A Long-Delayed Giant Bound
NASA is publicly chary about its Mars timeline, simply since the first term of erstwhile president George Westward. Bush, the agency has steadily worked toward a behemothic leap on the Martian surface by the stop of the 2030s. In 2020 NASA asked the National Academies of Sciences, Technology, and Medicine to study the technical challenges, benefits and risks of nuclear propulsion, with item 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’due south flight made humans a spacefaring species, NASA’s precursor, the National Advisory Committee for Helmsmanship, began a formal written report of nuclear propulsion as part of a crewed Mars trek. This investigation called for a 420-day expedition with 40 days at Mars. Other, more aggressive 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 role 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 even so ready to cut the terrestrial umbilical cord and truly “live off the country” in space.
The human being body tin can handle the journey, as evidenced past decades of data from crews living and working on space stations in low-Globe orbit. The electric current record for the longest continuous stay in space is held by the cosmonaut Valeri Polyakov. Thanks to a vigorous off-globe workout regimen, he was able to walk from his capsule subsequently landing despite having spent 437 days in muscle-wasting microgravity onboard the Soviet space station Mir. Upon returning to Globe, Polyakov’s get-go words to a young man 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 flying opportunities for a crewed mission, it would reduce the number of flights necessary to go the fuel for such a trip into Earth’s orbit.
Those fuel requirements are considerable. The International Space Station, painstakingly built via more iii dozen launches across a decade’s time, is approximately 420 metric tons. A chemical propulsion organisation necessary for a circular trip to Mars would require the very expensive chore of lofting somewhere between more than twice to about ten times as much tonnage from Earth. Consider that the mightiest of NASA’s rockets—the Space Launch Organisation (SLS), which has yet to even fly—is slated to deport 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 every bit SpaceX’due south in-development and all-reusable Starship, that single-launch mass limit will increase to more than than 100 metric tons, and the price per launch should collapse. Notwithstanding, the financial calculus of a chemically fueled Mars mission would withal be daunting.
In contrast, an coordinating Mars mission using nuclear propulsion would require sending up a total mass of between 500 and i,000 metric tons. Launching the equivalent of a single space station—maybe two—is plausible. After all, we have done it before.
NASA is presently pursuing not 1 merely 2 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 written report by the space bureau.
Nuclear thermal propulsion implemented on the interplanetary calibration would essentially be a ferry or transfer phase—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 similar a chemical propulsion system, although the combustion sleeping room—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 information technology through the nozzle to generate thrust. The process, viewed externally, looks nigh identical: a rocket engine diggings fire.
Nuclear electrical propulsion, on the other hand, works a lot like a nuclear power establish on World, in which fission reactions are used (via an intermediate step such equally driving a turbine) to generate electricity. That electricity, in turn, tin can ability an electric propulsion system similar to (but far stronger than) the solar-powered ion thrusters on NASA’south Dawn, a spacecraft that explored the asteroid Vesta and dwarf planet Ceres.
In that location are trade-offs to each arroyo. The greatest challenge of nuclear thermal propulsion is that information technology 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, probable sourced from on-orbit storage tanks that carry major engineering science 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.” Afterwards 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 fifty-fifty years, building fantastic speeds over time. It is a more complex system than its thermal counterpart in many ways. And information technology is less adult: the calculated performance levels for nigh-term designs are far below what would be necessary for a crewed mission to Mars. The ability produced past a nuclear electrical propulsion organization’south reactor must be converted multiple times (rather than just beingness absorbed and dissipated past propellant diddled out the back of a rocket). Conversions tin simply be done with efficiency percentages ranging from the mid-30s to 40.
The residuum of that thermal energy must somehow exist dealt with: nowadays concepts call for massive radiators to dissipate the backlog heat into space. The nuclear electric spacecraft would besides require a short, sharp kick from an old-fashioned chemical propulsion system to help it escape Earth’due south orbit and another to enter and depart orbit effectually Mars.
Past and Future
In function considering of its relative simplicity, nuclear thermal propulsion is the clear favorite among Mars mission planners—and U.S. politicians. This was the arroyo that netted the $110-million endorsement of congressional appropriators in July 2021 and that the NASA-sponsored National Academies report flagged equally nigh plausible for enabling a 2039 crewed mission to the Scarlet Planet.
Nuclear thermal propulsion also has the reward 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 historic period. One assuming early on endeavour traces to a 1955 Air Force effort known every bit Project Rover, which sought to build a nuclear thermal upper stage for intercontinental ballistic missiles. Only chemical propulsion soon proved sufficient for that job, and so Rover was absorbed into NASA, where information technology became the Nuclear Engine for Rocket Vehicle Application (NERVA) program. In the tardily 1950s, the DoD started work on the Systems for Nuclear Auxiliary Ability (SNAP) program, an attempt to launch space nuclear reactors to power long-duration missions such equally spy satellites.
