Introduction

As noted in the 2022 Architecture Concept 

Review Systems Analysis of Architecture Drivers 

white paper, exploration architectures are 

heavily influenced by the order in which driving 

questions are answered. Decisions in one part 

of the architecture will ripple through other 

parts of the architecture and beyond, often in 

ways that are not intuitively obvious. 

Making one key decision before fully 

understanding the cascading impacts of that 

decision across the end-to-end architecture 

can limit the architecture’s flexibility or utility. 

The essential question is: of all the important 

decisions to be made, which should be decided 

first? 

The practical utility of this approach is to 

understand which decisions lay in the critical 

path of others. To make good choices, it is critical 

to visualize and manage the complex web of 

interrelated decisions and their flow-down 

impacts. This approach allows for deliberate 

and informed progress. 

Ensuring the flow-down impacts of far-reaching 

decisions are carefully traced, assessed, and 

weighed will help NASA make lasting decisions 

that have the most flexibility and value. This is 

a critical factor in the effort as once these and 

other priority decisions are made they have 

lasting impact on the architecture. Subsequent 

changes will be costly in both time and money 

given the long timelines for development of 

new human capabilities (5 to 15 years, similar 

to aircraft).

This white paper describes the initial set of 

human Mars decisions that the agency has 

identified as high-priority architectural drivers.

Mapping Key Architecture Decisions

A “key” architecture decision is defined as 

a decision whose outcome so profoundly 

influences the architecture that it requires very 

high-level review. For example, deciding how 

many crew members an architecture 

must accommodate influences virtually every 

aspect of the architecture. It requires high-level 

consideration and consensus between multiple 

programs and projects. 

An example at the other end of the spectrum 

is deciding handrail color or style. Even though 

the decision may affect many elements, it is best 

categorized as an engineering decision that will 

not require the same level of scrutiny. 

NASA architecture teams have developed a 

systems engineering-driven process to:

1. identify key architecture decisions needed, 

2. determine relationships between decisions 

(including dependencies and flow-down 

impacts), 

3. and develop a recommended logical order 

in which to make these decisions. 

NASA is developing a model-based environment 

to manage this complex web of information. 

The process and rationale are described in 

the Exploration Systems Development Mission 

Directorate’s Moon to Mars Architecture 

Definition Document, Section 2.3.1 Key Mars 

Architecture Decision Drivers. 

To develop the catalog of key Mars architecture 

decisions, NASA subject matter experts have 

begun a bottom-up review of heritage Mars 

architecture studies. Analyzing decades of 

documents, these experts identified the most 

influential factors in designing the initial human 

exploration campaign for Mars. 

Next, they began decomposing the agency’s 

blueprint objectives for exploration using a topdown approach. This resulted in use cases and 

functions that can then be mapped to needed 

architecture decisions. 

Together, these two approaches provided 

more thorough insight, simultaneously helping 

refine objectives, uses cases, and functions. The 

resulting initial analysis — which is still ongoing 

National Aeronautics and

Space Administration

2023 Moon to Mars Architecture Concept Review 1white paper 2023 Moon to Mars Architecture

Key Mars

Architecture Decisions
2023 Moon to Mars Architecture Concept Review 2

Figure 1. Sample 

trade space 

representation, which 

can be constructed 

using the bottom-up 

approach

— identified nearly 100 candidate key decisions for the 

Mars architecture, though the count was slightly reduced 

during subsequent agency-wide review and refinement.

As part of this effort, NASA also developed an initial 

model of architecture decision relationships. Through the 

frequency or dependency linkages illustrated in Figure 

1, the agency extracted seven key decisions for priority 

analysis. 

The seven decisions presented here represent NASA’s 

initial focus for architecture integration efforts for 

an initial human exploration campaign for Mars. The 

complete model — including linkages to remaining lunar 

architecture decisions — continues to be developed and 

refined.

Seven Priority Human 

Mars Architecture Decisions 

NASA’s initial modeling effort isolated seven key human 

Mars architecture decisions, detailed below and shown 

in Figure 2. These are the recommended starting point 

for planning the initial human exploration campaign for 

Mars. 

While the agency will prioritize these seven decisions 

first, analysis and mapping of the remaining catalog of 

key architecture decisions will continue in parallel. NASA 
2023 Moon to Mars Architecture Concept Review 3

will report progress and results at annual Architecture 

Concept Reviews and document them in yearly revisions 

to the Moon to Mars Architecture Definition Document.

