What Is NASA’s Mars Human Mission Reference Model?

A Blueprint for the Red Planet

When the U.S. National Aeronautics and Space Administration (NASA) plans to send humans to a new destination, like Mars, it doesn’t just build a rocket and hope for the best. The process begins decades earlier with a “mission reference model,” also known as a Design Reference Architecture (DRA).

A reference model is not a funded, approved mission. It’s not a specific spacecraft with a name and a launch date. Instead, it’s an incredibly detailed, “living” engineering study. It’s a foundational blueprint that outlines one or more complete, end-to-end ways to accomplish the goal.

The purpose of a DRA is to:

• Identify the major engineering and scientific challenges.
• Compare different strategies (e.g., which propulsion system to use, what kind of orbit to take).
• Define the technologies that don’t exist yet and must be invented.
• Estimate the total mass that must be launched from Earth.
• Establish a baseline “story” of the mission, from launch to landing to return.

Over the years, NASA has produced several Mars reference models. One of the most detailed and influential is the Design Reference Architecture 5.0 (DRA 5.0), completed in 2009. While some of its specific hardware (like the Constellation program’s rockets) is no longer in development, its core strategies continue to be the foundation for all modern human Mars planning, including the Artemis program that uses the Moon as a stepping stone.

This article explores the components of this reference model, painting a picture of the immense challenge and the ingenious solutions required to send astronauts on a round-trip journey to the red planet.

The Unprecedented Challenge of Mars

A human mission to Mars is not just a “bigger Apollo.” The difficulties are exponentially greater. The reference model must account for several fundamental problems that define every choice.

Distance, Time, and Launch Windows

The Moon is, on average, 239,000 miles away. The Apollo program astronauts made the trip in about three days.

Mars is, at its very closest, 34 million miles away. Due to the constantly changing orbital mechanics of two planets moving around the Sun, the distance is usually much greater. A one-way trip, using the most efficient chemical or nuclear thermal propulsion systems, takes between six and nine months.

This distance creates the problem of launch windows. Unlike the Moon, which is always available, a mission can only launch from Earth to Mars during a small window that opens roughly every 26 months. This is when the two planets are in the correct alignment for an efficient Hohmann transfer orbit. If a mission misses this window, it must wait over two years for the next one.

Mission Classes: Sprint vs. Marathon

This 26-month cycle forces planners to choose between two main types of missions, which the reference model studies in depth.

• Opposition-Class Mission: This is the “sprint.” Astronauts use a very high-energy, fast trajectory to get to Mars. They stay on the surface for a very short time (perhaps 30 to 90 days) before the planets move out of alignment, forcing them to take another high-energy path home. The total mission time is shorter (around 1.5 to 2 years), but it requires immensely powerful propulsion and much more propellant.
• Conjunction-Class Mission: This is the “marathon.” Astronauts use the most efficient, lowest-energy trajectory (like a Hohmann transfer) to get to Mars, which takes longer (7-9 months). Once they arrive, they must wait on the surface for the planets to realign for an efficient return trip. This wait time is over 500 days. The total mission duration is long (about 2.5 to 3 years), but it requires far less propellant and gives the crew a year and a half on the surface to conduct science.

The NASA reference models have consistently favored the conjunction-class mission. The mass savings from the low-energy transfers are enormous, and the long surface stay makes the entire endeavor more scientifically worthwhile.

The Hostile Environment

During the 6-to-9-month journey, the crew is in deep space, outside the protection of Earth’s magnetic field. They face two types of radiation:

1. Galactic Cosmic Rays (GCRs): High-energy particles from distant supernovae that can penetrate the ship’s hull and the human body.
2. Solar Particle Events (SPEs): Sudden, intense bursts of radiation from our own Sun.

On Mars, the thin atmosphere and lack of a global magnetic field offer little protection. An astronaut on the Martian surface would receive a significant radiation dose.

The Martian environment itself is hostile. The atmosphere is 95% carbon dioxide, the average temperature is -80°F (-62°C), and the planet is subject to massive dust storms that can blot out the sun for months.

The Tyranny of Mass

The single biggest problem in space exploration is mass. Every kilogram launched from Earth requires a massive amount of propellant. A 3-year round trip to Mars for a crew of four to six people requires an astonishing amount of “stuff”: the habitat, the crew, food, water, oxygen, computers, science gear, rovers, and, most ofall, the fuel needed to get home.

Early estimates showed that a single Mars mission would require launching over a million pounds to Low Earth Orbit (LEO). This would require dozens of launches of the most powerful rockets ever built, just for one mission.

