Missions to Mars and other locations in deep space present numerous challenges, most of which stem from the distances involved. Using conventional propulsion methods (chemical rockets or Hall-effect thrusters) transits to Mars can take six to nine months. This makes the prospect of resupply missions impractical and emergency rescues impossible. On the one hand, multiple efforts are addressing these issues by ensuring that crewed missions are as self-sufficient as possible.
However, efforts are being made to develop advanced propulsion systems that reduce transit times. This includes nuclear propulsion concepts, which NASA began researching again in 2016 for its proposed “Moon to Mars” mission architecture. In a recent paper, two aerospace innovators reviewed key nuclear-electric propulsion concepts, their advantages, and challenges. In the end, they conclude that nuclear propulsion has the potential to revolutionize space exploration and make humanity “multiplanetary.”
The study was conducted by Malaya Kumar Biswal M, the Founder and CEO of Acceleron Aerospace Sciences, and Ramesh Kumar V, the Founder and CEO of Grahaa Space. The paper describing their findings was recently presented at the 2025 Lunar and Planetary Science Conference (2025 LPSC), which took place from March 10th to 14th in Woodlands, Texas. To break it down, long-duration missions to Mars come with many hazards for astronaut health. These include long-term exposure to microgravity, which leads to muscle atrophy, bone density loss, and many other health concerns.
There’s also the danger of long-term exposure to solar and cosmic radiation, leading to elevated risks of cancer. As mentioned, the long distances and transit times between Earth and Mars make resupply missions impractical. If astronauts suffer serious injury, it will take far too long to evacuate them. This is why all plans for missions to Mars include proposals for in-situ.-resource utilization (ISRU) and bioregenerative life support systems (BLSS) to reduce dependence on Earth.
However, since all the associated hazards stem from long distances and limited launch windows, efforts are also being made to reduce transit times via advanced propulsion. During the Space Age, NASA and the Soviets studied nuclear propulsion to enable missions to locations beyond Low Earth Orbit (LEO) and the Moon. Since then, research has focused on two primary methods: nuclear-thermal propulsion (NTP) and nuclear-electric propulsion (NEP).
Nuclear-thermal propulsion consists of a nuclear reactor heating hydrogen propellant and channeling it through nozzles to produce acceleration (delta-v). Nuclear-electric propulsion consists of nuclear reactors generating electricity to power a thruster, typically ion or Hall-effect thrusters.
Nuclear Electric Propulsion
However, as Biswal and Kumar indicate in their study, there are also two types of nuclear-electric concepts: Radioisotope Electric Propulsion (REP) and Fission Electric Propulsion (FEP). Whereas REP utilizes the heat generated from the natural radioactive decay of isotopes to produce electricity, FEP relies on nuclear reactors to generate power through controlled nuclear fission reactions. Each has its share of advantages that make it ideally suited to specific mission profiles.
For example, REP systems typically produce about 1 kW of power, sufficient for powering instruments and low-thrust propulsion systems like ion engines. They are known for being compact and reliable, making them ideal for small- to medium-scale missions. They have a proven track record thanks to missions like the Voyager probes and the Curiosity and Perseverance rovers. FEP is scalable, flexible, and more powerful, typically generating 8 to 10 kW. This makes it more suited to long-range exploration of the Main Asteroid Belt and outer Solar System.
Both systems are being researched for future missions to Mars and the outer Solar System. Some notable examples include the Kilopower Reactor Using Stirling TechnologY (KRUSTY) reactor, developed in 2018 by NASA. This reactor resulted from the Kilopower program to develop reactors that could continuously provide 1 to 10 kW of power for twelve to fifteen years. This reactor would leverage the heat generated by Uranium-235 to generate heat that would power Stirling converters.
The reactor test demonstrated its ability to provide reliable power for extended periods, making it a pivotal milestone in advancing nuclear propulsion and power systems for space missions. These systems are compact and efficient and have many applications, including powering space habitats, life support systems (LSS), and onboard instruments on multiplanetary missions.
Potential Mission Profiles
Biswal and Kumar list several examples of missions a crewed nuclear-electric spacecraft could execute. In all cases, kilowatt reactors can maintain a steady supply of power where solar power is limited or unavailable. This includes lunar surface operations, where solar power is unavailable during 14-day lunar nights. Kilopower reactors are also vital to NASA’s plan to create a program of “sustained lunar exploration and development,” which includes scientific research, habitation, and mining.
For missions to Mars, fission power could provide reduced transit times and heavier payloads, allowing for greater capability and safety. It could also provide sustainable energy for surface habitats, life support systems, and in-situ resource utilization technologies (ISRU) on Mars. Beyond Mars, fission power systems could enable missions to study the gas giants and their systems of moons—such as astrobiology missions to Europa, Enceladus, Titan, and other “Ocean Worlds.”
Beyond the gas giants, nuclear-electric spacecraft could explore the icy bodies and dwarf planets that make up the Kuiper Belt. Fission power would be especially useful given the low-temperature conditions and negligible solar energy available. Forerunner missions like the New Horizons probe have demonstrated the effectiveness of this technology. Lastly, high-power nuclear systems could enable long-duration missions to interstellar space, as exemplified by the Voyager probes.
Limitations
Naturally, nuclear systems also have their share of challenges, which Biswal and Kumar address. These include a higher initial mass compared to traditional systems, which can lead to increased launch weight and higher launch costs. Scaling the technology to higher power levels (>100 kWe) is challenging and may require significant advancements in materials, heat management, and power generation systems before they are ready.
With a nuclear system, there’s also the need for radiation shielding and protocols to ensure mission safety. Not only do crews need to be shielded from harmful radiation, but strict safety standards must be maintained when handling nuclear fuel and other hazardous materials. These considerations increase the time, cost, and complexity of mission planning.
Last but not least, nuclear-electrical propulsion has a limited operational history compared to solar power systems, such as Solar-Electric Propulsion (SEP). This increases the overall level of uncertainty and makes the technology seem riskier than conventional methods. Due to their complexity, nuclear-electric systems also require longer development times and time-consuming fixes.
Nevertheless, Biswal and Kumar believe the pros far outweigh the cons, and some of these challenges can be overcome. For instance, chemical rockets have a greater thrust-to-weight ratio, making them an option for initial deployment. Assembling the spacecraft in orbit is also a possibility, especially with the assistance of the International Space Station (ISS) and its proposed successors.
And given the range of possible missions REP and FEP propulsion could enable, the investment and challenges are certainly worth it. In addition to advancing exploration, this technology could lead to passenger missions, ferrying settlers to the Moon, Mars, and beyond. With a human presence on these bodies, humanity will have become a “multiplanetary” species.
Further Reading: USRA