(iv) A space probe uses the decay of 238 Pu as a source of power/heat. If the decay occurs by a emission, 338 Pu 234 U + a + 5.49 MeV, 94 92 J with a mean life of 128 years, what mass of 238 Pu is required to ensure a supply of at least 1 kW of heat for 50 years? (Assume that the kinetic energy released is largely converted into heat, and that the decay rate of 234U is negligible.) 92