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<tbody>
<tr class="odd">
<td colspan="12"><strong>JSC-39502</strong></td>
</tr>
<tr class="even">
<td colspan="12"><p><strong>CREW AND THERMAL SYSTEMS
DIVISION</strong></p>
<p><strong>NASA–LYNDON B. JOHNSON SPACE CENTER</strong></p>
<p><strong>HOUSTON, TEXAS</strong></p></td>
</tr>
<tr class="odd">
<td colspan="2"></td>
<td colspan="8"><p><strong>Advanced Life Support<br />
Systems Modeling and Analysis<br />
Reference Missions Document</strong></p>
<p><strong>Document Number</strong> <u>CTSD-ADV-383</u>
<strong>Date</strong> <u>December 01, 1999</u></p></td>
<td colspan="2"></td>
</tr>
<tr class="even">
<td colspan="12"></td>
</tr>
<tr class="odd">
<td></td>
<td colspan="3"><strong>PREPARED BY:</strong></td>
<td colspan="7">A. E. Drysdale</td>
<td></td>
</tr>
<tr class="even">
<td></td>
<td colspan="3"><strong>PREPARED BY:</strong></td>
<td colspan="7">S. Maxwell</td>
<td></td>
</tr>
<tr class="odd">
<td></td>
<td colspan="3"><strong>PREPARED BY:</strong></td>
<td colspan="7">A. J. Hanford</td>
<td></td>
</tr>
<tr class="even">
<td></td>
<td colspan="3"><strong>PREPARED BY:</strong></td>
<td colspan="7">M. K. Ewert</td>
<td></td>
</tr>
<tr class="odd">
<td></td>
<td colspan="3"><strong>APPROVED BY:</strong></td>
<td colspan="7">C. H. Lin</td>
<td></td>
</tr>
<tr class="even">
<td></td>
<td colspan="3"><strong>APPROVED BY:</strong></td>
<td colspan="7">D. L. Henninger</td>
<td></td>
</tr>
<tr class="odd">
<td></td>
<td colspan="3"><strong>APPROVED BY:</strong></td>
<td colspan="7">A. F. Behrend, Jr.</td>
<td></td>
</tr>
<tr class="even">
<td colspan="12">Number of pages: <u>___34__</u></td>
</tr>
<tr class="odd">
<td colspan="12"><strong>REVISIONS</strong></td>
</tr>
<tr class="even">
<td colspan="3"></td>
<td colspan="2"></td>
<td colspan="4">APPROVALS</td>
<td colspan="3"></td>
</tr>
<tr class="odd">
<td colspan="3">DATE</td>
<td colspan="2">AUTHOR</td>
<td>SECTION</td>
<td>BRANCH</td>
<td>DIVISION</td>
<td></td>
<td colspan="3">REV. LETTER</td>
</tr>
<tr class="even">
<td colspan="3"></td>
<td colspan="2"></td>
<td></td>
<td></td>
<td></td>
<td></td>
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</tr>
</tbody>
</table>

JSC FORM 151C (Rev Sept 86) NASA-JSC

**DRAFT (01 December 1999)**\
\
\
\
\
\
\
***Advanced Life Support\
Systems Modeling and Analysis\
Reference Missions Document***

Alan E. Drysdale, Ph.D.\
Sabrina Maxwell\
*The Boeing Company\
Kennedy Space Center, Florida 32815*

Anthony J. Hanford, Ph.D.\
*Lockheed Martin Space Operations\
Houston, Texas 77058*

Michael K. Ewert\
*National Aeronautics and Space Administration\
Lyndon B. Johnson Space Center\
Houston, Texas 77058*

December 1999

Contents

[**[1]{.smallcaps}** **[Introduction 1]{.smallcaps}**](#introduction)

[1.1 Goal 1](#goal)

[1.2 Approach 1](#approach)

[1.3 Scope 1](#scope)

[1.4 Mission Infrastructure and Associated Costs
1](#mission-infrastructure-and-associated-costs)

[1.5 Applicable Documents 1](#applicable-documents)

[1.6 Authority and Revision Control 1](#authority-and-revision-control)

[2 Orbiting Research Facility: International Space Station Mission
1](#orbiting-research-facility-international-space-station-mission)

[2.1 International Space Station Life Support System Hardware
1](#international-space-station-life-support-system-hardware)

[2.2 Advanced Life Support Straw Man Architecture
1](#advanced-life-support-straw-man-architecture)

[3 Independent Exploration Mission: Mars Combo Lander Architecture
1](#independent-exploration-mission-mars-combo-lander-architecture)

[3.1 Applying International Space Station Life Support Hardware
1](#applying-international-space-station-life-support-hardware)

[3.1.1 Mars Transit Vehicle 1](#mars-transit-vehicle)

[3.1.2 Combo Lander Vehicle 1](#combo-lander-vehicle)

[3.1.3 Mars Ascent Vehicle 1](#mars-ascent-vehicle)

[3.1.4 Mission Summary Using International Space Station Hardware
1](#mission-summary-using-international-space-station-hardware)

[3.2 Advanced Life Support Straw Man Architecture
1](#advanced-life-support-straw-man-architecture-1)

[3.2.1 Mars Transit Vehicle 1](#mars-transit-vehicle-1)

[3.2.2 Combo Lander Vehicle 1](#combo-lander-vehicle-1)

[3.3 Alternate Formats for Life Support System Architecture
1](#alternate-formats-for-life-support-system-architecture)

[3.3.1 In Situ Resource Utilization 1](#in-situ-resource-utilization)

[3.3.2 Bioregeneration 1](#bioregeneration)

[4 Concentrated Exploration Mission: Mars Split Mission Architecture
1](#concentrated-exploration-mission-mars-split-mission-architecture)

[4.1 Applying International Space Station Life Support Hardware
1](#applying-international-space-station-life-support-hardware-1)

[4.1.1 Surface Habitat Lander 1](#surface-habitat-lander)

[4.1.2 Predeployed Surface Assets 1](#predeployed-surface-assets)

[4.1.2.1 Mars Ascent Vehicle 1](#mars-ascent-vehicle-1)

[4.1.2.2 Inflatable Habitat 1](#inflatable-habitat)

[4.1.2.3 Pressurized Rovers 1](#pressurized-rovers)

[4.1.2.4 In Situ Resource Utilization Facilities
1](#in-situ-resource-utilization-facilities)

[4.1.3 Earth Return Vehicle 1](#earth-return-vehicle)

[4.1.4 Mission Summary Using International Space Station Hardware
1](#mission-summary-using-international-space-station-hardware-1)

[4.2 Advanced Life Support Straw Man Architecture
1](#advanced-life-support-straw-man-architecture-2)

[4.2.1 Surface Habitat Lander 1](#surface-habitat-lander-1)

[4.2.2 Earth Return Vehicle 1](#earth-return-vehicle-1)

[4.3 Alternate Formats for Life Support System Architecture
1](#alternate-formats-for-life-support-system-architecture-1)

[4.3.1 In Situ Resource Utilization 1](#in-situ-resource-utilization-1)

[4.3.2 Bioregeneration 1](#bioregeneration-1)

[5 Extended Presence: Evolved Mars Base
1](#extended-presence-evolved-mars-base)

[5.1 Applying International Space Station Life Support Hardware
1](#applying-international-space-station-life-support-hardware-2)

[5.2 Advanced Life Support Straw Man Architecture
1](#advanced-life-support-straw-man-architecture-3)

[5.3 Alternate Formats for Life Support System Architecture
1](#alternate-formats-for-life-support-system-architecture-2)

[5.3.1 In Situ Resource Utilization 1](#in-situ-resource-utilization-2)

[5.3.2 Plant Growth Using Natural Lighting
1](#plant-growth-using-natural-lighting)

[6 Other Considerations 1](#other-considerations)

[6.1 Contingencies 1](#contingencies)

[6.1.1 International Space Station Contingency Approach
1](#international-space-station-contingency-approach)

[6.1.2 Advanced Life Support Straw Man Contingency Approach
1](#advanced-life-support-straw-man-contingency-approach)

[6.1.3 Contingency Issues 1](#contingency-issues)

[6.2 Alternative Mission Scenarios 1](#alternative-mission-scenarios)

[7 Potential Additional Scenarios 1](#potential-additional-scenarios)

[8 References 1](#references)

[9 Appendices 1](#appendices)

[9.1 Appendix A: Acronyms and Abbreviations
1](#appendix-a-acronyms-and-abbreviations)

Tables

[Table 1.3.1 A List of Life Support Subsystems
1](#__RefHeading___Toc467051662)

[Table 1.3.2 A List of Life Support Subsystems Based on the Advanced
Life Support Project 1](#__RefHeading___Toc467051663)

[Table 1.4.1 Infrastructure Equivalencies for Mission Environments
1](#__RefHeading___Toc467051664)

[Table 2.1.1 International Space Station Life Support System Equivalent
Mass Summary 1](#__RefHeading___Toc467051665)

[Table 3.1.1 Independent Exploration Mission Life Support System
Equivalent Mass Summary 1](#__RefHeading___Toc467051666)

[Table 3.2.1 Exploration Mission Advanced Life Support Straw Man Systems
1](#__RefHeading___Toc467051667)

[Table 3.3.1 Composition of the Martian Atmosphere
1](#__RefHeading___Toc467051668)

[Table 4.1.1 Concentrated Exploration Mission Life Support System
Equivalent Mass Summary 1](#__RefHeading___Toc467051669)

[Table 5.1.1 Evolved Mars Base Life Support System Equivalent Mass
Summary 1](#__RefHeading___Toc467051670)

[Table 5.2.1 Extended Duration Advanced Life Support Straw Man Systems
1](#__RefHeading___Toc467051671)

[Table 6.1.1 International Space Station Contingency Approach
1](#__RefHeading___Toc467051672)

[Table 6.1.2 International Space Station Contingency Analysis
1](#__RefHeading___Toc467051673)

# Introduction

## Goal

This document provides the Advanced Life Support (ALS) Project with
several reference missions that are likely scenarios for near-term human
exploration and development of near-Earth space. It expounds mission
details to allow meaningful system-level analyses of proposed life
support system (LSS) designs. As such, this document is a supplement for
other analysis reference documents.

Another specific goal here is to define realistic technology options for
each of four long-duration missions. It will document two approaches to
providing life support for these missions. The first approach applies
currently existing, flight-qualified hardware, including equipment
planned for the International Space Station (ISS). The second approach,
referred to as the "Advanced Life Support Straw Man Architecture,"
applies technologies under development within the ALS Project. These
architecture scenarios for the missions presented are not intended to be
a definitive selection of technologies, but rather a starting point for
analyses studies.

## Approach

The technologies recommended for the ALS straw man architecture
associated with each mission are those that initially appear to be the
most economical, in terms of overall system mass, after considering a
variety of current and near-term technologies. For this evaluation,
technologies were drawn from the state-of-the-art based on ISS
technology (Carrasquillo, *et al.*, 1997, and Wieland, 1998), ALS
systems using mostly physicochemical (PC) technologies (ALS-PC) (Lin,
1997a and 1997b), and four ALS bioregenerative (ALS-BIO) options based
on the current BIO-Plex biomass production system (BPS), two versions of
an improved BPS, and a BPS using in situ resource utilization (ISRU) and
natural sunlight.

The current state-of-the-art for long-duration life support is embodied
in the ISS environmental control and life support system (ECLSS)
(Carrasquillo, *et al.*, 1997, and Wieland, 1998). Much of the hardware
for the ISS ECLSS has been developed and ground-tested to a fairly high
maturity. This approach is documented for comparison with ALS
approaches.

The ALS Project is exploring technologies that are expected to provide
significant improvements over the ISS technologies, reducing overall
mass, power, cooling, and crew time requirements, for specified levels
of performance. The Systems Modeling and Analysis Project Element (SMAP)
has the task of recommending the optimal set of technologies for each
advanced mission. The technologies selected will depend critically on
the mission parameters, the range of technologies deemed ready for
flight, and on the infrastructure costs. This document defines realistic
initial ALS system designs for the missions considered. Future analyses
will improve and refine these ideas.

The approach taken for comparing options is to collect data on mass,
volume, power and cooling, crew time, and resupply for the various life
support subsystems. This data is fairly independent of the mission. For
each mission, technology scenarios have been developed, resizing
components for the number of crew, taking into account mission operating
environments. Equivalent system masses (ESM) were then calculated for
each option using the selected infrastructure equivalencies.

So far as possible, contingency provisions are separated from nominal
provisions. This makes the contingency assumptions more visible and,
hopefully, more completely defined.

