NASA's latest car-size Mars rover, Perseverance, rocketed off Earth last week, kicking off a seven-month voyage through deep space to Mars.
The robot is designed to spend three years exploring the red planet's surface, hunting for signatures of ancient alien microbes, stashing Martian soil samples for future return to Earth, deploying the first-ever interplanetary helicopter, and paving the way for human explorers with a variety of experiments.
However, a device called a multi-mission radioisotope thermoelectric generator, or MMRTG, could power Perseverance for more than 14 years thanks to a unique nuclear material called plutonium-238, or Pu-238. The material has powered NASA spacecraft for decades, including some for close to half a century.
Pu-238 is a byproduct of nuclear weapons production. Unlike its sister chemical, plutonium-239 (which makes up the fissile cores of bombs), half of any amount decays within about 87 years. On a spacecraft, Pu-238's decay gives off lasting warmth that helps safeguard fragile electronics. Most importantly, wrapping Pu-238 with thermoelectric materials that convert heat to electricity, forms a bewilderingly long-lasting power source.
The space agency used to have just only 37 lbs of Pu-238 left to put inside a spacecraft — enough for another two or three spacecraft.
But NASA and the US Energy Department have resurrected Pu-238 production capabilities, helping provide enough material for Perseverance and future missions.
To tide you over until the Perseverance reaches Mars and begins its alien hunt, here are the 16 greatest Pu-238-powered US space programs of the past and present — plus more that have yet to launch.