• Post category:Space Technology
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In our previous post we discussed about the need of nuclear energy in space applications. Today we will understand in brief 2 important nuclear technologies:

Radioisotope Thermoelectric Generator

A radioisotope thermoelectric generator (RTG, RITEG) 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.

The design of an RTG is simple by the standards of nuclear technology: 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.

Diagram of an RTG used on the Cassini probe
Diagram of an RTG used on the Cassini probe

RTGs were used with Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Galileo, Ulysses, Cassini, New Horizons, and the Mars Science Laboratory. RTGs were used to power the two Viking landers and for the scientific experiments left on the Moon by the crews of Apollo 12 through 17.

Radioisotope heater unit

Radioisotope heater units (RHU) are small devices that provide heat through radioactive decay. They are similar to tiny radioisotope thermoelectric generators (RTG) and normally provide about one watt of heat each, derived from the decay of a few grams of plutonium-238—although other radioactive isotopes could be used.

Diagram of a radioisotope heater unit
Diagram of a radioisotope heater unit

Most lunar and Martian surface probes use RHUs for heat, including many probes that use solar panels rather than RTGs to generate electricity. Examples include the seismometer deployed on the Moon by Apollo 11 in 1969, which contained 1.2 ounces (34 grams) of plutonium-238; Mars Pathfinder; and the Mars Exploration Rovers Spirit and Opportunity. RHUs are especially useful on the Moon because of its lengthy and cold two-week night.

Virtually every deep space mission beyond Mars uses both RHUs and RTGs. Solar insolation decreases with the square of the distance from the Sun, so additional heat is needed to keep spacecraft components at nominal operating temperature.

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