Human Mars Mission Science Priorities

NASA’s Moon to Mars strategy identifies science as one 

of three pillars upon which the agency’s blueprint for 

sustained human exploration throughout the solar 

system is built. As a foundational aspiration, it can trigger 

the cycle of national capability and inspiration and build 

the value system of human exploration upon benefit 

to humanity. The blueprint identifies objectives in five 

different science disciplines: 

• Lunar/planetary science

• Heliophysics science

• Human and biological science 

• Physics/physical science

• Applied science 

Accomplishing any portion of these objectives will 

require resources in virtually all aspects of the mission, 

including crew time, dedicated payload mass delivered 

to the surface, dedicated payload mass returned from 

the surface, communication throughput, and power. 

Arguably, the science conducted on the surface of Mars 

— at the farthest end of the human transportation and 

communications systems in consideration through the 

next decade — will have the greatest impact on the scope 

and scale of the architecture. Therefore, science priorities 

warrant the earliest possible attention. 

Recent history demonstrates the importance of making 

this decision earlier rather than later. NASA’s Artemis 

exploration campaign was directed to establish initial 

operations in the lunar South Pole region, with a focus on 

acquiring volatile resources thought to be found there. 

That limited focus may be incompatible with high-priority 

lunar science objectives uniquely addressed at other 

locations. 

Establishing foundational science priorities built on 

broad input from the science community early in the 

architecture definition process may help mitigate 

disruption or delay to implementation of an initial human 

exploration campaign for Mars. 

Initial Human Mars Segment Target State

A decision about the vision — or “target state” — for 

NASA’s initial human exploration campaign for Mars is 

fundamental to developing an architecture that enables 

that vision. Architecture elements and concepts of 

operation will vary greatly depending on the desired end 

state. 

For example, a series of focused science exploration 

missions to different landing sites would favor one 

architecture. Establishing a permanent, fixed base from 

which astronauts could conduct many surface missions 

supporting diverse and evolving exploration activities 

would favor a very different architecture. 

Figure 2. Priority Human Mars Architecture Decisions
Note that the scope of this key decision is limited to 

defining a vision for the initial Humans to Mars campaign 

segment. A separate decision will define subsequent 

human Mars campaign segments. The ideal end state for 

the vision is an architecture that meets NASA’s highest 

priority objectives, with flexibility to expand to meet new 

needs or goals as they emerge.

Initial Human Mars Segment Mission Cadence

The initial state for human exploration of Mars will 

establish the “right” for “architecting from the right,” but 

other questions remain: 

• How many unique missions are necessary during the 

initial segment? (These could include robotic science, 

cargo delivery, or precursor demonstration missions.)

• Will there be crewed orbital or fly-by precursor 

missions to Mars, or will the first crewed mission land 

on the surface of the Red Planet? 

• What additional resources are needed to balance the 

cadence of initial Mars missions with ongoing nearEarth and lunar surface operations? 

Historically, human exploration spaceflight programs have 

established a campaign of test flights, demonstrations, 

and crewed missions that build up to a desired end state. 

Depictions of these gradual buildups can aid stakeholders 

in strategic planning and investment forecasting for the 

initial human exploration campaign for Mars.

Mars Loss of Crew Risk Posture

Robotic exploration projects typically establish a loss of 

mission risk posture, but human spaceflight programs 

must also develop an understanding of the overall loss of 

crew risk. Loss of crew risk posture is a useful guidepost 

in making risk-informed architecture decisions. For 

example, whether to prioritize technologies that enable 

faster round-trip human missions as one means to 

mitigate crew health and performance concerns. 

As the architecture becomes more defined, a formal 

agency-level safety reporting threshold will be established 

for each design reference mission to achieve human 

rating certification. However, establishing a risk posture 

guidepost early in the architecture development process 

will help avoid disruptions and reworks during the later 

certification phase. 

Number of Crew to Mars Surface per Mission

Crew complement is the most common study constraint 

across all architectures and elements. Crew complement 

selection has implications for habitable vehicle and 

element volume, life support system design, and crew 

support systems for health and performance (such 

as medical, exercise, and food systems). It also has 

ramifications for logistics needs (including science and 

mission utilization, food, clothing, medical supplies, etc.), 

which inform campaign launches and cadences. 

