitude. J Appl Physiol, 1970; 28:775-778.

29. Schobersberger W, Hoffman G, Gunga HC. [Interaction of hypoxia and hemostasis: 

hypoxia as a prothrombotic factor at high altitude?] Wien Med Wochenschr. 2005 

Apr;155(7-8):157-62 [Article in German]

30. Shvartz E. Advantages of a low-oxygen environment in space cabins. Aviat Space 

Environ Med, 1990; 61(3):272-6.

31. Sonnefeld G, Shearer WT. Immune function during spaceflight 2002; 18(10):899-903.
122

32. Taylor GR. Overview of spaceflight immunology. J Leuk Biology, 1993; 54(3):179-188.

33. United States Naval Flight Surgeon's Manual: Third Edition 1991: Chapter 1: Physiology 

of Flight Oxygen Toxicity

34. Ward MP, Milledge JS, West JB. High Altitude Medicine and Physiology. University of 

Pennsylvania Press, Philadelphia, 1989, pgs. 67-96.
123

Appendix F:

Exploration Atmospheres Workshop

November 1-3, 2005

Houston

November 1-3, 2005
124

1. Executive Summary 

The Exploration Atmospheres Workshop was conducted to validate the Exploration 

Atmospheres Working Group’s (EAWG) recommendations for human-occupied atmospheres 

and for additional research and technology development to enable NASA to specify exploration 

atmosphere requirements. Both NASA-affiliated and select external experts were in attendance 

to review the EAWG plans, process and analysis methods and results. A second goal for these 

participants was to provide recommendations concerning these review areas with respect to 

atmosphere design.

To structure the workshop discussion, eight questions were derived prior to the workshop that 

guided reviewer comments such that these objectives could be achieved within the timeframe of 

the workshop. Accordingly, alternate plenary and breakout sessions were arranged to discuss 

and evaluate the EAWG background material, technical white papers on major risks and analyses 

performed by the EAWG with respect to candidate atmosphere design points. The results of 

these sessions led to guided plenary discussions and two series of breakout reports, both of which 

are furnished in this report.

A key result of this process was the segmentation of suggested design atmosphere points by 

vehicle and mission class, including the Crew Exploration Vehicle (CEV), the lunar and Martian 

landers and habitats, and the Mars transit vehicle. Revised recommendations for oxygen partial 

pressure and total atmosphere pressure were developed during the workshop and proposed by the 

attending EAWG members for consideration as the formal recommendation of the working 

group. 

A driving principle was developed by the combined group (EAWG members and reviewers) for 

selection of preferred design points for each mission. This principle emphasized the importance 

of facilitating surface extravehicular activities (EVA) by mitigating decompression sickness risk 

and by shortening the necessary pre-breathe protocol time, in order to enhance mission 

productivity (mitigating mission operations risk). The participants arrived at atmospheric design 

points by revisiting the permissible lower limit on the operating space for partial pressure of 

oxygen (mitigating hypoxia risk), and the permissible upper limit on fractional concentration of 

oxygen (mitigating flammability risk) via the utilization of materials qualified at higher oxygen 

fraction levels than currently used.

A significant topic of discussion was over the latter issue, since it was felt that setting an 

operating point above 30% oxygen would be possible, but might incur costs in time, money, and 

available materials & equipment that would be difficult to support in the near term. This was not 

resolved by the workshop for all missions, but a solution was proposed for the nearest term 

vehicle, the CEV, by determining that Shuttle standards for atmospheres and materials were 

appropriate for its role as a transport vehicle with only contingency EVA requirements.

EAWG process issues were also addressed by the reviewers. These included the utility of the 

Fault Tree Analysis (FTA) and Analytical Hierarchy Process (AHP) methodology in helping to 

make atmosphere design point selections. The response from most reviewers was that the FTA 

process was not an appropriate fit to the atmosphere selection issue, given the current uncertain 
125

state of knowledge of many of the risk factors. The AHP process had a mix of evaluations on its 

value in selecting points. Generally, the reviewers expressed that their time was not best spent 

evaluating the processes, but by applying their collective knowledge to discuss with the EAWG 

members the underlying risk issues and atmosphere design point selection. Other reviewers 

questioned the selected risk factors themselves, the weighting of these factors and the selection 

of atmosphere design points (e.g. no points selected by the EAWG resided in the trade space 

region below the hypoxic boundary). Overall, the reviewers suggested that AHP and FTA 

analyses were appropriate vehicles to begin conceptualizing the various risks and factors 

involved in determining a design point, however, they may have been introduced too early in the 

process. The AHP analysis was better received than the FTA and was advocated for use again 

by the EAWG as a post-workshop activity with respect to the new design points.

Finally, the reviewers discussed the best way to present the results of the design point 

recommendations to Constellation management. The majority suggested that specific points be 

proposed, inside a range or “band” of acceptable values with a discussion of the risks over that 

range, to present a rational trade space to program management. The importance of the EAWG 

reaching a consensus on its recommendations was emphasized in order that they may be 

presented with the strongest possible technical and engineering support.

2. Workshop Objectives 

As NASA plans human exploration missions and develops new spacecraft, space suits, and 

habitats, it must specify environmental requirements for the human-occupied systems. One such 

requirement is the definition of the human habitation atmosphere - pressure and constituent 

gases. Human-occupied environments must provide a safe atmosphere. Total atmospheric 

pressure and constituent gas composition must be consistent with human physiology, the space 

environment, the spacecraft or habitat design, and with space exploration mission objectives. 

Given new mission requirements, it is necessary to assess the optimum design parameters for the 

human habitable atmospheres (spacecraft cabins, planetary surface habitats, airlocks, space suits, 

and pressurized rovers).

This workshop was formed to validate the EAWG's recommendations for human-occupied 

atmospheres and the additional research and technology development to enable NASA to specify 

exploration atmosphere requirements. Both NASA-affiliated and select external experts were 

invited to participate in the workshop with the goal of addressing the following objectives:

• Validate the EAWG plan, process and analysis methods, and results

• Bring additional ideas into the process - including constraints, potential technical 

solutions, impacts, process, etc.

• Identify areas where additional research is needed

These objectives were accomplished through the review of white papers, mission assumptions, 

and research and technology needs prepared by the EAWG. In addition, the participants 
126

reviewed the results of the EAWG’s ranking of candidate atmosphere designs and a specific 

reference design point.

3. Workshop Development 

3.1 Planning Team

The Exploration Atmospheres Workshop planning team was comprised of both NASA civil 

service and contractor staff. Complementary areas of expertise and capabilities were represented 

in the team’s membership as indicated in the profiles below. Planning team members 

representing the EAWG included -

• Donald Henninger, Ph.D. - Chair, EAWG, Chief Scientist, Crew and Thermal Systems 

Division, NASA Johnson Space Center

• Paul Campbell - Deputy Chair, EAWG, Office of Bioastronautics Exploration Research 

& Technology, NASA Johnson Space Center. 

The Technology Integration Agent (TIA) provided project management, technical, and 

administrative support for the workshop. TIA is a technology solutions provider charged to 

identify, assess, and recommend technologies ranging from low-TRL R&T projects to 

commercially available products based on specific capability requirements, application, and 

targeted timeframe for use. TIA supports the analysis of requirements and technology 

assessments for which it utilizes multiple mechanisms (workshops, focus groups, seminars) 

aimed at infusing internal and external (industry, academia, other government) technical 

expertise. The following TIA staff members supported the EAW activities as members of the 

planning team -

• Julianna Fishman is a project manager with Lockheed Martin Space Operations at the 

NASA ARC. 

• Michael Krihak, Ph.D. is currently a technical advisor to NASA ARC and provides 

expertise in the areas of materials science, biotechnology, MEMS, optics and analytical 

sciences. 

• Katy Souza is a planning operations representative with Lockheed Martin Space 

Operations at the NASA ARC.

• Fritz Stawitcke, Ph.D. is currently a technical advisor to NASA ARC and provides 

expertise in medical measurement and life support technologies.

• Dionna Suess is an administrative assistant with Unconventional Concepts Inc. at the 

NASA ARC.

