Introduction

The problem of transporting humans to Mars is a daunting one, yet the slate never stays blank for long. Since the dawn of spaceflight, there have been no shortage of proposals attacking the problem from every conceivable angle. It is tempting to view Mars transportation as a strictly technical challenge; it does not take long for inspired minds to begin scratching out their ideas on napkins. However, if we are to succeed in this boldest of adventures, we must first step away from the whiteboard and take a moment to reflect on our motivations and goals. To answer the question properly, we have to start by defining and understanding the problem at hand. Only then can we thoughtfully plan the mission our vehicle must complete, and study the technologies that may enable our aspirations.

Architecting from the Right

At NASA, the Exploration Systems Development Mission Directorate (ESDMD) is the group responsible for high-level direction, planning, definition, and management of human spaceflight ventures. The structure that relates a program’s mission, objectives, strategies, and relevant technologies is referred to as “architecture.” In particular, ESDMD is responsible for developing NASA’s “Moon to Mars” architecture, which includes the Artemis program.

In January of 2023, ESDMD convened to review and compile the key points of NASA’s work in the previous year. The results of this Architecture Concept Review, also called ACR22, have been made available to the public at NASA.gov/MoonToMarsArchitecture/. The key deliverable from this conference is the Architecture Definition Document, or ADD, which is rich with insights into the current state of the art in NASA’s Mars mission plans.

So, what does the ADD tell us about the problem of Mars transportation?

NASA highlights six simple questions which lie at the core of the discussion. Who do we send? What do we do? Where do we go? When do we go? How do we get there and back? Why do we go? These questions are closely intertwined, and the answer to each one strongly influences how we address the others.

The Apollo program is frequently placed on a pedestal in the space community, as a triumph of national focus and willpower. Yet in the ADD, NASA calls the Apollo program “a cautionary tale for Mars.” The Apollo program’s top priority was its answer to “When,” with Kennedy’s challenge of landing on the Moon before 1970. Its answers to the remaining questions fell into place accordingly: whatever gets us there soonest. Critically, this strategy proved unsustainable.

Nevertheless, some Mars mission proposals, from NASA and commercial entities alike, make “When” their highest priority: as soon as possible. To date, this approach has yet to bear meaningful results.

In the ADD, NASA chooses a different approach: defining its architecture by starting with “Why.” Three critical pillars answer this question: Inspiration, Science, and National Posture. Metrics like cost and schedule are not driving factors; instead, they are merely the means to achieve NASA’s guiding principles. Starting from what we hope to achieve in the end, we can then work “backwards” to plan our strategy. This is called “architecting from the right,” and is the way NASA is currently developing its Mars mission plans.

The ADD also outlines a thorough list of objectives, forming a checklist that NASA can use to measure the success of its Moon to Mars program. One of the most notable is Recurring Tenet 3: Crew Return, which requires missions to “return crews safely to Earth while mitigating adverse impacts to crew health.” Simply put, in NASA’s eyes, a Mars mission architecture is not complete unless it enables its crew to come home.

Some proposals have suggested that a Mars mission should be a one-way trip; others rely heavily on new infrastructure to enable a return journey. However, when cost and schedule are not drivers, there is no reason to send humans to Mars without doing our due diligence to ensure they can return safely to Earth. We can afford to spend time and energy developing the systems that will best support our needs, rather than compromising on a less capable approach. In effect, we are trading increased technical risk in the development phase for reduced mission risk in the operational phase.

The crew return requirement has strong implications for Mars transportation. Everything needed to support the full mission, from departure to return, must either be prestaged in advance or carried aboard the crew vehicle. This approach tends to favor larger vehicles, more complex mission profiles, and more advanced technologies. A complete Mars transportation system must safely deliver the crew both to and from the red planet, supporting their needs and those of the mission along the way.

