A Nuclear Spacecraft That Could Get Humans to Mars and Back in Under a Year
NASA engineer Kurt Polzin and General Atomics engineer Robert Schleicher have proposed a new type of nuclear-powered spacecraft that could shorten a round trip to Mars by up to two years. The design, called the synchronal bimodal nuclear rocket (S-BNR), was described in a paper presented at the American Institute of Aeronautics and Astronautics and covered by IEEE Spectrum.
The idea centers on a single nuclear reactor that can work in two different ways. In one mode, it heats up a propellant directly to create a powerful burst of thrust, similar to how heating water produces steam that can drive a turbine. This mode is useful for the big pushes needed to leave Earth orbit and to slow down when arriving at Mars. In the other mode, the reactor generates electricity to power ion thrusters, which accelerate tiny particles to very high speeds. Ion thrusters are extremely fuel-efficient but produce gentle push, making them better for long, steady cruising between planets.
The reason this dual approach matters is that neither mode works well on its own. The high-thrust mode is powerful but not very fuel-efficient, capping out at about twice the efficiency of a conventional chemical rocket. The ion-thruster mode is far more fuel-efficient but too weak to quickly escape Earth's gravity or brake into Mars orbit. By using the same nuclear reactor for both, the spacecraft gets the best of both approaches without carrying two separate reactor systems, which would add a lot of weight.
There is also a practical benefit. A nuclear thermal rocket only produces power while it is actively heating propellant. During long coasting phases between planets, it would generate no electricity at all. But a crewed mission needs constant power for life support, communications, and keeping the spacecraft at a safe temperature. The S-BNR's electric mode fills that gap without needing a separate power source.
NASA's shortest existing plan for a crewed Mars round trip requires 620 days in space plus 30 days on the Martian surface. The S-BNR design aims to bring total crew transit time down to 335 days or less. That reduction matters because prolonged exposure to deep-space radiation and weightlessness is one of the biggest obstacles to sending humans to Mars. Halving the transit time does not eliminate the risk, but it sharply reduces how long astronauts are exposed to those hazards.
The technology has a long history. Researchers recognized as early as 1946 that nuclear reactors could serve as rocket engines. In the 1960s, U.S. programs called NERVA and Rover conducted ground tests that demonstrated much of the underlying technology. Despite that progress, the United States has launched only one nuclear reactor into space, a prototype called SNAP-10A, orbited in 1965. The two propulsion approaches in the S-BNR have been studied extensively since then but never flown together as a crewed spacecraft system.
The broader context is that NASA has tried several propulsion designs for Mars missions over the past two decades, including conventional rockets and solar-powered electric systems, none of which has progressed to actual flight hardware. The S-BNR proposal remains a paper design. No flight reactor has been built or tested under this configuration, and the U.S. has not orbited a nuclear reactor in over 60 years. Testing a space-rated nuclear propulsion system would require not just engineering work but also regulatory approval for launching radioactive material, which has historically been politically and procedurally difficult.
In my view, the S-BNR is best understood as a well-reasoned design argument rather than a spacecraft you will see launched soon. What makes it notable is not the idea of nuclear propulsion itself, which has been studied for eight decades, but the specific combination of two propulsion modes from a single reactor core. That combination directly targets the two things that matter most for getting humans to Mars: total travel time and a steady supply of electricity. If the design advances to ground-based prototype testing, the key question will be whether one reactor can reliably switch between modes without either function degrading over the course of a Mars mission.
The 335-day transit target is aspirational and depends on design parameters that no hardware has yet validated. But the direction of progress in nuclear propulsion has been consistent: each new proposal integrates systems more tightly and reduces the number of separate components a Mars spacecraft must carry. The S-BNR continues that trend. Whether it becomes the design that finally sends humans to Mars, or another well-engineered concept that never leaves the ground, depends on funding, testing, and policy decisions that have not yet been made.