In addition to the planning team, other individuals played important roles as plenary session speakers, breakout session technical leads and 

facilitators, recorders, note takers, and support staff who oversaw workshop operations. Workshop technical and support staff are listed below in 

Table 3.1-1. Participating members of the EAWG are listed in Table 3.1-2.
127

Table 3.1-1 Workshop Technical and Support Staff

Name Affiliation Role

Anderson, Molly Code EC2, NASA JSC AHP Analyst

Collins, Cynthia Lockheed Martin Space Operations Recorder/Note taker

Cook, Dan Bastion Technologies Carpool Coordinator

Davis, Jeffrey Code SA, NASA JSC Plenary Speaker

Deckert, George SAIC FTA Analyst

Duffield, Bruce ESCG AHP Analyst

Hoffpauir, Kellie USRA, Division of Space Life 

Sciences

Workshop Operations

Hopmeier, Michael Unconventional Concepts, Inc. Facilitator

Kovo, Yael Lockheed Martin Space Operations Website Design & 

Development

Miller, Gina Lockheed Martin Space Operations Recorder

Miller, Suzanne Bastion Technologies Recorder/Note taker

Okimura, Takeshi Lockheed Martin Space Operations Website Design & 

Development

Pacetti, Gail USRA, Division of Space Life 

Sciences

Workshop Operations

Schanafelt, Carol Wyle Laboratories Note Taker

Shaw, Kim Bastion Technologies Recorder

Snyder, Tim Lockheed Martin Space Operations Facilitator

Stephenson, Lisa Wyle Laboratories Recorder

Thigpen, Eric SAIC FTA Analyst

Thorton, Clint Code NX22/GHG, NASA JSC FTA Analyst

Watkins, Bobby Code AG, NASA JSC Plenary Speaker

Watts, James ESCG AHP Analyst

Wren, Kiley Lockheed Martin Space Operations Facilitator
128

Table 3.1-2 Participating EAWG Members 

Name Affiliation Role

Barghouty, Nasser Code XD41, NASA MSFC Plenary Speaker/Technical Lead

Buderer, Melvin Code SK, NASA JSC Participant

Campbell, Paul Lockheed Martin Space 

Operations

EAWG Deputy Chair/ Plenary 

Speaker

Chambliss, Joe Code EC1, NASA JSC Participant

Conkin, Johnny NSBRI Plenary Speaker/Technical Lead

Ewert, Michael Code EC2, NASA JSC Participant

Fitzpatrick, Daniel Code SD37, NASA JSC Participant

Galbreath, Gregory Code ES2, NASA JSC Participant

Gernhardt, Michael Code CB, NASA JSC Plenary Speaker/Technical Lead

Henninger, Donald Code EC1, NASA JSC EAWG Chair/ Plenary Speaker

Jones, Jeff Code SD2, NASA JSC Participant

Khan-Mayberry, 

Noreen

Code SF23, NASA JSC Participant

Lange, Kevin Jacobs Sverdrup Technology Plenary Speaker

Lawson, Michael Code EC1, NASA JSC Participant

Lin, Chin Code EC2, NASA JSC Participant

Morin, Lee Code CB, NASA JSC Participant

Patrick, Jeff Code XA, NASA JSC Plenary Speaker/Technical Lead

Pedley, Michael Code ES4, NASA JSC Plenary Speaker/Technical Lead

Powell, Michael Code SK, NASA JSC Plenary Speaker/Technical Lead

Ruff, Gary Code PTO0, NASA GRC Plenary Speaker/Technical Lead

Trevino, Robert Code EC5, NASA JSC Plenary Speaker/Technical Lead

Urban, David Code RUC0, NASA GRC Participant

Waligora, Jim USRA Participant

3.2 Questionnaire and Pre-Work Material

Given the multidisciplinary nature of the EAWG’s task to recommend optimal atmosphere 

design points, a considerable amount of technical resource material was prepared in advance of 

the workshop to support the efforts of the EAWG. To promote efficient use of the reviewers’ 
129

time and expertise, a significant portion of the resource material that was presented and 

discussed at the workshop was provided as read-ahead material in advance. Select materials, 

including white papers and summary charts on particular session topics, were made available as 

downloadable files via the workshop website. Additional NASA references, such as the 

Bioastronautics Roadmap and references on the Vision for Space Exploration and NASA 

standards, were provided. The titles of read-ahead material provided to workshop participants 

are summarized in Table 3.2-1.

Table 3.2-1 Workshop Read-Ahead Material

Overview Section Title

EAWG Process Review EAWG Plan, Process, and Product

Mission Systems Assumptions, Candidate Atmosphere 

Options, Trade Space

Analytical Hierarchy Process - An Introduction

Fault Tree Analysis - An Introduction

EAWG White Papers Ambulation During Extravehicular Activity on Moon and 

Mars as a Risk Factor for Decompression Sickness

Diluent Gas Options

Dual Pressure EVA Suit

The Effect of Long-Term Partial Pressure Oxygen Exposure

Materials Flammability Control for Constellation Program: 

Impacts for Enriched Oxygen Environments

Space Radiation Shielding Materials

Additional NASA References Bioastronautics Roadmap

Bounding the Spacecraft Atmosphere Design Space for 

Future Exploration Missions

Human Systems Integration Standards

NASA Flammability and Offgassing Standards 

The Vision for Space Exploration

A list of eight questions was developed to facilitate participant review and discussion of EAWG 

work to date. These questions were each addressed in specific workshop sessions to allow for 

focused reviewer input on particular issues. These questions are presented in Table 3.2-2.
130

Table 3.2-2 Workshop Questions and Associated Sessions

Question Content Workshop Session

Q1 Do the EAWG planned products directly support the 

exploration mission and systems architecture?

Plenary Session - Discussion 

of EAWG Results to Date

Q2 Is the EAWG process properly constructed to provide 

recommendations to ESMD?

Plenary Session - Discussion 

of EAWG Results to Date

Q3 What other risks are not identified by these white paper 

topics?

Breakout Session #1 – White 

Paper Review

Q4 What mission assumptions should be added or 

modified and how would these assist the EAWG in 

making its recommendations?

Breakout Session #1 – White 

Paper Review

Q5 How well has the EAWG's analysis process supported 

the conclusions to date?

Breakout Session #2 – Review 

& Consensus on Results

Q6 Do additional data exist in any of the areas that would 

support or change the conclusions of the EAWG?

Breakout Session #2 – Review 

& Consensus on Results

Q7 *What should EAWG's recommendations to 

Constellation Program be?

Plenary Session - Discussion 

on EAWG Recommendations

Q8 *How can EAWG most effectively proceed from 

this point?

Plenary Session - Reviewer 

Discussion on EAWG Process

A brief worksheet consisting of the 8 questions was provided to participants who were requested 

to use the worksheet prior to the start of the workshop to capture their initial thoughts on the 

material provided so that over the course of the workshop their resulting input would be as 

complete and thorough as possible. In addition to questions, the worksheet also provided space 

for participants to suggest other resources (e.g. websites, publications, subject matter experts) 

that the EAWG may consider as it works toward making its final recommendations.

Questions 7 and 8 were modified prior to the start of the last plenary discussion by the EAWG 

chair. For reference, the original questions are listed below. The questions remained as stated 

below in the worksheet used by participants.

Q7: What are the most promising atmosphere designs?

- Are there important candidate design points that EAWG did not consider?

- What are their pros and cons relative to each other?

Q8: Has the process to rank/evaluate the candidate design points supported the conclusions?

- Is EAWG membership scoring of the candidate designs adequate? 

- Should the risk weighting be changed?

3.3 Reviewer Selection

NASA subject matter experts and external subject matter experts with extensive expertise in 

technical areas tied to the space environment were selected as reviewers of the EAWG’s work to 

date. Representatives of the Phase I prime contractors for the Crew Exploration Vehicle were 

also reviewers. The individuals selected had expertise in several medical, operational, and 
131

engineering disciplines and collectively were able to provide a substantive review of the range of 

EAWG working products. The following technical discipline areas were assembled:

• Active Thermal Control 

• Combustion and Material Flammability

• Environmental Control and Life Support 

• EVA Operations

• Physiology

• Safety

• Space Medicine

• Structural/Mechanical Design and Analysis

• Systems Engineering.

The names of the reviewers who attended the workshop are provided in Table 3.3-1 along with 

their area of expertise and affiliation. 
132

Table 3.3-1 Workshop Reviewers

Name Expertise Affiliation

Bauer, Peter Space Medicine Code SD, NASA JSC

Bleisath, Scott EVA operations Code DX, NASA JSC

Bolton, Paul Flight Controller. Operation of shuttle 

environmental control and active 

thermal control systems.

JSC Mission Operations 

Directorate

Chapline, Gail Technical Assistant Director for 

Development

Code EA, NASA JSC

Davis, Jeff Space Medicine Code SA, NASA JSC

Fernandez-Pello, 

Carlos

Combustion, Material flammability University of California -

Berkeley

Hirsch, David Flammability Honeywell-WSTF

Jernigan, Mark Life Support & Habitation Code SA2, NASA JSC

Joshi, Jitendra Life Support, Monitoring and Control 

and Systems

ESMD, NASA HQ

Kerwin, Joseph Physiology and operations: Apollo, 

Skylab, Shuttle

Former Astronaut

Kennedy, Kriss Habitation Systems Engineering Code EC, NASA JSC

Lewis. John Life Support Systems Code EC, NASA JSC

Linteris, Gregory Fire Safety; Microgravity Combustion NIST

Maples, Whitney ISS Life Support; ISS flight control Code DF, NASA JSC

Marmolejo, Joey EVA Systems Code EC, NASA JSC

McCandless, Bruce Former Astronaut Lockheed Martin

Myers, Jerry Computational Physiology, Biofluids NASA GRC

Pilmanis, Andrew Hypobaric/hyperbaric physiology and 

medicine; Altitude decompression 

sickness, hypoxia, pressure suits

Wyle Laboratories

Prisk, Kim Pulmonary physiology University of California –

San Diego

Rotter, Henry Life Support, Active Thermal Control & 

Analysis, and Fluids

JSC Engineering 

Directorate

Rouen, Mike EVA Systems Code EC, NASA JSC

Schmidt, Patricia Boeing

Schultz, Denise Space Station Safety Operations JSC S&MA Directorate

Scull, Tim Design and development of 

Environmental Control and Life 

Support (ECLS) hardware.

Hamilton-Sundstrand

Thurman, Randy ECLS & TCS subsystems design for 

space vehicles.