## Scope

The LSS is defined broadly to include not only the traditional ECLSS
functions of providing clean air and water, but also to encompass
interfaces with and impacts of other subsystems that impact the
traditional ECLSS functions. For example, a clothes laundry requires a
non-trivial amount of water while a plant growth system provides both
oxygen and edible biomass that can be processed into food. A complete
list of life support subsystems, based on the organization of the ISS
ECLSS, is provided in Table 1.3.1. To make analysis simpler, the
organization here groups functions which depend on similar parameters.
Thus, the "cabin" includes both the habitat gases and the gas leakage as
both depend on the overall habitat volume. Table 1.3.2 provides a
similar classification of subsystems from the ALS Project's
Bioregenerative Planetary Life Support Systems Test Complex (BIO‑Plex).

[]{#__RefHeading___Toc467051662 .anchor}Table 1.3.1 A List of Life
Support Subsystems

  -------------------- --------------------------------------------------
  **Subsystem**        **Description or Scope**

  Air                  Maintains composition, including circulation. This
                       category includes the air revitalization system,
                       the temperature and humidity control system, and
                       the trace contaminant control system.

  Cabin                Provides pressured volume and accounts for gas
                       leakage

  Clothing             Provides clean clothes for the crew.

  Extravehicular       Provides life support consumables, oxygen and
  Activity (EVA)       water, and carbon dioxide removal for
  Support              extravehicular activities.

  Food                 Provides food for the crew. If plants are
                       employed, this system also regenerates air and
                       water.

  Radiation            Provides radiation protection for the crew if
                       necessary.

  Waste                Disposes of and/or recycles solid wastes,
                       including products from other systems.

  Water                Provides potable and hygiene water for the crew.
  -------------------- --------------------------------------------------

[]{#__RefHeading___Toc467051663 .anchor}Table 1.3.2 A List of Life
Support Subsystems Based on the Advanced Life Support Project

  -------------------- ------------- ------------------------------------------
  **Subsystem**        **Acronym**   **Comments**

  Air Revitalization   ARS           The ARS maintains the vehicle cabin gases,
                                     including the overall composition and
                                     atmospheric pressure.

  Water Recovery       WRS           The WRS provides water at the appropriate
                                     purity for crew consumption and hygiene.

  Biomass Production   BPS           The BPS provides raw agricultural products
                                     to the FPS while regenerating air and
                                     water. This subsystem is not present in a
                                     solely PC LSS.

  Food Processing      FPS           The FPS transforms raw or bulk
                                     agricultural products into foodstuffs.

  Solids Processing    SPS           The SPS handles solid waste produced
                                     anywhere in the LSS, including packaging,
                                     human wastes, and brines from other
                                     subsystems such as the WRS. The SPS may
                                     sterilize and store the waste, or reclaim
                                     LSS commodities, depending on the LSS
                                     closure and/or mission duration.

  Thermal Control      TCS           The TCS is responsible for maintaining
                                     cabin temperature and humidity within
                                     appropriate bounds and for rejecting the
                                     collected waste heat to the environment.

  Integrated Control   ICS           The ICS provides appropriate control for
                                     the LSS.

  Human Accommodations HAS           The HAS is responsible for the crew cabin
                                     layout, crew clothing, and the crew's
                                     interaction with the LSS.
  -------------------- ------------- ------------------------------------------

This document defines one technology set for each of four mission
scenarios: an orbiting research facility, which is the ISS mission, an
exploration format which is independent of previous or future site
selection, which is the Mars Combo Lander Architecture, an exploration
format which provides surface infrastructure and a concentrated
examination of a single site, which is the Mars Split Mission
Architecture, and an Evolved Mars Base. The Evolved Mars Base mission
considers only the nominal, or steady state, base operation. The Base
construction phase is not defined, but evolution from infrastructure
emplaced by a Mars Split Mission Architecture is a possible scenario.

Some other options are also of interest, notably using the same transit
vehicle for both the outbound and return trip, like the Mars Combo
Lander, but visiting the same site several times, as in the Mars Split
Mission format. These alternative scenarios are discussed further in
Section 6.2.

## Mission Infrastructure and Associated Costs

The ESM for any mission depends on the crew size, the mission duration,
the mission architecture, and the technology used. Here it is assumed
that a technology's ESM scales linearly with crew size and mission
duration for the ranges of interest. More specifically, crew size varies
from 4 to 9 individuals while mission duration varies from 180 days, for
transit to or from Mars, to almost 15 years, for the life of a Mars
Base. Mission LSS architecture affects the number of ship-sets of
equipment required and contingency planning. Infrastructure
equivalencies define the cost, in mass, for using supporting
infrastructure with a particular technology. Equivalencies used in
evaluating technology applicability are shown in Table 1.4.3. For each
ALS straw man architecture, the most cost-effective technologies, based
on ESM assessments, are generally assumed.

The infrastructure equivalencies arise from a variety of sources. Mars
volume equivalencies assume an inflatable primary structure, with a
water radiation shield in the transit case. The ISS volume equivalency
is for the ISS common module machined out of aluminum. Solar power is
assumed for Mars transit and operations in low Earth orbit. The
equivalency for ISS power depends on whether the associated power
requirements are continuous, or if power is only required while the
vehicle is in sunlight. Continuous power using solar photovoltaic cells
for generation requires some form of energy storage. For ISS, batteries
are used for storage. Dust storms and the high penalty for power storage
makes solar power generation on the surface of Mars impractical, so
nuclear power is assumed for surface operations. Nuclear power becomes
highly cost effective for larger capacity systems. Based on data in
Cataldo (1998), the power generation mass as a function of power
delivered to the user may be expressed as a linear function.

$\text{Power  System  Mass   } = \text{   1}\text{.}\text{94 MT   } + \text{   0}\text{.}\text{0268  }\frac{\text{MT}}{\text{kW}} \times (\text{User  Power  Delivered})$

where MT denotes metric tons. Crew time availability varies according to
the time allotted for crew health maintenance. The difference in
available crewmember hours [^1] per week (ch/wk) reflects the need for
exercise to minimize effects of weightlessness during transit and
orbital mission phases [^2]. The gravity on Mars is assumed to be
adequate for maintaining crew fitness using fewer hours of dedicated
exercise time.

[]{#__RefHeading___Toc467051664 .anchor}Table 1.4.3 Infrastructure
Equivalencies for Mission Environments

+-------------------+------+------+------+--------+-------------------+
| **\               | *    |      |      | **Crew | **\               |
| \                 | *Equ |      |      | Time   | \                 |
| Mission           | ival |      |      | A      | Source**          |
| Environment**     | enci |      |      | vailab |                   |
|                   | es** |      |      | ility\ |                   |
|                   |      |      |      | \[ch/  |                   |
|                   |      |      |      | wk\]** |                   |
+-------------------+------+------+------+--------+-------------------+
|                   | *    | **Po | **   |        |                   |
|                   | *Vol | wer\ | Cool |        |                   |
|                   | ume\ | \[k  | ing\ |        |                   |
|                   | \[m  | W/kg | \[k  |        |                   |
|                   | ³/kg | \]** | W/kg |        |                   |
|                   | \]** |      | \]** |        |                   |
+-------------------+------+------+------+--------+-------------------+
| **Mars Transit**  | > 0  | > 0  | > 0  | > 59   | Drysdale and      |
|                   | .062 | .012 | .043 |        | Hanford (1999)    |
+-------------------+------+------+------+--------+-------------------+
| **Mars Surface**  | > 0  | > 0  | > 0  | > 66   | Drysdale and      |
|                   | .480 | .012 | .015 |        | Hanford (1999)    |
+-------------------+------+------+------+--------+-------------------+
| > **ISS (Low      | > 0  | > 0  | > 0  | > 59   | Drysdale (1998)   |
| > Earth Orbit;\   | .015 | .002 | .006 |        |                   |
| > Continuous      |      |      |      |        |                   |
| > Power)**        |      |      |      |        |                   |
+-------------------+------+------+------+--------+-------------------+
| > **ISS (Low      |      | > 0  |      |        | Drysdale (1998)   |
| > Earth Orbit;\   |      | .013 |      |        |                   |
| > Power in        |      |      |      |        |                   |
| > Sunlight        |      |      |      |        |                   |
| > Only)**         |      |      |      |        |                   |
+-------------------+------+------+------+--------+-------------------+

## Applicable Documents

The following documents are considered to be of some importance, and if
there are conflicts between these documents and this baseline document,
these other documents will have precedence. The most recent releases of
these documents are:

-   Behrend, A. F., Jr., *et al.* (1999) "Advanced Life Support Project
    Plan," JSC‑39168 (CTSD‑ADV‑348), Revision B, National Aeronautics
    And Space Administration, Johnson Space Center, Houston, Texas

-   Drysdale, A. E., and Hanford, A. J., (1999) "Advanced Life Support
    Systems Modeling and Analysis project Baseline Values and
    Assumptions Document," JSC‑39317 (CTSD‑ADV‑371) National Aeronautics
    and Space Administration, Johnson Space Center, Houston, Texas.

-   Ewert, M. K., and Hanford, A. J., (1998) "Systems Modeling and
    Analysis Project Plan," JSC‑39149, National Aeronautics and Space
    Administration, Johnson Space Center, Houston, Texas

Another document related to this document is:

-   Drysdale, A. E. and Hanford, A. J. (1999) "Advanced Life Support
    Research and Technology Development Metric Document," JSC‑39503
    (CTSD‑ADV‑384), National Aeronautics and Space Administration,
    Johnson Space Center, Houston, Texas.

## Authority and Revision Control

This document will be revised as needed if significant changes are
required. Change control authority will reside with the SMAP Manager. As
currently envisioned, other documentation will update the ALS straw man
architecture designs presented below as current and emerging life
support technologies become more cost-effective and reliable through
research and development or through more efficient architecture designs.
Further, additional reference missions may be added to future revisions
of this document to reflect changes in NASA's plans and goals. Suggested
changes should be provided to:

M. K. Ewert, Manager\
Systems Modeling and Analysis Project Element\
NASA Johnson Space Center\
Mail Code EC2\
2101 NASA Road One, Houston, TX 77058\
\
E-mail: michael.k.ewert1@jsc.nasa.gov

# Orbiting Research Facility: International Space Station Mission

ISS will provide a permanently habitable facility in low Earth orbit for
conducting scientific research across various disciplines, including
biomedical and micro-gravity research. It has a planned life of ten
years and the United States Operating Segment (USOS) ECLSS is generally
sized for a crew of six, with supplies for four.

The ISS will support a crew of three at Permanent Human Capability
(Phase 2) and six following Assembly Complete (Phase 3). The life
support system is described in Reuter and Reysa (1997) and Wieland
(1998). ECLSS components are distributed among the various modules.
Design responsibility is similarly spread among the various partners.
Components and design philosophies differ according to the design
responsibility. Initial components are a Russian responsibility.
Additional ones are an American responsibility. As international partner
elements are added, international responsibilities are added, though
primarily for temperature and humidity control, fire detection and
suppression, and for experiment-support such as vacuum systems. Some
items, such as solid oxygen generators, are unique to one partner. This
document primarily addresses USOS components.

## International Space Station Life Support System Hardware

Life support aboard ISS is based on a 90‑day resupply schedule, with a
45‑day contingency cache. Oxygen is supplied by water electrolysis and
pressurized-gas storage. High efficiency particulate air (HEPA) filters
remove particulates. A four-bed molecular sieve using zeolite 13X and
silica gel carbon dioxide removal assembly (CDRA) removes carbon
dioxide. Water is removed from the air by the condensing heat exchanger
in the common cabin air assembly (CCAA) and the desiccants in the CDRA.
Higher molecular weight compounds are removed by the trace contaminant
control system (TCCS) using activated carbon and a high temperature
catalytic oxidizer (HTCO) converts low molecular weight compounds to
water vapor and carbon dioxide. Human solid waste collection is provided
by a removable and reusable canister in the commode that collects the
waste for return to Earth. A trash compactor will be used for other
solid waste, which will also be returned to Earth. Urine is stabilized
with Oxone and sulfuric acid before delivery to the urine processor
(UP). The UP removes most of the water, nominally 95%, for recycling
while the remaining brine is returned to Earth. Grey water, including
effluent from the UP and cabin condensate, is processed through the
water processor (WP), producing potable water for the crew to drink,
hygiene use, and use in the oxygen generation assembly (OGA). The WP
uses multifiltration to remove particles and solutes from the water. A
volatile removal assembly (VRA) oxidizes low molecular weight alcohols
in the water. Iodine is added as a biocide.

Clothing on ISS is supplied from the ground. Before flying, each
crewmember selects a set of clothes from an approved list, and those
clothes are packaged and shipped to the ISS. Dirty clothes are returned
and laundered on the ground. The allowance is currently 1.7 kg/cd
(Branch, 1998). A stowage factor of 1.5 is assumed. Thus, the total
clothing mass for a crew of six would be 2.6 kg/cd, or 5,585 kg/y.

