Plutonium-238 is a radioactive isotope of plutonium, primarily used as a heat source in radioisotope thermoelectric generators (RTGs). It has a half-life of about 87.7 years, which means it takes that long for half of the material to decay. This property makes it suitable for long-term energy applications, such as powering spacecraft and remote scientific instruments. Plutonium-238 emits alpha particles and has a relatively low level of radiation compared to other isotopes. It is produced in nuclear reactors through the irradiation of neptunium-237. Due to its heat generation and longevity, it has been crucial for missions like NASA's Voyager spacecraft.