Boeing

Webb, James T. Decompression sickness (research, 

some NASA funded); Hypoxia 

(research, training)

USAF Retired

Zipay, John Structural/Mechanical Design and 

Analysis, Systems Engineering

Code EX, NASA JSC
133

4. Workshop Results

4.1 Overview of Plenary Presentations

Overview presentations were made on the first day of the workshop to introduce participants to 

the approach and strategy utilized by the EAWG in developing supporting documentation and 

ensuing recommendations for human habitable atmospheres. EAWG members, NASA JSC 

management, and invited experts presented a range of topics to describe how the charter and 

work of the EAWG fit into the Exploration effort; the risk areas and multiple other 

considerations and factors associated with atmosphere design; assumptions made and the process 

and analytical tools used to build and support decisions; and how the EAWG’s product will be 

used by the Constellation program. 

The EAWG presented results of their analyses of nine potential atmosphere design points 

spanning the considered atmosphere trade space, plus a candidate reference design point referred 

to as "Point X". Point X is nominally 8.5 psia, 32% oxygen and meets NASA’s hypoxia 

standards, while minimizing decompression sickness (DCS) risk and pre-breathe time to 

facilitate surface EVA's.

A structured plenary discussion followed this overview to capture participant comments related 

to this session and to answer questions clarifying the role of reviewers and the workshop 

objectives.

4.2 Plenary Discussion of EAWG Results to Date - Q1 & Q2 

This session was structured to answer Q1 and Q2. The questions and a summary of discussion 

points are below.

Q1: Do the EAWG planned products directly support the exploration mission and systems 

architecture? Is the content appropriate? Will the delivery be timely?

Q2: Is the EAWG process properly constructed to provide recommendations to ESMD? Are 

the AHP and FTA appropriate tools for this analysis? How should the EAWG present results 

to ESMD to be most useful?

• Multiple comments that the goal is to define trade space and recommended design points 

within it to Constellation. Should describe cost, benefit, and risk functions inside the 

trade space and describe why going outside it is bad. Need to layout the trades considered 

to recommend future trades to Constellation beyond the one we're making now.

• Need to design now for a progression of vehicles and missions. Most urgent need is the 

early CEV design points. LSAM is secondary.

• Need to clearly document assumptions, such as no airlock on CEV, what contingency 

EVA means for CEV, etc.

• Capture development issues and operational issues separately.
134

• Need to make requirements clear.

• Need to present realistic costs (Schedule, Funding).

• Need to capture expertise of this group.

4.3 Breakout Session #1 -Review & Discussion of White Papers, 

Mission Assumptions, and Research Areas - Q3 & Q4

A review of the white paper topics is presented in this section. Each subsection includes three 

types of information: report-out charts, highlights of the report-out comments from the plenary 

session, and breakout session comments of note. Since the viewgraph summaries generated by 

each breakout group were the intended products of the focus group, the reproduced text of the 

report-out charts is provided first. The actual presentation of the report-out charts may be found 

in the appendix. Since the main points of the breakout groups are provided by the report-out 

charts, the highlights and breakout session summaries offer additional information that is not 

already presented by the report-out charts.

Also, each breakout group differed in size and in the expertise areas of the participants. As a 

result, each breakout group had its own working dynamics. Due to the variation in the way each 

group approached the questions, submissions to the data collection matrix in some instances 

were not entered in the tabular form that was anticipated by the templates prior to the workshop. 

Nonetheless, each of the groups responded with the appropriate set of report-out presentations 

that addressed the following questions -

Q3: What other risks are not identified by these white paper topics?

 - What research, technologies, relevant experience, issues, etc. should be added for 

consideration?

 - Are there risk mitigations or technologies available that may change the importance of 

some risks?

Q4: What mission assumptions should be added or modified and how would these assist the 

EAWG in making its recommendations?

 - Does the ESAS Report require changes in our assumptions?

A list of each group’s participants is below in Table 4.3-1.
135

Table 4.3-1 Breakout Session #1 Participant Lists

Group # 1 Flammability

Technical Lead: Michael Pedley

Facilitator: Tim Snyder

Recorder: Kim Shaw

EAWG Members: David Urban

Reviewers: Carlos Fernandez-Pello, David Hirsch, Gregory Linteris, Whitney Maples, Randy 

Thurman, Denise Schultze

Group 2 – Space Radiation

Technical Lead: Nasser Barghouty

Facilitator: Mike Hopmeier

Recorder: Lisa Stephenson

EAWG Members: Greg Galbreath, Chin Lin

Reviewers: Jitendra Joshi, John Zipay

Group 3 – Variable Pressure EVA Suit

Technical Lead: Robert Trevino

Facilitator: Mike Krihak

Recorder: Gina Miller

EAWG Members: Jeffrey Patrick, Molly Anderson

Reviewers: Peter Bauer, Scott Bleisath, Lee Morin, Andrew Pilmanis, Mike Rouen

Group 4 – EVA Ambulation

Technical Lead: Michael Powell

Facilitator: Kiley Wren

Recorder: Suzanne Miller

EAWG Members: Johnny Conkin, Mike Lawson, Mike Lawson, Mel Buderer

Reviewers: Joe Marmolejo, Jim Waligora, Patricia Schmidt

Group 5 – Atmosphere Gases

Technical Lead: Michael Gernhardt

Facilitator: Fritz Stawitcke

Recorder: Cynthia Collins

EAWG Members: Gary Ruff, Noreen Khan-Mayberry, Daniel Fitzpatrick, Jeff Jones

Reviewers: Paul Bolton, Bruce McCandless, Jerry Myers, Kim Prisk, James Webb, Tim Scull, 

Joe Chambliss
136

4.3.1 Flammability/Fire

Report-Out Charts

Tables 4.3.1-1 and 4.3.1-2 are summaries of the responses to questions #3 and #4, respectively, 

that were provided by the Flammability/Fire breakout group. Table 4.3.1-3 captures additional 

comments that were considered significant by the breakout group itself.

Table 4.3.1-1 Flammability Report-Out Chart for Question #3

Risk Additional Research Technologies, Relevant, 

Experience, Issues

Extramural Risk 

Mitigations or 

Technologies

Water Based Fire 

Extinguishers

Issues with high-voltage power systems -----

Crew Clothing Off-the shelf, flammable outer clothing not 

suitable above 30% oxygen, custom clothing 

raises issues with wearability, clothing 

supplies, laundering, etc.

-----

Trash Spontaneous 

Ignition

Need to ensure that trash cannot self-heat to 

ignition in enriched oxygen

-----

Flammability of 

Dust on filters

Need to ensure flammable dust protected from 

ignition sources and cannot self-ignite

-----

Pyrolysis Products Recommend controlling primarily through fire 

response (breathing masks, ECLS cleanup, etc.) 

rather than materials selection, but need to 

ensure fire response can address successfully

-----

Table 4.3.1-2 Flammability Report-Out Chart for Question #4

Assumption Assumption assistance in EAWG 

recommendations

Effect of ESAS 

Report

Humidity Control/Static 

Electricity

Need to ensure minimum humidity 

controlled to eliminate static sparks as 

ignition sources, especially at high oxygen 

concentration (ISS 30-70% is fine)

-----

No air cooling of electronics At high oxygen concentrations, lower total 

pressure reduces air cooling effectiveness. 

Also, coldplated sealed boxes provide 

much more effective flammability control.

-----

Use of COTS hardware Current mindset on use of COTS will have 

to change for oxygen above 30%. Reduced 

crew friendliness 

-----

Fire Detection technology Need more small distributed sensors and 

better (different) sensor technology

-----
137

Table 4.3.1-3 Additional Comments – Flammability Report-Out

Other Considerations Open Issues

Probably None of these items considered closed

Report-Out Comments from Plenary Session – Summary

No comments were documented.

Breakout Session Notes/Matrix Summary

Q3: What other risks are not identified by these white paper topics?

This question focused on other risks not well addressed by the white paper topic. From the 

discussion, emphasis was placed on the higher oxygen concentration leading to a greater risk of 

flammability since the fire suppression capability will decrease. Concerns were also raised about 

the clothing and potential flammability hazard in greater oxygen concentrations. Another 

concern was the effect of dust build-up on HEPA (High Efficiency Particulate Air) filters and 

water vapor becoming a fire hazard in the presence of electronics.

The detection of a fire itself was also posed as a concern and that an increase in the number of 

detectors (small, distributed network ideally) and different detectors (from the ones currently 

used on the International Space Station (ISS) should be considered. Finally, the toxicity issues 

of pyrolysis products were also stated as being currently important.

Q4: What mission assumptions should be added or modified and how would these assist the 

EAWG in making its recommendations?

The goal of this question was to identify the mission assumptions that would assist the EAWG in 

making its recommendations. The plenary discussion identified the use of humidity/static 

control, a reduced pressure atmosphere (less air cooling), the use of COTS (commercial off-theshelf) hardware, and a cabin atmosphere of greater oxygen content as issues. 
138

4.3.2 Space Radiation Shielding

Report-Out Charts

Table 4.3.2-1 presents the results of the Space Radiation Shielding Materials report-out for 

question #3. The summary of assumptions is provided in Table 4.3.2-2 and additional 

noteworthy comments by the breakout group are presented in Table 4.3.2-3.

Table 4.3.2-1 Space Radiation Shielding Materials Report-Out Chart for Question #3

Risk Additional Research 

Technologies, Relevant, 

Experience, Issues

Extramural Risk Mitigations 

or Technologies

1> Extended 

exposure chemical 

breakdown

Technology development, material 

maturation

Environmental shielding and 

isolation

2> In situ repair Repair techniques, damage 

detection

Design for reparability; look at 

existing repair techniques 

(commercial or military)

3> Verification of 

shielding 

performance

Define environment, develop 

analytical tools, and in situ 

monitoring and testing

Look at existing verification 

strategies (micrometeorite, 

DSTB)

4> Knowledge of 

gaps and capabilities 

of multifunctional 

and mechanical 

properties

Extensive development and tests 

for primary structure applications

Improved design tools

5> Systems level 

compatibility and 

integration

Improved system modeling and 

integration

Improved design tools
139

Table 4.3.2-2 Space Radiation Shielding Materials for Question #4.