ISS food will be supplied every 90 days by exchanging the pressurized
logistics module (PLM). Because solar photovoltaic panels provide
electrical power, no extra water will be generated onboard [^3]. Most of
the food planned for ISS will be refrigerated, frozen, or
thermostabilized and will not require hydration before serving. Many
beverages are supplied dehydrated. Food will be heated to serving
temperature in a microwave/convention oven. Food shipped within a PLM
will be packaged in Russian food lockers. One locker contains provisions
for 3 cd (1.76 kg/person/day), and has a mass of 7 kg.

Extravehicular activity (EVA) support is not generally considered to be
part of the LSS. However, it does have significant interfaces with the
LSS. EVA support itself is not included in life support in this
document, but interfaces with the LSS, such as provision of oxygen,
water, and food, and removal of carbon dioxide, are included. EVA
following assembly of ISS is assumed to average one 2-crew-member
excursion twice per month. While EVA will be necessary for maintenance,
the majority of ISS tasks will be conducted inside the vehicle.

ISS life support system data has been calculated for a crew of six to
maintain consistency with other mission scenarios presented in this
document. All identified contingency support data has been removed, and
numbers are given for a mission duration of ten years in Table 2.1.4.

[]{#__RefHeading___Toc467051665 .anchor}Table 2.1.4 International Space
Station Life Support System Equivalent Mass Summary

  ---------------- ----------- ------------ ----------- ------------ ------------ -------------
  **Subsystem**    **Mass\     **Volume\    **Power\    **Cooling\   **Total\     **Total\
                   \[kg\]**    \[m^3^\]**   \[kW\]**    \[kW\]**     Crew Time\   Logistics\
                                                                     \[ch\]**     \[kg\]**

  Air              1,321       72.15        6.02        6.02         163          18,771

  Cabin            388         300.00       0.00        0.00         0            2,465

  Clothing         0           0.00         0.00        0.00         0            55,845

  Extravehicular   70          0.14         0.01        0.01         0            2,138
  Activity support                                                                

  Food             0           3.00         0.00        0.00         10,950       85,629

  Radiation        0           0.00         0.00        0.00         0            0

  Waste            77          0.09         0.76        0.76         900          4,391

  Water            448         2.00         0.65        0.65         295          9,795

  **Totals**       **2,304**   **377.38**   **7.44**    **7.44**     **12,308**   **179,034**

                                                                                  

  **Equivalent     **2,304**   **25,158**   **3,542**   **1,219**                 **179,034**
  System\                                                                         
  Mass \[kg\]**                                                                   
  ---------------- ----------- ------------ ----------- ------------ ------------ -------------

Note that data for logistics and crew time have been calculated over the
total mission duration of ten years. Total ESM, corrected for life
support system (LSS) crew time, is 226.4 metric tons (MT).

## Advanced Life Support Straw Man Architecture

Though ISS technology is mature and provides a set of baseline
technologies for all missions presented here, some technology options to
reduce ISS life support operating cost are potentially available.
Possible regenerative ALS technologies for ISS include a carbon dioxide
reduction system (CRS), a biological water processor, an improved CDRA,
and an advanced TCCS. These options would benefit ISS by decreasing
annual resupply and power requirements compared with the baseline ECLSS
technologies.

# Independent Exploration Mission: Mars Combo Lander Architecture

The Mars Combo Lander mission architecture employs two vehicles: a Mars
Transit Vehicle and a Combo Lander Vehicle (Conley, *et al.*, 1999). A
single Mars Transit Vehicle is used for the outbound and return trips
and a separate landing vehicle, the Combo Lander, contains a surface
habitat and an ascent vehicle. This approach eliminates the benefit of
repeated use of the same site, where redundancy or common use can reduce
the overall cost of multiple missions. However, surface site selection
is independent of any previous or following missions. Thus, multiple
trips to Mars will allow explorers to visit any site, targeting
exploration opportunities to satisfy demands for scientific information.
However, the only infrastructure at Mars that might be reused from one
mission to the next are orbital facilities, such as communication
satellites or a spare transit vehicle stored in Martian orbit as a
contingency measure.

The transit vehicle launches from the surface into low Earth orbit.
After outfitting, the transit vehicle boosts to high Earth orbit to
await transfer of the crew. Similarly, the Combo Lander launches into
low Earth orbit, outfits with propulsion stages, and also boosts into
high Earth orbit. In both cases, the transit vehicle and Combo lander
use energy efficient electrical propulsion and lengthy transfer orbits
to reach high Earth orbit. Both vehicles are serviced for the voyage to
Mars and the crew is delivered to the transit vehicle by a taxi flight
just before departure. The voyage to Mars will nominally take 180 days.
The Mars Transit Vehicle and Combo Lander both enter into a low Mars
orbit through aerocapture, and rendezvous.

The crew transfers to the Combo Lander and descends to land on Mars.
During the surface mission, nominally 600 days, the transit vehicle
waits untended in low Mars orbit. The Combo Lander surface habitat is
integrated with the ascent vehicle. When the ascent vehicle leaves Mars,
the surface habitat is destroyed. Following a second rendezvous in Mars
orbit, the crew transfers to the transit vehicle and returns to Earth.
Again, the interplanetary voyage will nominally require 180 days.

## Applying International Space Station Life Support Hardware

Excluding differences in overall mission duration and EVA frequency, the
Mars Combo Lander LSS hardware using ISS ECLSS technologies is identical
to that presented in Section 2.1. As noted above, this mission
architecture uses two long-duration vehicles. The crew travels to and
from Mars in the Mars Transit Vehicle. The Combo Lander houses the crew
for the descent to Mars and during the surface stay. A separate
short-duration ascent vehicle returns the crew to Martian orbit to
rendezvous with the Mars Transit Vehicle for the trip back to Earth.

### Mars Transit Vehicle

The Mars Transit Vehicle is expected to transport the crew to and from
Mars so it will carry a full set of LSS hardware. Each voyage between
Earth and Mars is expected to nominally require 180 days. EVA is not
anticipated except as a contingency, so no allowance is provided on the
transit vehicle. This vehicle uses an aluminum exterior shell. The crew
cannot return to Earth early with the current or foreseeable near-term
propulsion capabilities. Because the rendezvous with the Combo Lander
may fail to occur, the Mars Transit Vehicle must provide contingency
capability, notably a stock of food, for 600 days in Martian orbit to
maintain the crew until the next transportation opportunity to Earth is
available. Using these assumptions, the Mars Transit Vehicle LSS will
have an equivalent mass of 30.1 MT using ISS ECLSS technologies. The
contingency stock of food and LSS consumables, for an additional
600 days of crew habitation, adds an additional 29.2 MT. If unused,
these contingency provisions would be jettisoned before initiating the
return to Earth.

### Combo Lander Vehicle

The Combo Lander Vehicle will journey to Mars without a crew. After a
rendezvous with the Mars Transit Vehicle in Martian orbit, the crew will
transfer to the lander to descend to Mars. On Mars, the lander will
provide a crew habitat for the surface operations for a nominal duration
of 600 days. Thus, the Combo Lander Vehicle will use an inflatable shell
and carry a full set of LSS hardware. During surface operations, the
crew is expected to perform extensive EVA operations, averaging one
2-crew-member EVA per weekday, or five such sorties every seven days.
Using these assumptions, the Combo Lander Vehicle LSS will have an
equivalent mass of 41.1 MT using ISS ECLSS technologies.

### Mars Ascent Vehicle

While the Mars Ascent Vehicle is essential to mission success, the crew
occupies it for a very brief period as they ride from the surface of
Mars to the Mars Transit Vehicle. As such, it is likely that the ECLSS
for the Mars Ascent Vehicle will use consumable life support
technologies like those employed for the Space Transportation System.
Thus, the Mars Ascent Vehicle is not considered further here.

### Mission Summary Using International Space Station Hardware

For a single mission using the Mars Combo Lander architecture, the
overall ESM for the LSS, divided by functional area, is summarized in
Table 3.1.5 below.

[]{#__RefHeading___Toc467051666 .anchor}Table 3.1.5 Independent
Exploration Mission Life Support System Equivalent Mass Summary

  ---------------- ----------- ------------ ----------- ------------ ------------- ------------
  **Subsystem**    **Mass\     **Volume\    **Power\    **Cooling\   **Total\      **Total\
                   \[kg\]**    \[m^3^\]**   \[kW\]**    \[kW\]**     Crew Time\    Logistics\
                                                                     \[ch\]**      \[kg\]**

  Air              2,642       144.30       12.04       12.04        42.9          4,937

  Cabin            776         600.00       0.00        0.00         0.0           648

  Clothing         0           0.00         0.00        0.00         0.0           14,688

  Extravehicular   70          0.14         0.01        0.01         0.0           5,346
  Activity support                                                                 

  Food             0           6.00         0.00        0.00         2,880.0       22,522

  Radiation        1,744       1.29         0.00        0.00         0.0           0

  Waste            154         0.18         1.52        1.52         236.7         1,155

  Water            896         4.00         1.30        1.30         77.6          2,576

  **Totals**       **6,282**   **756.91**   **14.87**   **14.87**    **3,237.2**   **51,872**

                                                                                   

  **Equivalent     **6,282**   **6,874**    **1,266**   **653**                    **51,872**
  System\                                                                          
  Mass \[kg\]**                                                                    
  ---------------- ----------- ------------ ----------- ------------ ------------- ------------

Note that data for logistics and crew time have been calculated over the
total mission duration. Total LSS ESM, corrected for LSS crew time, is
71.2 MT plus 29.2 MT for contingency (See Section 3.1.1). For three
missions to a single site using this mission architecture, the total LSS
ESM is 213.6 MT plus 87.6 MT for contingency.

## Advanced Life Support Straw Man Architecture

The proposed ALS straw man architecture for exploration missions is a
mix of Space Transportation System (STS), ISS, and ALS equipment as
summarized in Table 3.2.6. Air regeneration uses solid amine vacuum
desorbed (SAVD), Sabatier, and solid water polymer electrolysis (SPWE)
equipment. Water recovery includes a rapid cycle biological water
processor (BWP) to degrade organic contaminants, reverse osmosis (RO) to
remove inorganic contaminants, and an air evaporation subsystem (AES) to
reclaim water from the RO brine. Post treatment includes a Milli-Q
system for ammonia removal, an aqueous phase catalytic oxidation
subsystem (APCOS) as a final post-treatment step, and addition of iodine
to control bacteria. Initially, waste is collected, sterilized, and
stored. Later the waste is disposed of overboard or stored to provide
radiation protection, depending on the mission protocol for waste
disposal. An oxidation process, such as an incinerator, is not listed as
a technology option because reclamation of resources is neither
necessary nor economical at this level of food closure (See Gertner,
1999a). Heat is collected by use of a water/glycol loop, using fixed
cold plates where possible, and rejected by use of lightweight
body-mounted radiators.

[]{#__RefHeading___Toc467051667 .anchor}Table 3.2.6 Exploration Mission
Advanced Life Support Straw Man Systems

