Rocket engines are used for propulsion on spacecraft, but the electronics has to be powered also. Frequently solar panels are used for generating electricity. But what happens if a space probe is so distant from the Sun that it's just a pinprick of light like any other star?
The Seebeck effect
If you heat the end of a piece of wire, it establishes a thermal gradient from the hot end to the cold end. This causes charge carriers (electrons or "holes") to move from one end of the wire to the other, resulting in a potential difference or voltage between the two ends of the wire. This is called the Seebeck effect. Devices that make use of this effect are called thermocouples and there's one in your oil or gas central heating boiler for measuring the temperature inside the burner chamber. A thermocouple is made of two wires of dissimilar materials twisted or bonded together at one end and can measure temperatures of over 1000 degrees C.
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| A thermocouple measuring circuit. Image courtesy Wtshymanski, CC BY-SA 3.0 via Wikimedia Commons |
Thermopiles
Now as you may be aware, if you connect lots of batteries in series, e.g. AA cells end to end, the voltages add up. So for instance in a transistor radio, four AA, 1.5 V cells give 6 V for powering the radio. The electromotive force (EMF) or voltage generated by a single thermocouple is typically small, of the order of tens of microvolts per degree of temperature difference between its hot and cold end. If hundreds or thousands of thermocouples are connected together in series, they can generate useful voltages. This arrangement is called a thermopile. If all the junctions between the wires are made hot, and alternating junctions are kept cold, the device can generate electricity to power things. Thermopile powered radios, running on oil lamps, were used in remote areas of Russia in the past where electricity and batteries were unavailable.
Radioisotope thermoelectric generators (RTG, RITEG)
A radioisotope thermoelectric generator is a thermopile that uses a radioisotope source to heat the hot junctions of a thermopile while the other junctions are kept cool. The radioisotope source is typically materials such as uranium or plutonium. RTGs can generate hundreds of watts and were used in the Voyager 1 and Voyager 2 spacecraft, launched in 1977. The Perseverance Rover on Mars is powered by a 110 W RTG using Plutonium-238 as a heat source.
References
NASA. (n.d.). Mars 2020: Perseverance Rover - NASA Science. NASA. https://science.nasa.gov/mission/mars-2020-perseverance
Rowe, M. (2020, October 23). Thermocouples: Simple but misunderstood. EDN. https://www.edn.com/thermocouples-simple-but-misunderstood/
Thermo-electric generators. (n.d.). http://www.douglas-self.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm