NASA Chooses Nuclear Fission Power for Humans on Mars

NASA released its latest set of Moon to Mars Architecture products on December 13th, sharing developments from this year’s planning cycle. Among the results are two new spacecraft, ten new white papers on a range of topics, and detailed explanations for how decisions are made. 

While all of these products are valuable to NASA’s future plans, one of them is extremely significant in the history of human spaceflight. NASA has made its first key architecture decision on the branching road to Mars: nuclear fission will power the first human missions on the Martian surface. This choice is in itself a milestone for NASA, and the advantages offered by nuclear power will shape the road to Mars in far-reaching ways.

A True Decision For Mars

Before diving into the technology itself, it’s important to realize that this marks the first time NASA has ever committed to any “plan” for sending humans to Mars. The agency is no stranger to the web of decisions involved in Mars planning, and has studied the problem extensively throughout its history. Yet even NASA’s most recognizable mission concepts, like its 2009 Design Reference Architecture 5.0, are only that—concepts and studies. Like worked examples in a math textbook, previous “plans” have merely been instructive, meant to illustrate how a mission takes shape around a given set of assumptions. To quote the introduction to DRA 5.0,

The strategy and implementation concepts that are described in this report should not be viewed as constituting a formal plan for the human exploration of Mars. Instead, this report provides a vision of one potential approach to human Mars exploration […] .

More recently, NASA’s 2021 Strategic Analysis Cycle (SAC21) produced a concept for a minimalist, short-stay architecture, notably requiring its crew to live out of a pressurized rover on the surface. Yet again, the authors of the publication clearly state in the abstract:

It is important to note that NASA does not have a formal human Mars program and no decisions have been made; the architecture described here is intended to fill in an often-overlooked corner of the trade space, helping to complete the menu of options available to decision-makers […] .

It is incorrect to represent any of these past studies as NASA’s “plan” for sending humans to Mars. However, the opening statements from the SAC21 paper are now out of date. A formal human Mars program now exists, and the first key decision has been made, heralding NASA’s commitment to exploration of the red planet.

The Moon to Mars Program was established following the 2022 NASA Authorization Act with the explicit charge “to achieve the goal of human exploration of Mars,” and its responsibilities include defining requirements and making decisions for the first crewed mission to the surface. During its 2023 Architecture Concept Review (ACR23), NASA identified surface power generation technology as one of the first such decisions that needed to be made, because it so thoroughly influences the shape of early missions. Further information about this analysis process can be found in our earlier article on the Humans to Mars segment of the program.

The language in the ACR24 products clearly frames the selection of fission surface power as the choice made for initial human missions to Mars—not an example, nor merely a recommendation, but a formal decision. NASA’s first decision.

Power generation is notable among NASA’s identified “priority” decisions in that it concerns a specific technology. The remainder of these questions are more nuanced, ranging from choosing a crew size to defining the agency’s loss of mission risk posture. On the other hand, the trade space for power generation is very well understood, to the point that selecting fission was practically a foregone conclusion. So, what makes the nuclear option so attractive?

The Power of Fission

At a glance, it may seem surprising that a technology as apparently complex as nuclear fission could prevail over solar panels, which have a storied history of use on Mars. But while solar power has served NASA well for small, robotic missions, human-class missions turn the trade space on its head.

When it comes to power generation, the key difference between robotic missions and human exploration is scale. Consider Opportunity, a robotic Mars rover which lasted 14 years on solar power alone. At their peak output, Opportunity’s solar panels produced just 140 watts of power—barely enough to power a domestic light bulb. In the White Paper explaining their selection of nuclear power, NASA points out that even the most spartan crewed surface mission would require about 10,000 watts (10 kilowatts), pushing a hundred times the output of Opportunity’s panels.

