MARS — NASA plans to launch the Space Reactor-1 Freedom mission by December 2028, aiming to explore Mars using nuclear propulsion. The agency describes the spacecraft as "the first nuclear-powered interplanetary spacecraft."

The mission will use a fission reactor to generate electricity for its thrusters. Fission reactors split atoms to release heat, which is converted into electricity. Nuclear propulsion could reduce travel times for Mars missions and decrease astronauts' exposure to cosmic radiation during transit.

The White House has established a National Initiative for American Space Nuclear Power. NASA also intends to deploy a small nuclear reactor on the Moon by 2030 for its lunar exploration program. This reactor would address challenges posed by the Moon's approximately 29.5-Earth-day cycle, which includes roughly two weeks of darkness.

Historically, radioisotope power systems have generated electricity from the heat released by the natural decay of plutonium-238. Later Apollo missions utilized radioisotope thermoelectric generators (RTGs) to power scientific experiments on the Moon. These RTGs also power the Mars rovers Curiosity and Perseverance, as well as the twin Voyager spacecraft, which continue to communicate from interstellar space.

The United States previously launched the SNAP-10A fission reactor into orbit during the Cold War; it remains the only U.S. fission reactor successfully launched. Researchers at the Massachusetts Institute of Technology are studying the performance of various materials and reactor designs under harsh space conditions.

International law does not prohibit the use of nuclear power sources in space, though the Outer Space Treaty of 1967 prohibits placing nuclear weapons in orbit or on celestial bodies. The United Nations adopted Principles Relevant to the Use of Nuclear Power Sources in Outer Space in 1992, developed by the Committee on the Peaceful Uses of Outer Space (COPUOS).

These UN principles call for safety assessments before launch and for notification and international assistance in the event of re-entry risks. They also recognize state responsibility and liability for launching states. The principles state that reactors should not become "critical" before reaching their operating orbit or interplanetary trajectory. In 2009, COPUOS and the International Atomic Energy Agency developed a broader safety framework offering guidance on launch authorization, emergency preparedness, and end-of-service phases. Both the UN principles and the 2009 safety framework are non-binding, leaving safety assessments and launch authorizations largely to individual states.