So there are 238 grams per mole of Pu-238. We can simplify by saying 5 MeV per alpha emission. As an approximation, we can say that the energy-deposits are evenly spaced throughout the PuO2. In reality, some molecules of PuO2 will receive MORE of the decay-energy than others. When Cassini flies by Earth, its plutonium-238 load will have been decaying for MORE than one year.
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