The 1977 Soviet satellite Kosmos 954 was supposed to monitor ocean traffic using radar—a technology that works best at short distances. For this reason, the craft traveled in Earth’s low orbit, where solar panels alone could not provide consistent power. And so, the satellite was equipped with a small, efficient, yet powerful nuclear reactor fueled by approximately 50 kg of weapons-grade uranium 235. Within weeks of its launch, Kosmos 954 veered from its path like a drunkard on a walk. The Soviets tried to eject its radioactive core into a higher orbit by way of a safety system designed for that purpose. But the safety system failed. In January 1978, Kosmos 954 burst into the Western Canada skyline, scattering radioactive dust and debris over a nearly 400-mile path. The cleanup and recovery process, which took nearly eight months and started in the subarctic winter, found that virtually all of the satellite fragments were radioactive, including one that was “sufficient to kill a person or number of persons remaining in contact with that part for a few hours.”
Now that the United States has set a goal of a human mission to Mars by 2039, the words “nuclear” and “space” are again popping up together in newspaper headlines. Nuclear propulsion systems for space exploration—should they materialize—are expected to offer significant advantages, including the possibility of sending spacecraft farther, in less time, and more efficiently than traditional chemical propulsion systems. But extreme physical conditions on the launchpad, in space, and during reentry raise questions about risk-mitigation measures, especially when nuclear materials are present. To realize the goal of a nuclear-propelled, human mission to Mars, scientists must overcome significant challenges that include—but go beyond—the technical. That is, any discussion about such an uncommon journey must also consider relevant medical, environmental, economic, political, and ethical questions.
Why not travel to Mars on a chemically propelled spacecraft? Spaceships that use chemical propellants benefit from tremendous thrust to get the job done. However, they also need to carry fuel and oxidizer to power that incredible upward or forward movement. For example, NASA succeeded in traversing the approximately 240,000 miles from Earth to the moon on chemically propelled spacecrafts. It has also flown rovers to Mars, which averages 225,000,000 miles away from Earth, by way of chemical propulsion. But a human-crewed trip from Earth to the Red Planet on a chemically fueled spaceship would require 1,000-4,000 tons of fuel—an impractical amount.
“There is no perfect propulsion system for all missions out there. You have to select the appropriate propulsion system for the mission that you’re trying to achieve,” Kareem Ahmed, a mechanical engineer at the Propulsion and Energy Research Lab at the University of Central Florida, said.
Nonetheless, SpaceX, the American aerospace manufacturer founded by Elon Musk, seeks to overcome the challenge of sending a chemically propelled rocket to Mars by building infrastructure to refuel in space and manufacture propellant on the surface of Mars. Transferring cryogenic propellants—”gases chilled to subfreezing temperatures and condensed to form highly combustible liquids”—in zero gravity represents a nontrivial challenge. Also, the company will need a power source on Mars to manufacture propellant. SpaceX did not respond to a request for comment.
“I’d caution [SpaceX] to think about the amount of infrastructure in space and infrastructure on the surface of Mars that’s necessary to be successful,” Roger Meyers said. Meyers is a consultant to NASA who co-chaired the National Academies of Sciences, Engineering, and Medicine’s committee that wrote the report, Space Nuclear Propulsion for Human Mars Exploration, released earlier this year. “I applaud the effort and hope it’s successful. But given what we know today, it would not be prudent [for NASA] to down select to one technology or another. We should be pursuing multiple pathways at an appropriate level in order to make sure that we’re ready.”
Even if a spacecraft were able to refuel with a chemical propellant in space or magically carry enough chemical propellant for the journey to Mars, the long transit time would present a hazard to the crew. To minimize the amount of fuel required, the astronauts would need to chart the shortest path between the two planets. Such a path, which relies on orbital mechanics, occurs only once every 26 months. The complete journey, including time for the round-trip flights and waiting time on Mars for optimal planetary realignment, could require more than three years, which would expose the crew to a significant amount of cosmic radiation and increase their lifetime risk of cancer. Also, to state the obvious, the more time astronauts spend in space, the more time there is for something to go wrong.