Nomenclature

Mars vehicles go by many names. Older literature, like NASA’s Constellation-era Design Reference Architecture, refers to such a system as a “Mars Transfer Vehicle,” or “MTV.” However, later documents, including those in ACR22, have shifted away from this term, instead referring to a Mars vehicle as a “Deep Space Transport,” “DST,” or even simply a “transport.” For clarity, I will use the newer terms in the rest of this article, in an attempt to stay on top of the trend.

Mars Mission Profiles

So far, we have established that NASA’s high-level goals favor a Deep Space Transport which can provide complete, end-to-end Mars transportation for our crews. As we continue to architect from the right, we must next consider the mission profile that our DST will follow.

Choosing a trajectory between Earth and Mars is a complex, open-ended problem with major consequences for DST design. Shorter transit times require higher-energy trajectories, which in turn demand more propellant and greater DST mass. On the other hand, longer transit times allow for lower-energy trajectories, which tend to reduce DST mass. Science goals drive the desire for long stays at Mars. Meanwhile, principles like Crew Return emphasize human health, which means minimizing the time astronauts spend in the hazardous deep space environment. These opposing goals set the stage for the reference mission trade space.

Reference missions provide a blueprint for a Mars flight. They are a map of our journey, or a concept of operations, defining a timeline and a trajectory that the DST will follow between Earth and Mars. Traditionally, the trade space is bounded by two reference missions at either extreme.

A “conjunction-class” mission leverages the natural alignment of Earth and Mars to take advantage of a minimum-energy transfer each way. This allows for the lowest possible vehicle mass, but results in the longest transfer times. Notably, the crew must loiter on or around Mars for 500 days, far beyond current human spaceflight experience. Total mission times may be as high as three years.

On the other end of the scale, “opposition-class” missions use a minimum-energy transfer for only one leg of the mission, either to or from Mars. Transit in the other direction uses a high-energy trajectory that carries the DST deep into the inner solar system, often including a Venus flyby. This requires the highest vehicle mass, but allows the crew to bypass planetary alignment constraints, staying in the Mars system for only around 30 days. Total mission times approach two years, mostly spent in transit.

The ADD designates this high-energy opposition-class trajectory as Reference Mission 0 (M0), not to exceed 760 days in duration. The low-energy conjunction-class trajectory is named Reference Mission 2 (M2), capped at 1,100 days in duration. However, these missions are not the only two options for a Mars trajectory. In reality, the trade space is a continuum, with a variety of intermediate options available. To emphasize this point, the ADD also defines Reference Mission 1 (M1), with a duration of approximately 850 days. The continuity of this space is important to bear in mind as we explore vehicle designs. Some options may appear optimized for one mission type or another, but there is a substantial amount of overlap as the mission duration varies. Intermediate trajectories like M1 may represent the ideal compromise between shorter transit times and longer surface stays.

Here we see more benefits to grounding our architecture in “Why” rather than “When” or “How.” Starting with a specific timeline, trajectory, or propulsion technology would place severe constraints on the practicality of different mission options. By first prioritizing our overarching goals, we can ensure that the full swath of mission types remain available to us, which we can select from at our discretion to inform the “When” and “How.”

Propulsion Systems

With an understanding of the mission profiles available to us, we are finally prepared to dive into the design of the DST itself. Like any spacecraft, a DST will need a myriad of systems, from power and communications to crew habitation systems. However, one of the most demanding features of a DST is its propulsion system.

Crewed DSTs will easily set the record for the greatest mass ever delivered to Mars, and they will be among the first payloads to be returned to Earth. This makes high-performing propulsion systems a necessity. Additionally, the ability to accomplish missions with less propulsion mass relaxes constraints in other critical areas, such as crew size. Thus, a capable propulsion system reduces overall mission risk.

Propulsion options for DSTs generally fall into four major categories. These options each have their own strengths and weaknesses, and hybrid proposals exist which seek to employ the best of two or more systems.

As a brief refresher, propulsion in space has two basic components: a reaction mass and an energy source. Reaction mass is any material which is “thrown” by the vehicle, propelling it forward according to Newton’s third law. The energy source provides the means to “throw” the reaction mass. These key areas are where the four DST propulsion schemes differ.

