On Aug. 25, 2012, about 11 billion miles (18 billion kilometers) away from the sun, the NASA probe Voyager 1 left the heliosphere, boldly going where no object had gone before. By crossing that boundary, Voyager 1 traveled beyond the solar system and entered interstellar space, a historic first.
Look at the bottom row of a (traditional) periodic table and you'll find the element that made this cosmic adventure possible: plutonium.
What Is Plutonium?
First identified in the 1940s, plutonium has been used for both creative and destructive purposes. The late physicist John Goffman once called plutonium "the element of the lord of hell." A linguist might be inclined to agree.
But first a bit more about this element. Every atom of plutonium contains 94 protons. By contrast, there are only 92 protons per uranium atom and 93 in each neptunium atom.
"Plutonium was discovered by Glenn Seaborg and co-workers at Berkeley Laboratory (CA) in late 1940," says Peter C. Burns, a chemist at the University of Notre Dame, in an email.
Ten years earlier, astronomers had observed a new dwarf planet near Neptune. To honor the Roman god of the underworld, it was dubbed "Pluto." And plutonium derives its name from that heavenly body.
Originally, Seaborg and company were able to produce plutonium by using a cyclotron particle accelerator at Berkeley. With this device, particles called "deuterons" were fired at a uranium sample. The experiment created a small amount of neptunium, which then became plutonium through a decaying process.
The first weighable plutonium sample was created at the University of Chicago Aug. 20, 1942. By that point, some parties had recognized the element's military potential.
Plutonium atoms always come with 94 protons. But the neutron count can vary, and chemists refer to these variations as "isotopes." Uranium has isotopes as well. One of these, called uranium-235 (U-235), was soon identified as a potential fuel source for atomic bombs. Shortly after its discovery, plutonium entered the conversation as another way to power nuclear weapons. The Atomic Age was about to begin.
Today, for all practical purposes, there are two kinds of plutonium: reactor-grade and weapons-grade. Plutonium was the key ingredient behind "Fat Man," the nuclear bomb that decimated Nagasaki, Japan, in 1945, killing tens of thousands of people and effectively ending World War II.
Plutonium and Weapons
Plutonium used for military purposes is recovered from uranium fuel that has been irradiated for two to three months in a plutonium production reactor. It takes about 22 pounds (10 kilograms) of nearly pure plutonium-239 isotope (Pu-239) to make a bomb. That type of bomb requires 30 megawatt-years of nuclear reactor operation, with constant fuel changes and reprocessing of the 'hot' fuel, according to the World Nuclear Association. That's why "weapons-grade" plutonium is made in special reactors that increase the concentration of the higher isotopes of plutonium.
The first atomic bomb explosion on Earth took place July 16, 1945. It was in New Mexico, and it was strong enough to be felt 100 miles (160 kilometers) away. It was part of the Manhattan Project's top-secret "Trinity Nuclear Test" at the Alamogordo Bombing Range. The device in question had a plutonium core; no uranium-based nukes were deployed for the experiment.
Subsequently, the U.S. dropped a U-235 nuclear bomb over the Japanese city of Hiroshima Aug. 6, 1945. Three days later, the U.S. dropped a second bomb nicknamed "Fat Man" on Nagasaki. Just like the weapon tested in New Mexico that summer, the Nagasaki bomb relied on plutonium.
"[It] will never be known for certain how many people died as a result of the atomic attack on Nagasaki," reports the U.S. Department of Energy's official website. According to their best estimate, "40,000 people died initially, with 60,000 more injured." Over the coming months and years, the ultimate death total may have climbed to 140,000 or more. The Nagasaki Peace Park hosts an annual ceremony to honor their memories every August.
The biggest issue today with the weapons-grade plutonium stockpile is what to do with it. The U.S. is estimated to currently have 96.6 tons (87.7 metric tons) of plutonium — and a storage problem. Much of it is currently stored in a building at the Savannah River Site in South Carolina.
Plutonium and Power
Today more than one-third of the energy produced at nuclear power plants comes from plutonium. The United States, however, doesn't have any facilities that rely on plutonium for energy.
The most common plutonium isotope formed in a nuclear reactor is Pu-239, which is created by neutron capture from depleted uranium (U-238). When fissioned, Pu-239 can have as much energy as enriched uranium (U-235), which is also used in nuclear weapons.
Historically, another plutonium isotope, Pu-238, was used to power the batteries in some commercial pacemakers. Those medical devices went out of style as lithium-powered alternatives hit the market.
But in the final frontier, plutonium remains a valuable commodity.
Plutonium and Deep Space
"The most significant, lesser-known use of plutonium is for power generation during space exploration," Burns says. "Plutonium-238 emits a lot of heat when it undergoes radioactive decay, and this heat can be used in a thermoelectric generator to produce electricity."
Pu-238 has many qualities that make the isotope very attractive to engineers working for space agencies. For starters, you don't need much of it to generate a whole lot of heat, which can then be converted into electricity.
Then there's the half-life, a metric that tells you how long it'll take half of the atoms in a given radioactive isotope to decay and transform into something else. With a respectable half-life of 88 years, Pu-238 can keep rovers and space probes running for decades on end.
Far away from the sun, in places where the star's rays are weak and dim, solar-powered satellites aren't going to perform that well. Meanwhile, Mars rovers that depend on sunlight (like the now-defunct Opportunity Rover) have to contend with the dust from passing storms that can smother their panels and impede battery function.
For these reasons, Pu-238 is a great fit for both Martian and deep-space exploration. So far, Pu-238 has powered at least 30 U.S. space vehicles. The Perseverance Rover that touched down on the Red Planet in February 2021 has a generator fueled by Pu-238. So do far-flung spacecrafts like Voyager 1 and Voyager 2, which have been touring the solar system ( and beyond) since 1977.
Plutonium and Toxicity
Plutonium is radioactive, though you'll likely never be exposed to it. Robert M. Hazen at the Carnegie Institution for Science says there "are no natural sources" of plutonium. "It has to be made through breeder reactors, so all plutonium in use on Earth is human made," he explains via email.
It can be released into the environment, though, via an industrial plant, or from a container, however the levels of plutonium in air, water, soil and food are extremely low. However, if you are exposed, it would likely be through breathing in radiated aerosols or skin contact. And many factors will determine whether exposure will harm you, including how much, how long and how you came in contact with the plutonium.
When you breathe it in, some plutonium will get trapped in your lungs and will move to your bones and liver. If you swallow it via food, a trace amount can also spread to your bones and liver. If you touch plutonium, very little — if any — will enter your body, but it can burn the skin that came into contact with it. So while it is a radioactive element, plutonium is far from being "the most toxic substance known to man," as activist Ralph Nader once proclaimed.