Introduction

As noted in the 2022 Architecture Concept 

Review “Mars Transportation” white paper, 

the distance between Earth and Mars changes 

constantly as the two planets revolve around 

the Sun. Regardless of their relative positions, 

traveling to Mars requires significantly more 

energy than lunar missions. However, the 

distance between the planets is only part of 

the story. This white paper explains how gravity 

wells, combined with the distance and desired 

transit duration between them, serve as a mass, 

and potentially cost, multiplier for a round-trip 

human Mars mission. 

Escaping from a Gravity Well

A gravity well is one way to visualize the 

gravitational pull exerted by a large body in 

space. The “depth,” or strength, of a given 

gravity well is a function of the planetary body’s 

mass, with the bottom of the well terminating 

on the body’s surface. For example, Mars is 

smaller and less massive than Earth, so Mars’ 

gravity well is shallower than Earth’s gravity 

well; the Moon is even less massive than Mars, 

so the Moon’s gravity well is much shallower 

than either Earth’s or Mars’ gravity wells, as 

depicted in Figure 1. 

National Aeronautics and

Space Administration

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

Round-Trip Mars Mission

Mass Challenges

Figure 1. Relative “Depths” of the Moon, 

Earth, and Mars Gravity Wells
2023 Moon to Mars Architecture Concept Review 2

Gravity wells help visualize part of the mass challenge 

that a round-trip human Mars mission poses. Most 

people can appreciate that climbing up a taller hill (or 

climbing out of a very deep well) requires more physical 

exertion than climbing up a smaller hill (or climbing out 

of a very shallow well). Consider the exertion required to 

“climb” from a planetary body’s surface to orbit, but with 

exertion measured in kilograms (kg) of rocket engines 

and propellant instead of calories burned. 

For example, visualizing the depth of Earth’s gravity well 

versus the Moon’s helps explain why Apollo astronauts 

required the large Saturn V rocket just to escape Earth’s 

gravity well and reach the Moon, but could escape from 

the Moon’s gravity well and return to Earth with a much, 

much smaller vehicle.

Ascent from the surface of a gravitational body not only 

requires the thrust necessary to counteract gravity and 

ascend to a target altitude, but also that the spacecraft 

match the orbital velocity of the target orbit. Proximity 

to a gravitational body determines the gravitational pull 

that body exerts on the spacecraft. 

For circular orbits, which have a near-constant orbital 

altitude, the gravitational pull will be constant. For elliptical 

orbits, the gravitational pull will vary over the course of 

the orbit as the distance between the spacecraft and 

gravitational body changes. Highly elliptical orbits, which 

are extremely elongated (e.g., lunar-distance high-Earth 

orbit (LDHEO) or the 5-sol Mars orbit) spend a significant 

percentage of their orbital period at distances far from 

the gravitational body, meaning that the “average” depth 

of these orbits is near the top of the gravity well. Orbits 

near the top of the gravity well generally require less 

effort to escape than orbits closer to the bottom of the 

well. 

However, depth in a gravity well is not the only factor to 

consider when evaluating the relative difficulty of escaping 

a gravitational body. Escaping requires a spacecraft to 

achieve enough kinetic energy — the energy due to its 

orbital velocity — to overcome the gravitational pull. 

Like the gravitational pull, orbital velocity increases and 

decreases over the course of a period in an elliptical orbit. 

Although gravitational pull is reduced at farther distances, 

departure burns from elliptical orbits are typically done 

near closest approach, where the difference between the 

kinetic energy of the spacecraft and the kinetic energy 

required to escape is at a minimum. 

Interplanetary Transit and Capture

Reaching Mars requires not only the energy to climb 

out of Earth’s gravity but additional energy to transit the 

distance between Earth and Mars. While Figure 1 is useful 

for visualizing the relative “depths” of gravity wells, it does 

not capture the changing distance between gravity wells. 

Both Earth and Mars orbit the Sun, but travel at different 

velocities, so the distance between them is constantly 

changing. Over the course of their 780-day synodic 

period, this separation varies by between 56 and 400 

million kilometers, but a spacecraft cannot simply travel 

in a straight line between them. Instead, a spacecraft must 

traverse in parabolic paths (Figure 2) shaped by the Sun’s 

gravity and the desired transit time between Earth and 

Mars. The transit time between the planets determines 

the distance traveled and the amount of energy that 

must be expended to accomplish the mission. Fast 

transits between Earth’s and Mars’ gravity wells can be 

more expensive (in terms of energy) than escaping their 

respective gravity wells.

In addition to distance, the relative velocity of the planets, 

mission duration, and orbital stay time all influence the 

interplanetary energy required to reach Mars. A vehicle 

departing Earth must expend energy to accelerate toward 

Mars — and then expend more energy to match Mars’ 

speed once it arrives at Mars vicinity. A vehicle must slow 

down as it gets closer to a planet before it can “fall” into 

the planet’s gravity well. If the vehicle is going too fast, it 

can easily “skip” over the gravity well, much like a fastmoving golf ball skipping over a golf hole.

