ARCHITECTURE CONCEPT 

REVIEW 2022 White Paper

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National Aeronautics and 

Space Administration

MARS TRANSPORTATION

BACKGROUND

As discussed in the Systems Analysis of Architecture Drivers white paper, the systems engineering 

process identifies several example trades and design drivers to selecting an architecture. These 

trades reflect the tightly coupled dependency on the order and weighting applied answering 

the fundamental questions of Who, What, Where, When, Why, and How to meet the Moon 

to Mars objectives. One of the most visible and significant portions of the Mars architecture 

is the Earth to Mars transportation system. While numerous previous studies, analyses, and 

trades have been conducted with respect to this system, most were highly constrained with 

initial conditions or assumptions made in the absence of objective clarity that did not permit 

accurate or meaningful “apples to apples” comparisons, making selections between concepts 

difficult. It is NASA’s intention to utilize the Moon to Mars Goals & Objectives as a foundation 

to establish the key architectural needs, use cases, and functions, conduct assessments to the 

performance of multiple solutions, and ultimately establish a roadmap of system, mission, and 

technology demonstrations to enable informed architectural decisions. This effort and planning 

will be communicated through the Architecture Definition Document and future revisions. 

The purpose of this paper is to orient the greater community to the breadth, variations, and 

associated risks of the range of Earth-to-Mars transportation systems under study.

INTRODUCTION

Spacecraft traveling either to the Moon or Mars do not travel in a straight line; rather, they 

must travel along arced paths shaped by the relevant gravity wells. A single round-trip journey 

between Earth and Mars will put about 1.8 to 2 billion kilometers on a Mars transportation 

system’s odometer, which is different than the straight-line distance between the two planets. 

To put this distance in context, the recent Artemis I mission between Earth and the Moon put 

about 2.2 million kilometers on Orion’s odometer.

The distance and velocity that the Moon orbits Earth varies by a relatively small amount. However, 

both Mars and Earth orbit the Sun at different velocities, so the distance between the two planets 

is constantly changing, hence the energy required to travel between Earth and Mars is also 

constantly changing. This variation leads to some mission opportunities requiring 20 to 60% 

more transportation energy to complete the interplanetary transits between Earth and Mars for 

the same mission duration than in other calendar years. Figure 1 shows the cyclical variation in 

energy during interplanetary round-trip transits between Earth and Mars. In this particular analysis, 

Lunar Distance High Earth Orbit (LDHEO) is used as the starting point for Earth departure staging 

for Mars missions, and a 5-sol orbit is used for Mars arrival staging and departure.

Transportation energy is a function of many variables, including the distance and relative 

velocity of the planets as well as the mission duration and orbital stay time. This is shown in 

the comparison between two example mission types in Figure 1. The conjunction class Mars 
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mission on the left is a minimum energy style mission, where the outbound and inbound portions of the 

interplanetary trajectory are optimized for the best planetary alignment between Earth and Mars, resulting in a 

roundtrip mission duration of ~1,000 days, with more than 300 days loiter time in Mars vicinity while awaiting 

the next optimal planetary alignment for the return journey. The opposition class Mars mission shown on the 

right shortens the total mission duration to ~760 days or less but requires more energy to achieve and shortens 

the Mars vicinity loiter time to about 30- 50 days. It is important to note that Mars missions are not restricted to 

a binary choice between these two options as the mission duration can be varied across a continuous space.

The interplanetary energy required for the Mars mission may be a major component of achieving the roundtrip 

mission, but the energy required to get in and out of the planet’s gravity well cannot be overlooked. Figure 

2 shows an Earth-Moon-Mars energy map for missions to the surface of the Moon or Mars. Both missions 

have unique challenges, and systems designed for one may not be directly applicable to the other as they 

have different energy and mission needs. Missions to Mars generally have a higher energy need, require much 

longer system service life, and have more stringent departure window constraints.

