Science

NASA’s ‘Nuclear Option’ May Be Crucial for Getting Humans to Mars

The pathway to humans on Mars lies through the atom, split.

Far from Earth, whether in the void or on another world, power is life. A steady, strong flow of electricity is as crucial for operating computers and engines as it is for assuring access to corporeal necessities such as light and heat, breathable air and potable water, and preparation or even growth of food. And one of the most potent and reliable ways to get all those vital kilowatts is via nuclear fission—something aspiring astronauts realized long before anyone ever reached space (or developed nuclear weapons, for that matter). Yet more than 60 years into the space age, nuclear fission for spaceflight remains mostly a dream. Now, however, as NASA pursues its Apollo-esque Artemis program to build a crewed lunar outpost (with an eye toward eventual human landings on Mars), a rare alignment of technology, funding and political will is on the verge of making spaceborne nuclear reactors a routine reality.

In 2020 the White House gave NASA a 10-year deadline to deliver a 10-kilowatt nuclear power system to the surface of the moon. The project is now a top priority of the agency’s Space Technology Mission Directorate. And in July 2021 congressional appropriators earmarked $110 million for NASA to advance development of a new nuclear rocket suitable for sending cargo and crew on interplanetary voyages. NASA had not even asked for the money.

The reason for this sudden urgency is simple: Without nuclear power, the space agency’s stated goal of establishing a moon base by the end of the decade—let alone putting boots on Mars—becomes difficult, if not impossible, to achieve.

Surprisingly, no fundamental technology breakthroughs are required to build a nuclear reactor for spaceflight applications. (In fact, the U.S. already did so once—and so far only once—with the Air Force’s development and launch of a working prototype in 1965.) Instead the difficulty lies in navigating the complex web of regulations that surrounds all things nuclear and in ensuring any chosen approach for nuclear power beyond Earth does not needlessly limit NASA to just the lunar surface or any other lone deep-space destination. Ideally, the power of the atom can be harnessed not only for crewed missions to the moon and Mars but also for robotic exploration throughout the solar system.

“The goal going in is make sure that what we use on the moon from a fission reactor standpoint is also directly applicable for use on the surface of Mars,” says Michael Houts, manager of nuclear research at NASA’s Marshall Space Flight Center.

Fission, he explains, is a pretty simple process. “It’s literally just the right materials in the right geometry,” Houts says. “That’s why, once it was discovered, we very quickly had systems able to self-sustain a chain reaction.” This differs completely from the radioisotope thermoelectric generators (RTGs) that power NASA’s Mars rovers, the New Horizons mission to Pluto and beyond, and the Voyager spacecraft now in interstellar space. RTGs merely convert the heat released from naturally decaying plutonium into electricity. Fission reactors are far more powerful and versatile, splitting atoms from uranium fuel and channeling the released energy into propulsion and electricity production.

“There are no physics breakthroughs needed, no miracles necessary. But just like terrestrial systems, you’re going to need to have some really good engineering,” Houts says.

An illustration of a fission-based nuclear power system deployed on the lunar surface. Credit: NASA

A Long-Delayed Giant Leap

NASA is publicly cagey about its Mars timeline, but since the first term of former president George W. Bush, the agency has steadily worked toward a giant leap on the Martian surface by the end of the 2030s. In 2020 NASA asked the National Academies of Sciences, Engineering, and Medicine to study the technical challenges, benefits and risks of nuclear propulsion, with particular emphasis on a notional nuclear-propelled cargo launch to Mars in 2033 that would precede a human mission in 2039.

In logistic terms, what such a mission would look like has scarcely changed since the 1950s. Three years before Yuri Gagarin’s flight made humans a spacefaring species, NASA’s precursor, the National Advisory Committee for Aeronautics, began a formal study of nuclear propulsion as part of a crewed Mars expedition. This investigation called for a 420-day expedition with 40 days at Mars. Other, more ambitious proposals have examined lengthier surface sojourns on Mars stretching to around 500 days, but the classic mission profile has remained the dominant vision for crewed Mars exploration, driven in part by celestial mechanics and reasons of survival: To conserve fuel, both Earth and Mars must be properly aligned in their orbit. And technologically speaking, humans are not yet ready to cut the terrestrial umbilical cord and truly “live off the land” in space.

The human body can handle the journey, as evidenced by decades of data from crews living and working on space stations in low-Earth orbit. The current record for the longest continuous stay in space is held by the cosmonaut Valeri Polyakov. Thanks to a vigorous off-world workout regimen, he was able to walk from his capsule after landing despite having spent 437 days in muscle-wasting microgravity onboard the Soviet space station Mir. Upon returning to Earth, Polyakov’s first words to a fellow cosmonaut reportedly were “We can fly to Mars.”

