In 1977, the Voyager 1 spacecraft left Earth on a five-year mission to explore Jupiter and Saturn. Thirty-six years later, the car-size probe is still exploring, still sending its findings home. It has now put more than 19 billion kilometers between itself and the sun. Last week NASA announced that Voyager 1 had become the first man-made object to reach interstellar space.
The distance this craft has covered is almost incomprehensible. It’s so far away that it takes more than 17 hours for its signals to reach Earth. Along the way, Voyager 1 gave scientists their first close-up looks at Saturn, took the first images of Jupiter’s rings, discovered many of the moons circling those planets and revealed that Jupiter’s moon Io has active volcanoes. Now the spacecraft is discovering what the edge of the solar system is like, piercing the heliosheath where the last vestiges of the sun’s influence are felt and traversing the heliopause where cosmic currents overcome the solar wind. Voyager 1 is expected to keep working until 2025 when it will finally run out of power.
None of this would be possible without the spacecraft’s three batteries filled with plutonium-238. In fact, Most of what humanity knows about the outer planets came back to Earth on plutonium power. Cassini’s ongoing exploration of Saturn, Galileo’s trip to Jupiter, Curiosity’s exploration of the surface of Mars, and the 2015 flyby of Pluto by the New Horizons spacecraft are all fueled by the stuff. The characteristics of this metal’s radioactive decay make it a super-fuel. More importantly, there is no other viable option. Solar power is too weak, chemical batteries don’t last, nuclear fission systems are too heavy. So, we depend on plutonium-238, a fuel largely acquired as by-product of making nuclear weapons.
But there’s a problem: We’ve almost run out. “We’ve got enough to last to the end of this decade. That’s it,” said Steve Johnson, a nuclear chemist at Idaho National Laboratory. And it’s not just the U.S. reserves that are in jeopardy. The entire planet’s stores are nearly depleted.
The country’s scientific stockpile has dwindled to around 36 pounds. To put that in perspective, the battery that powers NASA’s Curiosity rover, which is currently studying the surface of Mars, contains roughly 10 pounds of plutonium, and what’s left has already been spoken for and then some. The implications for space exploration are dire: No more plutonium-238 means not exploring perhaps 99 percent of the solar system. In effect, much of NASA’s $1.5 billion-a-year (and shrinking) planetary science program is running out of time. The nuclear crisis is so bad that affected researchers know it simply as “The Problem.”
But it doesn’t have to be that way. The required materials, reactors, and infrastructure are all in place to create plutonium-238. In fact, the U.S. government recently approved spending about $10 million a year to reconstitute production capabilities the nation shuttered almost two decades ago. In March, the DOE even produced a tiny amount of fresh plutonium inside a nuclear reactor in Tennessee.
The only natural supplies of plutonium-238 vanished eons before the Earth formed some 4.6 billion years ago. Exploding stars forge the silvery metal, but its half-life, or time required for 50 percent to disappear through decay, is just under 88 years.
Fortunately, we figured out how to produce it ourselves — and to harness it to create a remarkably persistent source of energy. Like other radioactive materials, plutonium-238 decays because its atomic structure is unstable. When an atom’s nucleus spontaneously decays, it fires off a helium core at high speed while leaving behind a uranium atom. These helium bullets, called alpha radiation, collide en masse with nearby atoms within a lump of plutonium — a material twice as dense as lead. The energy can cook a puck of plutonium-238 to nearly 1,260 degrees Celsius. To turn that into usable power, you wrap the puck with thermoelectrics that convert heat to electricity. Voila: You’ve got a battery that can power a spacecraft for decades.