In theory, nuclear propulsion for space travel will offer two significant advantages over chemical propulsion. First, since nuclear systems are much more efficient, the amount of fuel required for the journey to Mars is practical. Second, without a need to traverse the shortest path, the flight could take off from Earth and Mars anytime—without delay. The latter would reduce the length of the roundtrip journey and the crew’s exposure to radiation.
Still, attaching what amounts to a nuclear reactor to a human-occupied spaceship is not without risks.
Is the idea of sending nuclear materials into space new? The idea of sending nuclear materials into outer space is not new. And unlike Kosmos 954, many instances have been successful. Since 1961, NASA has powered more than 25 space missions with nuclear materials. The only other practical power option—solar power—is often unavailable in dark, dusty, far-off corners of the solar system.
Likewise, the Atomic Energy Commission launched a nuclear-thermal rocket propulsion research and development program in 1955. Around that time, the Los Alamos and Lawrence Livermore Laboratories also initiated exploratory research on nuclear propulsion for rocket engines. When NASA was established in 1958, the United States shifted its nuclear-propulsion research focus from missile applications to lunar and planetary missions. Regrettably, funding and interest in the programs dried up in the 1970s, but not before fundamental research and testing had been executed—and deemed successful.
What new plans does the United States have for sending nuclear materials to space? The National Academies’ report released earlier this year recommended that NASA “commit within the year to conducting an extensive and objective assessment of the merits and challenges of using different types of space nuclear propulsion systems and to making significant technology investments this decade.” The report offers a roadmap for developing two different kinds of propulsion systems—nuclear electric and nuclear thermal—for human missions to Mars.
A nuclear electric propulsion system bears some resemblance to a terrestrial power plant. That is, first a fission reactor generates power for electric thrusters. That power positively charges the ions in the gas propellant, after which electric, magnetic, or electrostatic fields accelerate the ions. The accelerated ions are then pushed out through a thruster, which propels the spacecraft.
Alternatively, in a nuclear thermal propulsion system, the reactor operates more as a heat exchanger in which a fuel such as liquid hydrogen is first heated to very high temperatures—up to 4,600 degrees Fahrenheit—that is then exhausted through a rocket nozzle to produce thrust.
“For nuclear thermal propulsion, the challenge is: temperature, temperature, temperature,” Anthony Calomino, a materials and structure research engineer at NASA’s Langley Research Center, said. “There are not many materials that can survive those kinds of temperatures.” The potential payoff, should NASA meet the challenge, will be significant: Nuclear thermal propulsion systems are expected to twice as efficient as traditional chemical propulsion systems.
“It’s like a Corvette versus a Prius. Both will get you from A to B, but a Prius gets you there more efficiently while a Corvette will get you there quickly using more gas,” Ahmed said of the difference between nuclear and chemical propulsion systems for space travel.
While nuclear electric propulsion systems do not require extreme temperatures, they face a different hurdle. Nuclear electric systems have six subsystems, including a reactor, shield, power conversion, heat rejection, power management and distribution, and electric propulsion systems. The operating power of all of these subsystems will need to be scaled up by orders of magnitude—and in such a way that they continue to work together—before they are ready for space.
“For nuclear electric propulsion, the challenges are developing a power reactor for space operation. It’s going to be very different than what we do here on earth ,” Calomino said. Still, nuclear electric propulsion systems are also expected to be much more efficient than traditional chemical propulsion systems.
“It’s really important to invest in both technologies to get to the point where we have enough data to down select,” Meyers said. “Making a decision too early is not smart if you’re trying to manage the risk.”
Earlier this year, the Defense Advanced Research Projects Agency (DARPA) awarded three multimillion-dollar contracts to companies for the first phase of a project designed to test nuclear-thermal propulsion systems.
Why might US funding be directed at one technology over another? Though the National Academies’ report recommends researching both kinds of nuclear propulsion systems, funding to support nuclear thermal research has been more forthcoming than that for nuclear electric.
“It has to do with politics and senators wanting to fund certain centers,” Myers said. “They’re advocating for work in their districts, just like they should be. I would not say it’s a well-informed decision. I would say it’s a let’s-get-this-potentially-big-program-into-my-district decision.”
How are the risks of nuclear-propelled spacecraft mitigated? To be sure, engineers have learned a lot since the crash of Kosmos 954. The scientific community and US government have identified some non-negotiable mitigation measures to protect the crew or, in the event of a launch failure or accident, people on Earth.