A Legacy of Mars Planning

Ideas for sending humans to Mars are not new. In 1952, Wernher von Braun wrote “Das Marsprojekt,” a detailed technical plan that imagined a fleet of ten massive ships carrying 70 people. While visionary, it was far beyond the technology of its time.

After the Apollo program, NASA conducted various studies, but no serious plan took shape until the 1989 Space Exploration Initiative (SEI). This led to a “90-Day Study” that produced a baseline mission, but its estimated cost was so high (over 400 billion dollars) that it failed to gain political support.

Throughout the 1990s, NASA engineers went back to the drawing board, leading to the first “Design Reference Mission.” The goal was to find a new, more sustainable architecture. This effort evolved over 20 years, culminating in DRA 5.0.

Core Strategies of the Mars Reference Model

The modern reference model is built on several key strategies designed to solve the mass and time problems. These revolutionary ideas are what make a Mars mission feasible.

Strategy 1: Split-Mission Architecture

The old “battlestar” model (like von Braun’s) put the crew and all their supplies, landing craft, and return fuel onto one giant ship. This made the ship incredibly heavy, complex, and risky.

The DRA uses a split-mission architecture. The mission is “split” into two parts: cargo and crew.

1. Cargo Mission: A slow, uncrewed, robotic cargo ship is launched first, on a low-energy trajectory. It carries the heaviest components: the surface habitat, the exploration rovers, the power systems, and – most importantly – the Mars Ascent Vehicle (MAV), the crew’s rocket for getting off Mars.
2. Crew Mission: The crew doesn’t launch from Earth until the next launch window, 26 months later. They travel on a separate, lighter, and faster ship.

This approach has a significant safety benefit. The crew does not leave Earth until they have visual confirmation from orbiters that their “ride home” (the MAV) and their new home on Mars (the habitat) have landed safely and are fully operational.

Strategy 2: In-Situ Resource Utilization (ISRU)

This is the single most important concept in the reference model. “In-Situ Resource Utilization” is a simple idea: live off the land.

Remember the “tyranny of mass”? The heaviest single item on the mission list is the propellant needed for the Mars Ascent Vehicle to launch the crew from the Martian surface back into orbit. Carting all that rocket fuel from Earth to Mars is incredibly inefficient.

The ISRU strategy proposes to manufacture the rocket propellant on Mars.

The Martian atmosphere is 95% carbon dioxide (CO2). The cargo lander brings a small nuclear reactor (like the Kilopower concept) for power and a chemical processing plant.

1. The plant sucks in the Martian atmosphere.
2. It uses electricity to split the CO2 into carbon monoxide (CO) and Oxygen (O2). The oxygen is cryogenically cooled and stored as liquid oxygen (LOX), which is the oxidizer for the rocket.
3. The cargo mission also brings a small tank of liquid hydrogen (H2) from Earth.
4. This hydrogen is combined with the Martian carbon dioxide in a Sabatier reaction, which produces two things: water (H2O) and Methane (CH4).
5. The methane is chilled and stored as liquid methane, which is the fuel for the rocket. The water is recycled, and its hydrogen is used again in the process.

Over the 26 months between the cargo landing and the crew’s arrival, this automated plant slowly and steadily fills the propellant tanks of the Mars Ascent Vehicle. For every 1 kilogram of hydrogen brought from Earth, the ISRU plant can generate over 13 kilograms of high-grade methane/oxygen rocket propellant.

This strategy is not science fiction. An instrument on the Perseverance rover called MOXIE (Mars Oxygen ISRU Experiment) has already successfully proven the first step: it has generated pure oxygen from the Martian atmosphere.

Strategy 3: Nuclear Thermal Propulsion (NTP)

For the long transit between planets, the reference model favors Nuclear Thermal Propulsion (NTP).

A standard chemical rocket (like the ones that launch from Earth) works by mixing a fuel and an oxidizer and igniting them. It’s a controlled explosion that provides a massive amount of thrust for a short time.

A nuclear thermal rocket works differently. It uses a compact nuclear reactor to heat a single, lightweight propellant (like liquid hydrogen) to extreme temperatures. The superheated hydrogen gas then expands and shoots out of a nozzle at very high speeds.

NTP isn’t as high-thrust as a chemical rocket, so it can’t launch from Earth. But in the vacuum of space, it is up to twice as efficient. It gets twice the “gas mileage” (known as specific impulse). This high efficiency means the ship can complete the journey to Mars in just six months, instead of nine. This shorter trip time is a major benefit, as it significantly reduces the crew’s exposure to deep-space radiation and the negative health effects of weightlessness.

Strategy 4: Supersonic Retro-propulsion

The final major challenge is landing. Safely landing a multi-ton habitat or crew vehicle on Mars is notoriously difficult. The atmosphere is thick enough to burn up a spacecraft that enters too fast, but it’s too thin to slow a heavy vehicle down with parachutes alone.

