Nuclear Powered Spacecraft, Part 2

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Nuclear Powered Spacecraft

Part 2

Nuclear-powered spacecraft use nuclear energy for power or propulsion, enabling missions beyond the reach of solar power and reducing travel times. Common types include radioisotope thermoelectric generators (RTGs) for consistent power, nuclear thermal rockets (NTRs) which use a reactor to heat a propellant, and nuclear electric propulsion (NEP) systems that convert reactor heat into electricity to power ion thrusters. While some early systems have been in space, renewed focus is on developing more powerful systems for deep-space exploration. 

We have already wrote about NEP and NTR systems in previous article Nuclear powered spacecraft (Part 1). 

Many nuclear-powered spacecraft are currently operational, primarily using radioisotope thermoelectric generators (RTGs) for electricity and heat in deep space, or on the planetary surface. These include the Voyager 1 and 2 probes, the New Horizons spacecraft, and the Curiosity and Perseverance Mars rovers. 

Voyager schema, source: NASA

Current and Past Operational Missions with Nuclear Power 

The majority of operational nuclear-powered spacecraft use RTGs, which convert heat from the natural decay of plutonium-238 into electricity. 

  • Voyager 1 and 2: Launched in 1977, both spacecraft still operate using their original RTGs and are in interstellar space, continuing to send back data.
  • New Horizons: This probe, which flew by Pluto in 2015, uses an RTG to power its systems as it travels out of the solar system, where solar power is insufficient.
  • Mars Rovers (Curiosity and Perseverance): Both rovers use Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) to provide consistent power and heat, allowing them to operate over a wide range of conditions on the Martian surface for many years.
  • Other deep space probes: Past missions like Pioneer 10 and 11, Galileo, Ulysses, and Cassini also used RTGs during their operational lives.
  • Radioisotope Heater Units (RHUs): Many other spacecraft and landers use smaller RHUs to keep their sensitive instruments warm in the cold environment of space. 
Diagram of an RTG used on the Cassini probe, source: Wiki

A radioisotope thermoelectric generator (RTG) is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioactive material into electricity by the Seebeck effect. This type of generator has no moving parts and is ideal for deployment in remote and harsh environments for extended periods with no risk of parts wearing out or malfunctioning.

RTGs are usually the most desirable power source for unmaintained situations that need a few hundred watts (or less) of power for durations too long for fuel cells, batteries, or generators to provide economically, and in places where solar cells are not practical. RTGs have been used as power sources in satellites, space probes, and uncrewed remote facilities such as a series of lighthouses built by the Soviet Union inside the Arctic Circle.

The design of an RTG is actually quite simple. The main component is a sturdy container of a radioactive material (the fuel). Thermocouples are placed in the walls of the container, with the outer end of each thermocouple connected to a heat sink. Radioactive decay of the fuel produces heat. It is the temperature difference between the fuel and the heat sink that allows the thermocouples to generate electricity.

The primary radioactive material used in modern RTGs is plutonium-238 (Pu-238), specifically in the form of plutonium oxide. Other isotopes used in specific applications have included strontium-90 (Sr-90), polonium-210 (Po-210), and americium-241 (Am-241).

The choice of radioisotope depends on several factors, including half-life, power density (watts per gram), radiation type, and availability. An ideal RTG fuel is typically an alpha-emitter, as alpha radiation has a very short absorption length and can be contained with minimal shielding, converting its energy efficiently into heat within the device itself.

Table of Isotopes, their properties and main applications; source: Wiki


RTGs and fission reactors use very different nuclear reactions. In case of nuclear power reactors (including the miniaturized ones used in space), they perform controlled nuclear fission. The rate of the reaction can be controlled with control rods, so power can be varied with demand or shut off (almost) entirely for maintenance. However, care is needed to avoid uncontrolled operation or even nuclear accident. However, in contrast Chain reactions do not occur in RTGs. Heat is produced through spontaneous radioactive decay at a non-adjustable and steadily decreasing rate that depends only on the amount of fuel isotope and its half-life. In an RTG, heat generation cannot be varied with demand or shut off when not needed and it is not possible to save more energy for later by reducing the power consumption. 

Radioisotope Thermoelectric Generators are essential and reliable power sources for deep space and remote missions where solar power would fail. They offer decades of consistent energy for probes like Voyager and Curiosity, but they face future challenges with limited Plutonium-238 (Pu-238) supply. 


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