+-----------------------------+-----------------------------------------+
| **System**                  | **Comments**                            |
+-----------------------------+-----------------------------------------+
| **Air**                     |                                         |
+-----------------------------+-----------------------------------------+
| > Improved Carbon Dioxide   | Removes carbon dioxide from cabin air   |
| > Removal Assembly          | using solid amine or carbon bed         |
|                             | technology.                             |
+-----------------------------+-----------------------------------------+
| > Sabatier                  | Carbon dioxide reduction                |
+-----------------------------+-----------------------------------------+
| > Solid Polymer Water       | Oxygen production based on electrolysis |
| > Electrolysis              | of water.                               |
+-----------------------------+-----------------------------------------+
| > Catalytic Oxidation       | An advanced trace contaminant control   |
|                             | technology.                             |
+-----------------------------+-----------------------------------------+
| > Avionics Air Assembly     | Air-cooling for avionics.               |
+-----------------------------+-----------------------------------------+
| > Anti-microbial condensing | Humidity and temperature control.       |
| > heat exchanger            |                                         |
+-----------------------------+-----------------------------------------+
| > Reusable filters          | Particulate and microbe control.        |
+-----------------------------+-----------------------------------------+
| > Intermodule ventilation   | Support and control air exchange        |
| > fans & valves             | between modules.                        |
+-----------------------------+-----------------------------------------+
| > Atmosphere composition    | Monitors the atmospheric composition    |
| > monitors                  | for major constituents.                 |
+-----------------------------+-----------------------------------------+
| > Portable fire             | Cabin fire suppression.                 |
| > extinguishers             |                                         |
+-----------------------------+-----------------------------------------+
| > Smoke detectors           | Fire detection.                         |
+-----------------------------+-----------------------------------------+
| **Cabin**                   |                                         |
+-----------------------------+-----------------------------------------+
| > Software operated         | Atmospheric pressure control.           |
| > pressure control          |                                         |
+-----------------------------+-----------------------------------------+
| > Bottled gases (oxygen and | Initial cabin pressurization and        |
| > nitrogen)                 | leakage make-up.                        |
+-----------------------------+-----------------------------------------+
| **Clothing**                |                                         |
+-----------------------------+-----------------------------------------+
| > Clothing                  | An initial supply of crew clothing.     |
+-----------------------------+-----------------------------------------+
| > Laundry                   | To clean and recycle crew clothing.     |
+-----------------------------+-----------------------------------------+
| **Extravehicular Activity   |                                         |
| Support**                   |                                         |
+-----------------------------+-----------------------------------------+
| > ISS technology            | Only EVA consumables will be provided   |
|                             | by the ECLSS. EVA equipment will be     |
|                             | provided by other systems.              |
+-----------------------------+-----------------------------------------+
| **Food**                    |                                         |
+-----------------------------+-----------------------------------------+
| > ISS Food System           | Prepackaged food modified for           |
|                             | long-duration shelf stability.          |
+-----------------------------+-----------------------------------------+
| > Salad Machine             | To provide a mass of fresh food with    |
|                             | insignificant caloric content through   |
|                             | fresh salad crops.                      |
+-----------------------------+-----------------------------------------+
| **Radiation**               |                                         |
+-----------------------------+-----------------------------------------+
| > Bladder-less tanks        | To hold water for protection during     |
|                             | transit.                                |
+-----------------------------+-----------------------------------------+
| > Regolith bags             | To hold regolith in place on top of the |
|                             | surface crew habitat.                   |
+-----------------------------+-----------------------------------------+
| > Excavation equipment      | To fill regolith bags with regolith for |
|                             | surface vehicles.                       |
+-----------------------------+-----------------------------------------+
| **Waste**                   |                                         |
+-----------------------------+-----------------------------------------+
| > Bladder-less tanks        | Urine storage.                          |
+-----------------------------+-----------------------------------------+
| > ISS Human Waste           | Urine and feces collection.             |
| > Management System         |                                         |
+-----------------------------+-----------------------------------------+
| > ISS Compactor             | Non-human solid waste collection and    |
|                             | storage.                                |
+-----------------------------+-----------------------------------------+
| > Solid Waste Processor     | Stabilization and treatment, as         |
|                             | necessary, of solid wastes.             |
+-----------------------------+-----------------------------------------+
| > Waste storage containers  | Solid waste disposal. Modules are       |
|                             | stored or jettisoned as appropriate to  |
|                             | the mission protocol and phase.         |
+-----------------------------+-----------------------------------------+
| **Water**                   |                                         |
+-----------------------------+-----------------------------------------+
| > Biological Water          | Primary water processor to remove       |
| > Processor                 | organic compounds.                      |
+-----------------------------+-----------------------------------------+
| > Reverse Osmosis           | Water processor to remove inorganic     |
|                             | compounds.                              |
+-----------------------------+-----------------------------------------+
| > Air Evaporation System    | Reclaims water from reverse osmosis     |
|                             | brine.                                  |
+-----------------------------+-----------------------------------------+
| > Milli-Q                   | Initial water post-processing step.     |
+-----------------------------+-----------------------------------------+
| > Aqueous Phase Catalytic   | Final water polishing step.             |
| > Oxidation                 |                                         |
+-----------------------------+-----------------------------------------+
| > Bladder-less tanks        | Water storage.                          |
+-----------------------------+-----------------------------------------+
| > ISS Water Monitor         | Water contaminant monitoring.           |
+-----------------------------+-----------------------------------------+
| > Iodine Microbial Check    | Microbial control.                      |
| > Valve                     |                                         |
+-----------------------------+-----------------------------------------+

Food systems for the exploration mission ALS straw man design are
similar to ISS technology. Prepackaged food with an extended shelf life
will provide almost all of the crew's nutritional needs. A salad machine
will provide fresh salad vegetables to vary the diet and to demonstrate
plant growth technologies for an exploration mission. However, the salad
machine will not provide significant air regeneration nor will it be
tasked with any water recovery functions.

Nontraditional life support functions, such as clothing and radiation
protection will differ from previous formats for the exploration mission
ALS straw man design. The clothing system will use significantly fewer
clothes than the corresponding ISS approach. To provide the crew with
clean clothing, an aqueous laundry will be provided. The interfaces with
the water recovery system are to provide clean water and cleanup dirty
water. The radiation protection system, which is not necessary for ISS,
will use water, waste products, or a combination of these products to
provide a safe haven for the crew during periods of energetic solar
radiation. On the surface of Mars, local regolith can also be used to
provide a radiation barrier to shelter the crew.

### Mars Transit Vehicle

The Mars Transit Vehicle is a pure spacecraft. As such, the technologies
employed must be insensitive to changes in local gravity, including
microgravity. In general, the technologies presented as part of the
Exploration Mission ALS Straw Man design in Table 3.2.6 do not have any
known theoretical limitations that would prohibit their use in
microgravity. However, radiation protection for the Mars Transit Vehicle
must rely solely on commodities brought from Earth, such as water, or
those generated from supplied commodities, such as waste products.

### Combo Lander Vehicle

The Combo Lander Vehicle, though it will spend most of its human-tended
life on Mars, must also employ microgravity-insensitive technologies for
at least some functions because the crew will descend to the surface in
this vehicle. However, due to the short duration of the transit from
orbit to the surface of Mars, surface regolith could be utilized for
radiation protection once the Combo Lander is fully emplaced. Likewise,
components of systems which are not immediately required during the
descent maneuver, such as possibly the water recovery system or the
salad machine, could use gravity-sensitive technologies as they may only
operate once the vehicle is on the surface of Mars.

## Alternate Formats for Life Support System Architecture

While earlier comments have focused on more traditional approaches to
life support for the Mars Combo Lander Architecture, this section
discusses a few alternative approaches. In situ resource utilization for
life support commodities might provide contingency stocks of life
support commodities and allow the crew to "live off of the land," while
growing significant quantities of higher plants might provide a larger
fresh food component to the crew's diet.

### In Situ Resource Utilization

While ISRU is an integral part of other missions, the Mars Combo Lander
Architecture stresses providing all that the crew will need within the
mission vehicles when they depart from Earth. However, ISRU might
provide an additional life support capability. This would require
additional equipment and power, but might reduce the total mission cost
while providing the same level of reliability.

The composition of the Martian atmosphere is detailed in Table 3.3.7
(Smith and West, 1983), assuming a total pressure of 0.6 kPa. After some
modification, these commodities can, as a minimum, provide a cabin
atmosphere. Without any reactants from Earth, an ISRU plant could obtain
oxygen, and carbon monoxide, from the decomposition of carbon dioxide.
Additionally, inert gases, such as nitrogen and argon, could be
separated from the other atmospheric constituents. The Martian
atmosphere also contains the best documented source of water available
at any surface site, and utilization of such water might also provide a
contingency source. However, there are questions as to the economics of
water production, due to the low concentrations in the Martian
atmosphere.

[]{#__RefHeading___Toc467051668 .anchor}Table 3.3.7 Composition of the
Martian Atmosphere

  ------------------------ --------------- --------------- ---------------
  **Component**            **Percentage\   **Partial       **Assumed Mass
                           (by Volume)**   Pressure\       Leverage**
                                           \[kPa\]**       

  Carbon Dioxide (CO~2~)   95.32           0.572           47

  Nitrogen (N~2~)          2.7             0.016           106 [^4]

  Argon (Ar)               1.6             0.0096          

  Oxygen (O~2~)            0.13            0.00078         19

  Carbon Monoxide (CO)     0.07            0.00042         

  Water Vapor (H~2~O)      0.03            0.0018          
  ------------------------ --------------- --------------- ---------------

### Bioregeneration

Due to the Spartan approach here, bioregeneration at a level to provide
significant food closure does not appear compatible with the Mars Combo
Lander Architecture, except as a biomass production demonstration and/or
to provide a fresh food supplement for the crew's diet. While a larger
biomass production chamber might be employed to provide air and water
revitalization in addition to food, such a system is probably not
cost-effective for a mission of the duration proposed by the Mars Combo
Lander Architecture without significant reductions in the associated
costs.

# Concentrated Exploration Mission: Mars Split Mission Architecture

The Split Mission architecture is a well-documented approach for landing
people on Mars and returning them safely to Earth (Hoffman and Kaplan,
1997, and Drake, 1998). Three missions are assumed, all landing at the
same location on Mars in order to build up an infrastructure that will
provide a safer site than any other place in the Solar System except for
Earth itself.

For each mission, two flights preposition equipment around and on Mars
during the transportation opportunity before the crew transits to Mars.
A cargo flight lands on Mars carrying a Mars Ascent Vehicle, an in situ
resource utilization (ISRU) plant, and an inflatable habitat. A second
flight prepositions an Earth Return Vehicle in a stable Martian orbit.
At the next Mars transportation opportunity, a Surface Habitat Lander
transports the crew from Earth to the surface of Mars, rendezvousing on
the surface with the prepositioned surface assets. On this same
transportation opportunity, the two flights with the prepositioned
assets for the next crew also transit to Mars and arrive while the first
crew is conducting surface operations. Thus, the first crew could, if
necessary, use the assets originally intended for the second crew for
contingencies. As with the previous Mars exploration scenario, transit
to or from Mars nominally takes 180 days, while the surface mission is
nominally 600 days. Following the surface mission, the crew ascends to
Martian orbit in the Mars Ascent Vehicle and rendezvous with the Earth
Return Vehicle. The Earth Return Vehicle transports the crew back to
Earth.

The second crew departs from Earth while the first crew is returning.
The second crew voyages to Mars and lands their Surface Habitat Lander
at the site prepared by the first crew, integrating their vehicle into
the existing infrastructure. Thus, each successive mission will expand
the habitable volume. The cargo flights for the second and third crews
each bring a pressurized rover in place of the inflatable habitat
manifested for the first crew's cargo flight.

## Applying International Space Station Life Support Hardware

The Mars Split Mission LSS hardware using ISS ECLSS technologies is
similar to the material presented in Section 2.1 once the differences in
overall mission duration, environment, and EVA frequency are considered.
As noted above, this mission architecture uses four vehicles. Three
vehicles transit to Mars during the first Mars transportation
opportunity. An inflatable Habitat Module and the Mars Ascent Vehicle
land on Mars, while the Earth Return Vehicle is placed in a stable
Martian orbit. At the second Mars transportation opportunity, the crew
travels from Earth to the surface of Mars in the Surface Habitat Lander.
This vehicle houses the crew for the descent to Mars and during their
surface stay. A separate ascent vehicle returns the crew to Martian
orbit to rendezvous with the Earth Return Vehicle for the trip back to
Earth.

### Surface Habitat Lander

The Surface Habitat Lander transports the mission crew from Earth to the
surface of Mars and provides the crew with a primary habitat during
surface operations. As noted above, the Surface Habitat Lander volume,
which is housed in an aluminum shell, is augmented on the surface by an
inflatable habitat and/or additional vehicles from previous missions. As
the Surface Habitat Lander is an independent vehicle, it carries a full
suite of regenerable LSS hardware. Further, it carries food for the
outbound voyage and for surface operations, which totals 780 days of
supplies. As for the previous mission, EVA will be used only for
contingency during interplanetary transit phases, while during surface
operations the crew is expected to average one 2-crew-member EVA per
weekday. Using these assumptions, the Surface Habitat Lander will have
an equivalent mass of 57.8 MT using ISS ECLSS technologies.

### Predeployed Surface Assets

Several important mission assets are predeployed on the Martian surface
using robotic cargo missions on the Mars transportation opportunity
before each crew transit. These assets include a Mars Ascent Vehicle,
which arrives on Mars with dry fuel tanks. The Mars Ascent Vehicle is
fueled on the surface with fuels from an ISRU plant operating on the
Martian atmosphere with hydrogen feedstock to produce methane and
oxygen. The first cargo flight also brings an inflatable module to
expand the first crew's living volume. The second and third cargo
flights each provide a pressurized rover.

#### Mars Ascent Vehicle

As noted in Section 3.1.3, the Mars Ascent Vehicle is essential to
mission success. However, under the Split Mission Architecture the crew
only occupies it while they ride from the surface of Mars to the Earth
Return Vehicle. The Mars Ascent Vehicle may also provide the crew with
direct return to Earth's surface as a re-entry capsule. Due to the
limited operational lifetime, it is likely that the ECLSS for the Mars
Ascent Vehicle will use consumable life support technologies similar to
those employed for the Space Transportation System. Thus, the Mars
Ascent Vehicle is not considered further here.

#### Inflatable Habitat

While the inflatable habitat manifested for the first crew's cargo
flight will provide crew living volume, it will probably derive most or
all it's life support functions from the initial Surface Habitat Lander.
As such, the inflatable habitat is not considered here.