Figure 2. Earth and Mars Orbit the Sun 

at Different Distances and Velocities for 

an Example 850-day Roundtrip Mission
2023 Moon to Mars Architecture Concept Review 3

How fast a spacecraft travels is a function of the desired 

transit time between planets; faster requires more transit 

energy but reduces trip time. For the sake of minimizing 

crew exposure to the space environment, faster is better 

(for the crew), but faster comes with an enormous energy 

penalty that results in increased propulsion system and 

propellant mass. 

Minimum-energy missions utilize optimal planetary 

alignment for each leg of the interplanetary transit, 

resulting in a long (300 days or longer) loiter period at 

Mars and a round-trip mission duration of about 3 

years. Trip time can be reduced by optimizing planetary 

alignment for only one leg of the mission, paired with 

a short loiter period at Mars, for a round-trip duration 

on the order of about 2 years, but at the expense of 

additional interplanetary energy expended on the nonoptimal leg. 

Shortening transit times between bodies generally 

increases the propulsive energy the transit vehicle must 

deliver. Short transits require acceleration to a higher 

energy state and consequently approach their target with 

higher excess velocity. Longer transits provide more time 

to obtain minimum energy transfers through optimal 

planetary alignment. The 2022 Architecture Concept 

Review “Mars Transportation” white paper used an 850-

day round-trip mission (Figure 2) to compare several 

transportation options.

The energy required to capture into a body’s gravity 

well is generally applied quickly at the point of closest 

approach to reduce the relative velocity of the capturing 

spacecraft. Although the body continues to exert its 

gravitational force on the spacecraft while in orbit, 

pulling it toward the surface, the translational velocity of 

the spacecraft keeps it in orbit. A de-orbit burn to arrest 

this translational velocity and slow down further allows 

the body’s gravitational force to pull the spacecraft down 

to the surface. Typically, crewed and uncrewed landing 

systems remove orbital energy following the de-orbit 

burn to maintain a safe landing velocity. 

One Way v. Round-Trip Missions

All robotic Mars missions to date have been one-way, 

so they have only had to exert enough energy to climb 

out of Earth’s gravity well and push the payload to Mars 

vicinity. Once the robotic payloads arrived at Mars, 

they “fell” into the Mars gravity well, often bypassing 

orbit capture, with additional energy expended to slow 

down for a soft landing. Robotic missions are afforded 

the option to bypass orbit capture and decelerate while 

following a direct path to the surface because they can 

withstand more force during the “fall” into Mars’ gravity 

well and they typically do not have to rendezvous with 

anything in Mars orbit prior to descending to the surface. 

The first part of a round-trip human Mars mission is 

similar to a one-way robotic mission: the crewed vehicle 

and cargo need to escape Earth’s gravity well, transit to 

Mars, capture into the Mars gravity well, and then deorbit to initiate the “fall” to the Martian surface — with a 

little bit more energy expended to slow down for a softer 

landing on the Martian surface. However, unlike the 

one-way robotic missions, the humans need to return to 

Earth. To do this, they will need enough energy to climb 

back out of the Mars gravity well, push the crew, their 

return cargo, and their vehicle back to Earth, and then 

capture back into Earth’s gravity well. This means more 

than double the amount of energy is needed for a human 

Mars surface mission compared to a one-way robotic 

mission. 

Figure 3 shows the mass impact of traversing 1 kg of 

payload from Earth launch through a full round-trip 

mission versus delivering 1 kg of payload from Earth 

launch to the Martian surface. For the round-trip mission, 

ascent from the Martian surface is a mass driver that 

ripples through the earlier stages of the mission.

Gear Ratios

The mass required to launch any given payload out of 

Earth’s gravity well, transport it to Mars, slow it down, 

descend, and land it on Mars is a mass multiplier, 

sometimes called a “gear ratio.” This ratio provides 

a numerical representation of climbing in and out of 

planetary gravity wells. Gear ratio is defined as the 

change in the initial mass of the vehicle when a unit of 

payload (inert mass) is added. In other words, how much 

more mass is needed to deliver 1 additional kg of mass to 

a given point in the journey. 

The relationship between the initial and final mass of a 

spacecraft is a function of ∆V (“delta V,” the change in the 

velocity of the spacecraft to modify its kinetic energy), 

specific impulse of the propulsion system (how efficiently 

the propellant is converted into thrust), and propellant 

mass fraction (proportion of the vehicle mass that is 

propellant). Gear ratio will grow exponentially as the 

propellant mass fraction increases. Missions with higher 

energy requirements, like short-duration crewed surface 

missions to Mars, will have higher propellant mass 

fractions, and therefore higher gear ratios, than a Mars 

science payload performing a one-way, conjunction-class 

transit. 

