Plutonium-238, having 94 protons and 144 neutrons in its nucleus, was the first discovered plutonium isotope. This happened when mad genius scientists like Glenn Seaborg decided to bombard uranium-238 atoms with heavy hydrogen.

Pu-238 is radioactive, and it is a very powerful alpha particle emitter. After its initial dark history (being used in human experiments and for weapon applications), it found its way into peaceful applications as the heat source of radioisotope thermoelectric generators (RTG), which use radioactive materials and thermocouples to directly convert decay heat to electrical energy. Essentially, RTGs are batteries, which can run while there is still radioactive material left. Considering the half-life of Pu-238 (87.7 years) that is a long time. In addition, the alpha decay has high energy, resulting in high power to weight ratio. In RTGs some Pu-238 made it to space, to the Moon and beyond. One example is the Mars rover Curiosity. This rover has explored the Mars surface since 2012, powered by about 5 kg of our nuclide. At the time of writing this, it is still running on the same initial fuel.

Pu-238 also became part of cardiac pacemakers, guaranteeing their recipients decades of continuous operation (beating any other battery-powered pacemakers). Unfortunately, it turned out that cremating people with Pu-238 inside them is not necessarily an environmentally friendly action, thus the production of plutonium based pacemakers was cancelled.