Naturally, higher power consumption leads to larger, heavier power generation systems, and this is the first area where nuclear fission shines. Each of the International Space Station’s vast solar arrays generates about 30 kilowatts of power, yet is 35 meters long and impossibly delicate. Arrays like these are relatively simple to deploy in microgravity, but would need much more structural reinforcement even to lay out across the ground on Mars. Conversely, a single fission reactor producing about 10 kilowatts could be just a few meters long, comparable in size to a human. Fission surface power can achieve much higher power densities—more output for a given size and mass—than solar arrays, allowing them to scale easily to meet greater needs.

Furthermore, solar panels on Mars are hard-pressed to provide consistent power, even compared to their use on Earth. Mars’ thin atmosphere is full of clinging dust which gradually coats every surface. Seasonal dust storms suspend enough material in the sky to blot out the sun, and one of these is what ultimately claimed Opportunity. Even during clearer weather, Mars’ distance from the sun means it receives less than half the sunlight we do on Earth, doubling the size of solar arrays. This also limits where they can be practically deployed on Mars; each winter, Opportunity’s drivers would strive to park the rover high on a hillside, tipped towards the sun in the hopes of surviving the darkness to come. Even surviving a single, freezing night on Mars is a gamble when your power source is tied to the sun. By contrast, nuclear power is available anytime, anywhere, and in any weather, without needing constant cleaning or repositioning.

Altogether, NASA cites consistent availability, reliability, and scalability as key reasons for choosing nuclear power over other options. Solar and other technologies have their place, and will no doubt complement nuclear energy during early missions. Still, NASA’s planning will now baseline fission as the backbone of surface power on Mars, which has significant implications for the road ahead.

NASA’s Nuclear Future

Though selecting fission surface power is just the first of many choices to come, it handily illustrates NASA’s architecture planning process. Power generation was selected as a priority decision specifically due to its potential for long lead times in development; even with crewed Mars missions likely at least a decade out, nuclear reactors for use in space will need plenty of time to mature both their technologies and relevant policies.

NASA has long expected that nuclear power would prove crucial for future exploration, and accordingly has spent years funding projects to gradually mature reactor concepts. In 2018, the Kilopower project successfully operated a subscale demonstration reactor using a stirling heat engine to generate electricity. More recently, NASA has partnered with the Department of Energy to advance the concept further, awarding three industry contracts in 2022 to study 40-kilowatt reactor designs massing under six tons. Solicitation for a lunar surface demonstration is set to open in 2025, bringing fission surface power ever closer to flight.

Exploration of the Moon during the Artemis program offers an excellent opportunity to test fission power technologies before sending them to Mars. The Moon shares several of Mars’ environmental challenges, including a pervasive coating of hazardous dust and long, freezing nights. Intriguing science locked within permanently shadowed craters creates an ideal use case for nuclear power over solar, and industry designs for Artemis could evolve into those deployed on Mars. As NASA’s white paper on key decisions points out,

The narrowing window of opportunity to infuse Mars-forward considerations into […] decisions for Artemis makes this a timely activity.

NASA’s nuclear ambitions don’t end on planetary surfaces. As discussed in more depth elsewhere, the vessels that carry astronauts to Mars and beyond may well be powered by even larger reactors, and projects like DRACO and JETSON will help these concepts develop alongside surface power.

However, just as important as the technology is the policy associated with launching nuclear reactors into space. There is limited precedent for this; simpler radioisotope generators have powered many missions, but the US last launched a true reactor in 1965. In addition to its partnership with the DOE, NASA has consistently worked with the Nuclear Regulatory Commission to review safety for nuclear space missions, from ground processing through launch. These stakeholders have continued to provide input as new technologies such as fission surface power rise to prominence. As NASA takes a renewed interest in sustainable and ethical use of space, it will need to work with regulators to create new safety standards suited to the coming nuclear age.

By selecting fission surface power for its first crewed Mars missions, NASA has done more than affirm its commitment to that horizon goal. The Moon to Mars Program has broken the glass of an unseen barrier in the agency’s history: for the first time, NASA has begun to codify a formal plan to send humans to the red planet. As the strategic analysis cycle begins anew in 2025, further decisions will begin to take shape, set in motion by this measured first step forward.

Edited by Nik Alexander