Operationally, crew complement helps establish an 

upper limit for Mars entry, descent, landing, and 

ascent vehicle sizing (with flow-down impacts to ascent 

propellant management, including Mars surface 

infrastructure needs). It also helps establish a lower limit 

for crew availability to perform systems monitoring, 

maintenance and troubleshooting; science and utilization 

(particularly during surface extra-vehicular activities); and 

inspirational engagements with the public. The unique 

communications challenges at Mars — an environment 

where real-time communication with Earth is not possible 

— also have implications for task management and 

contingency responsiveness of a given crew complement 

during critical operations.

Number of Crew to Mars Vicinity per Mission

A companion to the Mars surface crew complement 

decision is deciding the total crew complement to Mars 

vicinity. This decision will have some similar considerations 

to defining crew complement to the surface, but also 

some unique constraint drivers. 

The number of crew to the vicinity of the Red Planet will 

have implications for Earth ascent and descent, Mars 

transit vehicle habitable volume, crew support systems 

sizing, and logistics manifesting. This decision may also 

influence Mars capture and parking orbit operations, 

with flow-down implications for task management 

and contingency response. For example, in “split crew” 

architectures, some crew might remain in Mars orbit 

while others descend and work on the surface, changing 

the crew’s physical availability to perform these functions.

Primary Mars Surface Power Generation Technology 

The scope of human exploration on Mars will depend 

largely on the amount of energy available. That energy 

will power crew life support systems, support surface 

element keep-alive functions, and make, move, or 

maintain critical ascent vehicle propellants. 

Solar energy has long been a reliable choice for in-space 

power applications. However, recent robotic science 

mission experience has brought solar power risks for 

Mars surface missions into sharper focus, particularly 

given the loss of crew risk if the surface power system 

were to fail during a human expedition with limited 

mission abort options. 

This particular architecture decision is limited in scope 

to power generation technique. Power load sizing and 

distribution technology selections are cataloged as 

separate decisions, though interdependencies with 

those decisions must be factored into power generation 

decision analyses. The narrowing window of opportunity 

to infuse Mars-forward considerations into lunar surface 

power implementation decisions for Artemis make this a 

timely activity. 

Future Work

During upcoming strategic analysis cycles, NASA 

architecture teams will continue to refine the modeling 

environment, assess various options within the solution 

space, and prioritize remaining decisions for the initial 

2023 Moon to Mars Architecture Concept Review 4
2023 Moon to Mars Architecture Concept Review 5

human exploration campaign for Mars. As the bottomup and top-down identification processes continue, 

additional needed decisions may be identified. Linkages 

to decisions for lunar exploration campaign segments 

that have not yet been made will be developed, analyzed, 

and prioritized. This insight will enable an informed and 

methodical approach to address the needs of the multidecadal vision that is the Moon to Mars Objectives.

Conclusions

Developing architectures to enable human exploration 

of the solar system will require hundreds of individual 

decisions by many different decision authorities across 

the agency. All of these decisions will be important, but 

there is a class of decisions that so profoundly influences 

the entire end-to-end architecture as to warrant the 

highest level of scrutiny. Ensuring the integrated impacts 

of far-reaching decisions are carefully traced, assessed, 

and weighed will help decision authorities make lasting 

decisions that are resistant to implementation delays, 

disruptions, or costly relitigation. 

Through a methodical process, NASA has identified a 

set of seven Mars architecture decisions to start with. 

However, the agency will continue to define and map 

the full catalog of key decisions, reporting progress at 

annual Architecture Concept Reviews and updating the 

Architecture Definition Document with architecture 

decisions as they are made.

Key Take-Aways

The order in which key decisions are made heavily influences 

exploration architectures. Every decision is important, but 

not every decision can be first. 

NASA endeavors to establish a logical order for decision 

making by modeling the decision trade space for human 

Mars exploration. This methodology will allow decisionmakers to understand the integrated impacts of each 

individual decision on the overarching architecture. 

Of the nearly 100 Mars architecture candidate decisions 

identified for analysis, NASA has identified seven key 

decisions to focus on first. 

While Mars serves as a test case for this approach, lessons 

learned will inform future decision-making for the Moon and 

subsequent human exploration enterprises. 

As architecture decisions are made, updates will be reflected 

in NASA’s Moon to Mars Architecture Definition Document.