#### Pressurized Rovers

Like the Mars Ascent Vehicle discussed above, the pressurized rovers
will probably use consumable life support technologies similar to those
employed for the Space Transportation System.

#### In Situ Resource Utilization Facilities

Because ISRU is not one of the ISS technologies, ISRU is confined, under
the present scenario, to providing only methane and oxygen for the Mars
Ascent Vehicle using a stock of hydrogen from Earth.

### Earth Return Vehicle

The Earth Return Vehicle is an independent vehicle with an aluminum
habitat module that houses the crew on the 180-day return voyage. Thus,
this vehicle carries a full suite of LSS hardware. Additionally, the
Earth Return Vehicle provides a contingency habitat for the crew should
they abort surface operations early and live in Martian orbit until the
next transportation opportunity to Earth is available. As such, the
Earth Return Vehicle must provide food and life support functions for a
complete 600 day stay in the Martian system. However, EVA support is not
provided because EVA is not planned for operations in space. Using these
assumptions, the Earth Return Vehicle will have an equivalent mass of
19.1 MT using ISS ECLSS technologies. The contingency stock of food and
LSS consumables, for an addition 600 days of crew habitation, adds an
additional 29.2 MT.

### Mission Summary Using International Space Station Hardware

For a three-mission series using the Mars Split Mission Architecture,
the overall ESM for life support systems, categorized by functional
area, is summarized in Table 4.1.8 below.

[]{#__RefHeading___Toc467051669 .anchor}Table 4.1.8 Concentrated
Exploration Mission Life Support System Equivalent Mass Summary

  ---------------- ------------ ------------ ----------- ------------ ------------- -------------
  **Subsystem**    **Mass\      **Volume\    **Power\    **Cooling\   **Total\      **Total\
                   \[kg\]**     \[m^3^\]**   \[kW\]**    \[kW\]**     Crew Time\    Logistics\
                                                                      \[ch\]**      \[kg\]**

  Air              2,642        144.30       12.04       12.04        42.9          4,937

  Cabin            776          600.00       0.00        0.00         0.0           648

  Clothing         0            0.00         0.00        0.00         0.0           14,688

  Extravehicular   70           0.14         0.01        0.01         0.0           5,346
  Activity support                                                                  

  Food             0            6.00         0.00        0.00         2,880.0       22,522

  Radiation        1,744        1.29         0.00        0.00         0.0           0

  Waste            154          0.18         1.52        1.52         236.7         1,155

  Water            896          4.00         1.30        1.30         77.6          2,576

  **Totals\        **6,282**    **756.91**   **14.87**   **14.87**    **3,237.2**   **51,872**
  (for Each                                                                         
  Crew)**                                                                           

  **Totals\        **18,846**   **2,271**    **44.61**   **44.61**    **9,711.6**   **155,617**
  (for Three                                                                        
  Crews)**                                                                          

                                                                                    

  **Equivalent     **18,846**   **36,528**   **3,798**   **1,959**                  **155,617**
  System\                                                                           
  Mass \[kg\]**                                                                     
  ---------------- ------------ ------------ ----------- ------------ ------------- -------------

Note that data for logistics and crew time have been calculated over the
total mission duration. Total ESM, corrected for LSS crew time, is
230.7 MT plus 87.6 MT for contingency.

## Advanced Life Support Straw Man Architecture

The ALS Straw Man design for the Mars Split Mission Architecture uses
the systems and technologies defined above in Section 3.2 for the Mars
Combo Lander Architecture. As both exploration missions are similar in
scope and duration, the expected technologies are similar. One
exception, however, does arise with the Mars Split Mission Architecture.
The life support systems on the first Surface Habitat Lander will
operate for all three crews at the landing site for a nominal total
duration of 1,800 days. This duration is sufficient that a large-scale
plant growth chamber might seem like a prudent investment for the first
landing vehicle. However, such early utilization of plant growth systems
is not consistent with the current mission documentation (Hoffman and
Kaplan, 1997, and Drake, 1998). As such, a highly physicochemical LSS is
presented here.

### Surface Habitat Lander

The Surface Habitat Lander carries the crew from Earth orbit to the
surface of Mars and then provides a habitat for the entire surface
mission. Therefore, the life support systems employed must function
efficiently for extended periods of time both in microgravity and on the
surface of Mars. While the technologies outlined in Table 3.2.6 do not
have any known gravity dependencies, this multi-segment mission is
extremely demanding. While it is possible that some radiation protection
could be provided at the site by using local regolith, the portion of
its mission in interplanetary space during the voyage to Mars is the
limiting environment and so defines the radiation protection requirement
for the Surface Habitat Lander. However, use of regolith for radiation
protection on the surface of Mars could allow material provided for
radiation protection in interplanetary space, such as water, to be used
elsewhere.

### Earth Return Vehicle

The Earth Return Vehicle is a pure spacecraft. Thus, like the Mars
Transit Vehicle mentioned in Section 3.2.1, the LSS technologies
employed must be insensitive to changes in local gravity, including
microgravity. Further, radiation protection for the Earth Return Vehicle
must rely solely on commodities initially included on the vehicle, such
as water, or those generated from supplied commodities, such as waste
products [^5]. Finally, because the Earth Return Vehicle will wait
inactive in Martian orbit for an extended period before the crew boards
for the trip to Earth, the systems selected must be highly reliable both
while crew-tended and while operating autonomously. Currently, the
technologies proposed in Table 3.2.6 do not have any known limitations
that prevent them from operating for the prescribed mission.

## Alternate Formats for Life Support System Architecture

While earlier comments have focused on more traditional approaches to
life support for the Mars Split Mission Architecture, this section
outlines a few alternative approaches. In situ resource utilization for
life support commodities is a natural extension of the ISRU capability
to provide fuels, while growing higher plants as an integral part of the
life support system is a significant, yet probably valid, departure from
more traditional approaches.

### In Situ Resource Utilization

The Mars Split Mission Architecture assumes using an ISRU plant to
manufacture rocket propellant for Mars ascent using hydrogen brought
from Earth and the Martian atmosphere. In fact, because the ISRU plant
is scheduled to arrive on Mars with the predeployed assets from the
cargo flight, a full load of fuel for the Mars Ascent Vehicle will be
manufactured and waiting before the mission crew leaves Earth. This ISRU
plant could also provide a cache of life support commodities, including
the gases and maybe water for a biomass production chamber. A plant
growth atmosphere can be produced by compressing the gas, removing the
majority of the carbon dioxide and trace contaminants, such as carbon
monoxide, and adding oxygen as needed. These could also be available
when the crew arrives to either provide contingency for a regenerative
LSS or to replace the regenerative LSS entirely and allow the crew to
live on the stored life support commodities. The contingency LSS ISRU
sizing requirements given in Hoffman and Kaplan (1997) and Drake (1998)
would make the life support commodities a significant driver for sizing
an ISRU system.

The composition of the Martian atmosphere is detailed in Table 3.3.7
above, assuming a total pressure of 0.6 kPa. These commodities can be
modified in various ways to produce an atmosphere for either plants or
people. Without any reactants from Earth, an ISRU plant could obtain
oxygen, and carbon monoxide, from the decomposition of carbon dioxide.
Additionally, inert gases, such as nitrogen and argon, could be
separated from the other atmospheric constituents. Conservatively, then,
ISRU could provide cabin gases for either plants or people. The Martian
atmosphere also contains the best documented source of water available
at any surface site. However, some other source of water would be
required to provide a significant mass of water in situ.

The natural partial pressure of carbon dioxide on Earth is about
0.04 kPa, while the optimal partial pressure of carbon dioxide for plant
growth chambers is about 0.12 kPa. Thus, the level of carbon dioxide in
the Martian atmosphere is excessive for plants even before it is
compressed. However, the optimum atmosphere for plant production, in
terms of productivity per unit cost of the atmosphere, has yet to be
defined. Conservatively, then, ISRU could provide cabin gases for either
plants or people. Further, ISRU could provide all atmospheric gases in
place of regenerative approaches or as a contingency supporting
regenerative approaches.

Considering just gas production, ISRU could fill any crew modules that
were delivered to the surface of Mars empty, or provide gases to refill
modules in a contingency. As a minimum, the gases to offset cabin
leakage could be provided. Finally, ISRU could supply gases lost during
EVA and airlock operations.

On Mars, water could be made available from the atmosphere, despite its
dryness, from permafrost located a meter or two below the surface, from
surface snow, from polar ice, or from subsurface water or ice deposits.
The acquisition cost would depend on the cost of extraction and
purification. Water could also be manufactured from atmospheric carbon
dioxide, if a source of hydrogen is available. Hydrogen can be supplied
from Earth in the form of liquid hydrogen. Using this format, a
comparable mass of liquid hydrogen is five times lighter than a
corresponding mass of water, even including tankage. One reaction to
produce water from hydrogen and carbon dioxide is exactly the reaction
already proposed to produce methane for the Ascent Vehicle.

### Bioregeneration

Plants naturally perform many life support functions. Humans require
food, water and oxygen, and release carbon dioxide and organic wastes.
Plants take up carbon dioxide, produce edible biomass, and release
oxygen and water. Inedible plant material and organic wastes can be
broken down by microbial action to recycle nutrients and purify water.
The efficacy of using plants to perform regenerative functions largely
depends on the availability of lighting, heat rejection, and, to a
lesser extent, on availability of volume, water, and cabin atmospheric
gases.

While the most obvious source of light for plant growth would be natural
sunlight, this source is diminished on the Martian surface due to Mars'
increased distance from the Sun and the incidence of Martian dust
storms. Thus, the naturally available photosynthetically active
radiation is reduced and alternative sources of lighting may be
required. However, a hybrid system that combines natural solar radiation
with some level of artificial lighting may be a viable alternative.

If a plant unit is at the same pressure as and adjacent to the crew
spaces, a single air loop can be used. The temperature and humidity
would normally be different for the crew, but the pressure and
atmospheric compositions for both plants and crew are compatible.
Another approach would segregate the atmospheres for the biomass
production chamber and the crew cabin. Mass transfer is more problematic
if different pressures are used for the crew and the plants. Further,
gas separation might be needed. While the additional equipment would not
be expensive, such an approach is more complex and therefore more
susceptible to faults and, though a lesser concern here, this approach
requires more power.

Growing crops in sufficient quantities to provide regenerative life
support functions requires significant volume. An inflatable module will
provide the greatest pressurized volume at the lowest cost. To provide
some estimates of the volume necessary for plant growth, an example is
useful. In this example, the biomass production module will provide 25%,
by dry mass, of the food for a crew of six. The average crop here is
assumed to produce 30 g of dry edible biomass per square meter of
growing area per day and grow to a height of 0.7 m. Assuming crew access
to the growth bays adds an additional fifty percent to the volume
required for plant growth, this rather modest chamber requires a volume
of 36 m³. Additional volume would be required for supporting subsystems.
The average crop growth rate here assumes full artificial lighting and,
for some crops, extended photoperiods to achieve this relatively high
productivity. Thus, if natural Martian lighting provides some or all of
the photosynthetically active radiation for crop growth, the average
productivity will decrease and the corresponding plant growth volume
will increase.

Plants provide the function of water regeneration by acting as a water
still. Plants take in water through their roots and release it as vapor
through their leaves. Tests with the Biomass Production Chamber at
Kennedy Space Center indicate that the water condensed following
transpiration from plants is quite clean even when hygiene water is used
as a source. Minor post-processing may be required.

# Extended Presence: Evolved Mars Base

An extended human presence on Mars requires a Mars Base using either
infrastructure established during an earlier exploration initiative,
such as the Mars Split Mission Architecture, or as an entirely new
project. The Evolved Mars Base does not consider the particular
circumstances and events leading up to a permanent human settlement on
Mars. Instead, it is designed to look at the nominal, or steady state,
operation of such a facility. In short, the facility is assumed to be
fully functional when the first permanent, non-assembly crew arrives on
Mars. While this mission scenario does not detail the process that
produced the base initially, that process could strongly influence the
overall base configuration. Rather, the scenario here is provided as a
case for study and not as a projection of future human missions.
However, this scenario assumes present or near-term technologies both
for life support and other applicable areas.

Like the other reference missions, the Evolved Mars Base assumes a crew
of six. A single suite of life support equipment supports the entire
base, which is housed in one or more inflatable modules. The crew will,
on average, conduct one 2-crew-member EVA each weekday. Nominally, a
transportation opportunity between Earth and Mars occurs once every
26 months. Assuming the first permanent crew members stay at least
600 days on Mars from the time they enter the Evolved Mars Base, and
that the last crewmembers leave when the seventh transportation
opportunity to Earth opens, the facility lifetime is 600 days plus
13 years, or 14.6 years. While transportation between Earth and a Mars
Base will not be trivial, the mission here does not consider such
transportation except for its impact on crew assignments and resupply
schedules. The Evolved Mars Base will support one or more pressurized
surface rovers, but these again are currently beyond the scope of this
mission.

