[petuniasevan]

I forgot to mention - the nuclear fusion process is very efficient, and since it converts hydrogen to helium, it has a LOT of "fuel" to work with.

Even nuclear fission (breaking off protons and/or neutrons from heavy elements to form lighter elements) is much more efficient than oxidation. Various space probes such as the Voyagers, Galileo, Viking, ALSEP, and Cassini all have a little reactor onboard for charging the batteries for the various equipment onboard. Radioisotope Thermoelectric Generators (RTGs) are still functioning aboard even the Pioneers 10 and 11. No other systems (e.g., batteries, fuel cells, etc.) can meet these mission requirements.

The Sun burns several hundred million tons of hydrogen per second, converting it to helium by nuclear fusion. The total mass of the sun is 2 x 1033 grams, out of which a small fraction of it is be used for nuclear reactions. If you calculate the age of the Sun based on the amount of gas consumed per second, you will find that the Sun can live for about 10 billion years.

[petuniasevan]

2 x 1033 grams

Forgot the subscript! OOPS! That should be 2 x 10₃₃ grams!

[MikeD]

RTGs aren't reactors. The heat generated by the decaying plutonium is used to create electrical power for the spacecraft. Any mission that goes past Mars must have RTGs due to the rapid decrease in solar intensity.

MD