The bulk of current human spaceflight experience is limited to six months or less in space, with just a few 

data points around the one-year mark, and almost all within the Earth’s magnetosphere. While International 

Space Station (ISS) research is helping to answer some questions about human health and performance 

during long duration spaceflight, we don’t yet understand all of the mechanisms involved, or the extent that 

multifactorial contributors to human system risks may limit which countermeasures would be effective or 

potentially introduce other stressors. Researchers believe that the combined effects of many stressors such as 

microgravity, radiation exposure, isolation, confinement, and closed environment are at issue, so many different 

countermeasures may be needed. Crew cabin mass/volume limitations and lack of on-demand resupply for 

medical countermeasures supplies further complicates planning. Some proposed countermeasures, such as 

active radiation protection or artificial gravity, would drive transportation mass and complexity. NASA’s Human 

Research Program has identified a plateau for some—but not all—adverse health and performance effects. In 

other words, by some metrics, human health and performance may continue to degrade or adapt the longer 

crew remain away from Earth, but we don’t yet know the extent of these changes, whether adverse effects are 

reversible, or what individual variation we’ll see across a larger cohort of astronauts. Health and performance 

issues have implications for a Mars crew’s ability to perform their surface mission and poses uncertainty for 

their long-term health after returning to Earth.

To mitigate crew health and performance risks, NASA has recently revisited “opposition” class missions that 

can shave a year or more off total mission duration by shortening Mars vicinity stay time, and not waiting for 

optimal planetary alignment before returning to Earth. 

Figure 1. Variation of roundtrip interplanetary energy needs between LDHEO earth and 5-sol Mars orbits
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Though this type of mission can significantly reduce crew time away from Earth, the penalty is significantly 

higher transportation energy (and propellant mass). Heritage chemical propulsion systems were historically 

considered impractical for opposition-class missions, but the emergence of reusable launch systems and 

in-space propellant depot concepts potentially changes the calculus. Alternatively, highly efficient nuclear 

propulsion systems can mitigate the potential for exponential propellant mass growth of all-chemical 

transportation but will require significant technology investments. In addition to crew health and performance 

risks, vehicle reliability and operational risks must also be considered when comparing different transportation 

and mission options.

NASA ANALYSIS APPROACH

Crew departure opportunity – As noted in Figure 1, transportation energy varies significantly over mission 

opportunities. Instead of designing a transport for the lowest energy opportunity (which only enables missions 

about every 15-20 years), or for the highest energy opportunity (resulting in an oversized vehicle for most 

missions), recent NASA analysis has focused on moderate energy opportunities, such as 2039, as an analysis 

reference. This is not to say that 2039 is the recommended mission date, only that 2039 is a representative 

energy opportunity for the purpose of evaluating transportation system performance.

Transportation architecture – NASA is evaluating four transportation propulsion systems (Figure 3): 

Nuclear Electric Propulsion/Chemical hybrid (NEP/Chem), Nuclear Thermal Propulsion (NTP), Solar Electric 

Propulsion/Chemical hybrid (SEP/Chem), and All-Chemical propulsion (All-Chem). For initial apples-to-apples 

performance analysis comparisons, a separable transit habitat, sized to accommodate 4 crew for up to 1200 

days is assumed for all four transportation systems, but an integrated transit habitat case is part of the AllFIGURE 2. Earth-Moon-Mars ΔV (energy) map
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Chem trade space, and the impacts of varying number of crew will be assessed. Launch cadence and costs 

for the Space Launch System (SLS) and emerging super-heavy Commercial Launch Vehicles (CLVs) are also 

being assessed for each transportation option.

Entry, Descent, Landing and Ascent (EDLA) architecture – in all analysis cases, NASA assumes surface 

mission cargo—most importantly a Mars Ascent Vehicle—will be pre-deployed prior to crew arrival to ensure 

crew are able to return. All four transportation systems are also being assessed for cargo transport use. Two 

different lander types (Figure 3) are also being analyzed: a “flat bed” lander with a lower payload capacity, but 

readily accessible cargo deck, and a vertical lander with higher payload capacity, sometimes referred to in 

previous analyses as a “mid L/D” lander. Again, for the sake of apples-to-apples comparison analyses, roughly 

the same cargo suite is assumed for both approaches, the difference being the number of landers required 

and some differences in cargo operations, particularly Mars Ascent Vehicle propellant loading.

Surface Mission – for the sake of apples-to-apples comparisons, initial analysis held the surface mission 

constant across all options for various in-space mission profiles, focusing on the minimum practical corner 

of the trade space: two crew to the surface for 30 days, with limited surface infrastructure. Release of the 

September 2022 Moon-to-Mars Strategy and Objectives provides guidance to expand surface mission 

analysis to include longer surface stay durations and expanded surface infrastructure, such as in-situ resource 

utilization. Architectural impacts of varying the number of surface crew is also being assessed.

