ARCHITECTURE CONCEPT 

REVIEW 2022 White Paper

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National Aeronautics and 

Space Administration

WHY NRHO: THE ARTEMIS ORBIT

INTRODUCTION

The programs and systems that today comprise Artemis have evolved from a series of 

agency initiatives. These systems, along with the Moon to Mars and Artemis drivers, 

have been determined through feasibility analyses and assessments of mission design 

and architecture conducted over many years. One of the key architectural features is 

the staging orbit from which Artemis and the programs, including Orion, Gateway, and 

the Human Landing System (HLS), will operate.

The key relevant drivers influencing the selection of the orbit can be summarized in 

three distinct areas:

•	 Cislunar Needs: Accessible and repeatable support for Lunar South Pole surface operations 

and exploration, and orbital capabilities to demonstrate Mars forward integrated missions 

and operations. These two fundamentals form the “where” and “why” drivers of the orbit 

selection in the architecture that are expanded on later in this paper.

•	 Vehicle Performance and Access: Capability and performance needs to access the 

cislunar orbit and return to Earth along with the transit to and from the lunar orbit to the 

South Pole to support the base Artemis goals.

•	 Environment and Operations: Integrated vehicle performance of both transportation 

and orbiting platform vehicles in the designated orbit environment when considering 

orbit maintenance, power, thermal, communications, and vehicle design for sustained 

operations in a relevant deep-space environment.

ORBIT ARCHITECTURE COMPARISON

In assessing Artemis architectures, multiple orbits have been considered and evaluated 

against the architectural goals and the performance needs. The following discussion 

provides a comparison for a selection of broad categories of orbits representative of 

the trade space. Actual orbits and variations assessed in studies number in the tens 

of orbits and alternatives. The significant orbits shown in this comparison include Low 

Lunar Orbit (LLO), an Elliptical Polar Orbit (EPO), Near-Rectilinear Halo Orbit (NRHO), 

an Earth-Moon L2 Halo, and Distant Retrograde Orbit (DRO). A diagram of the orbits is 

shown in Figure 1 to illustrate the relative size and orientation of each.

The relevant characteristics of each orbit are summarized at a high level as follows:

•	 LLO: approximately 100 km circular orbit with an inclination at or near 90 deg, 

orbit period of roughly 90 min

•	 EPO: polar oriented elliptical orbit with a Coplanar Line of Apsides, roughly 9 

hr orbit period

•	 NRHO: an L2 halo orbit oriented approximately 90 deg out of the Earth-Moon 
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plane and perilune near the Lunar North Pole, roughly 6.5 day orbit period

•	 L2 Halo: the class of L2 halo orbits ranging from 0 to 60,000 km from L2 with orbit periods 

of 8- 14 days

•	 DRO: Earth-Moon coplanar with extremely high stability, roughly 12 day orbit period

While the results of the analytical studies are not discussed in detail, an overall comparison of these 

orbits to the selected NRHO is provided at the end of the paper

ARCHITECTURE SELECTION: NEAR RECTILINEAR HALO ORBIT

One of the defining characteristics for the selection of the Artemis architectural orbit is the sustaining 

operation and extensibility to future missions. This need tends to motivate solutions that utilize halo 

orbits for the advantages of low-cost maintenance and stability over years of operation. Halo orbits 

generically are deviations about stable LaGrange points utilizing both Earth and Lunar gravity to 

maintain that stability. They can be thought of as resulting from an interaction between the gravitational 

pull of the two planetary bodies (Earth and Moon) and the Coriolis and motion of a spacecraft. These 

halo orbits exist in any 3-body system, e.g., an Earth-Moon-orbiting satellite system. Continuous 

“families” of both northern and southern halo orbits exist at each Lagrange point. The selected 

Artemis NRHO orbit is from one of the southern families of the L2 halo orbits and has a synodic 

resonance (revs to months) of 9:2. It is the lowest altitude NRHO with useful resonance and has an 

orbital period of 6.5 days (see Figure 2).

NRHO ASSESSMENT AND CHARACTERISTICS

For selection of this particular NRHO, several characteristics were examined. A brief discussion of the 

characteristics and how they manifest in the NRHO are provided.