## Applying International Space Station Life Support Hardware

The Evolved Mars Base LSS using ISS ECLSS technologies is similar to the
material presented in Section 2.1 except for differences in mission
duration and EVA frequency. This mission considers the nominal operation
of a continuously occupied research facility on the Martian surface with
an overall mission life of 14.6 years following completion of the
assembly sequence. Finally, while surface rovers and transportation
between Earth and Mars are important, they are not considered here.

For an Evolved Mars Base, the overall ESM for life support systems,
categorized by functional area, is summarized in Table 5.1.9 below.

[]{#__RefHeading___Toc467051670 .anchor}Table 5.1.9 Evolved Mars Base
Life Support System Equivalent Mass Summary

  ---------------- ----------- ------------ ---------- ------------ ------------ -------------
  **Subsystem**    **Mass\     **Volume\    **Power\   **Cooling\   **Total\     **Total\
                   \[kg\]**    \[m^3^\]**   \[kW\]**   \[kW\]**     Crew Time\   Logistics\
                                                                    \[ch\]**     \[kg\]**

  Air              1,321       72.15        6.02       6.02         239          27,488

  Cabin            388         300.00       0.00       0.00         0            3,610

  Clothing         0           0.00         0.00       0.00         0            81,778

  Extravehicular   70          0.14         0.01       0.01         0            47,627
  Activity support                                                               

  Food             0           3.00         0.00       0.00         16,035       125,393

  Radiation        200         0.00         0.00       0.00         0            0

  Waste            77          0.09         0.76       0.76         1,318        6,430

  Water            448         2.00         0.65       0.65         432          14,344

  **Totals**       **2,504**   **377.38**   **7.44**   **7.44**     **18,024**   **306,670**

                                                                                 

  **Equivalent     **2,504**   **786**      **647**    **496**                   **306,670**
  System\                                                                        
  Mass \[kg\]**                                                                  
  ---------------- ----------- ------------ ---------- ------------ ------------ -------------

Note that data for logistics and crew time have been calculated over the
total mission duration. Total ESM, corrected for LSS crew time, is
330.8 MT.

## Advanced Life Support Straw Man Architecture

The primary difference between the Extended Presence ALS Straw Man
design, outlined in Table 5.2.10, and the corresponding Exploration
Mission ALS Straw Man design (See Table 3.2.6) is the addition of a
plant growth module. More specifically, the plant growth module here
provides more than fifty percent, by mass, of the crew's diet with the
remainder provided as prepackaged food from Earth. In such a scenario,
the majority of the dietary carbohydrates and fiber might be derived
from crops grown in the plant growth chamber while spices, protein, and
oil are provided from Earth. Plant growth for this straw man design is
supported by artificial lighting drawing energy from nuclear power
generation. The chamber module is an inflatable structure.

Employing such a large plant growth chamber also provides complete
bioregenerative air revitalization and water recovery. When locally
grown crops provide more than roughly twenty five percent, by dry mass,
of the crew's diet, the plant growth chamber will provide enough fresh
water in the form of transpired moisture to provide the recommended
daily allowances of potable and hygiene water. Reverse osmosis is
provided to inhibit an accumulation in the nutrient solution of sodium
and other ions which are toxic to plants. In this situation, resources
from the RO brine are recovered within the solid waste processing system
using incineration. Further, if the locally grown crops provide more
than roughly fifty percent, by dry mass, of the crew's diet, the
photosynthesis reaction within the growing plants will consume carbon
dioxide and produce oxygen in sufficient quantities to support the human
crew. Finally, as the percentage of dry dietary mass provided by locally
grown crops increases beyond roughly fifty percent, there is sufficient
capacity to regenerate the crew's air. As the percentage of food
provided by local production increases still further, it becomes
economic to oxidize an increasing amount of solid waste products,
releasing carbon dioxide to support the additional plant growth without
supplemental support from physicochemical air revitalization components.
While somewhat unrealistic using current technologies and a limited crop
selection, as the production of locally grown crops approaches providing
all of the crew's dietary requirements and the food resupply approaches
zero, specific productivity of the overall system will decrease because
all crops are not equally efficient in terms of providing edible biomass
from the resources they consume. In fact, more efficient plants will
provide the initial crops for local food production within a
bioregenerative life support system. As greater closure is required,
less efficient crops are selected based on their nutritional
contribution to the overall diet rather than their efficiency in
producing edible biomass. However, the goal here is economic use of
resources rather than self-sufficiency.

Aside from the plant growth chamber, which replaces the physicochemical
air revitalization and water recovery components, but not all of the air
and water systems, the supporting technologies for the Extended Duration
ALS Straw Man design are identical to the systems listed for the
Exploration Mission ALS Straw Man design.

[]{#__RefHeading___Toc467051671 .anchor}Table 5.2.10 Extended Duration
Advanced Life Support Straw Man Systems

+-----------------------------+-----------------------------------------+
| **System**                  | **Comments**                            |
+-----------------------------+-----------------------------------------+
| **Air**                     |                                         |
+-----------------------------+-----------------------------------------+
| > Plant growth chamber      | Plants remove and reduce carbon dioxide |
|                             | in the cabin air and replenish oxygen   |
|                             | via photosynthesis.                     |
+-----------------------------+-----------------------------------------+
| > Catalytic Oxidation       | An advanced trace contaminant control   |
|                             | technology.                             |
+-----------------------------+-----------------------------------------+
| > Avionics Air Assembly     | Air-cooling for avionics.               |
+-----------------------------+-----------------------------------------+
| > Anti-microbial condensing | Humidity and temperature control.       |
| > heat exchanger            |                                         |
+-----------------------------+-----------------------------------------+
| > Reusable filters          | Particulate and microbe control.        |
+-----------------------------+-----------------------------------------+
| > Intermodule ventilation   | Support and control air exchange        |
| > fans & valves             | between modules.                        |
+-----------------------------+-----------------------------------------+
| > Atmosphere composition    | Monitors the air composition for major  |
| > monitors                  | constituents.                           |
+-----------------------------+-----------------------------------------+
| > Portable fire             | Cabin fire suppression.                 |
| > extinguishers             |                                         |
+-----------------------------+-----------------------------------------+
| > Smoke detectors           | Fire detection.                         |
+-----------------------------+-----------------------------------------+
| **Cabin**                   |                                         |
+-----------------------------+-----------------------------------------+
| > Software operated         | Atmospheric pressure control.           |
| > pressure control          |                                         |
+-----------------------------+-----------------------------------------+
| > Bottled gases (oxygen and | Initial cabin pressurization and        |
| > nitrogen)                 | leakage make-up.                        |
+-----------------------------+-----------------------------------------+
| **Clothing**                |                                         |
+-----------------------------+-----------------------------------------+
| > Clothing                  | An initial supply of crew clothing.     |
+-----------------------------+-----------------------------------------+
| > Laundry                   | To clean and recycle crew clothing.     |
+-----------------------------+-----------------------------------------+
| **Extravehicular Activity   |                                         |
| Support**                   |                                         |
+-----------------------------+-----------------------------------------+
| > ISS technology            | Support for EVA will depend on EVA      |
|                             | technology, however EVA consumables     |
|                             | will be supported by other systems.     |
+-----------------------------+-----------------------------------------+
| **Food**                    |                                         |
+-----------------------------+-----------------------------------------+
| > Plant growth chamber      | Provides higher crops that can be       |
|                             | processed into foodstuffs.              |
+-----------------------------+-----------------------------------------+
| > Food processing equipment | Produces food items from edible         |
|                             | biomass.                                |
+-----------------------------+-----------------------------------------+
| > Bulk storage              | Stores edible biomass and food          |
|                             | products.                               |
+-----------------------------+-----------------------------------------+
| > ISS Food System           | Prepackaged individual and bulk food    |
|                             | modified for long-duration stability to |
|                             | provide necessary nutrients not         |
|                             | economically available in food derived  |
|                             | from crops grown in the plant growth    |
|                             | chamber.                                |
+-----------------------------+-----------------------------------------+
| **Radiation**               |                                         |
+-----------------------------+-----------------------------------------+
| > Regolith bags             | To hold regolith in place on top of the |
|                             | surface crew habitat.                   |
+-----------------------------+-----------------------------------------+
| > Excavation equipment      | To fill regolith bags with regolith for |
|                             | the surface elements.                   |
+-----------------------------+-----------------------------------------+
| **Waste**                   |                                         |
+-----------------------------+-----------------------------------------+
| > Bladder-less tanks        | Urine storage.                          |
+-----------------------------+-----------------------------------------+
| > ISS Waste Management      | Urine and feces collection.             |
| > System                    |                                         |
+-----------------------------+-----------------------------------------+
| > ISS Compactor             | Non-human solid waste collection and    |
|                             | storage.                                |
+-----------------------------+-----------------------------------------+
| > Solid Waste Processor     | Stabilization and incineration, as      |
|                             | necessary, of solid wastes.             |
+-----------------------------+-----------------------------------------+
| > Waste storage containers  | Solid waste disposal. Containers are    |
|                             | stored or jettisoned as appropriate to  |
|                             | the mission protocol.                   |
+-----------------------------+-----------------------------------------+
| **Water**                   |                                         |
+-----------------------------+-----------------------------------------+
| > Plant growth chamber      | Primary water processor for urine and   |
|                             | hygiene waste water streams. Removes    |
|                             | organic and inorganic contaminants from |
|                             | water stream, transpiring clean water.  |
+-----------------------------+-----------------------------------------+
| > Reverse Osmosis           | Water processor to remove inorganic     |
|                             | compounds.                              |
+-----------------------------+-----------------------------------------+
| > Aqueous Phase Catalytic   | Final water polishing step.             |
| > Oxidation                 |                                         |
+-----------------------------+-----------------------------------------+
| > Bladder-less tanks        | Water storage.                          |
+-----------------------------+-----------------------------------------+
| > ISS Water Monitor         | Water contaminant monitoring.           |
+-----------------------------+-----------------------------------------+
| > Advanced Microbial        | An advanced microbial control           |
| > Control                   | technology that does not use a plant    |
|                             | toxin like iodine.                      |
+-----------------------------+-----------------------------------------+

## Alternate Formats for Life Support System Architecture

Section 5.2 presents a moderately conservative approach to the life
support system architecture for an Evolved Mars Base. The most
significant costs associated with using crops for a significant
percentage of the critical life support functions are pressurized volume
to house the plant growth chamber, masses in the form of air, water, and
nutrients to support plant growth, photosynthetic flux to permit
photosynthesis, and crew time for facility operation and maintenance.
All of these can be addressed to some extent.

### In Situ Resource Utilization

While nutrients in the form of salts will probably come from Earth, ISRU
is highly likely to provide the initial atmospheric and water masses for
a plant growth chamber. As noted in Section 4.3.1, ISRU can provide
cabin atmospheric gases at a fraction of the cost of bringing such
commodities from Earth. Separating water from the local environment in
significant quantities may be more complex. The Martian atmosphere is
relatively humid, but is so cold that it carries little water in
absolute terms. One remedy is to bring liquid hydrogen from Earth and
produce water using Martian carbon dioxide. This approach is expected to
require only one fifth as much mass as bringing water from Earth, even
including all costs for tankage and thermal conditioning. A second
remedy would use locally available water. Mars has significant amounts
of water in permanent polar ice caps, permanent or seasonal permafrost,
seasonal snow, and possibly even subterranean liquid caches.
Unfortunately, the relative abundance and location of most of these
water sources is currently unknown or inconvenient for other activities.
Thus, use of such sources may or may not unreasonably restrict base site
selection, but overall this appears to be a viable approach for an
Evolved Mars Base and significantly more information should be available
to the site selection team than will be available for selecting
exploration mission sites.

ISRU could also provide minerals, plant-rooting substrates, and raw
material for structures that could be used to house plant growth.
Unfortunately the location of minerals on Mars is not well understood.
Substrates could be used to buffer the uptake of both water and minerals
by the plants, reducing the potential for spread of pathogens. However,
there are indications that Martian surface materials are quite toxic due
to the presence of peroxides and sulfur oxides. Furthermore, the
physical properties of readily accessible materials are relatively
unknown. A good plant substrate would be porous and have a particle size
on the order of a millimeter. Much of the surface of Mars is covered by
dust which is much finer than a millimeter. Several studies have
investigated building structures on Mars using local materials. However,
most of them do not provide sufficient engineering detail nor do they
consider the economic impacts involved to the level that would allow for
critical evaluation.

