Introduction

Lunar site selection is an iterative process that evolves as we learn about vehicle capabilities, 

objectives, and architecture use cases and functions. Selecting sites for lunar operations requires 

identifying locations that would enable stakeholders to address one or more of NASA’s Moon to 

Mars Objectives: in essence, “where we want to go,” balanced with locations where safe lunar 

landings can be conducted, or “where we can go.” 

Available capabilities will evolve throughout the Moon to Mars Architecture segments, as defined 

in the Architecture Definition Document,[1] which will affect the relationship between “where we 

want to go” and “where we can go.” As Artemis missions progress from the Human Lunar Return 

segment through Foundational Exploration and Sustained Lunar Evolution segments, mission 

planning will benefit from increased access to reusable infrastructure on the lunar surface and 

in orbit, as well as a better understanding of the lunar environment (for a detailed description of 

Moon to Mars exploration segments, refer to NASA’s Architecture Definition Document). 

Human Lunar Return missions will need to find safe landing locations close to the intended 

destination of surface operations as new systems are tested for the first time. Subsequent 

missions will benefit from the lessons learned during the Human Lunar Return segment, improving 

awareness of the lunar surface and environment and enabling more accurate landings, the ability 

to traverse longer distances across the Moon, and longer duration missions. 

These improvements will relax the need for proximity between safe landing locations and intended 

targets of interest for surface science operations. As the architecture evolves, “where we want to 

go” will influence requirements for new systems, leading to an architecture that can reliably send 

astronauts to locations of interest.

National Aeronautics and

Space Administration

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

Lunar Site 

Selection

Objectives Traceability

The Moon to Mars Objectives define the 

locations that NASA and its partners will need 

to access on the lunar surface or in lunar orbits 

in order to address our goals.[2] Therefore, 

traceability to these objectives determines 

“where we want to go.” 

Some objectives can be addressed simply 

through access to lunar orbits or the surface 

in general, without location-specific needs 

(e.g., observations of the human response to 

the lunar environment or gravity transitions). 

However, some objectives require access to 

specific environmental conditions or physical 

locations on the lunar surface, such as access 

to lunar volatiles in persistently or permanently 

shadowed regions or locations near multiple 

diverse terrain types, which would enable us to 

study the history of the Moon. 

Progression through the architecture segments 

will likely result in an evolution of emphasis on 

different objectives. For instance, objectives that 

require longer stays and increased capabilities 

will benefit from favorable conditions, such 

as sustained access to greater-than-average 

amounts of sunlight to reduce thermal 

variability or to enable better power generation. 

As missions progress throughout the segments, 

NASA must achieve a balance between visiting 

previously unexplored terrain and developing 

routine and repeatable presence at select 

locations. 

Lunar Conditions

Human Lunar Return activities will focus on 

conducting safe lunar landings and returning 

crews to Earth while conducting science in 

a region of the Moon that has not yet been 

explored by astronauts. These early Artemis 

missions will test new systems in new 

environments and establish a path for more 

capable missions to follow in later exploration
2023 Moon to Mars Architecture Concept Review 2

segments. As these new systems are tested, the initial 

landings will need to identify relatively flat terrain, with 

only small blocks and impact craters that are within the 

lander’s hazard tolerance. This type of terrain is also of 

value for initial extravehicular activities, or spacewalks, 

as the capabilities of new suits and surface tools are 

tested. Awareness of the physical characteristics of a 

potential landing site requires adequate data for site 

characterization. Each lander will have a unique tolerance 

for surface roughness or obstacle size; knowing if those 

obstacles are present requires proper data. NASAacquired lunar data is made publicly available via the 

Planetary Data System[3] (the Lunar Reconnaissance 

Orbiter team provides a useful tool for accessing the 

data).[4] The highest resolution image data for the Moon 

has a resolution of roughly a single meter, but this 

resolution is not universally available across the polar 

regions. Therefore, data availability (data collected prior 

to or during a landing) and surface characteristics affect 

site selection. 

Lunar lighting must also be taken into consideration; 

early landings will be conducted at times for which the 

landing site is largely sunlit throughout the entire mission. 