NASA’s current goal for a Mars mission calls for a round trip of about two years. Nuclear propulsion would be a critical enabler. In addition to increasing the number of flight opportunities for a crewed mission, it would reduce the number of flights necessary to get the fuel for such a trip into Earth’s orbit.

Those fuel requirements are considerable. The International Space Station, painstakingly built via more than three dozen launches across a decade’s time, is approximately 420 metric tons. A chemical propulsion system necessary for a round trip to Mars would require the very expensive task of lofting somewhere between more than twice to nearly 10 times as much tonnage from Earth. Consider that the mightiest of NASA’s rockets—the Space Launch System (SLS), which has yet to even fly—is slated to carry a mere 95 metric tons to space at $2 billion per launch. If—or when—the SLS is superseded by more capable and cost-effective rockets such as SpaceX’s in-development and all-reusable Starship, that single-launch mass limit will increase to more than 100 metric tons, and the price per launch should plummet. Even so, the financial calculus of a chemically fueled Mars mission would still be daunting.

In contrast, an analogous Mars mission using nuclear propulsion would require sending up a total mass of between 500 and 1,000 metric tons. Launching the equivalent of a single space station—maybe two—is plausible. After all, we have done it before.

Hard Choices

NASA is presently pursuing not one but two classes of atomic-powered rocketry: nuclear thermal propulsion and nuclear electric propulsion. Either of these approaches could pair with nuclear surface power—the third key fission technology under study by the space agency.

Two illustrations of NASA nuclear propulsion concepts.
Two illustrations of NASA nuclear propulsion concepts. The space agency is developing technologies for spacecraft using nuclear electric propulsion (top) as well as nuclear thermal propulsion (bottom). Credit: NASA (top) and NASA (bottom)

Nuclear thermal propulsion implemented on the interplanetary scale would essentially be a ferry or transfer stage—a smaller nuclear-powered rocket that would dock with other transport elements in orbit before pushing its separately launched payload onward. Such an arrangement operates much like a chemical propulsion system, although the combustion chamber—where a rocket’s fuel and oxidizer mix and ignite, producing hot exhaust forced from the rocket nozzle—is replaced with a nuclear reactor that heats a cryogenic propellant, blasting it through the nozzle to generate thrust. The process, viewed externally, looks virtually identical: a rocket engine blasting fire.

Nuclear electric propulsion, on the other hand, works a lot like a nuclear power plant on Earth, in which fission reactions are used (via an intermediate step such as driving a turbine) to generate electricity. That electricity, in turn, can power an electric propulsion system similar to (but far stronger than) the solar-powered ion thrusters on NASA’s Dawn, a spacecraft that explored the asteroid Vesta and dwarf planet Ceres.

There are trade-offs to each approach. The greatest challenge of nuclear thermal propulsion is that it is a high-performance reactor operating at a high temperature, reaching circa 2,500 degrees Celsius—an unnerving prospect for astronauts and materials engineers. The reactor would also require immense volumes of cryogenic propellant, likely sourced from on-orbit storage tanks that carry major engineering challenges of their own. But the approach’s focused intensity has an upside: “The propulsion system only needs to run for a few hours total,” Houts says. “You get all your [work] done very quickly.” After that, the spacecraft has all the speed it needs for a trip to Mars or home.

Nuclear electric propulsion, meanwhile, runs at lower temperatures and power levels, but it must operate continuously for months or even years, building fantastic speeds over time. It is a more complex system than its thermal counterpart in many ways. And it is less developed: the calculated performance levels for near-term designs are far below what would be necessary for a crewed mission to Mars. The power produced by a nuclear electric propulsion system’s reactor must be converted multiple times (rather than just being absorbed and dissipated by propellant blown out the back of a rocket). Conversions can only be done with efficiency percentages ranging from the mid-30s to 40. The rest of that thermal energy must somehow be dealt with: present concepts call for massive radiators to dissipate the excess heat into space. The nuclear electric spacecraft would also require a short, sharp kick from an old-fashioned chemical propulsion system to help it escape Earth’s orbit and another to enter and depart orbit around Mars.

Past and Future

In part because of its relative simplicity, nuclear thermal propulsion is the clear favorite among Mars mission planners—and U.S. politicians. This was the approach that netted the $110-million endorsement of congressional appropriators in July 2021 and that the NASA-sponsored National Academies report flagged as most plausible for enabling a 2039 crewed mission to the Red Planet.