Assumption Assumption assistance in EAWG 

recommendations

Effect of 

ESAS Report

1> Mission duration Shielding requirements may depend on 

mission duration assumptions and affect 

atmosphere recommendations

-----

2> Limit and design of 

component lifetime

Optimal habitat design -----

3> Exposure limits Primary design driver for shielding 

requirements

-----

4> No secondary source of 

shielding for CEV and 7-

day Lander

Atmosphere of CEV and Lander independent 

of shielding

-----

5> Assume that the only 

interaction between 

atmosphere and radiation 

shielding design is 

flammability

Material containment for flammability control 

with respect to point X

-----

6> Early requirements 

freeze

Enhances confidence in design activity postSRR

-----

Table 4.3.2-3 Additional Comments – Space Radiation Shielding Report-Out

Other Considerations Open Issues

Only natural radiation environment assumed; no man-made sources e.g. 

nuclear for surface power generation, are being considered.

-----

Report-Out Comments from Plenary Session – Summary

One comment provided during this session focused on the radiation shielding design for GCR 

(galactic cosmic rays) and SPE (solar particle event). It was noted that shielding against GCR 

will shield SPE, however, the shielding for the reverse is not true. During extended missions 

(longer than 6 months) on the Moon and/or Mars, most of the radiation dose is estimated to be 

from GCR exposure. Thus, SPE radiation exposure may not be as dangerous as GCR. 

Conversely, a short duration mission (< 1 month) such as a lunar excursion the astronaut would 

be mostly exposed to SPE. It was also noted that SPE may not be as dangerous as GCR.

Breakout Session Notes/Matrix Summary

Q3: What other risks are not identified by these white paper topics?

The responses to this question are well-captured in the Report-Out Charts. Additional comments 

were reported as follows:
140

<Risk 2> – in-situ repair

• Restoration to original structural capability

<Risk 3> – Verification of shielding performance

• What is the strategy to verify performance?

<Risk 4> – Knowledge of gaps and capabilities of multifunctional and mechanical properties

• Secondary structure applications are more realistic in the near term

Q4: What mission assumptions should be added or modified and how would these assist the 

EAWG in making its recommendations?

The mission assumptions requested by this question were reproduced in the Report-Out Charts. 

No other comments from the breakout session itself were recorded.

4.3.3 Variable Pressure EVA Suit

Report-Out Charts

The tables assembled by the Variable Pressure EVA Suit report-out group include Table 4.3.3-1, 

Table 4.3.3-2 and Table 4.3.3-3. These tables summarize the discussions of risks and 

assumptions for questions #3 and #4, respectively. 

Table 4.3.3-1 Variable Pressure EVA Suit Report-Out Chart for Question #3

Risk Additional Research 

Technologies, Relevant, 

Experience, Issues

Extramural Risk Mitigations 

or Technologies

Variable secondary 

regulator

Emergency system is "on our 

back"

-----

Operational regulation Adds weight Air Force using DCS 

computer for calculating risk

Assumption that if cabin is 

Point X, 0 pre-breathe 

would be required when 

donning a suit at 6 psig

Theoretical models must be 

validated by research

-----
141

Table 4.3.3-2 Variable Pressure EVA Suit Report-Out for Question #4

Assumption Assumption assistance in 

EAWG recommendations

Effect of ESAS 

Report

Engineer out to low probability of 

Type II DCS

----- -----

Habitat pressure will be higher than 

suit

Reduce weight -----

Operational scenarios will keep 

EVA close to rover/habitat 

----- -----

Table 4.3.3-3 Additional Comments –- Variable Pressure EVA Suit Report-Out

Other Considerations Open Issues

Having the lower secondary operating pressure is 

acceptable for failure of the primary system alone, but is 

not acceptable for DCS.

Programmatic decision is required early on to possibly 

use the suit as a hyperbaric chamber. Any suit 

development would be impacted by a late decision (e.g., 

structural loads, variable pressure considerations)

Automation of regulation would be 

the optimal system, however, this 

automation could affect the 

robustness and reliability of the suit.

-----

Report-Out Comments from Plenary Session – Summary

Suit design to accommodate pressures above 6 psig was identified as a possible measure for 

emergency hyperbaric treatment during an EVA. Concerns were expressed, however, that the 

suit integrity may be compromised if operating at those elevated pressures. It was suggested that 

a more rugged suit should be designed for future lunar or Mars missions to permit normal use 

after repeated hyperbaric treatments.

The attendees agreed on the need for more than two regulator set points. However, an upper 

limit of 3 or 4 set points was a distinct possibility with current regulators. If more set points are 

needed, then a new regulator design would need to be employed. 

Breakout Session Notes/Matrix Summary

Q3: What other risks are not identified by these white paper topics?

Additional comments to the risks discussed during this session were in reference to a variable 

secondary regulator and operational rules. With regards to the former, the variable secondary 
142

regulator would need to be within 1-2 psig of the primary regulator. For the latter, the Air Force 

was identified as having a DCS computer for calculating risk, but it was noted that this computer 

would need to be modified for space exploration missions. 

Q4: What mission assumptions should be added or modified and how would these assist the 

EAWG in making its recommendations?

One assumption that was addressed was regarding engineering a low probability of Type II DCS 

risk into the suit design. Even though a very low probability may be achieved, one reviewer 

expressed that the complete elimination of such a risk cannot be accomplished.

One other question was whether it will be acceptable to use an emergency oxygen supply 

pressure that increases the risk of DCS. This comment was in regards to the mission assumption 

that the operational scenarios will keep EVA close to a rover or habitat.

Other Notes

Discussion during this session also addressed three other topics related to a variable pressure 

EVA suit. The first topic was defining the variable pressures and that these pressures would 

actually be discrete points that are achieved within the suit. A second item was determining the 

lowest operating suit pressure. With the current regulators, a suit pressure of 3.5 psig could be 

achieved with confidence. When factoring other suit parameters such as glove dexterity, 

operating at 3.5 psig versus 4.3 psig would not provide much advantage. In conclusion, the EVA 

breakout group agreed that a 4.3 psig nominal suit operating pressure was most appropriate.
143

4.3.4 EVA Ambulation Effects

Report-Out Charts

This session reported its results with two tables, one each for questions #3 and #4. Table 4.3.4-1 

lists five additional research suggestions and Table 4.3.4-2 identifies seven mission assumptions 

to guide ambulation risk assessments. 

Table 4.3.4-1 Ambulation Effects Report-Out Chart for Question #3

Risk Additional Research Technologies, Relevant, Experience, 

Issues

Extramural Risk 

Mitigations or 

Technologies

<Risk 1> Look at historical data from Apollo training and surface ops for 

DCS

-----

<Risk 2> Other non NASA data involving different types of exercise (e.g., 

World War II) for DCS

-----

<Risk 3> Need to perform chamber tests with ambulation and determine 

which kinds of ambulation best simulate lunar surface activity 

(climbing, swinging arms, upper body)

-----

<Risk 4> May need to test for pulmonary shunt and patent foramen ovale 

and consider with regard to bubble arterialization.

-----

<Risk 5> Conduct tests on well hydrated subjects. -----

Table 4.3.4-2 Ambulation Effects Report-Out for Question #4

Assumption Assumption assistance in EAWG 

recommendations

Effect of ESAS 

Report

<Mission Assumption 1> Consider different atmospheres for LSAM & CEV -----

<Mission Assumption 2> Consider use of pressurized rover -----

< Mission Assumption 3> Design point X seems reasonable. -----

< Mission Assumption 4> Ambulation significantly increases risk of DCS -----

< Mission Assumption 5> Surface activities must be considered (upper body 

movement)

-----

< Mission Assumption 6> Variable pressure suit might be smart option -----

< Mission Assumption 7> Try to design out the risk of DCS as much as 

possible in the design of the pressurized 

environments

-----
144

Report-Out Comments from Plenary Session – Summary

It was noted that helium has advantages over nitrogen, since it is more soluble and has more 

rapid washout from body tissues. The mode of EVA ambulation in partial gravity was proposed 

as an R&T topic (e.g. – is lunar loping different from gaits used in ground testing?). NASA 

exploration EVA ground testing will begin in 2006 to study different ambulation types.

Breakout Session Notes/Matrix Summary

No additional comments from session.

4.3.5 Atmosphere Gas Constituents

Report-Out Charts

Table 4.3.5-1 represents the working group’s suggestions, segmented by mission, for partial 

pressure of oxygen (pO2), diluent gas options, and total cabin pressure. Table 4.3.5-2 contains 

additional data and suggestions to address both questions #3 and #4.

Table 4.3.5-1 Atmosphere Gas Constituents Group Results

 

CEV CEV/LSAM Outpost

Hypoxia Use 

existing 

Shuttle 

standards.

- Push back on limits to 

increase design space.

- Want a nominal range of 

5,000 to 8,000 feet, including 

sensor error.

- Push back on limits to increase 

design space.

- Want a nominal range of 5,000 

to 9,000 feet, including sensor 

error.

