The RTG (Radioisotope Thermoelectric Generator) is an incredible device responsible for, among other things, powering the most distant spacecraft ever launched. RTGs are some of the best and most useful nuclear batteries, relying on heat transfer to produce electrical power from the decay of radioactive elements. These generators are a great example of why nuclear technology is necessary for the exploration of the solar system.

RTGs are used to power electronics in extremely remote and harsh environments. They have been used to power remote lighthouses, experiments on the Moon, vehicles on Mars, and numerous spacecraft exploring the outer planets.

This rendering of the Curiosity Mars rover shows its power supply: a radioisotope thermoelectric generator. You can see it as the large cylinder protruding out the back of the vehicle.
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Thermocouples and power from heat

RTGs derive energy from radiation using devices called thermocouples. A thermocouple is a loop of conductive material, with one half being one material and one half being another - essentially two different kinds of wire attached together at the ends. If one junction is heated up to a higher temperature than the other junction, a temperature gradient across them appears and an electrical voltage is produced via the Seebeck Effect(See here for my demo and explanation of this).

This means that you can generate electricity using thermocouples or similar devices. However, the source of this energy comes from the transfer of heat - we need a heat source.

An example of how a thermocouple works
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For RTGs, that heat source is a radioactive element. In radioactive decay, various kinds of radiation are released, with radiation particles having very high initial kinetic energies. When radiation is stopped in a shielding material, all of the radiation kinetic energy is converted to ordinary thermal energy. If the source of radiation is strong enough to deposit more thermal energy into the material than is lost via heat transfer mechanisms, then the material will heat up. This temperature shift can then be converted into electricity via a thermocouple!

Keeping Cool

But that a thermocouple can't work using just a high temperature: It needs a temperature gradient. This means that we need a way to keep the second end of the thermocouple colder than the radioactive source in order to generate electricity. If the entire thermocouple is allowed to heat up to the same temperature as the radioactive material, no energy/current will be generated.

This problem is particularly large in space. Surrounded by the vacuum of space, the RTG cannot be cooled via convection or conduction due to the lack of atmosphere (although it can transfer heat to the rest of the spacecraft). The only way for things to cool off in space is via blackbody radiation.

Blackbody radiation is the emission of electromagnetic waves from an object with non-zero temperature. Right now, your internal temperature causes you to brightly glow. You can't see this glow because it's deep into the infrared band of the electromagnetic spectrum. The frequency ("color") of the electromagnetic radiation emitted depends on the temperature of the object (for example, incandescent filament light bulbs glow because of thermal radiation emitted in the visible part of the spectrum (visible light). Since all electromagnetic waves contain energy, thermal radiation removes energy from an object and cools it down. As a side note, thermal radiation happens to be the source of the sun's light, which is again in the visible spectrum due to the solar surface's high temperature.

Emission curves for blackbody radiation at various temperatures. Wavelength determines the color of visible light, and the photon energy and frequency for all electormagnetic waves.
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The more surface area an object has, the more thermal radiation it will emit. This means that a piece of paper in space will cool down more quickly than a crumpled up piece of paper will, all else held constant. RTGs use this principle to remove excess heat from the outside of the device, allowing the thermocouples to generate energy continuously.

Most RTGs use large metal fins on the outside of the generator to add surface area. The entire metal frame acts as a heatsink, and takes heat away from the thermocouple assembly. This heat can then be radiated out into space via the relatively large surface area provided by the cooling fins. Thanks to the fins, the thermocouples keep their temperature gradient and current keeps flowing out of the generator indefinitely.

An example of a RTG design. Notice the large cooling fins present to add surface area to the outer metal.
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Now we've covered the basics of the RTG: Generate heat via radioactive decay, cool the outer shell of the device via high surface-area thermal radiation, and extract electrical energy using thermocouples and the Seebeck effect. Now I'd like to present a little about how exactly we can produce that heat.

RTG Fuel Materials

As previously mentioned, the heart of the RTG is a radioactive element. While all three primary radiation types (alpha, beta, gamma) will heat up materials, alpha radiation is by far the easiest to stop while also carrying (typically) the most energy per particle. Because of this, alpha radiation is the prime candidate for heating things up with ionizing radiation. Since alpha particles can typically be stopped with paper, almost none of them will escape the radioactive material itself, meaning that all of the decay energy from every decaying atom is deposited directly into the material as heat, without using significant shielding.

Of course, shielding is still necessarily to block the gamma component of most alpha decays, but these gamma rays don't contribute much to the overall heating because their energy is much lower than the alpha radiation.