Figure 1. Alternative orbit architecture examples.
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Vehicle Access and Performance

The first rendezvous orbit characteristic assessed is the accessibility of both Earth transiting vehicles 

(Orion) and the lunar access (HLS) and performance costs of both for staging through that orbit. A 

simplified diagram of approximate performance cost and transit time to support Orion access while 

performing both Lunar Orbit Insertion (LOI) and Trans-Earth Injection (TEI) operations is shown in 

Figure 3. The access to the lunar South Pole is a significant driver in landing system designs. The 

ability to support sustained long-duration surface exploration must consider the performance costs of 

aborts and contingency operations. For the NRHO, the weekly repeatability and relative predictability 

of operations can be considered a significant advantage. As shown in the diagram, delta-velocity 

costs for NRHO and the Global LLO, which would be required to sustain extended operations and 

support contingency aborts, result in similar overall performance. Not shown in this diagram are orbit 

access costs for L2 Halo or DRO, which result in even higher orbit to surface costs that would be 

incurred on any associated lunar landing system with minimal benefit of reduced TLI to orbit cost.

Figure 3. This diagram illustrates 

the approximate performance 

costs and transit times of various 

access options for lunar and surface 

operations. Lunar South Pole access 

is a driver in the overall architectural 

selection. The chart can be read by 

following a ‘path’ outbound in red to 

selected orbits, following the lunar 

surface and return paths in blue, 

and adding delta-velocity costs for 

each segment to estimate the total 

architectural needs. The Artemis 

Architecture utilizes the green paths 

for crewed missions.

Figure 2. Illustration of the family of NRHOs; the Artemis – Gateway selected NRHO is highlighted in white on the left image.
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ORBIT MAINTENANCE AND SUPPORT FOR SOLAR ELECTRIC PROPULSION

Consideration of the long-term sustainability 

of the orbit results in the need to identify an 

orbit with limited orbit maintenance costs. This 

minimizes the demand for resupply of propellant 

and intensive maintenance operations. On-going 

cost of maintenance in the NRHO is on the order 

of 10 m/s per year, which is significantly less 

than lower lunar orbits given the instabilities 

caused by the Moon gravitational instabilities. 

Further, placement at higher gravity well staging 

supports the demonstration of large scale 

Solar Electric Propulsion (SEP) capabilities, 

which is a key objective for Mars forward 

technology developments. NRHO insertion 

is both achievable for unassisted SEP and 

supports future relocation and Mars forward 

demonstration missions. Shown in Figure 4 is 

a diagram of the relative orbit position of the NRHO and comparatively the LLO with respect to the 

gravity well around the moon and the strength of the local gravity environment. This illustrates the 

relative access and maintenance costs with respect to orbit locations.

ENVIRONMENTAL PERFORMANCE CHARACTERISTICS

Sustainability and the achievability of a vehicle design that could support the long-term operations in 

the Artemis architecture also guided the exact phasing of the NRHO. At these distances, the shadow 

cast by the Earth can lead to multiple hour occlusions. The ability to support vehicle systems for power 

and survive the thermal environments posed by these durations were a primary consideration in the 

selection of the 9:2 resonance. This alignment of the orbit ensures a repeatable inertial orientation 

such that the vehicles will travel through apolune and below the Earth’s shadow, avoiding long eclipses 

entirely as shown in Figure 5. This allows for vehicle friendly thermal environments and reduces the 

quantity or mass of power storage systems required. Additionally, an advantage of the NRHO is the 

continuous Earth facing orientation that allows for uninterrupted communication as necessary for 

mission operations. As a staging orbit, this provides any NRHO-based vehicles a significant line of 

sight and coverage of the lunar South Pole to support telerobotic operations, communication relay, 

science observation, and other activities for the majority of the orbit.

Figure 4. Illustration of the relative gravity around the Moon and the 

location of both NRHO and Low Lunar Orbits to the gravity well.

