NASA is just days away from plunging its $3.26-billion Cassini mission into the clouds of Saturn. It will glow like a meteor for a minute or two, then never be seen again.
The mission has lasted two decades and explored the Saturn system for 13 years. But unlike the vast majority of spacecraft, Cassini hasn't relied on solar energy; sunlight is just 1% as strong at Saturn as it is at Earth. Powering a robot like Cassini with sunshine there would require solar panels the size of football fields.
What made Cassini possible is one of the rarest materials in the universe: plutonium-238.
Pu-238 is a byproduct of nuclear weapons production. But it's not a key ingredient in atomic bombs (unlike plutonium-239 and other fissile isotopes), and half of any amount decays within about 87 years. On a spacecraft, this decay gives off lasting warmth that helps safeguard fragile electronics. It also reduces the weight of a robot, allowing for heavier shielding where radiation fields are dangerous to electronics.
But most importantly, wrapping Pu-238 with heat-to-electricity converting materials, called thermoelectrics, forms a nuclear battery that lasts for decades.
On September 15, NASA will plunge its Cassini spacecraft into the clouds of Saturn on one final mission — and with it will vanish its 72 lbs of Pu-238.
The space agency has only 37 lbs 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.
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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.
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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.
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