Therefore, the initial Human Lunar Return landing site 

should be sunlit for approximately 6–6.5 days. As the 

architecture continues to develop, access to sunlight 

will allow Artemis missions to use long-lived, reusable 

infrastructure to generate solar power, optimize systems 

to account for expected thermal extremes, and maintain 

hardware and crew within certain temperature ranges. 

The Moon’s low axial tilt results in polar lighting conditions 

that can range from areas of continuous darkness to areas 

that are often sunlit (however, there is no known location 

in the South Pole region that is continuously sunlit). 

Generally, higher topography terrain will experience a 

longer duration of access to sunlight. Furthermore, any 

hardware that provides additional height off the surface 

will increase sunlight access. The architecture can take 

advantage of this characteristic as it evolves. 

Every location experiences a unique ratio or pattern of 

sunlight/darkness. These patterns can be predicted on 

the surface, but the ratio can vary significantly over short 

distances. Thus, the concept of a lunar day/night cycle at 

the poles is not consistent across the region and does not 

match our experience on the Earth, or even elsewhere on 

the Moon. 

Identifying initial locations with favorable lighting can 

restrict landing access to limited time periods throughout 

the year, and there will be times when a landing cannot be 

performed because the region will be in shadow (Figure 

1). Therefore, depending on when the mission launches, 

a desired landing site with gentle sloped terrain might 

not be in sunlight, and the period of darkness could be 

brief or extensive, lasting weeks or months. For a more 

detailed description of the lunar south polar lighting, 

refer to the 2022 Architecture Concept Review white 

paper “Why Artemis Will Focus on the Lunar South Polar 

Region.”[5]

Figure 1. Topographic maps of the lunar South Pole showing modeling lighting conditions during the summer season (left) 

and the winter season (right). Earth is to the top of the images. To see the full animated video of lighting conditions around 

the lunar south polar region please visit: NASA SVS | Illumination at the Moon’s South Pole, 2023 to 2030
2023 Moon to Mars Architecture Concept Review 3

The lunar seasonal cycle does not overlap with Earth’s 

seasonal cycle. The Moon will experience roughly 11 

seasonal cycles per 10 Earth years. This means that over 

a decade, the alignment between lunar season and Earth 

season will shift. The lunar summer will slip against the 

Earth calendar over a series of years and the best months 

of lighting at the lunar South Pole will not be the same set 

of months on the Earth. Therefore, lighting at a given site 

will shift throughout the Earth year over time. Increasing 

capability to land in all lighting conditions will enable 

additional site opportunities. 

Surface operations will require communications with 

personnel on the Earth. Prior to the establishment of 

communications infrastructure on or around the Moon, 

a Human Lunar Return landing site would likely need to 

depend on direct-to-Earth communications. This means 

that the Earth must be visible in the lunar sky from the 

landing site. 

The Apollo missions landed on the Earth-facing side of 

the Moon, so the Earth was always visible in the lunar 

sky. However, the Earth is never visible from the far side 

of the Moon. The poles are located along the edge of the 

visible surface (disc) of the Moon as viewed from Earth, 

between the Earth-facing side and far side of the Moon 

(limbs of the Moon). Thus, much like lighting conditions, 

visibility of the Earth can vary (Figure 2). 

The farther a location is past the pole toward the far 

side, the less likely the Earth is to be visible (and may 

only be visible from high-elevation terrain). Similarly, lowelevation terrain on the Earth-facing side of the Moon 

near the poles might also experience periods of time 

without direct Earth visibility. Additional architecture 

capabilities, such as communication relays, will enable 

more site selection options. As the exploration campaign 

progresses, surface mobility is likely to increase as well. 

As a result, planning for lighting and communications 

will not only need to account for landing, but also for 

traversing the lunar surface. 

Mission planning will benefit from over five decades 

of lunar data collection. Although lunar conditions in 

the South Pole region are different from past Apollo 

experience, these conditions are repeatable and 

predictable. While no single location constantly — or 

even routinely — has ideal lighting and Earth visibility 

conditions, we can identify landing sites that are available 

over specific periods. As the architecture evolves through 

each exploration segment, lighting and communications 

considerations can be addressed to enable better access 

to locations of interest.