### Plant Growth Using Natural Lighting

An approach to reducing the high cost associated with artificial
lighting might use natural lighting. Artificial lighting costs are
associated not only with the mass of lighting equipment, but artificial
lighting also requires significant equipment to provide power and cool
the lighting arrays. While crops grown for food on Earth almost
exclusively use natural lighting, this resource on Mars is more limited
due to Mars' greater distance from the Sun and the ever-present
atmospheric dust. Local dust storms and the more severe global dust
storms reduce the fraction of natural sunlight reaching the surface
further.

Natural sunlight can be collected for plant growth in two ways. One way
is to concentrate the light falling over a wide area of surface into a
more intense beam for plant growth (See Schwartzkopf, 1991, for an
example of such a system). Unfortunately, the ubiquitous Martian dust
scatters a percentage of the incident sunlight, and though this
irradiation often reaches the surface, it does so as diffuse irradiation
that would rapidly render a system relying on specular reflections, such
as a mirror concentrator, ineffective. As the atmospheric dust loading
increases, the percentage of incident sunlight scattered also increases.
Therefore, light concentration systems, which efficiently concentrate
and transport only the direct beam component of sunlight, are less
effective on Mars than they are in free space.

Another approach is to use the available energy "as is," whether direct
beam irradiation or diffuse irradiation, by placing the crops within a
greenhouse. Because plants use any impinging solar irradiation within
the proper wavelengths, this approach avoids the issues associated with
concentrating the sunlight. However, as light intensities of naturally
available sunlight do not approach the intensities achieved with
artificial lighting or light concentration systems, the crops grow more
slowly so a greater volume is required to maintain the same production
of daily edible biomass. There may also be seasons, just as on Earth,
when it is difficult to grow plants using only natural sunlight.
Preliminary work by Gertner (1999b) indicates that growing plants with
natural lighting or a combination of natural and artificial lighting is
possible.

# Other Considerations

The information above identifies proposed life support systems for each
mission. However, each format presents just a single approach and even
these designs are not sufficiently robust to ensure a high level of
mission success. Thus, this section discusses approaches for contingency
as well as mission designs that provide variations for the previously
presented ideas.

## Contingencies

A wide variety of contingencies are important from a life support
perspective. Adequate contingency planning must be performed and
implemented for significant failure modes. Some significant concerns
already identified, listed alphabetically, are:

-   accidents

-   environmental hazards

-   equipment failure

-   excessive consumption

-   inadequate performance of life support and related systems

-   leakage

Given sufficient time, accidents are inevitable. Accidents, which
directly interfere with life support functions, include puncturing the
pressure vessel, fires, a release of toxins, and LSS equipment failure.
LSS equipment must be designed to make accidents unlikely, and to limit
the severity of accidents when they do occur. A technology that is
inherently hazardous will require additional safety precautions, which
will generally increase the technology's ESM compared to competing
technologies.

Environmental hazards on Mars that could interfere with life support
functions, aside from the surface being generally uninhabitable by human
beings, would include dust, meteoroid strikes, radiation, and thermal
cycling. Dust is almost ubiquitous on Mars. Pathfinder measured
deposition rates of 0.3% coverage per day (Appelbaum, *et al.*, 1997)
locally at its site in the northern hemisphere. Pathfinder landed during
summer in the northern hemisphere, so the local weather at the landing
site throughout the 30-day primary mission was characteristic of the
clear weather on Mars. Further, the dust may be toxic or corrosive,
possibly containing high levels of sulfur trioxide. Dust accumulation
will probably occlude solar collectors, windows, and radiators. It blows
high into the air, sometimes forming planet-wide dust storms during fall
and winter in the northern hemisphere. The thin Martian air will provide
some protection from meteoroid strikes, but Mars is closer to the
asteroid belt and a greater frequency of strikes might be anticipated
than would be seen on Earth with a similar atmosphere. The Martian
atmosphere will also provide some protection from radiation, though,
again, less than on Earth. Unlike Earth, Mars lacks a strong magnetic
field to provide radiation protection. The temperature on Mars ranges
from the freezing point of carbon dioxide up to slightly above the
freezing point of water. This cycling will stress equipment, and make
EVA demanding.

Equipment failure is highly probable on missions of extended duration.
While designs should focus on preventing failures, total prevention is
unlikely, so failure events must be dealt with in planning. Appropriate
strategies include redundancy, repair, and replacement. Redundancy,
while costly, can be used where continuous functionality is critical or
equipment access is problematic. Because the functions provided by many
life support systems are not extremely time-critical, repair is likely
to be appropriate. Repair will require spares, tools, consumables, and
appropriate data and instructions. Replacement of larger units can be
more costly than repair, as replacements of these larger units are
involved, but replacement should be less expensive than extensive
redundancy.

Consumption of life support commodities will depend on the physical
workload of the crew, as well as possible luxury consumption and
wastage. Workload can be approximated from the planned tasks and
anticipated difficulty of performing them. However, Mars is a new
environment and predictions may be inaccurate due to unanticipated
additional tasks or unanticipated effort to perform the listed tasks.
While the crew's behavior and capabilities during training may be well
known and documented, such information may not predict crew behavior in
an unusual environment for a long duration mission. Isolation may, for
example, result in overeating. Procedural errors and equipment failures
may lead to commodity wastage.

Another issue is that plant production systems may not perform as well
in the new environments such as on Mars. To offset any shortages in the
production of food or other life support commodities during early
missions, the initial plant growth chambers design should support a
greater capacity than just the anticipated nominal load. Additionally,
stocks of consumable life support commodities could be used to offset
any shortages in production from the plant growth chambers. As systems
are operated in new and hostile environments, performance cannot be
guaranteed. To ensure life support, actual equipment must have the
capability to increase its output and offset shortages.

Leakage cannot be addressed by regenerative life support alone, as the
lost gas must be replaced either by resupply or local production.
Shipment from Earth is costly. Permanent gases in pressure vessels
require an additional 36% to 64% mass penalty. ISRU costs depend on the
commodity and the source. The composition of and the ability to process
the Martian atmosphere are fairly certain. Other potential sources of
LSS commodities are not as certain. While large holes in the vehicle
hull may not be immediately life threatening, they need to be closed off
to prevent unacceptable losses of consumables. Further, as seen on Mir,
such holes may be difficult to locate. While a holed inflatable
structure will eventually sag as gas is lost, it will not do so rapidly
even when supporting a considerable mass of regolith for shielding so
long as a suitable material is selected.

Actual life support designs will most likely use a mixture of approaches
to ensure a high availability of life support functions during long
duration missions. Contingency approaches will maximize chances of
success and minimize the cost of doing so. Different technologies might
provide robust redundancy for critical functions. Thus, a mix of
supplied commodities, PC, bioregeneration, and ISRU are likely to
provide life support contingency capability as well as baseline
capability.

### International Space Station Contingency Approach

International Space Station plans a 45-day contingency supply of all
life support systems commodities (Leonard, The Boeing Company, personal
communication). ISS carries three oxygen generators, two of which
operate at all times. ISS contingency plans would not be economically
feasible for long duration missions. This is particularly true, as the
criterion for the 45-day contingency duration is a missed resupply. Such
an event for a Mars mission would result in a delay of 26 terrestrial
months. The specific life support contingency approach for ISS (Leonard,
The Boeing Company, personal communication) is presented below in
Table 6.1.11. Table 6.1.12 provides mass estimates for ISS contingency
approaches.

[]{#__RefHeading___Toc467051672 .anchor}Table 6.1.11 International Space
Station Contingency Approach

  ---------------- ------------------------------ -----------------------
  **Function**     **Redundant Regenerative       **Resupply Approach**
                   Approach**                     

  Carbon Dioxide   Additional CDRA                45-day supply of LiOH
  Removal                                         canisters.

  Oxygen           Additional oxygen generator    45-day supply of
  Generation                                      bottled oxygen

  Trace                                           45-day supply of TCCS
  Contaminant                                     consumable beds
  Control                                         

  Food                                            Additional 45-day
                                                  supply of food

  Water                                           45-day supply of water

  Electrical       Additional short-term energy   
  Energy / Power   storage in batteries           

  Thermal Control  Thermal control loops are      
                   redundant. Both external       
                   thermal control loops are      
                   sized to transport 80% of the  
                   full ISS heat load. Heat       
                   acquisition devices for        
                   critical systems are served by 
                   two thermal control loops.     

  General          Over-sized equipment           

  Catastrophic     Abort to Earth at any time     
  Failure Safety   using crew return vehicles.    
  ---------------- ------------------------------ -----------------------

[]{#__RefHeading___Toc467051673 .anchor}Table 6.1.12 International Space
Station Contingency Analysis

  ------------------ ----------- ---------------------------------- ---------------
  System             Mass\       Rationale                          Reference
                     \[kg\]                                         

  Air                317.8       0.84 kg/cd for consumption for     
                                 45 days\                           
                                 + 91 kg tankage                    

  Carbon Dioxide     466         22 LiOH canisters                  Lowen, personal
  Removal                                                           communication

  Cabin Pressure     1,307       777 kg for a cabin                 
                                 repressurization + 530 kg tankage  

  Clothes            432.9       1.6 kg/cd for 45 days              

  EVA Support        6.1         3.06 kg per 8 hours of EVA per     
                                 crewmember for one, 2-crewmember   
                                 EVA excursion.                     

  Food               621         2.3 kg/cd for 45 days              

  Radiation          0           No additional radiation protection 
  Protection                     is necessary                       

  Waste Disposal     TBD         Waste disposal depends on          
                                 availability of Progress           
                                 spacecraft                         

  Water              1,890       7 kg/cd for drink, food            
                                 preparation, hand/face washing,    
                                 and urinal flushing for 45 days    

  **Total**          **5,041**                                      
  ------------------ ----------- ---------------------------------- ---------------

Masses are calculated for a crew of six. EVA support would be limited to
emergency repair only. The mass given is for one 2-crewmember EVA with a
duration of 8 hours.

### Advanced Life Support Straw Man Contingency Approach

A key difference between missions to ISS orbit and missions to Mars is
that the crew can always leave ISS on short notice using a crew return
vehicle, loiter as necessary for orbit phasing, and return to Earth.
Such an option is physically impossible from Mars, given the state of
our transportation technology. Thus, ensuring high overall reliability
and availability for the LSS is even more important.

The approach to contingencies for the Mars Combo Lander Architecture and
Mars Split Mission Architecture is as follows:

-   The basic life support system provides equipment and consumable
    commodities to provide complete life support for the crew. The
    equipment will be designed to be repaired. Spares are provided for
    critical pieces of equipment so that there is high confidence that
    no failures will occur that cannot be repaired on-site with the
    resources and time available. This approach is expected to include
    scheduled and unscheduled maintenance, and significant work will be
    required during design and qualification to estimate sparing, tool,
    and crew time requirements.

-   A contingency energy store to maintain critical life support systems
    for seven days without additional power generation will be provided.

-   Thermal control contingency is ensured by servicing mission-critical
    systems with multiple cooling loops and by sizing cooling loops to
    accommodate greater than nominal heat loads should one cooling loop
    fail.

-   A seven-day supply of critical life support commodities, such as air
    and water, is provided as an on-board cache. Such supplies should
    provide "open-loop" life support capability while the primary
    systems are repaired. Additionally, all long-duration vehicles are
    supplied with sufficient gases to repressurize the vehicle once
    following the complete loss of cabin atmosphere.

-   Preposition or provide on-board stores and equipment to allow
    production of all necessary emergency water and air from ISRU. If
    the equipment is prepositioned, it should produce and verify a cache
    of life support commodities before the crew is committed to the
    mission.

-   Clothing quantities are planned for the duration of the mission
    assuming use of a laundry. If the laundry fails, clothes could be
    laundered by hand if necessary.

-   Prepackaged food is provided for the entire mission plus any
    feasible extension, perhaps 5%, and for any reasonable long-term
    consumption rate, such as the nominal rate plus 20%. Thus, food is
    provided for 125% of the nominal mission. Emergency rations for 100%
    of the nominal mission are provided in the form of completely
    shelf-stable products and distributed as appropriate to each mission
    vehicle. In both mission scenarios, 600-days of contingency rations
    are provided on board of either the Mars Transit Vehicle or the
    Earth Return Vehicle in case the crew must abort to Martian orbit or
    they fail to rendezvous with the landing vehicle. This excess can be
    jettisoned prior to leaving Mars orbit.

-   Support for two, two-person, eight-hour contingency EVA excursions
    during transit.

This approach provides up to three strings of life support on Mars,
depending on the system, but only a single string of life support in
transit, though Lin (1997a) proposed a redundant CDRA and short term
emergency supplies would be available.

