Plutonium-238 has a mass number of 238. 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. In addition, the calculation above is very conservative, because it is limited to the first year of decay. When Cassini flies by Earth, its plutonium-238 load will have been decaying for more than one year.
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