Also extremely important is the life of the radioactive element. All radioisotopes have different, set half-lives that determine how quickly they will decay away. Shorter half-life means that you will get more power out of any given gram of material, but you will also obviously have a shorter lived RTG. For deep space missions, the ability of a RTG to last decades or more is absolutely essential when it takes 9 years just to reach Pluto.

The most viable radioactive isotopes for use in RTGs is Plutonium-238. This nucleus contains 94 protons and 144 neutrons. This material can only be produced via artificial nuclear reactions, and as such is extremely rare and doesn't occur in nature in significant enough amounts to matter. Pu-238 is actually becoming increasingly scarce, as it was a byproduct of nuclear weapons development which no longer goes on. With a half-life of around 87 years, Pu-238 lasts for long enough to power even the longest lived spacecraft.

This RTG onboard Curiosity on Mars contains Pu-238.
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Plutonium-238 is an excellent material for radioisotope thermoelectric generators because of how it decays. Pu-238 produces 5.6 MeV alpha radiation, which as mentioned previously is essentially the best radiation you can ask for when you want to heat something up, as essentially of it will be easily stopped in tiny amounts of shielding material. While gamma emissions do exist, they are extremely rare, so almost all Pu-238 decays result in only alpha (and a few X-rays from electrons knocked out by the alphas and decays) radiation. This means that the added risk of having a giant radiation source in order to generate electricity is massively mitigated, making Pu-238 nuclear batteries some of the safest nuclear batteries if properly built.

Since heat transfer typically scales with surface-area while radiation power production scales with volume/mass, macroscopic chunks of Pu-238 tend to heat themselves up automatically to the point where they glow:

Glowing Pu-238, powered by alpha radiation
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Pu-238 nuclear batteries have powered numerous spacecraft: New Horizons, the Curiosity rover, the Voyager probes, and Cassini, to name a few.

Other radioisotopes can also work well for building RTGs. Strontium-90, a moderate energy beta-emitter with a three decade half-life, was used to power several remote Arctic lighthouses in the Soviet Union. Sr-90's somewhat reasonable beta radiation energies makes it easy to shield the radiation and heat up the RTG. On the upside, Sr-90 is much easier to obtain than the rare Pu-238. This makes the Sr-90 RTG the "budget brand" radioisotope thermoelectric generator (not that they are cheap or easy to obtain by any means).

These defunct Sr-90 RTGs powered lighthouses in the far north.
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Another interesting possible RTG fuel is Americium-241 (Am-241). Yes, the same Am-241 that lives in your smoke alarm. It's very long half-life and similar decay mode to Pu-238 makes it a decent candidate for powering a RTG one day. However, its common 59 keV gamma byproduct means that an Am-241 RTG would require more shielding than a Pu-238 battery to mitigate external radiation. The reason your smoke alarm Am-241 doesn't produce very much heat or glow is because its surface area is massive compared to its tiny mass, meaning that heat is transferred out of the metal more quickly than the alpha decay can heat it up, causing no significant heating. A big chunk of Am-241 would glow just as the Pu-238 one does, although less so than Plutonium because of Am-241's longer half-life.

RTG Uses

I've already mentioned many uses for these interesting devices, but let's see some pictures! Here are some of the numerous ways RTGs have been used to power remote electronis in the past:

Cassini, the famed Saturn orbiter, used several RTGs to power itself far from the sun. This spacecraft recently crashed into Saturn as its mission ended. If you ever see a high-resolution images of one of Saturn's moons, thank Cassini.
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Voyager-2 currently uses a RTG to stay alive on the edge of the solar system itself. It has operated for 40 years on the same RTG, and is currently twice as far from the sun as Pluto after having flown by all four gas giants.
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A Pu-238 RTG allows the giant Curiosity Mars rover to continue to explorer the red planet to this day, without relying on solar panels that can be harmed by dust as the smaller Opportunity and Spirit rovers did. This picture is a real self-portrait, not a rendering, and was taken by Curiosity on the surface of Mars.
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RTGs also powered experiments on the surface of the moon during the manned Apollo lunar missions. This RTG is from the Apollo 14 mission.
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These Sr-90 RTGs were used by the USSR to power remote lighthouses on the coast of the Baltic sea.
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That's a basic overview of RTGs and how they work. RTGs are an absolute necessity for any spacecraft going past Jupiter, so it is important that governments sponsor peaceful production of Pu-238 to power these missions. I hope you were able to learn something new about these cool devices! Let me know if you have any questions, comments, or corrections.

Those who say nuclear power is dead are dead wrong. Even if we for some reason stop using nuclear reactor power on Earth (a poor choice in my humble opinion), nuclear technology is the only solution for exploring the majority of the solar system.

Thanks for reading!

Po-210 RTG
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