The Curiosity rover (one-ton) used a complex “Sky Crane” system. The reference model’s components, like a 40-ton habitat, are far too heavy for that. The solution is supersonic retro-propulsion.

After an initial deceleration from a heat shield, the lander would fire powerful rocket engines directly into the supersonic airflow. This is an extremely complex aerodynamic feat, but it’s the only known method to slow a massive payload down quickly enough for a soft landing. This is the same technique now used by SpaceX to land its Falcon 9 boosters, and it’s central to the Starship landing profile.

Anatomy of the Mission (DRA 5.0 Example)

Using these strategies, the reference model outlines a complete mission cadence.

The Launch Campaign

The mission begins with a series of launches from Earth. DRA 5.0 was based on the Ares V super heavy-lift launch vehicle. Today, this role would be filled by NASA’s Space Launch System (SLS) or a commercial alternative. Several launches are needed to put all the pieces into Low Earth Orbit (LEO) for assembly.

Phase 1: The Cargo Mission

Two years before the crew leaves, the first launch window is used. A large cargo lander is assembled in orbit and fired toward Mars on a slow, 9-month journey. It carries:

• The Surface Habitat (a large, pre-integrated “can”).
• The ISRU chemical plant.
• A nuclear fission power system (e.g., Kilopower).
• The empty Mars Ascent Vehicle (MAV).
• Two unpressurized rovers for the crew to use.
• One pressurized rover (a “mobile home” for long-distance science trips).

This vehicle lands autonomously on Mars. Once on the surface, it deploys its solar panels (or reactor) and begins the 500-day process of making rocket fuel.

Phase 2: The Crewed Mission

At the next launch window (26 months later), and only after NASA confirms the MAV is fueled and the habitat is safe, the crew mission begins.

1. A separate super-heavy rocket launches the Mars Transfer Vehicle (MTV) into LEO. This ship is powered by the NTP engines and has the crew’s transit habitat (their “home” for the journey).
2. The crew of four to six astronauts launches in an Orion spacecraft, docks with the MTV, and boards for the long trip.
3. The MTV fires its NTP engines, performing the Trans-Mars Injection (TMI) burn and sending the crew on a 6-month, high-speed path to Mars.

The Six-Month Voyage

Life aboard the MTV is cramped. The crew lives in a transit habitat, a small pressurized volume similar to a module on the International Space Station (ISS). They are in constant weightlessness, requiring daily exercise to combat bone and muscle loss. Their primary protection from a Solar Particle Event is a “storm shelter” in the center of the ship, where they can surround themselves with the ship’s water and food supplies, using that mass as shielding.

Arrival and 500 Days on the Surface

Upon reaching Mars, the MTV fires its engines for Mars Orbit Insertion (MOI), placing it in a stable orbit. The crew transfers from the transit habitat into the Mars Lander (which they brought with them) and performs the fiery Entry, Descent, and Landing (EDL) sequence, landing near their pre-deployed base.

The MTV/transit habitat remains in orbit, waiting for them.

For the next 500-plus days, the crew lives and works on Mars. They move into the large surface habitat, which is much more spacious than their transit ship. Their days are filled with science: geology, drilling for ice, searching for signs of past life, and atmospheric studies. They use the pressurized rover to conduct multi-week expeditions hundreds of miles from their base, wearing advanced spacesuits for EVAs.

The Ascent and Return

When the 500-day stay is over, the next Earth-return launch window is open.

1. The crew boards the Mars Ascent Vehicle (MAV), which is now fully fueled with Martian-made methane and oxygen.
2. The MAV launches from the surface – a historic moment – and carries the crew into Mars orbit.
3. They rendezvous and dock with their Earth Return Vehicle (the MTV) that has been waiting patiently in orbit for 1.5 years.
4. The crew transfers to the transit habitat, and the empty MAV is discarded.
5. The ship fires its engines one last time for the Trans-Earth Injection (TEI).

Splashdown

After another 6-to-9-month journey, the ship approaches Earth at blistering speed. The crew moves into the Orion capsule, which detaches from the large, radioactive transit vehicle (which is disposed of in deep space). The capsule alone enters Earth’s atmosphere and splashes down in the ocean, returning the crew after a nearly 3-year absence.

From Reference Model to Reality: The Evolvable Path

The DRA 5.0 was tied to the Constellation program, which was canceled in 2010. Since then, NASA’s strategy has shifted to the Evolvable Mars Campaign (EMC).

This is the “Moon to Mars” philosophy behind the Artemis program. The idea is not to go straight to Mars, but to use the Moon as a proving ground.

