Nuclear in Space

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Power Operational

RTGs

Radioisotope Thermoelectric Generators

Nuclear batteries that convert heat from radioactive decay directly into electricity. Powering deep space probes since 1961: where solar panels are useless.

Radioisotope thermoelectric generators (RTGs) use the heat produced by radioactive decay - typically plutonium-238 - and convert it directly to electricity using thermocouples (the Seebeck effect). They have no moving parts, work in extreme environments, and can operate for decades. NASA has used RTGs on over 40 missions including Pioneer 10 & 11, Voyager 1 & 2 (still operating 45+ years later), Cassini, Galileo, New Horizons, Curiosity, and Perseverance. Pu-238's 87.7-year half-life makes it ideal - it delivers steady power for decades and its alpha decay produces minimal penetrating radiation. The USA currently produces Pu-238 at Oak Ridge National Laboratory, having restarted production in 2015 after a 25-year gap.

Notable Missions / Programmes

Voyager 1 & 2 (1977: still operational in interstellar space)
Cassini Saturn mission (1997–2017)
New Horizons Pluto flyby (2006)
Curiosity Mars rover (2012)
Perseverance Mars rover (2021)

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Voyager 1, now more than 23 billion km from Earth, is still powered by its RTG - 47 years after launch. Its RTG has decayed to about 70% of its original power but still keeps the probe operating.

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Reactor Historical

SNAP-10A

SNAP-10A - First Space Reactor

The first (and only) US fission reactor operated in space - launched April 1965. Still in orbit today.

SNAP-10A (Systems Nuclear Auxiliary Power) was a small fission reactor launched by the US Air Force on April 3, 1965, aboard the Snapshot satellite. It operated for 43 days, generating about 500 watts of electricity, before an unrelated spacecraft failure ended the mission. The reactor is still in a 1,300 km orbit - well above the atmosphere - where it will remain for approximately 4,000 years, by which time the radioactivity of the core will have decayed to safe levels. The Soviet/Russian RORSAT and TOPAZ programmes flew over 30 nuclear-powered satellites, including some that shed radioactive debris - most notably Cosmos 954, which crashed into northern Canada in 1978.

Notable Missions / Programmes

Snapshot / SNAP-10A (USA, 1965)
RORSAT radar ocean reconnaissance satellites (USSR, 1967–1988)
Cosmos 954 reentry and crash (Canada, 1978)
TOPAZ reactor series (USSR/Russia, 1987–1992)

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The Soviet Cosmos 954 satellite crashed into northwest Canada in January 1978, scattering radioactive debris over 124,000 km². Canada spent C$14 million cleaning it up and billed the USSR. The USSR paid C$3 million.

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Reactor Development

Kilopower

Kilopower / KRUSTY

NASA and DoE's next-generation small fission reactor for Moon and Mars surface power - tested successfully in Nevada in 2018.

Kilopower is a small fission reactor project developed by NASA and the US Department of Energy to provide reliable power on the lunar and Martian surfaces, where solar power is unreliable (lunar night lasts 14 Earth days; Mars has dust storms blocking sunlight for months). The Kilopower Reactor Using Stirling Technology (KRUSTY) demonstrator was successfully tested in Nevada in March 2018, generating about 1 kW of electricity. Flight-ready versions would provide 1–10 kW each, with multiple units deployable for a surface outpost. Unlike RTGs, Kilopower uses uranium-235 fission - a full reactor - making it far more power-dense.

Notable Missions / Programmes

KRUSTY ground demonstration (Nevada, 2018)
Planned for future lunar Gateway station
Proposed for Mars surface power for crewed missions

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A 10 kW Kilopower reactor would fit inside a large suitcase and could power a small lunar habitat. NASA estimates that six 10 kW units would provide enough electricity for an early crewed Mars surface mission.

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Propulsion Development

NEP

Nuclear Electric Propulsion

Using a fission reactor to generate electricity to power ion thrusters - far more efficient than chemical rockets for deep space missions.