Gear ratio can be a convenient back-of-the-envelope 

multiplication factor to estimate propellant requirements 

or provide insight into the relative difficulty of a mission. 

A gear ratio will provide more straightforward insight for 

missions that utilize a single vehicle than it would for a 

complex mission with several propulsive elements. 

Many architectures split propulsive responsibility between 

several elements to maintain their individual masses 

within feasible limits. For multi-element architectures, 

gear ratio applicability is limited to the mission phases 

for which an element is actively providing the propulsion.
Additionally, many architecture elements duplicate 

mission phases, such as a cargo vehicle pre-positioning 

a lander and a later crewed vehicle that would both 

individually complete Earth departure and Mars orbit 

capture, leading to a compounding effect when looking 

for a whole architecture perspective. 

If the mission involves manufacturing Mars ascent 

propellant on Mars, that propellant is not “free.” 

The equipment needed to manufacture Mars ascent 

propellant will originate on Earth, so the full “cost” of that 

Mars ascent propellant will have to account for the mission 

mass required to launch the propellant manufacturing 

equipment out of Earth’s gravity well, push it to Mars, 

then slow it down at Mars so it can descend and land, 

adding to the gear ratio. 

Although a gear ratio can give insight into how inert 

mass added to a spacecraft can impact mission mass, 

it is a highly variable value that depends on mission 

parameters and spacecraft performance. Different 

missions across an architecture can have different gear 

ratios, which will reflect the varying mission parameters 

and spacecraft characteristics necessary to accomplish 

different missions. 

Comparing gear ratios can provide an idea of the 

relative propulsive difficulty required to accomplish 

different missions with the same propulsion system or 

the relative efficiency of different propulsion options 

when comparing them for a similar mission, such as the 

example 850-day round-trip mission. However, gear ratio 

should not be used as a stand-alone metric by which to 

assess architectures, as it does not convey the full scope 

of what an architecture is attempting to accomplish. 

Bringing more people or infrastructure to the surface of 

Mars will result in an architecture with a higher gear ratio 

than a science mission, but also adds capabilities to meet 

expanded surface objectives. 

Mass Multiplier Case Study

Gear ratios can be computed for each phase of a mission 

and show the initial mass required to move 1 kg of payload 

through that phase. Multiplying gear ratios for each 

phase results in the full gear ratio, or mass multiplier, for 

a given mission. 

Figure 4 provides two example cases that show the full 

mission gear ratio for an 1,100-day Earth-Mars roundtrip mission (left) versus an 850-day Earth-Mars roundtrip mission (right). For a single vehicle to complete the 

entire end-to-end 1,100-day mission would require 

approximately 10.6 kg of propellant mass for each kg 

completing the full round trip from LDHEO departure 

through return. Comparatively, an 850-day moderateduration round-trip mission has a full mission gear ratio 

of approximately 34.4, about 3 times that of the 1,100-

day conjunction-class mission. 

When compared with the relatively small gear ratios 

required to land payload on the surface, these large

round-trip gear ratios make using a single vehicle

2023 Moon to Mars Architecture Concept Review 4

Figure 3. Mass Impact of Delivering 1 kg of Payload Round-Trip vs. One-Way
2023 Moon to Mars Architecture Concept Review 5

without any prepositioned components (such as return 

propellant, a lander, or an ascent vehicle) a challenge. 

Mission designs frequently split the crewed mission, and 

its gear ratio, between multiple components to limit the 

mass of any one element — for example, a transit vehicle 

for moving between Earth and Mars gravity wells, a lander 

to descend to the surface, and an ascent vehicle with prepositioned propellant to return from the Martian surface 

to orbit. 

Figure 4. Gear Ratios for 850-Day and 1,100-Day Crewed Round-Trip Mars Missions

Key Take-Aways

Gravity well depth and the distance and desired transit time 

between gravity wells influence the total Earth-launched mission 

mass required for a particular payload. 

Though it is tempting to extrapolate lunar transportation system 

costs to Mars applications, the Mars gravity well is deeper, and 

much farther away, than the Moon’s gravity well, so the “cost” of 

a lunar architecture used at Mars cannot be directly translated 

without significant additional engineering analysis. 

A given mission’s total Earth-launched mass is often used as an 

analog for cost and can be useful in assessing the relative cost 

per kilogram of a given mission, as mission assumptions vary. 

Gear ratios can provide insight into how inert mass added to a 

spacecraft can impact initial mass, but vary significantly based on 

mission parameters and spacecraft performance. They should 

not be used as a stand-alone metric by which to assess mission 

architectures, as they do not convey the full scope of what an 

architecture is attempting to accomplish.