Diluent 

Gases

Use 

existing 

Shuttle 

standards.

- Nitrogen Baseline

- Helium offers significant 

decompression advantages if 

required due flammability 

issues. Drawback - Significant 

engineering challenges.

- Nitrogen Baseline

- Helium offers significant 

decompression advantages if 

required due flammability issues. 

Drawback - Significant 

engineering challenges.
145

Total 

Pressure

10psia to 

14.9psia

- 8-8.4psia (nominal 8.2psia), 

with a range of O2

concentration of 27.6% to 34%, 

nominal 30.7%

- 7.65-8.05psia (nominal 

7.85psia), with a range of O2

concentration of 27.3% to 34%, 

nominal 30.6%

Table 4.3.5-2 Additional Items Referencing Q3 and Q4 

• Human testing to validate pre-breathing protocols.

• Variable pressure suit to expand capability, increase EVA performance, and safety.

• If forced to Helium option, it will require additional pre-breathe studies, engineering 

evaluations.

• Argon is an undesirable pre-breathe gas. If engineering trade studies suggest benefit, then 

additional pre-breathe.

Report-Out Comments from Plenary Session – Summary

The use of helium as a potential diluent gas raised engineering issues, such as speech 

intelligibility, and thermal control of electronics. The reply was that these are all solvable. There 

were comments that a mixed gas (including helium) outweighs the cost of testing and evaluation. 

It was stated that tolerance on nominal operating ranges were based on a maximum cumulative 

error of 0.2 psia from the sensor and the controller.

The issue of upper limit on carbon dioxide was not addressed in the session. One participant said 

it was a partial pressure of 5 mmHg (0.1 psia). It was also noted that slight hypercarbia enhances 

hypoxia tolerance, however other issues might arise. It was asked whether any issues might arise 

from having crew members acclimated to 9000 feet, then going to sea level oxygen partial 

pressure to perform an EVA.

It was asked whether there have been any studies on living for long periods at lower total 

pressures. The reply was that most effects of altitude that have been studied are due to reduced 

pO2. However, lower total pressure means that food preparation and processing, as well as food 

growth, will be an issue (cooking at lower pressures, growing plants). The question was also 

raised of possible low pressure/low pO2 effects on metabolism (e.g., food absorption and 

nutrition, changes in cognitive capabilities). It was next asked if the proposed atmospheres 

eliminated dedicated pre-breathe (meaning time beyond that required for suit checkout). The 

answer was yes, subject to studies. Citations of research on going to increased hypoxia will be 

provided by Jeff Jones. EAWG had previously assumed a limit of 6000 feet, it is now proposed 

to go to a higher altitude equivalent. Although the habitats will be hypoxic, it was noted that 
146

EVA periods still will be conducted in slightly hyperoxic conditions, therefore hypoxia-induced 

performance reductions during EVA are not expected. 

Breakout Session Notes/Matrix Summary

It was stated that a key role for lunar missions will be to validate surface operations approaches 

before Mars missions. They should be designed with equipment and protocols that allow study of 

chronic vs. acute hypoxia exposure effects, in reduced gravity and total pressure.

It was pointed out that there will be additional engineering challenges to utilizing helium such as 

a higher leak rate and more challenging storage conditions.

4.4 Breakout Session #2 – Review & Consensus on Results and R&T 

Recommendations – Q5 & Q6

The results of the review and consensus with regards to the three sets of discipline-area factors 

evaluated by the EAWG are presented in this section. These discipline-areas are (1) 

Physiological and Medical Factors, (2) Mission Operations Factors, and (3) Vehicle-Habitat 

System Factors. These main categories are covered in three parts: report-out charts, highlights of 

the report-out comments from the plenary session, and comments captured in the data collection 

matrix as well as notes logged by note takers that were assigned to each breakout group. The 

viewgraph summaries generated by each breakout group were the intended products of the focus 

group and thus, the reproduced text of the report-out charts is provided first. The actual reportout presentations may be found in the Appendix. Since the main points of the breakout groups 

are provided by the report-out charts, the two sub-sections that summarize the plenary session 

highlights and breakout notes comprise the main thoughts and comments by the meeting 

attendees not already captured in the report-out viewgraphs. 

Also, each breakout group differed in size and in the expertise areas of the participants. As a 

result, each breakout group had its own working dynamics. Due to the variation in the way each 

group approached the presented questions, entries to the data collection matrix in some instances 

were not in tabular format as anticipated prior to the workshop. Similarly, remarks captured by 

the note takers were not assigned specifically to each question, but rather in a chronological 

account of the discussion that materialized. Overall, each of the groups responded with the 

appropriate set of report-out presentations that served to address the following questions: 

Q5: How well has the EAWG's analysis process supported the conclusions to date?

- What topics were not identified in the review process? How important are these identified 

topics (High, medium, low)?

- Are there any issues that are not adequately reflected in the findings?

Q6: Do additional data exist in any of the areas that would support or change the conclusions of 

the EAWG?

- What data need to be obtained before a further down-select of the candidate atmospheres 

is done?
147

- What R&T recommendations would you add? How important are these identified topics 

(High, medium, low)?

A list of each group’s participants is below in Table 4.4-1.

Table 4.4-1 Breakout Session #2 Participant Lists

Group 1 - Physiological & Medical

Technical Lead: Johnny Conkin

Facilitator: Fritz Stawitcke

Note Taker: Cynthia Collins

Recorder: Gina Miller

FTA Analyst: George Deckert

AHP Analyst: James Watts

EAWG Members: Daniel Fitzpatrick, Michael Gernhardt, Michael Powell, Jeff Jones

Reviewers: Peter Bauer, Bruce McCandless, Lee Morin, Jerry Myers, 

Andrew Pilmanis, Kim Prisk, Jim Waligora, James Webb

Group 2 - Mission Operations

Technical Lead: Jeff Patrick

Facilitator: Kiley Wren

Note Taker: Suzanne Miller

Recorder: Kim Shaw

FTA Analyst: Eric Thigpen

AHP Analyst: Molly Anderson

EAWG Members: Robert Trevino

Reviewers: Jitendra Joshi, Joey Marmolejo, Mike Rouen, Patricia Schmidt, Paul Bolton, 

Whitney Maples

Group 3 - Vehicle & Habitat

Technical Lead: Gary Ruff

Facilitator: Mike Krihak

Note Taker: Carol Schanafelt

Recorder: Lisa Stephenson

FTA Analyst: Clint Thorton

AHP Analyst: Bruce Duffield

EAWG Members: Chin Lin, Gregory Galbreath, Mike Lawson, Michael Pedley, 

David Urban, Noreen Khan-Mayberry, Nasser Barghouty

Reviewers: Carlos Fernandez-Pello, David Hirsch, John Lewis, Gregory Linteris, 

Henry Rotter, Tim Scull, Randy Thurman, John Zipay
148

4.4.1 Physiological and Medical Factors

Table 4.4.1-1 reports the group’s comments on the use of the FTA and AHP tools for risk 

analysis, in response to question #5. 

Table 4.4.1-1 Physiological and Medical Factors Report-Out Chart for Question #5

AHP Analysis - about “process”

• A methodology that ranked atmosphere preferences based on your assessment of relative 

risks of the variables involved.

• Ops and physiology subgroups preferred lower pressure and higher oxygen concentration.

• Materials / Flammability subgroup preferred higher pressure and lower oxygen 

concentration.

• The main contribution of this method was to quickly and objectively eliminate what

otherwise appeared to be reasonable atmosphere options.

• The external reviewers, in general, felt this method was an important component of the 

process that got us to Design Point X, and beyond.

• Several commented that it was not a good use of their time to evaluate “process”.

FTA Analysis - about “process”

• Hypoxia was not an issue since all atmosphere options were better than 6000 feet airequivalent.

• P(DCS) was provided for each candidate atmosphere, so this unique feature of the FTA 

was not necessary.

• Therefore, the FTA in our particular application was not helpful – also not a lot of time to 

get this complex analysis done.

The next three tables address question #6. Table 4.4.1-2 describes the group’s review of the 

EAWG pre-workshop list of 17 suggested R&T topics for physiological and medical factors. 

Table 4.4.1-3 contains additional comments on some issues that will arise if new diluent gas 

alternatives are considered. Table 4.4.1-4 represents an updated list of R&T needs (additions 

and modifications in red italics).
149

Table 4.4.1-2 Physiological and Medical Factors Report-Out Chart for Question #6

• We reviewed 17 R&T areas, improved some of the wording, and added two new topics.

• Lots of discussion over 19 topics, most was captured in our notes.

• In general, the external reviewers agreed with the priority ranking (H,M,L) of the topics. 

• Some felt that a few low priority topics could be removed from the list, but all will go 

forward since they serve as place holders for new research data.

• Specific data from Brooks AFB are available to address several topic areas, which would 

prevent unnecessary new research. 

Table 4.4.1-3 Notable Comment Physiological and Medical Factors Report-Out

• The first-time use of helium or argon in a NASA vehicle may not be as far-fetched as it 

first sounds if materials selection, radiation shielding, and flammability are big challenges 

at higher oxygen concentrations.

• There are significant engineering and physiological research issues with long lead time if a

trade study favors an inert gas in addition to nitrogen – tri-gas.

• The use of nitrogen, argon, and carbon dioxide on mars is likely a deciding factor to 

achieve a long-term presence on mars; maybe argon is just used on the return trip to earth. 
150

Table 4.4.1-4 Physiological & Medical R&T Needs

Additions and Modifications made on 11/2/2005 at Workshop in italics

Topic Priority

• Can an ISS-like pre-breathe protocol be used to extend the cabin atmosphere 

design space?

