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Why are the Voyager spacecrafts powered by thermoelectric generators instead of solar panels, if heat/energy transferred by radiation is independent from the distance between the source and the receiver in a vacuum?

Sunlight spreads out according to the square of the distance.

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Mars is only about 1.5 times as far from the sun as Earth, but it receives 44% as much sunlight. The Voyager spacecraft were built to study Jupiter, Saturn, Uranus, & Neptune, at 5.2, 9.5, 19.1, and 30 times the distance respectively.

As the spacecraft started the operational portion of their missions, they’d have been receiving 3.7% the sunlight as they would near Earth. That means that if a two square meter solar panel were sufficient to power the probes in Earth orbit (about right for the best technology of the time) they’d have needed 55 square meters of solar collecting surface near Jupiter, or a panel the size of a Walmart superstore by the time they reached Uranus. And Neptune? We can safely forget Neptune.

With today’s improved technology, the most distant spacecraft that uses solar power is Juno, in Jupiter’s orbit.