Total Mission Duration – To understand how the transportation architecture changes as a function of mission 

duration, each of the four transportation concepts is evaluated under three different reference missions, 

ranging from 870 to 1250 total days away from Earth (actual transit duration, plus rendezvous, staging, and 

crew launch opportunity time), resulting in 27 different mission concepts; design concepts and campaign 

analysis has been completed for about a third of these to date. Additional analysis cases that expand the 

surface mission will be added in a future analysis cycle.

PRELIMINARY ANALYSIS RESULTS

As mission duration decreases, the total energy required to perform a roundtrip Mars mission increases 

exponentially, significantly increasing Earth-launched mass. For both high thrust propulsion systems (NTP 

Figure 3. Mars transportation system options trade space
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and All-Chem), the total mass required is at the mercy of the exponential nature of the rocket equation. For 

the hybrid systems (SEP and NEP), for any given power level, there is a limitation to how much energy the 

low thrust system can produce. Thus, to enable higher energy missions, either the chemical part of the hybrid 

system must be more heavily utilized, or the power level must be increased. Mass curves for the low thrust 

systems are therefore also exponential in nature as mission duration is shortened.

Assuming active Cryogenic Fluid Management (CFM), a one-MegaWatt (MW)-class SEP/Chem, 2 x 12.5 

kilo-pound force (klbf) thruster NTP, or 1.8 MW NEP/Chem all weigh in at about 300 metric tons (t) Earth 

departure mass from high Earth orbit for the moderate 850-day transit duration (roughly in the middle between 

opposition and conjunction class). For reference, the fully assembled International Space Station (ISS) is about 

400t in low Earth orbit. Initial analysis of an All-Chem concept without active CFM (similar to designs being 

considered for lunar transportation) is nearly five times the mass of the other concepts. For the longer duration, 

conjunction class end of the trade space, a similar pattern is evident: for a 1000-day round-trip mission, the AllChem concept would shed the equivalent mass of an assembled ISS as compared to the moderate duration 

variant—but it would still be about five times the mass of similar duration SEP or NEP hybrid concepts, and 

nearly twice the mass of nuclear concepts flying the much shorter duration opposition class mission.

Addition of active CFM shows promise in bringing the All-Chem stack mass more in line with the other three 

options. NASA analysis found that active CFM could reduce the number of Earth- launched propellant tankers 

needed by 30% for an All-Chem transportation system flying the 850- day mission; active CFM could reduce 

the number of propellant tankers needed by 50% for an All-Chem concept flying a 1000-day conjunction 

class mission. In addition to significant stack mass savings, reducing the number of propellant launches can 

also reduce launch costs and shorten the in-space transport fueling timeline prior to Earth departure, which 

in turn relaxes launch cadence pressure and operational risk. Note that All-Chem concept analysis is not yet 

complete, but trends indicate that opposition-class All-Chem vehicle mass will be much higher than for other 

concepts. Additional analysis is required to pinpoint a mission duration length where All-Chem mass would be 

competitive with nuclear transportation mass at various power levels.

DISCUSSION

Historically, launched mass has served as a cost analog when comparing options, but if reusable launch 

systems promise to drive down Earth launch costs, transportation mass may not be a useful metric. If it 

was as simple as using our lunar all-chemical transportation system and just paying the cost to launch extra 

propellant mass, it could significantly decrease Mars transportation costs as compared to developing new 

technologies. But current lunar approaches cannot be used as-is for Mars. Even when the planets are as 

close as they ever get, the round-trip distance between Earth and Mars is about 950 times that to the Moon 

and back. What’s more, Mars gravity is twice Lunar gravity, requiring even more energy for Mars ascent and 

descent. As compared to their lunar counterparts, Mars transportation systems not only have to carry much 

more propellant, but they also need to maintain that propellant for much longer periods of time. Even a fraction 

of a percent boil-off loss per day will result in significant cryogenic propellant loss over a two to three-year 

round-trip Mars mission, so Mars transportation systems must either employ active CFM or carry significant 

additional propellant to offset losses. In considering Mars extensibility for All-Chem lunar transportation 

systems in development, it is important to note that active CFM is not required for the shorter duration Artemis 

lunar missions so Mars extensibility may require additional technology infusion and vehicle redesign.