Nuclear propulsion systems will not activate during launch. Despite the name, nuclear-propelled, human-crewed spacecrafts will have one big asterisk; they will be launched with chemical propulsion systems. The nuclear reactor will only operate once the vehicle has left Earth’s atmosphere. This design feature is intended to minimize the risk of releasing radioactive materials in the event of an accident on the launchpad.
“NASA’s priority is always safety first—not just safe for the astronauts but for the ground crews that support them as well as the environment,” Calomino said.
Nuclear propulsion systems on spacecraft will only operate beyond Earth’s atmosphere.
Should a nuclear-propelled spacecraft have an accident beyond Earth’s low orbit, it would remain in space rather than fall to Earth where it could harm people or the environment. Likewise, in the event of an accident, the radioactive debris would remain in orbit for tens of thousands of years, during which time it would decay.
“[Kosmos 954] showed the importance of using nuclear-safe orbits where you launch to thousands of kilometers rather than 200-300,” Myers said.
Shields will protect the astronauts from onboard radioactive materials. Nuclear propulsion systems will incorporate physical shields into their engineering designs, according to Calomino. In addition, the vehicle’s fuel tanks, which will be placed between the reactor and the crew, will provide additional protection.
“Especially for nuclear thermal propulsion, the fuel is hydrogen which is a great shield by itself,” Meyers said. Still, the calculation is made based on the lowest levels of fuel at the end of the mission, according to Meyers.
“We’re following design standards that are used here on Earth for [permissible radiation exposure],” Calomino said. “The bigger problem is protecting the astronauts against cosmic radiation.”
Nuclear propulsion systems will not use nuclear materials that could be diverted for illicit purposes. NASA is pursuing designs that are fueled by low enriched uranium. This approach is similar, though not identical to, terrestrial reactors. The uranium in use at commercial power plants is typically enriched up to five percent, which is insufficient for nuclear propulsion systems. For space travel, the uranium will need to be enriched up to 19.75 percent.
“[19.75 percent] is the highest enrichment that can still be classified as low enriched,” Meyers said. Enrichment that exceeds 20 percent could be used to build a nuclear weapon or improvised nuclear device.
“We are concerned with proliferation issues,” Calomino said.
Why is the United States planning to send humans to Mars anyway? Some argue that the scientific value of a human-crewed Mars mission could be captured by robots at a much lower cost and risk. Others think that humans, whose role in terrestrial climate change is apparent, should first rehabilitate Earth before colonizing other planets. Still others worry that human microbes could contaminate the Red Planet.
Indeed, a majority of Americans—63 percent according to a 2018 Pew Research Center survey—believe that NASA should prioritize monitoring Earth’s climate system. Only a minority—18 percent—said that NASA should prioritize sending humans to Mars.
But enthusiasts exist. Celebrated theoretical physicist and cosmologist Stephen Hawking (1942-2018) was nearly certain that Earth-bound humans would one day face a low-probability, high-impact catastrophe—from an asteroid strike, artificial intelligence, climate change, genetically modified virus, or nuclear war. To survive, he recommended that humans leave Earth and settle on other planets such as Mars.
Many government policymakers, scientists, and citizens agree—at least with the part about trying to get humans to Mars.
“Talk to a field scientist today. Ask if their job could be done by a robot,” Meyers said. “Robots should continue to do what they’re doing. But there are limits to what robots can do, and that’s where people come in.”
For some Americans, the argument in support of human missions to Mars boils down to this: To maintain competitiveness on an already-here, deep-space stage that may one day include international commercial markets and capabilities, the United States must invest in a transportation system now. If it does not, another nation will. And the investment at the moment is focused—for good reasons—on nuclear technologies.
“We should not continue to use old tools to get a job done that one day [makes] you no longer relevant on the world stage,” Calomino said. “It’s important for the United States to remain a primary and dominant player in space. It is the next frontier.”
But the United States expects to take at least until 2039 before it starts to conquer this new space frontier. Until then, world citizens are encouraged to engage in that celebrated, if somewhat battered, element of democratic societies: open debate, especially about the still-unanswered technical, medical, environmental, economic, political, and ethical questions related to a human journey to Mars on a nuclear-propelled rocket.
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