Chemical rockets burn fuel and oxidiser, reaching specific impulse around 300-450 seconds. Nuclear thermal rockets use a
small reactor to heat hydrogen, reaching 850-900 seconds efficiency about twice as good, according to the US Department
of Energy and NASA briefings. This means less fuel, more payload, or much faster journeys.
In the 1960s-70s, NASA’s NERVA programme tested nuclear thermal engines on the ground. Several full-scale tests achieved
around 825 seconds specific impulse and operated for hours without major incident. Engineers even pushed engines beyond
design limits to study failure, and they remained controllable. NERVA proved that nuclear rockets can work in practice,
In a nuclear thermal rocket, a compact reactor splits uranium atoms, releasing heat. Liquid hydrogen passes through the
hot core, turns into very hot gas, and rushes out through a nozzle to create thrust. No combustion is needed. Higher gas
temperature plus light hydrogen molecules mean faster exhaust and higher specific impulse than chemical fuel.
The chemical trajectories to Mars take about 7-9 months. NASA studies show a nuclear thermal stage could cut this to
roughly 3-4 months, depending on mission design. Shorter journeys reduce cosmic radiation exposure and limit time in
microgravity. This directly improves crew health and simplifies life-support planning for human Mars missions.
Nuclear electric propulsion uses a reactor to generate electricity, which powers ion or Hall-effect thrusters. These
engines produce low thrust but can run for months, achieving very high exhaust speeds. NASA and ESA analyses show such
systems are ideal for heavy cargo, deep-space probes, and long-duration robotic missions where efficiency matters more
than quick acceleration.
Hydrogen is extremely light, so when it is heated in a nuclear rocket it exits the nozzle at very high speed, raising
specific impulse. Nuclear thermal designs using hydrogen reach 850-900 seconds efficiency, versus 450 seconds for the
best chemical engines. Some advanced concepts suggest partially dissociating hydrogen into atoms to push performance
Modern nuclear space plans use low‑enriched uranium (LEU), not weapons‑grade fuel. NASA and the US Department of Energy
say LEU designs can meet mission needs while reducing proliferation risks. Reactors would only start once safely in
space. Launch vehicles would carry the reactor “cold”, cutting the risk of radioactive release if a launch fails.
New concepts like centrifugal nuclear thermal rockets (CNTR) spin molten fuel to the outer wall, heating hydrogen to
much higher temperatures and pushing theoretical specific impulse above 1,000 seconds. Hybrid designs such as
nuclear‑thermal‑electric rockets combine a thermal core for high thrust and electric thrusters for efficient cruise.
These ideas remain experimental but are being modelled in current research.
Nuclear propulsion raises concerns about launch accidents, orbital debris, and weaponisation of space. Past projects,
including Project Prometheus and earlier nuclear initiatives, faced political pushback and budget cuts despite technical
promise. International rules and strict safety standards would be essential if more countries start flying reactors into
orbit in the 2030s and beyond.
NASA, DARPA and industry partners are now funding demonstrator missions to test nuclear thermal propulsion in space
before 2035. If successful, nuclear rockets could make crewed Mars missions routine, speed up missions to Jupiter and
Saturn, and support permanent infrastructure in deep space. For now, chemical rockets dominate launches but nuclear
propulsion is moving from concept back towards flight testing.