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For satellites do they use solar or nuclear for the main power source? I do know that they have both but I do not know which is primary. for an example: Galileo, does it use solar or nuclear?

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    $\begingroup$ There are very few nuclear powered spacecraft in deep space (we generally don't call these satellites once they leave Earth orbit), and very, very, very few active nuclear powered satellites orbiting Earth, if any. Solar electric is the default option and there's always batteries for when the satellite goes into "eclipse" or night behind the Earth, or when a deep space spacecraft has to temporarily rotate away from the Sun or goes behind another planet. $\endgroup$
    – uhoh
    Nov 29 '21 at 23:21
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    $\begingroup$ You know that they have both? Are you sure? Do you have a source? Space probes close to Sun use solar panels, those going far away from Sun use nuclear. $\endgroup$
    – Uwe
    Nov 30 '21 at 0:31
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    $\begingroup$ It would be customary, in any question/statement, especially on an SE site, to back an "I know" by supporting evidence. Not everybody reading your question have the same knowledge as you. $\endgroup$
    – Ng Ph
    Nov 30 '21 at 10:35
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    $\begingroup$ Which satellites? $\endgroup$ Nov 30 '21 at 12:00
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    $\begingroup$ en.wikipedia.org/wiki/Galileo_(spacecraft)#Electrical_power $\endgroup$ Nov 30 '21 at 17:56
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Virtually all satellites use solar power as power source. The exception is the true nuclear reactors, exemplified by the Soviet RORSAT series, which needed more power for their radar surveillance.

Probes that head far away from Earth, and require significant power over a very extended period, tend to use radioisotope thermoelectric generators. Which is not a nuclear reactor but just a lump of red-hot radioactive material , with power extracted from it via thermocouples. Examples in this class consist of almost all of the probes that went further out than Mars, and many Mars rovers.

Additionally a great many satellites and probes use radioisotope thermal heaters, small lumps of radioactive material that generate a bit of heat to fend of the cold, but no power.

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