In short, a vehicle designed to fly to, land on, and ascend from the Moon simply can’t perform the same 

operation to Mars without refueling or modification to the design. As noted above, without active CFM, a large 

number of propellant tankers is required to fuel a single round-trip Mars crew mission. Even if these propellant 

launches are inexpensive, the complexity of fielding a significant number of individual launches, rendezvous, 

docking, and propellant transfer operations within a very specific deadline increases mission risk: the longer it 

takes to complete the entire in-space fueling operation, the more propellant will boil off before Earth departure. 

If a weather delay or launch pad issue causes the Mars transport to miss its narrow Earth departure window, 

it may have to loiter in Earth vicinity for several years until the next departure opportunity, and by then the 

propellant (and any other life-limited cargo) will have to be replaced, further adding to the mission cost.
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So why haul round-trip propellant, why not just carry enough to get to Mars and refuel there? Historically, 

NASA has been hesitant to send humans anywhere without on-board capability to return to Earth, although 

NASA’s Design Reference Architecture 5.0 (DRA5, 2009) did rely on surface refueling for a pre-deployed 

ascent vehicle to orbit. In that architecture the transport itself carried round-trip propellant, but the crew would 

not commit to the surface mission until after the pre-deployed ascent vehicle was confirmed fueled, thus 

mitigating the risk crew would be stranded on Mars in the event of an ISRU problem. The DRA5 architecture 

required about 40 kiloWatts (kW) of surface power, operating for more than a year prior to crew arrival, to 

robotically manufacture and liquefy enough oxygen propellant to boost a single-purpose ascent vehicle to 

Mars orbit. More recent concepts have gone beyond DRA5, proposing to fuel a combination ascent vehicle/

Earth return all-chemical transport on the Mars surface. Propellant loads will obviously depend on mission 

mode, ascent vehicle design, and Mars return cargo mass, but these concepts would require a leap from tens 

of kW to hundreds of kW surface power for propellant production. Expanding from atmospheric to regolith 

ISRU could reduce fueling time and enable other fueling strategies, but additional power would be required for 

the vehicles and equipment needed to collect, transport, and process Martian regolith or water ice.

For context, a one MW power capability would require 25 each of the 40-kiloWatt (kW) fission surface power 

units that NASA is currently developing, or perhaps tens of acres of solar arrays sized for seasonal solar 

variations and dust storm disruptions. It remains forward work to complete assessment of various surface 

mission cases and assess the number of landers required to deliver the power systems and equipment needed 

to robotically deploy and maintain the power infrastructure, plus robotic ice mining systems that could supply 

water ice to the ISRU plant. Atmospheric extraction could eliminate the need for robotic ice mining equipment, but 

the lower process efficiency will either require much more propellant production time, or much higher power, with 

flow-down impacts to operations and preparation timeline before the first crew return vehicle arrives on Mars.

FORWARD WORK

As this paper highlighted, design trades to address the architectural Who, What, Where, When, Why, and 

How come at a trade of cost, risk, and benefit. NASA will continue to analyze and publish the key findings of 

architectural trades and their effectiveness during annual updates to the Architecture Definition Document. 

This approach to refine and define a path to objective success informed by system, mission, and technology 

demonstrations will be reflected and communicated through the architectural process initiated by the Moon to 

Mars Goals & Objectives. NASA will continue developing transportation conceptual designs for the remaining 

cases, plus expanded surface mission concepts with heavier Mars surface infrastructure, to assess campaign 

impacts, technology development needs, and affordability.

KEY TAKE-AWAYS 

• Mars missions require significantly more energy (propellant mass), require much longer system service 

life, and have more stringent departure window constraints than the lunar missions that NASA is 

developing under the Artemis program, even for the most optimal planetary alignment and trajectories.

• Shorter total roundtrip duration missions potentially mitigate crew health and performance concerns 

but increase the energy (propellant mass & technology development) required for Mars missions.

• Four In-Space Transportation Systems are currently under consideration: Nuclear Thermal 

Propulsion, Nuclear Electric/Chemical Hybrid, Solar Electric/Chemical Hybrid, and All Chemical 

Propulsion.

• Active CFM reduces required propellant loads for all Mars propulsion systems.

• Both Moon and Mars missions have their unique challenges, and systems designed for one may not 

be directly applicable to the other. 

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.