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.
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.
SEE ALSO: The 25 most iconic photos of Earth from space
DON'T MISS: A forgotten war technology could safely power Earth for millions of years. Here's why we aren't using it
Transit satellite network
Physicist Glenn Seaborg discovered plutonium in 1940 and, just 20 years later, engineers used it to build nuclear batteries for spacecraft.
In 1960 the US Navy took over an experimental plutonium-powered satellite program called TRANSIT to guide their submarines and missiles from space.
The first satellite powered by plutonium, called Transit 4A (above), reached orbit on June 29, 1961.
http://www.youtube.com/embed/o9h2-TfaITo?rel=0
Width: 800px
Height: 450px
By 1988, dozens of similar spacecraft — four of them using nuclear batteries — made up a rudimentary satellite navigation network.
Each satellite beamed a unique radio signal. With multiple signals coming from different orbits, the Navy could easily track its submarines and other wartime hardware.
But space scientists hit a snag early on: Their data suggested that spacecraft slowed down or sped up over certain parts of Earth.
When researchers mapped the anomalies, they realized that some regions of the planet were far denser than they thought, and that the extra mass — and gravity — subtly affected spacecraft speed.
The map of the anomalies (above) became the first of Earth’s geoid, a representation of the planet’s true gravitational shape.
See the rest of the story at Business Insider