End-to-End Mission Availability

While the considerations above focus on the lunar 

surface environment, constraints, and operations, 

NASA assesses mission planning holistically. Building 

on lunar site conditions, developing end-to-end mission 

availability metrics requires incorporating when NASA’s 

Exploration Ground Systems, Space Launch System (SLS), 

and Orion spacecraft can launch the crew to rendezvous 

with Gateway and/or the Human Landing System, which 

would be located in near-rectilinear halo orbit, to perform 

the lunar surface sortie.[6]

Figure 2. Animations showing the same view of the Earth from a location near the South Pole. The degree to which the Earth 

is visible from this location changes over time, with the Earth being completely obscured at times throughout the year. To see 

the full animated video please visit: NASA SVS | Earth and Sun from the Moon’s South Pole
The Artemis enterprise’s unique multi-vehicle, multilaunch architecture also creates additional ground 

processing challenges. For Artemis III, Orion will 

rendezvous directly with SpaceX’s Starship Human 

Landing System. Rendezvousing with a prepositioned 

spacecraft creates additional constraints — mission 

planners must align the phasing of the target vehicle in 

lunar orbit with the window for Orion to intercept the 

Moon. 

From an Exploration Ground Systems/SLS/Orion launch 

availability perspective, the vehicle configuration (SLS 

Block 1 or Block 1B) faces unique mission availability 

challenges. Artemis III will be the last flight of the Block 1 

configuration. Artemis IV and beyond will use either the 

Block 1B or Block 2 configuration. 

For SLS Block 1, the vehicle launches to an intermediate 

elliptical low-Earth orbit to best position the upper stage 

to perform the trans-lunar injection, placing Orion on a 

trajectory to intercept the Moon. Given the necessary 

launch geometry, Exploration Ground Systems/SLS/

Orion can only achieve lunar orbit for roughly half of the 

Moon’s orbit around Earth, nearly centered around the 

Moon’s minimum lunar declination. 

Orion’s insertion into near-rectilinear halo orbit must also 

provide sufficient time for crew operations to prepare for 

the lunar surface mission. Thus, for Artemis III, mapping 

the intersection of available lunar landing sites with when 

the crew can launch and rendezvous with the Human 

Landing System is a critical component of mission 

availability. 

Furthermore, once in near-rectilinear halo orbit, the 

Human Landing System is expected to be viable to 

conduct a lunar landing for about 90 days, meaning 

that the crew must arrive within that window of time to 

use the Human Landing System for a landing. Carrying 

multiple landing site options maximizes the likelihood 

of a successful landing across a calendar year within the 

multitude of mission constraints, one being the Human 

Landing System vehicle lifetime. In later segments of 

lunar exploration, the infrastructure could evolve to relax 

constraints on landing site availability and enable the 

selection of a single site. 

For SLS Block 1B, the Exploration Upper Stage inserts 

into a circular low-Earth orbit. While this removes the 

performance constraint in the SLS Block 1 configuration, 

the new co-manifested payload capability can place 

additional performance demands on Orion. After the 

SLS Exploration Upper Stage performs the trans-lunar 

injection, Orion will be responsible for extracting the comanifested payload and ferrying it to near-rectilinear 

halo orbit. The mass of that payload can significantly 

affect mission availability. 

The mission designs for Artemis IV and beyond will 

also need to account for any timeline and consumable 

constraints. Mission availability for later Artemis missions 

will depend on the intersection of leveraging the range of 

the co-manifested payload capability and performing a 

lunar surface mission. 

While this is a core component of near-rectilinear halo 

orbit accessibility, the later Artemis missions do benefit 

from the presence of Gateway and a lunar relay. The 

presence of these elements will help alleviate the 

challenges of direct-to-Earth communications for the 

Human Landing System and other future surface assets, 

ultimately opening additional lunar site availability. 

In addition to all the nominal mission considerations 

above, protections for various contingency scenarios 

further restrict overall mission availability. The scope and 

coverage for these situations is a risk-informed decision 

that must maintain a delicate balance between the vehicle 

capabilities and protecting the crew. 