Nuclear thermal propulsion also has the advantage of a rich inheritance: The U.S. government—chiefly the Department of Defense—has been fitfully trying to get the technology flying since the dawn of the space age. One bold early attempt traces to a 1955 Air Force effort known as Project Rover, which sought to build a nuclear thermal upper stage for intercontinental ballistic missiles. But chemical propulsion soon proved sufficient for that job, so Rover was absorbed into NASA, where it became the Nuclear Engine for Rocket Vehicle Application (NERVA) program. In the late 1950s, the DoD started work on the Systems for Nuclear Auxiliary Power (SNAP) program, an effort to launch space nuclear reactors to power long-duration missions such as spy satellites.

Both projects achieved impressive results. SNAP led to the Air Force’s 1965 launch of SNAP-10A, the only U.S. fission reactor ever sent to space. The reactor functioned for six weeks in orbit. NERVA, meanwhile, successfully developed and tested nuclear thermal rockets on Earth. And the program was, for a time, central to NASA’s post-Apollo plans for Mars exploration. But the Nixon administration instead chose to pursue the space shuttle and canceled both projects in 1973. NERVA was briefly resurrected in the late 1980s by an Air Force–led effort, the Space Nuclear Thermal Propulsion program, but by the early 1990s interest had fizzled again.

Nuclear electric propulsion, too, had its brief moment in NASA’s limelight. In 2003 an initiative called Project Prometheus brought together NASA, the U.S. Navy’s submarine reactor program and the Department of Energy—this time to build a nuclear electric propulsion fleet for science missions. Spaceborne fission would enable a single spacecraft to explore multiple targets in the outer solar system and even beyond, where sparse sunlight profoundly limits solar power’s potential. Project Prometheus would have been nothing short of revolutionary: its reactor would have produced 200,000 watts of power for a spacecraft’s propulsion and instruments. (By comparison, the New Horizons probe operates on just 200 watts of power—that is, about two or three incandescent light bulbs’ worth.) NASA, however, snuffed out Prometheus after two years, citing budget concerns.

One might think all these past projects would be a huge boost for today’s push to develop atomic-powered rocketry, but their mercurial nature makes them of limited use.

“Historically, if you spend three or four years developing a nuclear propulsion system, and then you stop, and you come back a decade later, you’ve got to recapture a lot of knowledge,” says Shannon Bragg-Sitton, a leading nuclear engineer at the Idaho National Laboratory and co-author of the National Academies report. “The fact that we’ve been looking at both these systems since the 1950s doesn’t mean that we have 70 years of knowledge. It means that we started thinking about them then, and we made some efforts in each of them.”

NASA’s notional target date of 2039 for a crewed Mars mission might seem so far off that urgent action is not yet necessary, but Bragg-Sitton says the timing is deceptive. The tentative plan calls for nuclear-powered cargo flights to begin six years earlier, in 2033, to preposition materials on Mars and serve as dry runs for crewed transport. “We need to be ready to actually launch our first system for qualification with those supply missions,” she says. “Well, now the timeline is not as long as it sounded initially!” Ideally, she says, hardware designs for a flight in 2033 would be locked-in by 2027. That means the time is now to make critical decisions, chief among them comparing and choosing between nuclear thermal and nuclear electric propulsion.

“You can’t develop a nuclear system in a year or two—it’s just the way it is,” Bragg-Sitton concludes. “None of this is out of our reach. It just takes a lot of focus to get it done.”

But first, someone needs to let them do it.

The DRACO Wager

Getting approval to launch nuclear materials into space, it turns out, is at least as challenging as actually building a space-ready nuclear reactor or rocket. This is especially true if your fission system relies on highly enriched uranium—that is, uranium composed of 20 percent or more of the fissile isotope uranium 235. Only 1 percent of Earth’s naturally occurring uranium takes this form, which is prized by warhead designers and spacecraft engineers striving to make their creations as featherweight and powerful as possible. The more uranium 235 your nuclear fuel has, the smaller you can make your reactor—or your bomb, which is why the material is subject to such strict regulations.

For NASA, even a nuclear payload without highly enriched uranium has enormous hurdles to clear—namely a labyrinthine safety analysis process that often involves many other federal agencies and culminates in NASA’s administrator approving or rejecting a launch. If a rocket carries highly enriched uranium, however, it can only be launched after formal authorization from the White House. The additional stringency associated with this highest tier of approval can easily add several years and tens of millions of dollars to a project’s schedule and budget.