Both projects achieved impressive results. SNAP led to the Air Strength’s 1965 launch of SNAP-10A, the only U.S. fission reactor e’er sent to space. The reactor functioned for six weeks in orbit. NERVA, meanwhile, successfully adult and tested nuclear thermal rockets on Earth. And the program was, for a time, central to NASA’s post-Apollo plans for Mars exploration. Just 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 Strength–led effort, the Space Nuclear Thermal Propulsion program, but past the early 1990s interest had fizzled again.
Nuclear electrical propulsion, also, had its cursory 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. Projection Prometheus would have been nothing short of revolutionary: its reactor would accept produced 200,000 watts of power for a spacecraft’s propulsion and instruments. (By comparison, the New Horizons probe operates on merely 200 watts of power—that is, near two or 3 incandescent light bulbs’ worth.) NASA,
nevertheless, snuffed out Prometheus after two years, citing budget concerns.
Ane might think all these past projects would be a huge boost for today’s push to develop atomic-powered rocketry, just their mercurial nature makes them of limited use.
“Historically, if you spend three or 4 years developing a nuclear propulsion system, and so y’all finish, and you come back a decade afterwards, you lot’ve got to recapture a lot of cognition,” 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 accept seventy years of knowledge. Information technology ways that nosotros started thinking about them then, and nosotros made some efforts in each of them.”
NASA’due south notional target date of 2039 for a crewed Mars mission might seem so far off that urgent action is non yet necessary, but Bragg-Sitton says the timing is deceptive. The tentative programme calls for nuclear-powered cargo flights to brainstorm six years earlier, in 2033, to preposition materials on Mars and serve as dry out runs for crewed transport. “Nosotros need to be ready to actually launch our showtime organization 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 past 2027. That ways the time is
to make disquisitional decisions, primary among them comparing and choosing between nuclear thermal and nuclear electric propulsion.
“Yous tin can’t develop a nuclear system in a year or two—information technology’due south just the manner it is,” Bragg-Sitton concludes. “None of this is out of our accomplish. It just takes a lot of focus to become it done.”
But first, someone needs to let them do it.
The DRACO Wager
Getting blessing to launch nuclear materials into infinite, it turns out, is at least equally 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 xx percent or more of the fissile isotope uranium 235. Simply ane 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 y’all can brand your reactor—or your bomb, which is why the material is field of study 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, still, it can only be launched later formal authorization from the White House. The boosted stringency associated with this highest tier of approval can easily add together several years and tens of millions of dollars to a projection’s schedule and budget.
Observe a fashion to avoid using highly enriched uranium, then, and you may secure a far faster and cheaper path to the launchpad. In that location are, in fact, new designs for avant-garde high-power reactors that use big amounts of low-enriched uranium rather than small amounts of highly enriched fabric. But whether or non NASA ultimately pursues such an approach for its nuclear aspirations may be dictated by the piece of 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 ambitious even by Apollo standards. DARPA calls the arrangement the Sit-in 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 Globe-based trials, thus bypassing the presidential-tier launch blessing process and a rat’s nest of basis-testing ruby-red tape. This bold strategy arose from the bureau’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 projection could easily exist scuttled by shifting political priorities. Although DARPA’s accelerated programme 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 assistance, Dodson says, simply the task earlier the DRACO team remains “extremely challenging.”
Afterwards more than a half-century of starts and stops, says Air Force major Nathan Greiner, another DARPA projection managing director, launching a nuclear reactor would be a critical enabler. “Let’southward go this all the way beyond the end line—not just modest elements, not simply a reactor on the ground, but, no kidding, allow’s go build a spacecraft and put it in space,” he says. Such an “being proof” would then ease the way for NASA or the DoD in whatever future overtures to congressional appropriators. The question would no longer be “Does this engineering exist?” but rather “Exercise yous want more of it—or not?”
Let’southward Get Serious
Of course, DARPA alone cannot spark a spaceflight revolution. Nuclear propulsion for space exploration is a whole-of-government endeavour. At minimum, the Department of Energy volition need to make more depression-enriched uranium. I agency or another—most likely, several working together—will accept to develop orbital fuel depots to provide outbound missions with cryogenic propellants and will have to find ameliorate, safer ways to perform ground tests of interplanetary-scale propulsion systems. And and then NASA must actually build the rockets.
DRACO volition not get NASA and its astronauts all the style to Mars, Greiner says, “but this is going to accept information technology a hell of a long way forth that path.”
If nada else, today’southward push for nuclear power in space is a useful metric for measuring the seriousness of NASA’due south—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—just in this instance, the “onetime” manner 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 brand the case that tried-and-true chemical propulsion is easier or carries significantly less concrete—and political—take chances. 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 twoscore launches and at least $lxxx billion if the agency relies on the SLS.) And such a scenario assumes everything goes perfectly: sending help to a troubled coiffure on or around Mars would require dozens of boosted fuel launches, and chemic 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 arroyo? 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 brand that happen.