Figure 3. Mission availability coordinates across multiple 

considerations, including vehicle capabilities and lunar 

environmental and physical characteristics. All of these 

factors must be considered when planning site selection for 

lunar surface operations.

Site Selection Evolution

Lunar site selection considerations will evolve during the 

Foundational Exploration and Sustained Lunar Evolution 

segments. Both segments will involve an increase in 

capabilities to support lunar exploration from orbit 

and on the surface. Reusable hardware and surface 

infrastructure that can support longer stays and enable 

routine access to preferred locations and access to new 

locations will become key aspects of operations. Reusable 

surface assets are likely to be consolidated at one or 

more locations, which will have an impact on where we 

land, either to deliver new hardware or to use previously 

emplaced hardware. 

2023 Moon to Mars Architecture Concept Review 4
As in the Human Lunar Return segment, the timing of a 

launch and landing can lead to different lighting and Earth 

visibility conditions from different locations across the 

south polar region. However, our approach to landings 

will evolve as our knowledge of the lunar environment 

and terrain characteristics increases. 

For instance, the addition of communications capabilities 

will decrease the need for Earth visibility during landing 

or throughout a surface mission, and knowledge of the 

terrain and possible hazards for landers might lead to 

landing options in regions that are partially or entirely 

dark. 

As infrastructure is emplaced on the lunar surface, 

subsequent landings might need to be conducted at 

or around the same locations multiple times, meaning 

that site selection drives the mission. Returning to the 

same location will require relaxation of site accessibility 

constraints related to lighting, communications, and 

terrain awareness, which could be addressed through 

continued data acquisition for that location and 

contributions from the evolving architecture. 

New landing site characteristics might need to be 

considered. Hardware that remains on the surface could 

become an obstacle to future landings and surface 

operations; if that hardware remains in use, future 

landings will need to account for the plume surface 

interactions that landers create during descent and 

ascent. Furthermore, deployed hardware could become 

an obstacle for sun visibility for previously deployed 

elements. 

All partners operating on and around the Moon will need 

to consider these factors. As the architecture develops, it 

should use reusable infrastructure to relax some landing 

site constraints, thereby enabling mission planners to 

access locations of interest more dependably as missions 

progress. However, permanent infrastructure will also 

introduce important new considerations. 

Summary

Identifying lunar sites for landing and surface operations 

is an iterative process that considers vehicle capabilities, 

objectives, and architecture use cases and functions. Any 

mission must balance “where we want to go” with “where 

we can go” safely with our crew and other assets based on 

the capabilities available at that time. Site selection must 

account for characteristics such as surface roughness 

and slope, lighting, and, in early missions, visibility of the 

Earth. Mission planers require lunar data about these 

characteristics to match with vehicle capabilities. 

We must also consider the performance of multiple 

vehicles to enable spacecraft to reach Earth orbit, initiate 

the trans-lunar cruise, rendezvous with other previously 

deployed spacecraft in lunar orbit, and begin the descent 

to the lunar surface. Before we establish surface and 

orbital infrastructure to support these activities, early 

landing locations will be heavily influenced by when the 

crew launches from the Earth (Figure 3). As supporting 

infrastructure is emplaced and we learn about operations 

in the lunar south polar environment, mission planners 

will use the additional information to consider a broader 

range of sites to meet NASA’s Moon to Mars Objectives. 

2023 Moon to Mars Architecture Concept Review 5

Key Take-Aways

Physical and environmental lunar conditions will have a strong influence on site selection, 

including surface roughness and slope, lighting, and Earth visibility. 

Coupled with vehicle capabilities, the early Human Lunar Return mission sites will largely 

depend on when the crew launches. 

As supporting infrastructure is emplaced over time through the Foundational Exploration 

and Sustained Lunar Evolution segments, accessibility to sites of interest should increase and 

establish a stronger link to NASA’s Moon to Mars Objectives. 

References

1. NASA’s Moon to Mars Architecture Definition Document

2. NASA’s Moon to Mars Objectives

3. Planetary Data System

4. Lunar Reconnaissance Orbiter Lunar Quickmap

5. Why Artemis Will Focus on the Lunar South Pole Region

6. Why NRHO: The Artemis Orbit