In 1977, the Voyager 1 spacecraft left Earth on a four-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.
It’s a good start, but the crisis is far from solved. Political ignorance and shortsighted squabbling, along with false promises from Russia, and penny-wise management of NASA’s ever-thinning budget still stand in the way of a robust plutonium-238 production system. The result: Meaningful exploration of the solar system has been pushed to a cliff’s edge. One ambitious space mission could deplete remaining plutonium stockpiles, and any hiccup in a future supply chain could undermine future missions.
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 ejects an electron, it leaves behind a uranium atom while firing off a helium core at high speed. 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.
“It’s like a magic isotope. It’s just right,” said Jim Adams, NASA’s deputy chief technologist and former deputy director of the space agency’s planetary science division.
U.S. production came primarily from two nuclear laboratories that created plutonium-238 as a byproduct of making bomb-grade plutonium-239. The Hanford Site in Washington state left the plutonium-238 mixed into a cocktail of nuclear wastes. The Savannah River Site in South Carolina, however, extracted and refined more than 360 pounds during the Cold War to powerespionage tools, spy satellites, and dozens of NASA’s pluckiest spacecraft.
By 1988, with the Iron Curtain full of holes, the U.S. and Russia began to dismantle wartime nuclear facilities. Hanford and Savannah River no longer produced any plutonium-238. But Russia continued to harvest the material by processing nuclear reactor fuel at a nuclear industrial complex called Mayak. The Russians sold their first batch, weighing 36 pounds, to the U.S. in 1993 for more than $45,000 per ounce. Russia had become the planet’s sole supplier, but it soon fell behind on orders. In 2009, it reneged on a deal to sell 22 pounds to the U.S.
Whether or not Russia has any material left or can still create some is uncertain. “What we do know is that they’re not willing to sell it anymore,” said Alan Newhouse, a retired nuclear space consultant who spearheaded the first purchase of Russian plutonium-238. “One story I’ve heard … is that they don’t have anything left to sell.”
By 2005, according a Department of Energy report (.pdf), the U.S. government owned 87 pounds, of which roughly two-thirds was designated for national security projects, likely to power deep-sea espionage hardware. The DOE would not disclose to WIRED what is left today, but scientists close to the issue say just 36 pounds remain earmarked for NASA.
That’s enough for the space agency to launch a few small deep-space missions before 2020. A twin of the Curiosity rover is planned to lift off for Mars in 2020 and will require nearly a third of the stockpile. After that, NASA’s interstellar exploration program is left staring into a void — especially for high-profile, plutonium-hungry missions, like the proposed Jupiter Europa Orbiter. To seek signs of life around Jupiter’s icy moon Europa, such a spacecraft could require more than 47 pounds of plutonium.
“The supply situation is already impacting mission planning,” said Alice Caponiti, a nuclear engineer who leads the DOE’s efforts to restart plutonium-238 production. “If you’re planning a mission that’s going to take eight years to plan, the first thing you’re going to want to know is if you have power.”
Many of the eight deep-space robotic missions that NASA had envisioned over the next 15 years have already been delayed or canceled. Even more missions — some not yet even formally proposed — are silent casualties of NASA’s plutonium poverty. Since 1994, scientists have pleaded with lawmakers for the money to restart production. The DOE believes a relatively modest $10 to 20 million in funding each year through 2020 could yield an operation capable of making between 3.3 and 11 pounds of plutonium-238 annually — plenty to keep a steady stream of spacecraft in business.
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