Find a way to avoid using highly enriched uranium, then, and you may secure a far faster and cheaper path to the launchpad. There are, in fact, new designs for advanced high-power reactors that use large amounts of low-enriched uranium rather than small amounts of highly enriched material. But whether or not NASA ultimately pursues such an approach for its nuclear aspirations may be dictated by the work of another federal entity: the Defense Advanced Research Projects Agency wants to launch one of these new reactors to space by 2025 to power a proof-of-concept nuclear propulsion system—a timeline that would be aggressive even by Apollo standards. DARPA calls the system the Demonstration Rocket for Agile Cislunar Operations, or DRACO. The program’s murky origins involve DoD demands for some of its classified missions to have the capability to maneuver in space faster than would be possible through chemical propulsion.

DARPA’s gamble with DRACO is twofold: it seeks to reach the launchpad quickly by using a new type of reactor and by minimizing Earth-based trials, thus bypassing the presidential-tier launch approval process and a rat’s nest of ground-testing red tape. This bold strategy arose from the agency’s judgment that such tests are now virtually impossible to perform because of prohibitive regulations and inadequate infrastructure. One cannot, for instance, simply update and use the specialized facilities that supported NERVA testing—they were razed when the program ended. Building new test facilities is undesirable, too, because doing so would require billions of dollars and several years of work during which the project could easily be scuttled by shifting political priorities. Although DARPA’s accelerated plan calls for robust ground testing of DRACO’s smaller components, this does not include operating the full reactor at full power. Astoundingly, the very first time DRACO’s reactor would turn on would be in space.

“Starting the reactor is going to be entirely based on our predictions,” says Tabitha Dodson, a project manager for DRACO at DARPA. “We are going to put a lot of guesswork into our modeling and simulations before launching the engine, without ever having tested it on the ground.” Data from the NERVA tests of yore should help, Dodson says, but the task before the DRACO team remains “extremely challenging.”

After more than a half-century of starts and stops, says Air Force major Nathan Greiner, another DARPA project manager, launching a nuclear reactor would be a critical enabler. “Let’s get this all the way across the finish line—not just small elements, not just a reactor on the ground, but, no kidding, let’s go build a spacecraft and put it in space,” he says. Such an “existence proof” would then ease the way for NASA or the DoD in any future overtures to congressional appropriators. The question would no longer be “Does this technology exist?” but rather “Do you want more of it—or not?”

Technicians work on a test unit (center) in preparation for the April 1965 launch of SNAP-10A.
Technicians work on a test unit (center) in preparation for the April 1965 launch of SNAP-10A, the only U.S. fission reactor yet sent to space. A new U.S. effort, DARPA’s DRACO program, seeks to launch a second fission reactor by 2025. Credit: George Rinhart/Corbis via Getty Images

Let’s Get Serious

Of course, DARPA alone cannot spark a spaceflight revolution. Nuclear propulsion for space exploration is a whole-of-government effort. At minimum, the Department of Energy will need to make more low-enriched uranium. One agency or another—most likely, several working together—will have to develop orbital fuel depots to provide outbound missions with cryogenic propellants and will have to find better, safer ways to perform ground tests of interplanetary-scale propulsion systems. And then NASA must actually build the rockets.

DRACO will not get NASA and its astronauts all the way to Mars, Greiner says, “but this is going to take it a hell of a long way along that path.”

If nothing else, today’s push for nuclear power in space is a useful metric for measuring the seriousness of NASA’s—and the nation’s—lunar and Martian ambitions. In the context of human spaceflight, NASA has a well-known aversion to “new” (and thus presumably more risky) technology—but in this case, the “old” way makes an already perilous human endeavor needlessly difficult. For all the challenges of embracing nuclear power for pushing the horizon outward for humans in space, it is hard to make the case that tried-and-true chemical propulsion is easier or carries significantly less physical—and political—risk. Launching 10 International Space Stations’ worth of mass across 27 superheavy rocket launches for fuel alone for a single Mars mission would be a difficult pace for NASA to sustain. (That is more than 40 launches and at least $80 billion if the agency relies on the SLS.) And such a scenario assumes everything goes perfectly: sending help to a troubled crew on or around Mars would require dozens of additional fuel launches, and chemical propulsion allows very limited windows of opportunity for the liftoff of any rescue mission.

If, with a single technology, that alarmingly high number of ludicrously expensive launches could be cut down to three—while also offering more chances to travel to Mars and back—how could a space agency that was earnest in its ambitions not pursue that approach? No miracles are necessary, and regulators and appropriators seem to agree that the time has come.

As Polyakov said, “We can fly to Mars.” Splitting atoms, it seems, is now the safest way to make that happen.


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