Nuclear space propulsion and power

In plain language

Nuclear energy can be used in space in three different ways. A radioisotope generator turns heat from the natural decay of a radioactive material into electricity, without a chain reaction. A nuclear thermal rocket uses a reactor to heat a propellant, usually hydrogen, and pushes it out through a nozzle. A nuclear electric spacecraft uses a reactor to make electricity, which then powers electric thrusters.

Radioisotope generators have the longest flight record. NASA has used them on missions including Voyager, Cassini and the Mars rovers Curiosity and Perseverance. The United States flew one fission reactor, SNAP-10A, which launched in 1965 and produced electricity in orbit until a failure ended it after 43 days. Nuclear propulsion has been tested on the ground but has not yet flown. NASA plans a nuclear electric spacecraft, SR-1 Freedom, for a launch in late 2028.

Technical explanation

Radioisotope thermoelectric generators (RTGs) use the heat released by the decay of plutonium-238. NASA's FAQ says the conversion uses thermocouples based on the Seebeck effect and has no moving parts. Fuel supply is a constraint: NASA and DOE announced in December 2015 that Oak Ridge National Laboratory had produced 50 grams of new plutonium-238, the first new production in nearly 30 years.

Nuclear thermal propulsion (NTP) heats hydrogen propellant in a reactor and expels it through a nozzle. NASA describes NTP as giving high thrust and about twice the propellant efficiency of chemical rockets. Nuclear electric propulsion (NEP) uses reactor heat to generate electricity, which accelerates propellant electromagnetically. NASA describes NEP as low in thrust, so it accelerates a spacecraft over long periods, but with much higher propellant efficiency than NTP. The NTP ground-test programme NERVA ran from 1961 to 1973, and its reactors were tested in ground facilities, not in flight.

NASA's 20 October 2026 update describes SR-1 Freedom as a fission-electric mission to Mars. A closed Brayton cycle power-conversion system, supplied by Sierra Lobo, would convert reactor heat into electricity, and the reactor is designed by Idaho National Laboratory. NASA says the reactor will stay inactive until the spacecraft leaves Earth orbit. As of this profile, no flight results have been published.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Limitations and open problems

• Only radioisotope generators and one fission reactor (SNAP-10A, 1965) have flown in the sources reviewed here. No nuclear thermal or nuclear electric propulsion system has flown.
• A high-voltage failure in the Agena spacecraft's electrical system shut SNAP-10A down after 43 days, so the flight did not show long-term operation.
• NERVA ground tests are not flight tests, and the programme ended in 1973 without an operational nuclear rocket being deployed.
• Plutonium-238 supply limits how many radioisotope systems can be built. NASA and DOE reported 50 grams of new production in December 2015.
• NASA presents the late-2028 SR-1 launch as its plan, not a result. The reactor activation, safety reviews and regulatory compliance are still part of the programme.
• DARPA's programme record labels DRACO complete but gives no end date. The sources reviewed here do not explain why it ended, and that outcome does not mean all nuclear thermal development has ended.

Possible future uses

Timeline

1. 1955Project Rover begins, the start of the US nuclear rocket reactor and engine work that later became NERVA. Sources: [2]
2. 3 Apr 1965SNAP-10A launches from Vandenberg Air Force Base and produces more than 600 watts of electrical power in Earth orbit. Sources: [3], [2]
3. 1973The NERVA programme ends. Its ground tests covered sustained operation and restarts, but no engine was flown. Sources: [5], [2]
4. 6 Aug 2012The Curiosity rover, powered by a Multi-Mission Radioisotope Thermoelectric Generator, lands on Mars. Sources: [4]
5. 20 Oct 2026NASA publishes an SR-1 Freedom update reporting key contract actions. The planned nuclear electric mission to Mars targets a launch in late 2028. Sources: [1]

Futures this could help bring about

How far away?

A probe to another star

Science demonstrated · 1 of 8 steps done

How far away?

Travelling faster than light

Science unproven · 4 of 6 steps done

Glossary

Radioisotope thermoelectric generator (RTG)

A device that turns heat from radioactive decay into electricity, using thermocouples and no moving parts.

Thermocouple

A pair of different conducting materials that produces a voltage when one junction is hotter than the other.

Nuclear thermal propulsion (NTP)

A rocket that uses a nuclear reactor to heat propellant, which is then expelled through a nozzle to make thrust.

Propellant efficiency

How much thrust a rocket gets from each unit of propellant. Higher values mean less propellant is needed for a given change in speed.

Sources

1. government report
   SR-1 Freedom Advances with Progress on Nuclear Power Module
   NASA, 20 Oct 2026. Accessed 21 Oct 2026.
2. government report
   Space Nuclear Propulsion
   NASA Space Technology Mission Directorate, 21 Jul 2026. Accessed 21 Oct 2026.
3. government report
   System for Nuclear Auxiliary Power (SNAP) Overview
   US Department of Energy, 21 Oct 2026. Accessed 21 Oct 2026.
4. government report
   Radioisotope Power Systems FAQ
   NASA Science, 3 Apr 2025. Accessed 21 Oct 2026.
5. conference paper
   Nuclear Thermal Propulsion Ground Test History
   Harold P. Gerrish. NASA Technical Reports Server, 24 Feb 2014. Accessed 21 Oct 2026.
6. government report
   Demonstration Rocket for Agile Cislunar Operations
   DARPA, 21 Oct 2026. Accessed 21 Oct 2026.

Revision history

3 changes since this profile was created.