The Gateway lunar outpost, a small space station in orbit around the Moon, will serve as a testbed for the deep-space hardware needed for Mars. It’s where NASA can test advanced solar-electric propulsion, long-term life support systems, and the logistics of operating far from Earth.

By landing astronauts on the lunar surface, NASA can test its surface habitats, pressurized rovers, and new spacesuits in a relevant environment. The Moon is only a 3-day trip home, making it a much safer place to discover and fix problems before committing to the 3-year Mars journey.

The Commercial Disruption: SpaceX and Starship

The NASA reference model is no longer the only blueprint in town. The private company SpaceX is developing its Starship system, which is itself a complete, alternative Mars reference architecture.

The Starship model validates some of the DRA’s core assumptions (like using methane for ISRU and supersonic retro-propulsion for landing) but challenges others.

• Full Reusability: Starship is designed for full and rapid reusability, which could dramatically lower launch costs.
• Orbital Refueling: Instead of assembling many different modules in orbit, the SpaceX model involves launching one Starship and then refueling it in Low Earth Orbit with multiple “tanker” flights.
• Monolithic Design: Starship is an all-in-one vehicle. It serves as the launch vehicle, the in-space transit habitat, the Mars lander, and the Mars ascent vehicle. This is mechanically simpler than the DRA’s collection of separate, specialized spacecraft.

This parallel development by the commercial sector is accelerating the technologies needed for Mars, building upon the foundational “homework” that NASA’s reference models established over decades.

The Unsolved Problems

The Design Reference Architecture is as much a list of problems as it is a list of solutions. It identifies the major hurdles that must be overcome before any mission can launch.

The Radiation Showstopper

This remains the biggest medical unknown. While a “storm shelter” can protect the crew from temporary Solar Particle Events, there is no practical shielding against the constant, penetrating Galactic Cosmic Rays (GCRs).

The long-term health risks of a 3-year exposure are not fully understood, but they include a significantly increased lifetime risk of cancer, potential cardiovascular disease, and, most worryingly, damage to the central nervous system (CNS) that could cause cognitive impairment (“Mars brain”) during the mission. Solving this may require new shielding technologies or advanced medical countermeasures.

The Gravity Question: 1g, 0g, and 1/3g

We know that 6-12 months of weightlessness on the ISS causes severe bone density loss, muscle atrophy, and fluid shifts that can damage eyesight. Astronauts return to Earth weak and must be carefully rehabilitated.

A Mars-bound crew will endure 6-9 months of zero-g. When they land on Mars, they will not be in 1g. They will be in Mars’s 1/3g. Will they be strong enough to walk? Can they perform a physically demanding EVA in an emergency? We don’t know if 1/3g is enough for the human body to stay healthy, or if it will be a “worst of both worlds” environment. There is no way to test this long-term without going to Mars or the Moon (1/6g).

The Human Element

A small crew of four to six people will be confined in a space the size of an RV for three years. They will experience isolation unlike any in human history.

The communication delay with Earth (up to 22 minutes one-way) makes real-time conversation impossible. There is no “Houston, we have a problem.” The crew must be completely autonomous, able to handle medical emergencies (like surgery), complex repairs, and interpersonal conflicts on their own. The psychological resilience required is immense.

Planetary Protection

Finally, the mission must adhere to the Outer Space Treaty and strict planetary protection protocols. This is a two-way street.

1. Forward Contamination: The landers and habitats must be sterilized to avoid contaminating Mars with Earth microbes. This is essential if we are to search for signs of native Martian life; finding our own bacteria would corrupt the science forever.
2. Back Contamination: The crew and any samples they bring back must be strictly quarantined, as in the Apollo program. The reference model includes “breaking the chain of contact.” The crew launches from Mars in the MAV, docks with the return vehicle in orbit, and transfers their sealed samples. Only the Orionre-entry capsule, which never touched Mars, returns to Earth, ensuring no hypothetical Martian microbes can contaminate our biosphere.

Summary

The NASA Mars Human Mission Reference Model is not a single, static plan. It’s a “cookbook” that has evolved for decades, providing the essential recipes for human exploration. It identifies the key ingredients: In-Situ Resource Utilization (ISRU) to make fuel on Mars, Nuclear Thermal Propulsion (NTP) for a fast transit, and a split-mission architecture for safety.

More than a simple plan, the DRA is an “engineering problem-finder.” It highlights the massive, unsolved challenges in radiation, human health, and landing technology that must be solved before we can go. It provides the indispensable engineering and strategic backbone that informs all modern Mars planning, from the Artemis program to the ambitions of commercial partners. It’s the foundational “homework” that turns the dream of planting human boots on the red sand of Mars into a tangible, solvable engineering challenge.