Why Plutonium is used in space missions?

Ever wondered why Plutonium is used in space missions? this is a great article providing details and background information: http://ansnuclearcafe.org/2012/02/01/11193/ "Plutonium-238 is useful for radioisotope heat sources, and radioisotope power systems, because it decays radioactively, releasing a particularly useful form of radiation called alpha radiation.
Alpha radiation is simply energized and completely ionized helium atoms, which lose their energy in the form of heat when interacting with other matter. This energy loss mechanism is similar to how friction generates heat on a surface. Alpha radiation is generally not harmful to humans, provided its emitters are not inhaled or ingested; alpha particles can be stopped by the outermost layer of skin."