Propelling a spacecraft to Mars using nuclear energy? This concept is taking shape with NASA's SR-1 Freedom project. Scheduled for launch in 2028, this craft marks a turning point in interplanetary exploration.
But before talking about nuclear, we need to talk about electricity.
An illustration of NASA's SR-1 Freedom spacecraft, equipped with a nuclear electric propulsion system.
Credit: NASA
Electric propulsion, often referred to as ion thrusters, works by transforming a gas like xenon into ions to produce thrust. This force is very weak, but it adds up gradually, allowing for high speeds over long distances. Since the 1960s, this system has been used for many missions, first in Earth orbit, then towards other celestial objects, thus proving its reliability and value for long-distance travel.
However, in remote areas of the Solar System, solar panels become insufficient to power these engines. Radioisotope thermoelectric generators, or RTGs, provide a solution by harnessing the heat released from the radioactive decay of plutonium-238. These devices have allowed craft like the Voyager probes or Martian rovers to operate for decades, providing a stable energy source despite a harsh environment. Here, it is about supplying power for internal circuits, not for propulsion.
Nuclear electric propulsion, on the other hand, combines a fission reactor and an ion engine. The reactor generates electricity that is used to electrically charge the propellant gas. This approach delivers much greater power than solar systems, which facilitates the transport of heavier payloads and the exploration of regions where sunlight is too weak. It thus paves the way for more ambitious missions towards Mars or beyond.
Hall effect thrusters undergoing testing at Edwards Air Force Base in California, in March 2025.
Credit: U.S. Space Force Photo by AFRL The use of nuclear materials in space requires very strict safety measures. RTGs are for example enclosed in resistant materials like graphite and iridium to reduce risks in case of an incident. Despite some concerns expressed in the past, such as during the launch of the Cassini-Huygens probe which used this technology, missions equipped with these generators have so far proceeded without incident.
Tests of nuclear electric propulsion began as early as 1965 with the experimental satellite SNAP-10A, which remains to this day the only space reactor launched. After the abandonment of several projects subsequently, NASA is now reviving this technology with the SR-1 Freedom. If technical obstacles are overcome, this initiative could enable faster and more efficient travel, profoundly changing space exploration for the coming years and decades.