The four most-studied categories are chemical propulsion, Nuclear Thermal Propulsion (NTP), Solar Electric Propulsion (SEP), and Nuclear Electric Propulsion (NEP).

Chemical Propulsion

Chemical propulsion is the most familiar method, used by every rocket launch vehicle to this day. In chemical propulsion, the energy source is a chemical reaction involving one or more propellants. The products of this reaction are accelerated out through a nozzle, acting as the reaction mass.

Chemical propulsion has a single key advantage: experience. Every spacecraft in history has used chemical propulsion in its launch system, generating an immense wealth of knowledge and heritage hardware. As such, chemical propulsion easily achieves the lowest technical risk of any propulsion system. An additional, though smaller, advantage is that chemical propulsion generally provides higher thrust than other propulsion types. High thrust enables a DST to execute simple, brief maneuvers, minimizing the operational risk of its propulsion system, while also reducing the complexity of its trajectory.

The biggest disadvantage of chemical propulsion is its low efficiency compared to other propulsion types. Lower efficiency means that, for a given trajectory, a DST using chemical propulsion must carry more propellant mass than other options. This is most apparent with high-energy trajectories, such as Reference Mission 0, where an all-chemical DST would become impractically large. This brings chemical propulsion into conflict with the desire to minimize the time crews spend in deep space. Large DSTs may also require dozens of launches to support their construction, which puts strain on launch vehicles and ground systems.

An additional disadvantage of chemical propulsion is that the highest-performing propellants are cryogenic, meaning they must be stored at extremely low temperatures. Even with insulation, some propellant will “boil off” over time and be lost to space. This may be mitigated using new cryogenic fluid management technologies, including active refrigeration to reduce or eliminate boiloff. Still, these strategies are far from flight readiness, and so pose a major technical risk. Considerable work must be done before cryogenic propellants can be stored in space for years at a time.

Nuclear Thermal Propulsion

Thermal propulsion is similar to chemical propulsion in that its reaction mass is accelerated through a nozzle to produce thrust. However, unlike chemical propulsion, thermal propulsion separates its reaction mass, typically hydrogen, from its energy source. In Nuclear Thermal Propulsion, this energy is provided by a nuclear reactor operating at high temperature. The reaction mass is pumped through the core of the reactor, causing it to heat and expand through the nozzle to generate thrust. The propellant also cools the reactor as it flows through the core.

The primary advantage of NTP is its ability to combine reasonably high thrust with high efficiency. NTP can achieve approximately twice the efficiency of chemical propulsion, which reduces DST mass, enabling higher-energy trajectories, shorter mission times, and fewer launches for assembly. The moderately high thrust of NTP, though lower than chemical propulsion, provides similar benefits to operational risk and trajectory design.

A major disadvantage of NTP is its lower technological readiness and lack of flight experience. Although nuclear thermal engines have been built and tested in the past, none have flown, and developing modern designs poses a major challenge. NTP systems would require advanced ground test facilities to capture their irradiated exhaust, and producing their nuclear reactor fuel poses a significant cost and schedule risk. Advanced materials are required to withstand the high operating temperatures within the reactor. Finally, the use of cryogenic liquid hydrogen comes with its own challenges. As mentioned above, active refrigeration and fluid transfer systems for spacecraft remain unproven.

Electric Propulsion

Electric propulsion, like NTP, separates its reaction mass from its energy source. Electric propulsion systems work by ionizing their reaction mass, usually a noble gas like xenon, and then accelerating it using an electromagnetic field. Ionization and acceleration of the reaction mass both require large amounts of electrical power. With Solar Electric Propulsion, this power is provided by the sun through solar panels. In Nuclear Electric Propulsion, power is instead provided by an onboard nuclear reactor.

The single greatest advantage of electric propulsion is its extremely high performance. Electric propulsion systems can be many times more efficient than any other propulsion type, greatly reducing overall vehicle mass. This enables mission types such as Reference Mission 0, which use higher-energy trajectories to minimize overall mission time. Lower vehicle masses also allow DSTs to be constructed with only a handful of launches.

