National Aeronautics and

Space Administration

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

Overview

Throughout the history of human spaceflight, 

astronauts have never been more than a few 

days (and rarely more than a few hours) from 

Earth. Aborts for missions to low-Earth orbit or 

the International Space Station are relatively 

short. Aborts for lunar missions may be longer 

than aborts from Earth vicinity but are still 

measured in days. 

On the transit to Mars, mission abort is a much 

more complicated event because of the sheer 

distance between Earth and Mars. The distance 

and scale differences between missions to the 

Moon and Mars mean lessons learned from 

lunar mission aborts will have limited direct 

applicability for Mars. Depending on when an 

abort is initiated in a Mars mission timeline, 

the heliocentric nature of transit — in orbit 

around the Sun — may require many months to 

return to Earth, regardless of the transportation 

system selected. 

For transportation architectures that refuel in 

Mars vicinity, mission abort during outbound 

transit may not even be possible. In many cases, 

transit abort may not be a practical response to 

an emergency because the time to return the 

crew may exceed the crew’s ability to stave off 

the emergency. 

Early human Mars missions will also have 

limited abort options for descent to and ascent 

from the surface. 

• For descent — where abort means 

returning to orbit — Mars’ atmosphere and 

gravity will make it difficult to carry sufficient 

on-board propellant to initiate an abort for 

a human-scale payload.

• For ascent — where abort means returning 

to the surface — Mars will initially lack 

the specialized surface infrastructure and 

staffing needed to aid crew after the abort. 

Even a successful abort to the surface may 

leave crew stranded, far away from assets 

necessary for a safe return to Mars orbit. 

Both of these challenges will require an entirely 

new contingency operations paradigm relative 

to our flight experience nearer to Earth. 

Transit Abort Analysis

Due to the nature of celestial mechanics, abort 

maneuvers are inherently more challenging than 

nominal mission maneuvers. To understand the 

fundamental nature of these abort maneuvers, 

NASA evaluated three propulsion concepts 

for a crewed Mars mission. This initial scoping 

assessment assumes an example trajectory 

with a roundtrip duration of 850 days with a 

short stay in Mars vicinity. 

The three transportation propulsion concept 

scenarios analyzed were:

• A hybrid abort, where a low-thrust hybrid 

nuclear electric propulsion (NEP) and 

chemical propulsion system utilizes both 

stages to perform abort maneuvers.

• A NEP-only abort, where the hybrid NEP and 

chemical propulsion system jettisons the 

chemical propulsion stage and utilizes only 

the low-thrust electric propulsion system. 

• A ballistic abort, where a high-thrust 

propulsion system (e.g., a nuclear thermal 

propulsion [NTP] or all-chemical propulsion 

system) performs the abort maneuvers. 

In all three cases, the analyses assumed that 

the transportation systems depart Earth with 

only enough propellant for the expected roundtrip mission. Scenarios in which the propulsion 

system carries abort-specific contingency 

propellant were outside the scope of this initial 

assessment. 

Mars Mission

Abort Considerations
2023 Moon to Mars Architecture Concept Review 2

Figure 1. Summary of 

minimum duration transit 

abort options for three abort 

scenarios during an example 

850-day trajectory 

Figure 1 summarizes the minimum duration abort 

utilizing all remaining propellant as a function of the day 

abort is initiated for the three scenarios.

High-thrust, ballistic aborts using NTP or 

all-chemical systems provide an advantage 

if abort is initiated within a few days of 

departing Earth.

An abort using hybrid low-thrust systems can 

enable faster Earth return if initiated after 45 

days. 

A low-thrust hybrid system operating only on 

electric propulsion could still enable a faster 

return to Earth than high-thrust systems if 

abort is initiated between mission days 45 

and 75, even with the loss of its chemical 

stage. 

However, these differences may be inconsequential. An 

abort initiated beyond about 30 days of Earth departure 

will require a year or more to return to Earth in all cases. 

If a mission initiates an abort due to a life-threatening 

emergency like a failure of critical life support systems, 

a loss of crew is likely regardless of whether their 

transportation system is capable of returning to Earth by 

day 310 versus day 390.

To better understand these performance curves, it 

is necessary to understand the two classes of abort 

trajectories available:

• In a fast-transit abort, the spacecraft flies closer to 

the Sun to increase its relative velocity to Earth for a 

faster rendezvous. 

• In a slow-transit abort, the spacecraft increases its 

distance from the Sun to reduce its relative velocity to 

Earth, allowing Earth to catch up for the rendezvous. 

In this case, “fast” and “slow” are relative; even just a few 

weeks after departing Earth, the time required to return 

to Earth increases dramatically. An abort initiated on day 

30 of a Mars mission would result in a return to Earth 

nearly a year later — on mission day 300+ — regardless 

of propulsion system. This fundamental shift in abort 

definition from previous crewed exploration campaigns 

is one of the primary findings of this analysis. 

A ballistic abort utilizing high-thrust maneuvers and 

fast transit can return to Earth earlier than a hybrid 

propulsion abort. However, the availability of fast-transit 

abort is limited to about the first 40 days of the mission. 

After mission day 40, a ballistic abort is limited to slowtransit, while a hybrid propulsion system may pursue a 

fast-transit abort through day 75 if NEP-only and through 

day 140 if NEP and chemical. 

These estimates only consider this specific mission 

trajectory and these transportation system conceptual 
2023 Moon to Mars Architecture Concept Review 3

Reference

Chai, P. and M. Qu, Human Mars Mission Transit Abort Options for Ballistic High Thrust and Hybrid Transportation Systems, 

AIAA 2022-4374, AIAA ASCEND, October 15, 2022. https://doi.org/10.2514/6.2022-4374

Key Take-Aways

A Mars transit abort will not mean an immediate return to Earth, regardless of the chosen 

propulsion system. Mission aborts will be measured in months, not days.

At Mars, there will be limited ascent or descent abort options for early human exploration missions.

Abort planning for crewed Mars missions requires a fundamental shift in thinking regarding 

reliability, crew risk, contingency planning, and mission operations.

To meet NASA’s Moon to Mars Objectives, mission planners must develop a new operational 

paradigm. Mission abort alone will not be useful for crew risk mitigation.

Abort considerations will have flow-down impacts on vehicle design, redundancy and sparing 

strategies, and contingency planning. Understanding the complex interplay of these factors is the 

first step in developing a safe, reliable transportation system for crewed missions to Mars. 

designs. They may vary with other mission trajectories 

or concepts. The analysis shown here is not meant to 

generalize all abort scenarios. Abort capabilities are 

specific to a given scenario, with both abort availability 

window and return to Earth duration dependent on the 

exact situation that necessitated the abort. 

Fast-transit aborts would present significant challenges 

in thermal and radiation management, as the trajectory 

would push a mission significantly closer to the Sun than 

the expected mission trajectory. Spacecraft systems 

would need design modifications for these contingency 

scenarios to ensure crew safety. 

Hybrid propulsion systems have wider abort windows 

for slow-transit aborts compared to the ballistic option. 

However, these aborts are significantly longer in duration 

and may mitigate less risk to a crew. 

Risk mitigation for abort scenarios is a complex problem 

with many competing metrics. Without a holistic view 

of the integrated problem, it would be misleading to 

use abort as a discriminating factor between different 

transportation system options.

Additional details of this analysis may be found in the 

NASA-authored technical publication referenced below. 

Figure 2. Example “fast” transit (left) vs. “slow” transit (right) abort trajectories for an 

example 850-day short Mars vicinity stay–class roundtrip mission to Mars