1. The problem statement, all variables and given/known data A newspaper article stated that the NASA Galileo space probe to Jupiter 'contained 49 lb of plutonium to provide 285 watts of electricity through its radioactive thermonuclear generator (RTG)'. (Note: An RTG is a device for converting thermal energy produced by fission into electrical energy.) Assuming that the plutonium is 239Pu, which is built into a small nuclear reactor and that the efficiency of the RTG is 10 %, what is the maximum time for which the RTG will supply the required energy output? (Take the energy emitted for each nuclear disintegration of the 239Pu to be 32 pJ, NA = 6.0 * 1023 mol-1, 1 lb = 0.45 kg.) Answer: 6.2 * 1011 s. 2. The attempt at a solution We have the mass m = 22.05 kg, power P = 285 W, element 239Pu, energy emitted per disintegration E = 32 * 10-12 J and the Avogadro number with the fact that the RTG is 10 % efficient. Power P = Work done W / Time t where, as I understand, t is what we need to find. We know the energy emitted, but how many disintegrations are there? It says "for each disintegration", but how to find how many of them are there are what kind of disintegration a nuclear disintegration is? So, I think it is required to find the number of disintegrations, then calculate the total work done and then calculate time. Though I'm not sure about the 10 % efficiency. Efficiency = Power output / Power input. So for the final calculation it should be used 285 * 10 % = 28.5 W (like 28.5 W = 32 * 10-12 J * X number disintegrations / Y maximum time for which the RTG will supply the required energy output). Is this logic correct? How to find the number of disintegrations?