Electric propulsion also benefits from thorough spaceflight experience. Electric propulsion systems were first flown in 1964, and have been a staple of spacecraft ever since. Missions such as Deep Space 1, Hayabusa, DAWN, and BepiColombo have successfully used electric propulsion for long burns through interplanetary space.

All forms of electric propulsion suffer from one key disadvantage: very low thrust. Electric propulsion provides only a fraction of the thrust available with other systems, and this has major ramifications for vehicle and mission design. Conventionally, spacecraft trajectories consist of long, free-flying ballistic segments, broken up by short propulsive impulses that alter the vehicle’s path. Higher-thrust systems provide impulses lasting from a few seconds to several minutes. With electric propulsion, maneuvers may instead last for hours, days, or even months. This requires dramatic modifications to trajectory design.

To depart Earth or capture into orbit at Mars, DSTs using electric propulsion may need to perform long spirals, firing their engines for weeks on end as they gradually adjust their orbits. This introduces significant risks for the propulsion system, which must operate at high power during these long timescales. This also influences human health: long, low-thrust maneuvers increase the time the ship and its crew spends exposed to the deep space radiation environment.

Solar Electric Propulsion

SEP primarily benefits from its use of solar panels to provide electrical power. Solar panel technology is very well-understood and has a long history in spaceflight. However, the power needs for a DST would require the largest solar arrays ever flown. Very large Roll-Out Solar Arrays (ROSAs), similar to those on the International Space Station and the upcoming Gateway, provide one solution. More recently, the ADD describes Compact Telescoping Arrays, which are similar in concept to the older arrays on the ISS, with additional structure and rigging to provide stability. In either case, large arrays are delicate and vulnerable to loads induced by docking vehicles and thruster plumes.

SEP vehicles will require high-power electrical systems that can safely deliver power from their solar arrays to their electric thrusters. The ADD also notes that SEP alone may not be enough for higher-energy trajectories like M0, requiring augmentation from another propulsion type.

Nuclear Electric Propulsion

Reactor development is a major challenge for NEP systems. High-temperature materials, as well as nuclear fuel production and availability, introduce major risks. Unlike NTP reactors, which are cooled by propellant dumped overboard, reactors used in NEP are closed-loop and can only be cooled by radiating heat into space. NEP vehicles therefore require very large radiators, introducing several new challenges. Large radiators must be packed efficiently inside a launch vehicle fairing, necessitating a complex folding and deployment system. This also complicates the liquid cooling systems used to transfer heat to the radiator panels.

Finally, to convert heat into electricity, the onboard reactor must operate akin to a miniature power plant, using a working fluid in a closed thermodynamic cycle. The best way to accomplish this aboard a spacecraft is not yet known.

Comparison & Analysis

The benefits and challenges associated with these four propulsion types are often complementary. As a result, Mars mission studies frequently consider DSTs that employ a combination of these technologies. While chemical-only DSTs are typically viewed as impractical due to their immense size, smaller chemical stages may be combined with either of the low-thrust electric propulsion systems to create hybrid vehicles, with names like “NEP/Chem” or “SEP/Chem.” These utilize electric propulsion to provide the bulk of their performance, delivered during long interplanetary cruises. Meanwhile, chemical stages are used exclusively for the most demanding or time-critical maneuvers, such as capture or departure at Earth and Mars. This allows a hybrid DST to balance high overall performance with a relatively low mass. Additionally, offloading some of the performance to chemical propulsion relaxes requirements on the nuclear or solar components, reducing their development risk. NASA’s Mars Transportation white paper from ACR22 explicitly states that NEP and SEP are only in consideration in their hybrid forms, alongside NTP and all-chemical systems.