Nuclear electric propulsion (NEP) uses a fission reactor to generate large amounts of electricity, which powers ion or Hall-effect thrusters. Ion thrusters expel xenon gas at extremely high velocities, producing very low thrust but extraordinary fuel efficiency (specific impulse of 3,000–10,000 seconds, vs ~450 seconds for chemical rockets). This makes NEP ideal for long deep-space missions where the spacecraft can accelerate gradually over months. Russia flew the TOPAZ nuclear electric propulsion system on several satellites. NASA's Prometheus programme and more recently the NASA/DARPA DRACO programme are developing nuclear electric and nuclear thermal propulsion for cislunar and deep space missions, with a demonstration flight planned for 2027.

Notable Missions / Programmes

Soviet TOPAZ systems (1987–1992)
NASA Prometheus programme (2003, cancelled)
NASA/DARPA DRACO (Demonstration Rocket for Agile Cislunar Operations, 2027 target)

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A nuclear-powered spacecraft could reduce a Mars transit from ~7 months (chemical) to potentially 3–4 months. This is critically important for reducing crew exposure to cosmic radiation and the psychological toll of long transits.

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Propulsion Development

NTP

Nuclear Thermal Propulsion

A rocket that heats propellant (hydrogen) directly with a nuclear reactor instead of combustion - twice the efficiency of the best chemical rockets.

Nuclear thermal propulsion (NTP) passes hydrogen propellant through or around a nuclear reactor core, heating it to extreme temperatures (2,500–3,000°C), then expelling it through a nozzle to generate thrust. With a specific impulse of ~900 seconds (vs ~450 for liquid hydrogen/oxygen), NTP offers roughly twice the fuel efficiency of chemical rockets. The USA tested 20 NTP engines in the NERVA (Nuclear Engine for Rocket Vehicle Application) programme between 1959 and 1972, all without flying. The most powerful, Phoebus 2A (1968), ran at 4,200 MW thermal - one of the most powerful reactors ever built. The programme was cancelled when the Nixon administration cancelled the Mars mission it was designed for. Russia also developed and tested NTP engines. NASA/DARPA's DRACO programme aims to fly a demonstration NTP vehicle by 2027.

Notable Missions / Programmes

NERVA programme (USA, 1959–1972) - 20 engines tested, none flown
Soviet RD-0410 NTP engine (tested, never flown)
NASA/DARPA DRACO (2027 demonstration target)

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The NERVA Phoebus 2A reactor (1968) ran at 4,200 MW of thermal power - more than four times the output of a typical commercial power reactor - in a test stand in the Nevada desert. It was designed to power a crew to Mars by 1978.

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Propulsion Theoretical/Early Research

Fusion Drive

Fusion Propulsion

Using nuclear fusion reactions for spacecraft propulsion - theoretically capable of cutting Mars transit time to weeks. Still decades away from realisation.

Fusion propulsion concepts use the enormous energy released by fusing hydrogen isotopes to accelerate a spacecraft. Several approaches are being studied: Magnetized Target Fusion, Direct Fusion Drive (DFD, Princeton), and the Z-pinch concept. The theoretical performance is extraordinary - a fusion drive could achieve specific impulses of 10,000–100,000 seconds, enabling Mars transits in weeks rather than months. However, no fusion propulsion device has yet achieved the fundamental milestone of producing net energy (which terrestrial fusion programmes only achieved at NIF in 2022). The gap between laboratory fusion and a working space propulsion system remains enormous. Private companies like TAE Technologies and Princeton Satellite Systems are pursuing fusion propulsion with significant funding, but realistic timescales remain 20–40+ years.

Notable Missions / Programmes

Princeton Direct Fusion Drive (research stage)
DARPA FP3 programme (early research)
TAE Technologies / Helion Energy (civil power precursor)

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A fusion-powered spacecraft could theoretically reach Mars in 90 days, Jupiter in under a year, and could make interstellar precursor missions (to the heliopause at ~100 AU) feasible within a human lifetime.