Pu-238 is ideal for deep-space travel because of its long half-life of nearly 88 years, but it has been in short supply. Previously, the U.S. stockpile consisted primarily of Pu-238 produced at the Savannah River nuclear plant in the late 1980s, which has since been decaying away. But U.S. production of Pu-238 ended more than 30 years ago.
