When it comes to free energy, nothing beats the giant ball of plasma that is the sun. That's why engineers slap solar panels onto most spacecraft.
However, most of outer space is a dark, lonely void that can cripple even the hardiest electronics with circuit-snapping cold and powerful cosmic radiation.
To unlock the deepest secrets of space requires a bit of magic. And thanks to Cold War scientists, the US made lots of it: plutonium-238 (Pu-238), a byproduct of nuclear weapons production.
It's not a key ingredient in atomic bombs, unlike plutonium-239 and other fissile isotopes. But on a spacecraft, Pu-238's gradual decay gives off warmth that can safeguard fragile electronics.
More importantly, wrapping Pu-238 with heat-to-electricity converting materials, called thermoelectrics, forms a nuclear battery that lasts for decades.
In December 2015, Oak Ridge National Laboratory produced some of the first plutonium-238 since the end of the Cold War.
And in September 2017, NASA will plunge its nuclear-powered Cassini spacecraft into the clouds of Saturn on one last mission.
The space agency has only 37 lbs (16.8 kg) of Pu-238 left that's ready to put inside a spacecraft. That's enough to launch another two or three plutonium-powered spacecraft.
NASA and the US Energy Department are working hard to resurrect Pu-238 production capabilities, and they've shown recent progress, but the program is years behind schedule - and the material is our best and perhaps only way to explore most of deep space.
As NASA says goodbye to some of its longest-lived probes and sets its sights on future nuclear spacecraft, we review the 15 greatest plutonium-238-powered US space programs of the past and present.