Currently, NASA’s architecture studies appear to favor NEP/Chem and NTP options for short-duration, high-energy trajectories, while SEP/Chem and all-chemical DSTs are better suited for long-duration, low-energy trajectories. ACR22 in general has marked a shift towards shorter missions as a baseline, with 30-day surface stays preferred to the 500 days typical of older studies. Notably, NASA’s 2023 Mars Transportation Assessment Study (MTAS) focuses entirely on NEP/Chem and NTP, which it calls “the two most promising propulsion technologies.” Still, all four concepts may be viable depending on the exact mission type chosen, particularly in the “middle ground” space occupied by M1.

All DST concepts studied will require multiple launches for assembly, and each leverages a different strategy for aggregation in space. Assembling a vehicle in Low Earth Orbit (LEO) reduces launch vehicle requirements, maximizing the mass that can be delivered with each flight. However, LEO lies deep within the Earth’s gravity well, which means a DST would need to expend a lot of propellant just to escape from Earth’s influence. Assembling a vehicle far from Earth, such as in cislunar space, reduces the performance required for the DST itself, as the vehicle no longer needs to “climb out” of the gravity well. However, current launch vehicles cannot deliver as much mass to these locations as they can to LEO, and a greater number of launches would be required for assembly.

NTP and all-chemical architectures require the highest number of launches, and these are dominated by deliveries of cryogenic propellants. Because of the high mass and volume requirements for propellant deliveries, NTP and all-chemical DSTs are best-suited for assembly in LEO. If a moderate-energy Mars trajectory such as M1 is used, and DST masses are lower, it may be possible to push the aggregation point up to a “Medium” Earth Orbit (MEO). Furthermore, the high thrust offered by these systems enables them to more effectively depart from deeper in Earth’s gravity well.

NEP/Chem and SEP/Chem vehicles require significantly fewer total launches, with less mass dedicated to propellant. These DSTs can therefore be assembled in higher orbits or in cislunar space without straining current launch vehicle capabilities. This complements the propulsion system by requiring only a small boost from the chemical stage for departure. This allows the electric propulsion system to provide a majority of the vehicle’s overall performance. 

Looking to the Future

Every step of the decision-making process for DST design benefits from our strategy of architecting from the right. The fact that such a dizzying variety of technologies are in play is a testament to our ability to keep all options on the table. Instead of designing a Mars program based on specific capabilities, NASA will select capabilities to develop based on the needs of its program.

Many of these technologies are being matured as part of the Artemis program. High-power electric propulsion systems will be demonstrated aboard NASA’s Gateway space station in the near future. The integrated vehicle will spend months spiraling out towards the Moon using SEP, validating these systems for use in Mars vehicles. Meanwhile, small nuclear fission reactors are in development to power assets on the lunar surface, lending experience which may be extensible to NEP systems. Additionally, the crewed landers currently contracted for Artemis flights – SpaceX’s Starship and Blue Origin’s Blue Moon – will both rely on cryogenic fluid management to perform their missions, helping to reduce risk for future chemical and NTP systems. 

Beyond Artemis, Lockheed Martin has been contracted by DARPA to design, build, and fly a space vehicle using NTP, called DRACO, no later than 2027. Earlier this month, on November 8, 2023, Lockheed Martin announced that they had received another award, this time from the US Air Force Research Laboratory (AFRL), to mature technologies for NEP vehicles, including reactor development, power conversion, and electrical systems.

Each of these development projects represents an investment in the future of human spaceflight, and a critical piece of the Mars transportation puzzle. The successes, failures, and lessons learned from each of these programs will greatly inform NASA’s ultimate design decisions for a crewed Mars vehicle.

The vastness of the DST trade space is daunting, and each of these design areas is a world of study unto itself. And yet, even as this article is published, NASA’s mission architects are hard at work probing for the answers to these questions. Last week, ESDMD concluded its next Architecture Concept Review, called ACR23. A revised Architecture Definition Document, ADD Rev A, is expected to provide further details about NASA’s Mars mission plans upon its release in January of next year. The Space Scout team eagerly awaits the results of this review, and looks forward to gaining new insights into the anatomy of a Deep Space Transport.