Why NASA wants to test a nuclear rocket engine for a Mars mission

NASA and the US military plan to test a nuclear-powered rocket engine in space as early as 2027, potentially revolutionizing how humans travel the cosmos in the coming decades.

The two agencies will work on nuclear thermal propulsion(Opens in a new window) System, a technology NASA plans to use to send humans to Mars in the late 2030s. But the test mission – known as Demonstration Rocket for Agile Cislunar Operations or the DRACO program(Opens in a new window) – would not involve astronauts.

Proponents of nuclear-powered rockets have long emphasized the benefits: spacecraft can travel faster, carry heavier loads of people and cargo, and use fuel more efficiently than today’s chemical-based rockets. The idea has flared up recently, with a budget of $110 million for the project this year.

It’s not clear how much everyday people know about the technology and whether the project will renew concerns about nuclear disasters. NASA and defense leaders say the rocket will be safe for people on the ground and will ultimately make spaceflight safer for astronauts: Faster travel means they’re less exposed to harmful cosmic rays.

But at the American Institute of Aeronautics and Astronautics conference announcing the new collaboration Tuesday in Washington, DC, the moderator might have spoken for the collective conscience when he said, “Obviously, the public is going to be like, ‘You ‘Are you going to put a reactor over my head?’”

“We’re putting radioactive material into space, but we designed the whole process to be safe,” said Stefanie Tompkins, director of the Defense Advanced Research Projects Agency, the military’s research and development arm that oversees the overall program. “Part of the reason for doing it many ways in space is because it’s safer than having to do it on Earth.”

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How would nuclear missiles change space travel?

Experts described nuclear-powered missiles to Mashable as a leapfrogging technology, ranging from horse-drawn stagecoaches to steam locomotives or landline phones to smartphones. Right now, engineers rely on swinging around planets to get gravity boosts(Opens in a new window), the extra momentum needed to send spacecraft deep into the solar system. But nuclear power would free missions from having to wait sometimes years for the right timing of orbits.

“You’re opening up the outer solar system for scientific exploration. You don’t have to wait for the planets to align. You have launch opportunities every year,” said Dale Thomas, associate director of the University of Alabama at Huntsville Propulsion Research Center. “And by the way, since we’re talking about direct trajectories, you get there in about half the time.”

With nuclear-thermal propulsion, the journey to Mars could take as little as two months instead of nine, depending on the route, engineers said. And a reduction in voyage time would have the benefit of limiting the supplies needed for such a voyage.

Why did the US stop testing nuclear missiles?

The last United States nuclear thermal rocket engine tests took place more than 50 years ago. During this time, Los Alamos National Laboratory in New Mexico helped build nuclear missiles for NASA’s Project Rover program. The program ended in 1972(Opens in a new window) when President Richard Nixon made the policy decision to cut funding for manned missions to Mars and instead focus on the Space Shuttle and research in low Earth orbit.

A nuclear thermal propulsion system would pump liquid hydrogen through a reactor where uranium atoms would split and release heat. This process known as fission(Opens in a new window)would turn the hydrogen into a gas and eject it through a nozzle, creating thrust to propel a spacecraft.

“You open up the outer solar system for scientific exploration. You don’t have to wait for the planets to align. You have launch opportunities every year.”

The “game changer” in today’s technology development is a new form of uranium that is not considered a weapons-grade material. That opens the door for commercial space companies to work on spin-offs after the test mission, said Pam Melroy, NASA assistant administrator.

But this weakly enriched uranium(Opens in a new window) represents one of NASA’s greatest challenges. Materials in direct contact with the reactor fuel must withstand over 4,600 degrees Fahrenheit, about half the temperature of the sun’s surface.

Technicians prepare a Kiwi nuclear reactor nozzle for testing.
Photo credit: NASA

Are nuclear missiles safe?

In terms of safety, engineers say the nuclear systems would not be used at all on the launch pad. In fact, this is one of the most common misconceptions about technology. In a sort of hybrid approach, chemical rockets would lift the spacecraft off the ground. Then, once the ship climbed to an altitude in space between 400 and 1,300 miles – well above the International Space Station(Opens in a new window) – the nuclear-powered engines would take over. That’s critical to making sure the material is no longer radioactive when it re-enters Earth’s atmosphere, Melroy said.

If for some reason the chemical missile explodes and the nuclear-powered engine lands in the ocean, the nuclear hardware still can’t “go critical,” Thomas said. The space rocket system doesn’t work until it reaches orbit.

Perhaps the greatest safety risks arise during ground testing, a crucial step before launch. Engineers will need huge facilities, which don’t yet exist, to contain the exhaust fumes, a recommendation made in a 2021 independent report(Opens in a new window) NASA had applied to the National Academies to study nuclear propulsion. New construction or remodeling of existing facilities could cost billions of dollars.

A simulator at NASA’s Marshall Space Flight Center in Huntsville, Alabama, tests prototype nuclear rocket propellants.
Photo Credit: NASA/Mick Speer

Safe test facilities are a consideration that didn’t exist decades ago when engineers were working on earlier versions of these systems, said Roger Myers, an aerospace consultant who co-led the National Academies study.

“The US government fired nuclear rocket engines at night in the Nevada desert in the late 1960s and early 1970s, and they fired them in the open air,” Myers said. “We are much more careful today.”

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Radiation Exposure Risks for Astronauts

Perhaps surprisingly, experts say future astronauts would not be at a greater risk of radiation exposure because of the engine – their bigger concern would still be space radiation(Opens in a new window) from the sun. That comes from the design of the ship, Thomas said. NASA has received a research grant to model how a spacecraft could be designed to work with nuclear-powered rocket engines.

The crew would be in the front and the engine in the back, with a large hydrogen tank in between. Hydrogen is an exceptional neutron absorber. Translation: “It’s like your fuel is your shield,” said Thomas. “I’d say you’re actually in better shape with the nuclear thermal” missile than with an old chemical system.

“It’s like your fuel is your shield.”

Another bonus for astronauts is that nuclear-powered rockets give the crew the ability to abort a space mission. With chemical thrusters, once a spacecraft flew to Mars, there would be no going back until the planets lined up again.

The DRACO spacecraft will test a nuclear thermal rocket engine.
Credit: DARPA Illustration

Nuclear thermal vs nuclear electric missiles

Other types of nuclear propulsion(Opens in a new window) are not being developed in the DRACO collaborative program, such as nuclear electrical energy. A nuclear-electric missile would use a reactor to generate electricity like a small power plant. Myers believes NASA should invest in all of them because they have different advantages for space travel. But joining the Department of Defense in this nuclear thermal test project is smart to pool resources, he said.

All of this also has something for people who are down to earth. If scientists and engineers can develop a fuel and reactor that can withstand extremely high temperatures, it could lead to safer nuclear power plants on the ground.

“The reactor fuels that will work there, if you put them in a terrestrial reactor, will bulletproof that reactor,” he said.