Low

• What is the probability of DCS as a function of tissue ratio under microgravity, 

Moon and Mars partial gravity?

High

• What is the most effective pre-EVA Decompression Sickness (DCS) 

prevention strategy to include pre-breathe with various gases, exercise and 

other medical measures? 

High

• What are the appropriate screening procedures to minimize predispositions for 

DCS? 

Low

• What are the resources and techniques for early diagnosis of DCS signs and 

symptoms, including the use of Doppler U/S and other bubble detection 

technologies? 

Low

• What are the best methods for predicting DCS risk and for reducing the risk, 

based on understanding of the physiological mechanism for bubble formation 

and propagation, employing best available knowledge from flight and analog 

environment experience?

Medium 

(one vote 

for High)

• What are the most effective yet safe, and energy- and space-efficient means of 

managing DCS in the space flight milieu, including the use of hyperbaric 

oxygen delivery and other promising technology, and how might they be 

adapted for reduced-G operations? 

High

• What is the risk of DCS after an acute environmental insult - e.g., leaking 

module or damaged EMU, and what treatment or response options are 

available under these off-nominal situations? 

High

• What are the operational and medical impacts of off-nominal performance of 

DCS countermeasures? 

High

• What are the risk factors that can increase the likelihood of DCS, such as the 

presence of Patent Foramen Ovale (PFO)? 

High

• What is the likelihood of surviving an acute environmental insult severe 

enough to cause damage to the vehicle or spacesuit? 

Low

• Is it possible and what are the DCS risk mitigation options for interplanetary 

EVA (e.g., moon and Mars) given that a tri-gas breathing mixture including 

argon is present? 

High

• What is the role of individual susceptibility, age and gender on the risk of DCS 

during NASA operations involving decompression? 

Medium 

(one vote 

for High)

• What are the available and new technologies needed to provide DCS treatment 

options on the ISS and future habitats (or vehicles) beyond LEO (e.g., on the 

moon or Mars)? 

Medium

• What is the correlation between the detection/existence of gas phase creation in 

the bloodstream and development of clinically significant DCS? 

Low

• What are the combined effects of long-duration mild hypoxia, hypobaria, and 

hypogravity on the human body?

Low (new)

• Oxygen Partial Pressure, Diluents, and Gravity: Other diluents (other than N2) 

must be evaluated with respect to DCS, pre-breathe protocols, hypoxia risk, 

and other physiological effects.

Low (new)
151

11 / 02 / 05 additions

• What are best treatment strategies to manage ebulism? High

• Understand more about the expected advantages of doing EVAs once 

equilibrated to a hypobaric environment.

Medium

Report-Out Comments from Plenary Session – Summary

There was significant uncertainty and discussion on the risk posed by the flammability of hair, 

due to some conflict between space experience & other data. There were two new R&T issues 

proposed, one of which (ebulism treatment) does not affect atmosphere selection per se, but is a 

risk factor to refer forward to the Bioastronautics Roadmap.

Breakout Session Notes/Matrix Summary

The recorded session matrix includes numerous comments made during the discussion of each of 

the R&T needs entries, not reproduced here, but should prove useful for review by the 

Physiological & Medical subgroup of the EAWG.

4.4.2 Mission Operations Factors

Report-Out Charts

Tables 4.4.2-1 and 4.4.2-2 are the reproduced charts for Mission Operations Factors commentary 

on AHP and FTA analyses, respectively. Table 4.4.2-3 compiles the requested unidentified 

topics for both AHP and FTA. Finally, Table 4.4.2-4 provides the report-out chart for Question 

#6.

Table 4.4.2-1 Mission Operations Factors Report-Out Chart for Question #5, Part 1 - AHP

AHP Analysis - Analysis Process Support Conclusions

• Informative but preliminary. May help to downselect to other options.

• Perform another round of analysis and take into account ESAS results, cost, schedule, and 

risk.

• Reconsider relative to possible hypoxia limit change.

• Unclear how design point X resulted from AHP.

AHP Analysis - Issues Not Adequately Reflected

• ESAS results, cost, schedule, and risk and point X and Y (push hypoxia limit). Participants 

should provide input from program manager perspective rather than expert opinion.
152

Table 4.4.2-2 Mission Operations Factors Report-Out Chart for Question #5, Part 1 - FTA

FTA Analysis - Analysis Process Support Conclusions

• The complete FTA Models developed by other sub teams were not shared with entire team 

so it was an incomplete FTA Model.

• FTA Model that the Mission Op Sub Group developed only focused on mission success.

• FTA Model did not help with AHP assessment.

Table 4.4.2-3 Mission Operations Factors Report-Out Chart for Question #5, Part 2 for 

both AHP and FTA

Unidentified Topics Importance of Unidentified Topics 

(low, medium, high)

ESAS results, cost, schedule, and risk and point X and 

Y (push hypoxia limit). Participants should provide 

input from program manager perspective rather than 

expert opinion.

-----

Table 4.4.2-4 Report-Out Chart for Question #6

EAWG 

Conclusion

Existing Data 

to Support, 

Change 

Conclusions

Additional Data 

for Further 

Down-Select

Added R&T 

Recommendations

Importance of 

Identified 

Topics

(High, Med., 

Low)

----- See AHP 

Round 3 

comments 

above.

----- More cross-education 

across sub-groups 

(either by team 

formation or 

briefings).

-----

Report-Out Comments from Plenary Session – Summary

Several issues were raised during the plenary session for Mission Operations Factors. Some of 

the emphasized concerns included cost, and flammability. Even though cost was not a factor 

emphasized by the EAWG, it was nonetheless assumed to be an important factor in the way 

management will select a cabin atmosphere. A comment was made that building a CEV to 

handle a 40% oxygen atmosphere was not cost prohibitive, rather the other contents (e.g. 

portable equipment) that would be added to the vehicle could provide much of the cost increase 

related to materials certification. In short, the materials will need to be qualified in an 

atmosphere greater than 30% oxygen. There was also speculation that the CEV would need to 

support variable atmospheres and pressures for the different types of planned future missions as 

well as different radiation shielding. The risks of DCS and fire were also addressed. The DCS 

risk was conveyed to be a moderate probability with moderate consequence, whereas fire was 
153

considered a low probability with high consequence. It was commented that fires were probably 

going to happen, so an approach to mitigate the effects should be a consideration.

For presenting the findings to management or Constellation, it was suggested that other options 

in terms of cost should be provided along with the recommended atmosphere. In addition, these 

suggested changes to the atmosphere should be provided sooner rather than later because once 

the CEV design is locked-in, it will be difficult to re-engineer to accommodate higher oxygen 

content atmospheres and other gas constituents. Along these lines the final recommendation was 

advised to be an operating point, but additionally provide a broader operating band for alternate 

decisions. Finally, a third AHP after the workshop was suggested to further support the validity 

of the findings. 

Breakout Session Notes/Matrix Summary

Q5: How well has the EAWG's analysis process supported the conclusions to date?

This question focused on the evaluation of the AHP and FTA processes. In general, the results 

of the processes themselves were not well received and it was questioned whether or not outside 

reviewers should be responding to this question. A comment was captured stating that AHP is a 

better tool at this stage due to the lack of quantitative data. When the data exists, then FTA is the 

better tool. The consensus from the group was to wait on determining an analysis method. As a 

result, only a few comments were captured with respect to AHP and FTA.

AHP analysis was found to be informative but preliminary to help down select to other options. 

In addition, one reviewer suggested that a another round of analysis be performed that takes into 

account the ESAS results, cost, schedule, risk, and a possible hypoxia limit change. Finally, it 

was unclear to some reviewers how Point X resulted from the AHP process.

One reviewer questioned why hypoxia wasn’t weighted more heavily in the AHP process. 

Another participant expressed that FTA may be more useful later in the program. Finally, the 

application of mathematical models was suggested to have the capability to derive probabilities 

of loss of crew event for DCS, but it was also noted that such models would need to be validated 

in the future.

Q6: Do additional data exist in any of the areas that would support or change the conclusions of 

the EAWG?

Notes for this breakout session were all captured under Question #5, none of which were 

specifically emphasized as responses for this question.

4.4.3 Vehicle-Habitat System Factors

Report-Out Charts

Table 4.4.3-1 presents the results of the Vehicle-Habitat System Factors report-out for Question 

#5 that pertain to AHP analysis.
154

Analytical Hierarchy Process

• Can be a useful tool but there was insufficient time for the review panel to understand 

process, weightings, and implications (uncertain pay-off)

Table 4.4.3-1 Vehicle-Habitat System Factors Report-Out Chart for Question #5

Does the Analysis Process Support Conclusions?

• Uncertainty in how the expertise of the EAWG influenced the results

• Uncertainty in how the weighting influenced the results

Issues Not Adequately Reflected

• There should be more than four categories; they don’t address all the risks that a 

program manager would want to see (do these influence atmosphere selection)

• Specific impacts on systems should be included (life support, thermal, fire suppression, 

costs)

• No hypoxic design points were included in the original analysis (Points 1-9, X)

Table 4.4.3-2 presents the results of the Vehicle-Habitat System Factors report-out for Question 

#5 that pertains to Fault Tree Analysis.

Fault Tree Analysis

• Issues identified in the fault tree were relevant to assess flammability

• Analysis helps to focus thought on flammability risks for vehicle but ...