For a mission design with multiple crews using the same facility, such
as the Split Mission Architecture, only contingency and emergency
supplies and equipment that are actually used really need to be
replaced, assuming such commodities are shelf stable for the duration of
the overall mission. However, in actual practice this issue is more
complex because transit times between Earth and Mars are significant.
For example, using the Split Mission Architecture the first crew does
not leave Mars until after the second crew departs from Earth, making it
impossible for the first crew to completely account for any contingency
it might use on the surface before the second crew leaves.

The approach to contingencies for the Extended Mars Base is on a "per
crew basis." In other words, life support consumables and contingency
stocks are provided for each crew as it arrives.

-   The basic life support system provides equipment and consumable
    commodities to provide complete life support for the current crew.
    In addition to the primary air revitalization and water recovery
    system, which is the plant growth chamber, secondary systems that
    provide sufficient air and water for the crew are present. Spares
    are provided for critical pieces of equipment so that there is high
    confidence that no failures will occur that cannot be repaired
    on-site with the resources and time available.

-   Thermal control contingency is ensured by servicing mission-critical
    systems with multiple cooling loops and by sizing cooling loops to
    accommodate greater than nominal heat loads should one cooling fail.

-   A seven-day supply of critical life support commodities, such as air
    and water, is provided as an on-board cache. Such supplies should
    provide "open-loop" life support capability while the primary
    systems are repaired. Additionally, the base is supplied with
    sufficient gases to repressurize the base following the loss of
    cabin atmosphere.

-   ISRU equipment and feedstock, if necessary, are provided to supply
    the base with life support commodities for the crew using indigenous
    resources.

-   Clothing quantities are planned for the duration of the mission
    assuming use of a laundry. If the laundry fails, clothes could be
    laundered by hand if necessary.

-   Prepackaged food is provided for the current crew for that portion
    of their diet not supported by edible biomass from the plant growth
    chamber. To account for variations in biomass production and crew
    consumption, prepackaged food is added to allow the crew 125% of its
    nominal diet. Emergency rations for 100% of the nominal mission are
    provided in the form of completely shelf-stable products to guard
    against the failure of the primary food system. Although not a
    concern here and the transportation system is not addressed, the
    crew will probably also have prepackaged food for the duration of
    their stay in the Martian system waiting on board a transit vehicle
    in Martian orbit to allow an "abort to orbit" option.

This approach again provides up to three indefinite strings of life
support for the crew on Mars plus a fourth string with a week's
duration. To maintain such a system, scheduled and unscheduled
maintenance will be needed. Further, significant work will be required
during design and qualification to estimate sparing, tool, and crew time
requirements, although prior experience with the facility will provide
such information for later crews. Time-critical sub-systems, such as the
ARS equipment, and extremely important systems, will be duplicated.

The plant growth chamber is a special case in that it is too large to
duplicate completely. Most likely the plant growth capability will be
compartmentalized and oversized so as to maintain 100% of the rated
capacity even with one compartment not functioning. While changing the
production rate in a plant growth system to add capacity is at best a
slow process, other critical LSS equipment will have the ability to
provide more than just the nominal commodity production.

Maintaining adequate food production is the critical issue following a
failure in a bioregenerative life support system because typical staple
crops require two to three months to produce edible biomass. Further,
other fairly economic options for air and water regeneration are
available. Resupply and overproduction within the plant growth chamber
could maintain and replace contingency stores of food, bearing in mind
that transportation opportunities from Earth to Mars for resupply occur
only once every 26 months. A certain amount of buffering is possible by
modulating the rate of oxidation of inedible biomass and other waste
products. A bioregenerative LSS is in balance when the oxidation rate
for inedible biomass and the crew's metabolic rate provide sufficient
carbon dioxide to support any plant growth. However, the oxidation of
waste can be reduced to conserve oxygen while the carbon dioxide
required by the plants can be provided from other sources, such as ISRU.

### Contingency Issues

Several unanswered contingency issues include:

-   What are the radiation levels experienced by crews in Martian orbit
    should they abort from their surface site to a waiting transit
    vehicle? The solar intensity at Mars averages 43% of the value at
    Earth, although Mars does not have a magnetosphere. Some protective
    effect could be provided by using the contingency food as radiation
    shielding. The waste resulting from consuming the food would provide
    a similar level of protection as the original food, but the risk of
    biohazards due to inadequate stabilization is a concern. In any
    case, the level of additional protection would be less than ideal
    for such a long stay.

-   What are the gravitational effects on the crew of aborting to orbit?
    The 180-day transit is sufficient to raise concerns about
    physiological deconditioning in weightlessness. An additional 600
    days is certain to be of greater concern. However, the crew can
    spend several hours a day exercising, and the alternative of
    remaining on the surface with a major system failure would obviously
    be worse.

-   How long can food technology extend the effective life of food
    systems including prepackaged foodstuffs before the food loses
    significant nutritional value?

-   What additional capability arises from sending multiple missions to
    the same site? Which strategies for hazard management are most
    effective in such an environment?

## Alternative Mission Scenarios

Some other options are also of interest for these missions, notably
using the same transit vehicle for both the outbound and return trip,
like the Combo Lander, but visiting the same site several times, as in
the Split Mission format. Such a scenario would reduce the quantity of
equipment needed, simplify design of the transit vehicle, and provide a
simpler transition to a permanent base. The surface segment of such a
mission would correspond to the Split Mission scenario, though perhaps
of different duration.

# Potential Additional Scenarios

The scenarios addressed in detail above represent scenarios that have
attracted interest among mission planners. While many factors influenced
these mission selections from the many possible alternatives, these
missions currently have the most detailed supporting documentation and
the missions themselves represent a wide range of demanding situations.

Other scenarios that might involve long duration missions include, in no
particular order, a return to Luna or long-duration installations on
Luna, visits to asteroids, particularly Earth-grazers such as the Apollo
and Amor asteroids, a visit to the co-orbital asteroid Cruithne or other
destinations at high Earth orbits, and trips to the Martian moons. Trips
to comets are probably less likely due to the risk of debris strikes
from the comet itself and the high propulsive requirements associated
with rendezvous and return to Earth. Venus does not to date have any
moons while its surface is too inhospitable to be a likely target with
current or near-term technologies. Mercury and the outer planets are
probably too far away, requiring high energy propulsion and long transit
times. Further, these celestial bodies either exist in or generate too
much natural radiation to be likely destinations using current or
near-term technologies.

The more probable destinations will be similar to the cases described
above in terms of the life support systems, though mission parameters,
such as crew size, mission duration, and infrastructure equivalencies,
will differ. Such parametric variations could affect the technology
selection by favoring different technologies from those selected for the
missions above.

For visits to or bases on Luna, the main issues that differ from the
Martian scenarios and will impact life support technology selection are:

-   The long Lunar nights

-   The availability of life support commodities, notably hydrogen,
    nitrogen, and carbon

-   Rejection of life support thermal loads during the Lunar day

-   Wide environmental temperature swings

-   Dust.

For missions in near-Earth space, such as visits to near-Earth
asteroids, the co-orbital asteroid Cruithne, and high Earth orbit, or
sites on the Martian moons, the main issues that differ from the Martian
scenarios above and will impact life support technology selection are:

-   Weightlessness

-   Mission duration

-   Radiation protection

Life support commodities may be available at some near-Earth
destinations and on Luna, but the low gravity, high vacuum, and high
solar radiation flux will likely boil off volatile compounds at most
sites. The asteroids in the main belt, however, are expected to be rich
in useful life support commodities like water, methane, carbon dioxide,
and ammonia. The near-Earth missions are most similar to the Mars
transit scenario described in Section 3.2.1. In near-Earth space solar
power is available continually and heat rejection is relatively easy.
Operations on Luna are a notable exception as noted above. While plant
growth is attractive for missions with ready access to continuous
sunlight, most of these missions may be too short to benefit
significantly from plant growth modules. Longer missions will require
more radiation shielding than is noted here and either a form of
artificial gravity or more effective countermeasures than currently
exist for the physiological changes induced by weightlessness.

Current (shuttle) missions are restricted to low Earth orbit. While this
is below Earth's radiation belts, and thus relatively benign for
radiation, natural lighting is blocked by Earth's shadow for 40 minutes
out of every 90 minutes. Terrestrial plants do not react favorably under
such light cycling which limits the applicability of bioregeneration in
low Earth orbit. Additional investigation, however, could be useful.
Alternatively, one could employ continuous artificial lighting if energy
storage is utilized to provide power while the vehicle is in the Earth's
shadow, but this adds expense to the plant growth system.

Higher orbits than low Earth orbits would have higher radiation levels.
The radiation is markedly higher within the radiation belts, and
somewhat higher beyond them. Geosynchronous Earth orbits would be
potentially useful for satellite servicing, particularly if satellites
continue to grow in size. However, these are within the radiation belts
and are unlikely to be used extensively for long-duration crewed
missions any time soon. The Lagrange points, between the Earth and Luna
and between the Earth and Sun, are all above the radiation belts. These
points would have reduced propulsion requirements for station-keeping
compared to other orbits in near-Earth space. They have been suggested
as logical places for large-scale habitations for space manufacturing
and human colonies. For such facilities, bioregenerative life support
would be attractive as sunlight would be readily available, with only
occasional interruptions for an eclipse of the Sun.

Luna has local resources for radiation shielding in the form of
regolith. Luna also has some gravity, perhaps enough to allow extended
missions without permanently disabling the crew. It also is readily
accessible with current propulsion technology, with return to Earth in a
few days being a viable option from most surface sites. However, it does
have an unconventional diurnal cycle, compared to terrestrial standards,
and limited quantities of certain critical resources.

Bioregenerative life support could be attractive on Luna for longer
missions, but generally would require artificial lighting for most crops
during the Lunar night. Notably, however, lettuce would not require
significant artificial lighting because most of its vegetative growth
takes place during a 14 day period that corresponds to the length of the
Lunar day. Algae could also be grown during the Lunar day, but it is not
attractive as a major part of a traditional American diet in its natural
form.

If water is present at the Lunar poles, it would provide useful
materials, but it might still be difficult to access and exploit.
Conversely, investigating the Lunar water ice present at the poles and
possibly elsewhere on Luna might provide sufficient incentive to return
to Luna. Such missions might provide opportunities to test technologies
and equipment for the longer and more technically demanding missions
such as those to Mars.

# References

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# Appendices

## Appendix A: Acronyms and Abbreviations

AES air evaporation subsystem

ALS advanced life support

ALS-BIO bioregenerative\
advanced life support

ALS-PC physicochemical\
advanced life support

APCOS aqueous phase catalytic oxidation subsystem

Ar argon

ARS air revitalization subsystem

BIO-Plex Bioregenerative Planetary Life Support Systems Test Complex

BPS biomass production subsystem

BWP biological water processor

CCAA common cabin air assembly

cd crewmember day

CDRA carbon dioxide removal assembly

ch crewmember hour

CO carbon monoxide

CO~2~ carbon dioxide

CRS carbon dioxide removal subsystem

CTSD Crew and Thermal Systems Division at Johnson Space Center

ECLSS environmental control and life support systems

ESM equivalent system mass

EVA extravehicular activity

FPS food processing subsystem

H~2~O water

HAS human accommodations subsystem

HEPA high efficiency particulate air filter

HTCO high temperature catalytic oxidizer

ICS integrated control subsystem

ISRU in situ resource utilization

ISS International Space Station

JSC NASA Johnson Space Center

LiOH lithium hydroxide

LSS life support system

MT metric ton

N~2~ nitrogen

NASA National Aeronautics\
and Space Administration

O~2~ oxygen

OGA oxygen generation assembly

PC physicochemical

PLM pressurized logistics module

RO reverse osmosis

SAVD solid amine vacuum desorbed

SMAP Systems Modeling and Analysis Project Element

SPS solids processing subsystem

SPWE solid polymer water electrolysis

STS space transportation system

TBD to be determined

TCCS trace contaminant control subsystem

TCS thermal control subsystem

UP urine processor

USOS United States operation segment\
(of the ISS)

VRA volatile removal assembly

WP water processor

WRS water recovery subsystem

[^1]: A "crewmember hour" (ch) is defined as one hour of an individual
    crewmember's time. A "crewmember day" (cd) is defined as one day, or
    24 hours, of an individual crewmember's time.

[^2]: In principle, artificial gravity could be provided by rotating the
    habitat. In practice, this complicates the vehicle's design and
    operation, adding to the overall mission expense. Similarly, drugs
    could be used to prevent particular effects of weightlessness, such
    as decalcification of the bones. However, such drugs are not
    available at this time and are not considered.

[^3]: On board the Shuttle Orbiter, power is generated by a
    hydrogen/oxygen fuel cell, producing potable water.

[^4]: The leverage given is for an Argon/Nitrogen mixture.

[^5]: Waste products could replace other forms of consumable radiation
    protection, such as food. The driving issue that defines radiation
    protection is the occurrence of solar particle events, rather than
    the distance from the sun.