Just before the Artemis II crew began its historic slingshot around the moon this past April, NASA announced plans to build Space Reactor-1 Freedom. SR-1, the first-ever nuclear-reactor-powered interplanetary spacecraft, is meant to fly an uncrewed mission to Mars by the end of 2028, arriving in 2029. A successful mission would herald a new era of more efficient, potentially faster spaceflight—saving astronauts time and cutting their exposure to harmful cosmic radiation as they travel to Mars and beyond.
Running a nuclear reactor in space may sound like lunacy, but it’s not. The Soviet Union sent dozens of reactors into orbit, often to power spy satellites. And experts say there aren’t any major technical obstacles to making nuclear space propulsion happen.
The US has started—and terminated—several programs looking into it over the past 75 years. The latest casualty was DRACO, a collaboration between NASA and the Department of Defense, which ended in 2025. The project was canceled because of high experimentation costs, an inability to compete with conventional rocket propulsion on price, and difficulty ensuring that ground tests could be performed safely and effectively. (You are creating an incredibly powerful nuclear reaction, after all.) After much wavering, it now appears that nuclear-powered interplanetary travel is a $2 billion bet NASA is willing to make.
The agency is pursuing an approach called nuclear electric propulsion, which uses the neutrons produced in a self-sustaining, uranium-atom-splitting reaction to create heat and ultimately electricity. The electricity is used to ionize a gas, which is blasted out of the spacecraft with the help of magnetic fields, generating thrust. The approach “is very low thrust, but very efficient,” says Sebastian Corbisiero, the US Department of Energy’s national technical director of space reactor programs.
A presentation by Steve Sinacore, program director of NASA’s SR-1 program, offers some clues. So far, the concept art makes the craft look like a colossal fletched arrow. The back will house the power-and-propulsion system, while the tip will hold a uranium-filled nuclear reactor with a capacity of at least 20 kilowatts. (For context, a typical nuclear plant on Earth is about 50,000 times more powerful.) The “fletches” on SR-1 are large fins that help cool the reactor by radiating heat into space.
Like many other pieces of space hardware, the reactor will need to be put through its paces on the ground before it’s cleared for the rough rocket trip to space. Uranium isn’t tremendously dangerous by itself, but that can’t be said of the waste products that emerge when the reactor is running. If SR-1 works as planned, its journey to Mars will take about a year.
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