Figure 5. Three 

panel views for 

various reference 

frames and 

orientations of 

NRHO to the 

Earth and Moon 

such as Sun-Earth 

Rotating, EarthMoon Rotating 

Pulsating, and 

Moon-Centered 

Inertial.
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ARCHITECTURAL COMPARISON

A summary of the relative comparison of the key characteristics analyzed across the various orbit is in 

Table 1. These characteristics are normalized to NRHO to indicate the relative value as better or worse 

than NRHO. When taken as a whole, the NRHO was selected as the optimal Artemis architectural orbit 

to support the overall goals for sustained exploration, lunar South pole exploration, and achievable 

systems design. In particular, the NRHO provides the closest orbit to the Moon that balances both 

the long-duration orbital platform needs with the accessibility of the lunar South Pole.

SUMMARY AND CONCLUSION

A broad suite of assessments was conducted to determine the Artemis architectural orbit strategy. 

In particular, the driving considerations were identified through key exploration regions (lunar South 

Pole), leading to a sustainable and extensible architecture. These factors influenced the orbit selection 

to address how to support both long-term cislunar orbital platform capabilities and the accessibility 

between Earth, the staging orbit, and the Moon.

Analysis concluded that the NRHO was the optimal orbit to support the various and diverse goals of the Artemis 

campaign to support Moon to Mars development and operations needs. This orbit provides low-cost long-term 

stability, an environment achievable for vehicle designs, and the accessibility for transportation elements.

KEY TAKE-AWAYS

•	 NASA has studied numerous architectural options for staging orbits to support Artemis and 

future missions beyond the Moon; the Near-Rectilinear Halo Orbit provides a balanced approach 

to support the multiple objectives and goals of the architecture.

•	 NRHO is a highly stable orbit that offers repeatably and frequent access both to and from the

Earth and Moon with ideal environmental characteristics to support vehicle power, thermal, and 

communications capabilities.

Table 1. Artemis Architectural Orbit Comparison

CISLUNAR 

ORBITS

Crew Vehicle 

Access ΔV to/from 

Earth

Lunar Access 

(ΔV to/from 

Surface)

GATEWAY ORBIT FEATURES

Gateway 

Access ΔV

Orbit 

Maintenance RPOD Comm Cutouts

Power/

Thermal

Mars 

Forward

Low Lunar 

Orbit (LLO)

Equatorial High Infeasible/Short High Low/

Moderate Circular Orbit Moderate Most Challenging Minimal

Polar Highest 

Shorter Earth Return

Low/Short 

Duration

Moderate/

High

Elliptical Polar Orbit with 

Coplanar Line of Apsides (CoLA) Moderate/High Moderate Short Duration

Moderate/

High Moderate Challenging Moderate Challenging Minimal

Near Rectilinear Halo Orbit 

(NRHO) Moderate Moderate Medium Duration Moderate Minimal Near Linear Dynamics None Deep Space Equivalent Extensible 

Earth-Moon L2 Halo Low/Moderate

Longer Earth Return

Moderate

Long duration

Low/

Moderate Minimal Near Linear Dynamics None Deep Space Equivalent

Partial 

Extensibility

Distant Retrograde Orbit 

(DRO)

Low/Moderate

Longer Earth Return

High

Long duration

Low/

Moderate N/A Near Linear Dynamics Infrequent Deep Space Equivalent Minimal

Table normalized for comparison:

Better NRHO Worse

This white paper was developed as part of NASA’s 2022 strategic analysis cycle to address topics of frequent discussion. For 

the latest white papers or other architectural documents related to human missions to the Moon and Mars, please visit:

www.nasa.gov/MoonToMarsArchitecture. 

REFERENCES AND ADDITIONAL BACKGROUND

•	 R. Whitley and R. Martinez, “Options for Staging Orbits in Cislunar Space,” IEEE Aerospace Conference 2015, Mar. 2015.

•	 D. C. Davis, et. al., “Orbit Maintenance and Navigation of Human Spacecraft at Cislunar Near Rectilinear Halo Orbits,” 27th AAS/AIAA Space Flight 

Mechanics Meeting, Feb. 2017.

•	 N. Merancy, et. al., “Artemis Lunar Mission Availability & Design,” 73rd International Astronautical Congress (IAC), Paris, France, 18-22 September 2022.