Table 4.4.3-2 Vehicle-Habitat System Factors Report-Out Chart for Question #5

Does the Analysis Process Support Conclusions?

• May not be best format to capture what is needed in this group

• Best applied after vehicle has been designed

• Can’t assess probabilities at the high-level required without a design

Issues Not Adequately Reflected

• Death/Permanent injury due to fire is too general; there are many ways that a fire could 

injure a crew or impact a mission such as production of toxic products by fire or fire 

response, burns, equipment damage...

• Probabilistic Risk Assessment could be more useful to identify risks and focus 

attention on the highest risk issues during the design process

Q5: How well has the EAWG’s analysis process supported the conclusions to date?

FTA and AHP helped to focus thought on the issues but the output of these results do not 

make a strong case for the selection of an atmosphere. Discussions of the EAWG and 

white papers have been more useful.
155

What topics were not identified in the review process? How important are these identified topics 

(High, medium, low)?

• Topics not identified include effects on thermal control, life support systems, fire 

suppression, costs, etc. These would be the real drivers and are the areas where a 

program manager would want to see trade studies. 

Are there any issues that are not adequately reflected in the findings?

• Yes

Figure 4.4.3-1 shows the proposed trade space, Point Y, proposed during the workshop with 

respect to the other nine design points considered by the EAWG and Point X, which was the 

final design point recommended at the start of the meeting. The actual atmospheric design 

ranges are: 

Point X: 8.5 psia, 32% oxygen; 

Point Y: 8 – 8.4 psia, 27.6 - 34% O2; 7.65 – 8.05 psia, 27.3 – 34% O2.

Figure 4.4.3-1 EAWG Design Reference Points
156

In Figure 4.4.3-2 below, two design points (Point X and Point Y, where Point Y is the larger 

rectangle) were provided by the Vehicle-Habitat System Factors breakout group for proposed 

cabin atmosphere pressure and oxygen concentration. 

Figure 4.4.3-2 Historical and Workshop Candidate Design Points

Skylab Mercury/Gemini/Apollo

Early Apollo Design

Shuttle EVA Preparation

Shuttle/Mir/ISS

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Cabin Volume Percent Oxygen

Cabin Total Pressure, kPa

0

2

4

6

8

10

12

14

Cabin Total Pressure, psia

Normoxic Equivalent

Hypoxic Boundary

Historical Designs
157

Table 4.4.3-3 provides a list of R&T needs generated by the Vehicle-Habitat Systems Factors 

group. Each topic was given an assigned priority level of low, medium, or high. 

Table 4.4.3-3 Vehicle-Habitat System Factors R&T Needs

Topic Priority

• Flammability effects of pressure, gravity, oxygen mole fraction, diluent gas High

• Acceptable materials for clothing, windows, stowage foam, and radiation 

protection for O2% above 30%

High

• Ignition mechanisms for high oxygen concentration systems (flow friction and 

particle impact)

Medium

• Detection of fires in alternate environments and atmospheres must be evaluated High

• Development of fire suppression systems that are effective for oxygen 

concentrations > 30%; “No good story for putting out fires”

High

• Effect of atmospheric noise and voice communications Medium

• Effect of low atmosphere pressure on food preparation and processing Medium

• Methodology for maintaining configurations controls over long duration High

• Impact on fire risk for step increase in %O2, change in P; quantitative increase 

in risk between atmosphere with 30% and 36% O2; What is the threshold on 

flammability?

High

• Quantitative impact of high O2 on flammability, production of toxic products 

(gaseous and particulate), heat release

High

• Development and quantification of effectiveness of non-flammable coverings 

and coatings on flammable materials

High

• Methodology to assess total risk of fire to vehicle/habitat system High

• Food Processing: Development of a EMI acceptable microwave oven High

• Threshold for flammability of hair and skin Medium

• Material flammability during medical ops -----

• Burn treatment (external and inhalation) in reduced gravity -----

Discussion and Comments

Table 4.4.3-4 summarizes the discussion that related to the feasibility of a 30% O2 cabin 

atmosphere.
158

Table 4.4.3-4 Feasibility of 30% O2 Atmosphere

In the white papers and break-out sessions, we have said that increasing %O2 above 30% is 

feasible

• Design points have been developed that include higher %O2 because we have done it 

before

o Must have an answer for why not just go to 100% O2 and 5 psia

• Apollo vehicles were tested extensively before flight (before they truly assumed the risk of 

100% O2)

o Apollo was designed in an era of plenty of funding. Today, costs are a strong 

consideration.

o Risk has always been a driver but risk is mitigated through pre-flight testing

• “Smart” risks are acceptable but it is testing that makes the difference

• “No vehicle system since Apollo has undergone the same level of pre-flight testing.”

Discussion and Comments (continued)

You can’t accept “34% oxygen” without also immediately accepting the cost and schedule of the 

associated testing and development.

• Impact on thermal systems, life support equipment (new technologies), fire 

suppression, ... is unknown at this time

Additional concerns regarding a 30% O2 cabin atmosphere are presented in Table 4.4.3-5.

Table 4.4.3-5 Other Issues

Costs are immediate

• There is a step change in costs to go above 30% O2 (max)

o Contractors will have to add costs of re-testing materials and systems at these conditions

o This will be a consideration at PDR

• Uncertainty in how many materials will have to be tested but ...

o Configuration and component operating tests will be extensive

o Engineering issues can be addressed but development and testing is a critical

• Not accepting this cost and schedule impact is the same as setting the upper limit to 30% 

O2

Discussion and Comments (continued)

How do we get out of this box?

Table 4.4.3-6 summarizes possible answers to the question posed here.
159

Table 4.4.3-6 Considerations for Succeeding with a ≥30% Atmosphere

Understand the margin of safety associated with a specific change in atmospheric condition, 

design, etc.

• Current tests are pass/fail at a specific condition

o Don’t quantify a degree of failure to help assess incremental risk

• Propose a change in methodology to evaluate flammability of materials

o Toxicity of fire and pre-fire products (gaseous and particulates)

• Apply performance-based fire protection methodologies to fire protection on spacecraft

o Obtain data from material flammability testing at desired conditions

• Heat release, toxic products, flame spread rates, ignition sources ...

o Analyze impact on vehicle through entire fire scenario

• Detect and respond before system impact exceeds design envelope

o Requires testing (sub- and full-scale) and system simulation

Q6: Do additional data exist in this subgroup area that would support or change the conclusions of the EAWG?

Probably not

• What data need to be obtained before a further down-select of the candidate atmospheres is 

made?

− Development and certification costs associated with going above 30% oxygen

− Ability to accurately assess hypo-gravity material flammability

• Selection of materials and configurations

− Non-flammable covering and coatings

− Methodology for configuration control over the duration of a 2 – 3 year mission

Report-Out Comments from Plenary Session – Summary

The Vehicle-Habitat System Factors report-out raised several topics of conversation. One 

general comment was to justify a selection of 34% oxygen, above the 30% at which most 

materials are qualified for flammability issues, instead of just using 100% oxygen like was used 

in some past space missions, such as Apollo. The 30% barrier is important for newer classes of 

materials, however, that weren’t available on missions over 20 years ago. Another participant 

commented on cost from the point of view that questioned the cost of items that one can’t take 

with them when going to an atmosphere with over 30% oxygen. It was also noted that a 

comparison of the trade-off between DCS risk and pre-breathe efficiency costs should be taken 

into account. According to one participant, the cost at 30% oxygen content may be a 

significantly extended pre-breathe compared to 34% oxygen content. Since testing of materials 

has only been performed up to 30% oxygen, this oxygen concentration tends to be the abrupt cutoff value for flight qualified materials. It was noted that most of the expected increase in cost is 

in the new and different design approaches for the elevated oxygen content.
160

During this breakout report presentation, a new range of pressures and oxygen concentrations 

was presented and dubbed ‘Point Y.’ Since Point Y covered a range of oxygen concentrations 

from approximately 26% to 34% oxygen, one reviewer proposed a compromise to move to the 

left side of the box where values were below 30% oxygen content. Also, if such a design band is 

recommended then the vehicle will need to have variable pressure and atmosphere capability. 

In addition to the previously discussed topics, radiation shielding was recommended to be 

adequate for all types of missions. Also, life support issues were raised that may not have been 

investigated yet

An overall recommendation was to obtain more information on the goals of the Constellation 

program and try to align the EAWG recommendation with them.

Breakout Session Notes/Matrix Summary

Q5: How well has the EAWG’s analysis process supported the conclusions to date?

During the breakout session, AHP and FTA were found not to be beneficial to the 

recommendation according to the reviewers. For AHP analysis, having only 4 factors was 

questioned and that the analysis may be performed at too high a level. Concern was also 

expressed for flammability, which may have different costs associated with those risks. Also, 

the selection of the ten points for evaluation was identified as not having a point that lies in the 

hypoxic range. To better verify the analysis, it may have been beneficial to have one or two 

evaluation atmospheres that lie below the hypoxic boundary.

Other items brought forward included the ability for engineers to design systems for a predetermined atmosphere, if it is given to them in advance. Second, certain risks and system 

reliability may be greatly improved through comprehensive preflight subsystem and system-level 

testing, which may not be a feasible approach for the initial CEV (due to expected time 

constraints and funding issues).

The FTA model was suggested to possibly not be the best format to capture what the EAWG is 

seeking. Other factors were proposed to be built into the model that included toxic products 

produced by fire, an assessment of 30% oxygen atmosphere and the proper use of materials.

Q6: Do additional data exist in this subgroup area that would support or change the conclusions of the EAWG?

For assessing the EAWG recommendation, one reviewer was interested in determining what is 

the true impact of a 2% oxygen content increase in the vehicle (from 30% to 32%). The 

discussion also focused on flammability issues and ways to mitigate fire hazards such as painting 

items with an aluminum coating and to quantify the fire margin. It was also noted that personal 

items brought by astronauts such as toothbrushes and other paraphernalia need to be flight 

qualified, too. Furthermore, a request was made for NASA to consider a new standard 

methodology for fire testing that will yield oxygen concentration sensitivity data for each 

material.
161

Although not represented by an expert in the breakout session, food preparation issues were also 

highlighted by the group as a possible consideration for the CEV.

4.5 Reviewer Discussion on EAWG Recommendations – Q7 & Q8 

Q7: What should EAWG's recommendations to Constellation Program be?

Several reviewers felt that multiple sets of atmosphere recommendations should be made, 

in the manner of the matrix created by the Atmospheric Gases breakout group, to serve the 

differing needs of multiple missions and vehicles - CEV, LSAM, and habitat. The majority 

suggested that specific points be proposed, inside a range or “band” of acceptable values. 

They further suggested providing a discussion of the risks over the range, in order to 

provide a trade space for program management to use.

Another recommendation was to select atmospheres for the landers and habitats in order 

to facilitate frequent and efficient EVA’s, in particular by reducing the pre-breathe time. 

This led to suggested operating points that pushed the limits of acceptable hypoxia from 

the 5000 to the 9000-foot range, and lower total pressures. There was also a call to consider 

being less conservative about DCS risk, with the caveat that any new protocols needed to be 

studied in altitude chamber tests. It was noted that operation at oxygen levels equivalent to 

altitudes above 5000 feet will require a NASA hypoxia standard change.

Considerable debate took place over desirability vs. feasibility of operating with an oxygen 

concentration above 30%. The ‘con’ argument was that most modern materials typically 

used in the Space Shuttle and ISS are only qualified to pass flammability standards at that 

level, and the process of doing new qualifications would be costly and time-consuming, 

reduce the availability of COTS solutions, and increase the difficulty of specifying the CEV 

which is on a short timetable. The ‘pro’ arguments included a reduced EVA pre-breathe 

time, the present existence of technical solutions, and the assertion that it would be far 

more expensive and time-consuming to raise the oxygen limit later in the program. Several 

participants proposed that any new flammability testing be done over a range of oxygen 

concentrations, to provide trade space information.

It was stated that the CEV would not be used for routine EVA’s, so its atmosphere design 

could be compatible with existing ISS and Shuttle standards. The more challenging 

proposals for oxygen concentration and partial pressure were for the transit vehicles and 

habitats, which have more lead time available to set specifications, engineer solutions, and 

perform experimental validation of protocols. It was suggested that ground-based testing 

of proposed atmospheres and pre-breathe protocols begin as soon as possible, to validate 

existing models of performance and DCS risk levels.

Several reviewers supported some form of multi-pressure regulator for space suits, though 

cautions of added complexity & weight were made.

Q8: How can EAWG most effectively proceed from this point?
162

Several reviewers emphasized the importance of reaching a consensus of the EAWG on specific 

recommendations, in order to give them sufficient weight to influence Constellation 

management. Some thought that the AHP method should be redone on the new suggested 

operating points. However, a larger number felt that a more effective process was to continue the 

open and vigorous technical dialogue among experts, until a well-considered compromise 

position was reached that everyone could support. In general, they did not feel that their time 

was well spent critiquing the FTA and AHP methods at this workshop.

5. EAWG Draft Consensus 

The EAWG membership met during the last afternoon of the workshop to discuss the how the 

group would use the comments and suggestion made by the reviewers. The discussion was 

focused on arriving at a design point consensus recommendation segmented by vehicle. Table 5-

1 summarizes the EAWG’s draft consensus.

Table 5-1 Summary of EAWG Draft Consensus 

Operating Pressure 

Capability (psia)

pO2 Floor 

Capability/Permitted 

(mmHg)

O2 Max (%)

CEV / ISS (Block Ia) 10.2 – 14.7

(0-14.7)

115-128 30%

CEV / LSAM (Block II) 8.0 – 14.7

(0-14.7)

115-120 30% @ 8.5 psia

34% @ 8 psia

CEV / Outpost (Block 

III)

Moon / Mars

7.6 – 14.7 115-115 34%

MT Habitat 10.2 – 14.7 130 30%

Surface Suit 3.5 – 4.3 (nom.) 

8.4 (emergency)

Issues associated with these design points include:

• Validation and change of a human standard for hypoxia (literature review)

• Cost and time required for DCS studies will be high, but necessary for these proposals

• Acclimation during various mission durations

• Air cooling at lower pressures (electronics)

• Material flammability issues across the 30–34% range

• Multiple food preparation applications

• Leak rates and total air leakage mass over extended mission times.
163

6. Appendices 

The following documents may be found in the Exploration Atmospheres Workshop Report 

Appendix, a companion document to this report. 

• Workshop Agenda

• Workshop Participants

• Plenary Session Presentations

• Tuesday, November 1

o EAWG Process Overviews

o White Paper Overviews

• Wednesday, November 2

o EAWG Results Overview

o Breakout Session #1 Charter and Reports

o Breakout Session #2 Charter

• Thursday, November 3

o Breakout Session #2 Reports

o Reviewer Discussion on Recommendations

• Prework Instructions and Worksheet

• Completed Worksheets
164

7. Acronym and Abbreviation List

AFB – Air Force Base

AHP – Analytical Hierarchy Process

ARC – Ames Research Center

CEV – Crew Exploration Vehicle

COTS – Commercial Off-the-Shelf

DCS – Decompression Sickness

Doppler U/S – Doppler Ultrasound

DSTB – Deep Space Test Bed

EAWG – Exploration Atmospheres Working Group

ECLS - Environmental Control and Life Support

ECLSS – Environmental Control and Life Support System

EMI – Electromagnetic Interference

ESAS – Exploration Systems Architecture Study

ESCG – Engineering and Science Contract Group

ESMD – Exploration Systems Mission Directorate

EVA – Extra Vehicular Activity

FTA – Fault Tree Analysis

G – Gravity

GCR – Galactic Cosmic Ray

GRC – Glenn Research Center

HEPA – High Efficiency Particulate Air (filter)

HQ – Headquarters

ISS – International Space Station

JSC – Johnson Space Center

LEO – Low Earth Orbit

LSAM – Lunar Surface Access Module

MEMS – Micro Electromechanical Systems

mmHg – millimeters Mercury

MSFC – Marshall Space Flight Center

MT – Mars Transit

N2 - Nitrogen

NASA – National Aeronautics and Space Administration

NIST – National Institute of Standards and Technology

O2 – oxygen

P – Pressure

P(DCS) - Probability of Decompression Sickness

PDR – Preliminary Design Review

PFO - Patent Foramen Ovale

pO2 – Oxygen Partial Pressure 

psia – absolute pounds per square inch (ambient pressure)

psig – gauge pounds per square inch (suit pressure)

R&D – Research and Development
165

R&T – Research and Technology

S&MA – Safety and Mission Assurance

SAIC – Science Applications International Corporation

SPE – Solar Particulate Event

SRR – System Requirements Review

TCS – Thermal Control Systems

TIA – Technology Integration Agent

TRL – Technology Readiness Level

USAF – United States Air Force

USRA – Universities Space Research Association

WSTF – White Sands Test Facility
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1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

October 2010 Technical Publication

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Recommendations for Exploration Spacecraft Internal Atmospheres: The Final Report of the 

NASA Exploration Atmospheres Working Group

6. AUTHOR(S)

NASA Exploration Atmospheres Working Group*

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

REPORT NUMBERS

Lyndon B. Johnson Space Center

Houston, Texas 77058

S-1080

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

National Aeronautics and Space Administration

Washington, DC 20546-0001

TP-2010-216134

11. SUPPLEMENTARY NOTES

*NASA Johnson Space Center, Houston

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Available from the NASA Center for AeroSpace Information (CASI)

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Hanover, MD 21076-1320 Category: 18

13. ABSTRACT (Maximum 200 words)

The authors of this document recommend that, to enable missions to the moon and Mars, NASA’s Constellation vehicles, habitats, and 

spacesuits employ an integrated set of internal atmospheres rather than a single design for all elements. These atmospheres must 

mitigate the risks of fire, decompression sickness, and hypoxia while enabling crew excursions on planetary surfaces. It should be 

noted that the recommendations provided in this document are not requirements; instead, they reflect technical considerations only. To 

support the recommendations made herein, Constellation should institute a modified standard NASA flammability test as soon as

possible to generate ignition threshold data for key spacecraft materials. This modified test will allow the agency to identify materials 

at risk from increases in oxygen concentration, minimize potential impacts, and allow for development of sound requirements for 

landers and habitats. The Human Research, Technology Development, and Constellation Programs should also initiate activities to 

enable atmosphere designs for, respectively, human research, technology development, and program-specific studies to validate these 

recommendations for the Constellation habitable elements.

14. SUBJECT TERMS 15. NUMBER OF 

PAGES

16. PRICE CODE

cabin atmospheres; controlled atmospheres; life support systems; decompression 

sickness; hypoxia; fire resistance; habitats; Constellation Program 178

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