Radioisotope thermoelectric generator
Based on Wikipedia: Radioisotope thermoelectric generator
Somewhere in the outer reaches of our solar system, billions of miles from the Sun, Voyager 1 is still talking to us. It has been doing so since 1977. There are no gas stations out there, no solar panels that would work at that distance, no way to send a repair crew. And yet, every day, a trickle of electricity flows through its ancient circuits, keeping its instruments alive, its radio transmitter pointed toward Earth. The power comes from a device about the size of a garbage can, containing a few kilograms of plutonium-238 that has been quietly, ceaselessly generating heat for nearly half a century.
This is a radioisotope thermoelectric generator, or RTG. It is perhaps the most reliable power source ever invented by humans.
The Simplest Nuclear Device
An RTG works on a principle so simple it borders on elegant. Certain radioactive materials, as they decay, release heat. This is not a choice or a reaction that can be switched on and off. It is a fundamental property of the unstable atomic nuclei, happening whether anyone is watching or not, whether the device is being used or sitting in storage. The atoms simply fall apart according to their own schedule, and energy radiates outward.
If you place this hot radioactive material next to something cooler, you create a temperature difference. And here is where an odd quirk of physics comes into play: when you connect two different metals in a circuit, and one junction is hot while the other is cold, electricity flows. This phenomenon, called the Seebeck effect after the German physicist who discovered it in 1821, requires no moving parts. No turbines spinning, no pistons pumping, no gears grinding. Just heat flowing from hot to cold, and electrons coming along for the ride.
The thermocouples in an RTG are typically made of silicon and germanium semiconductors. Hundreds of them are arranged around the radioactive fuel, their hot ends touching the fuel container, their cold ends connected to fins that radiate heat into space or the surrounding air. The efficiency is terrible by modern standards, perhaps ten to fifteen percent. But efficiency is not the point.
The point is that nothing can break.
Inventing the Atomic Battery
The RTG was invented in 1954 at Mound Laboratories in Miamisburg, Ohio, by two scientists named Kenneth Jordan and John Birden. They were working on a problem that sounds almost quaint today: the Army Signal Corps needed a power source for remote equipment that could last for years without maintenance. Batteries ran down. Generators needed fuel. Solar cells barely existed. What if you could harness the steady, predictable heat of radioactive decay?
Their early work used polonium-210 as a heat source, a choice that tells you something about the pioneering spirit of the era. Polonium-210 is fantastically radioactive, with a half-life of only 138 days, meaning half of any given sample will decay in less than five months. It produces enormous heat for its weight but burns out quickly and is spectacularly toxic. A single microgram can kill a person. Later versions would switch to more practical fuels.
The United States Atomic Energy Commission funded further development, and by 1961, the first RTG flew into space. It was called SNAP 3B, a device containing just 96 grams of plutonium-238, about the weight of a small apple. It powered the Navy's Transit 4A satellite, part of the navigation system that would eventually evolve into GPS. The nuclear space age had begun.
The Voyagers and Their Siblings
If you wanted to design a power source for the harshest, most remote conditions imaginable, you could hardly do better than an RTG. There is no fuel to run out, no moving parts to jam, no computers to crash. The only thing that changes is the slow, predictable decline in output as the radioactive fuel decays.
This made RTGs the obvious choice for missions venturing beyond Mars, where the Sun becomes too dim for practical solar power. Pioneer 10 and 11, the first spacecraft to visit Jupiter and Saturn, carried RTGs. The Voyager probes, launched in 1977 and now in interstellar space, are still running on theirs. Galileo carried an RTG to Jupiter. Cassini brought one to Saturn. Ulysses used one to orbit the Sun at angles solar panels could not have survived. New Horizons flew past Pluto with an RTG humming quietly in its core.
The Curiosity and Perseverance rovers on Mars also use RTGs, though for different reasons. Mars does receive enough sunlight for solar panels, as the earlier Spirit and Opportunity rovers demonstrated. But solar panels limit where you can land, since you need areas with good sun exposure. They also limit when you can work, since dust storms can block the Sun for weeks. And they limit how long the mission can last, since Martian dust slowly coats the panels and reduces their output.
Curiosity landed in a deep crater. Perseverance works through the Martian winter. Both can drive at night, when the most interesting atmospheric observations happen. Neither worries about dust storms. The plutonium just keeps decaying, oblivious to the weather.
The Moon's Nuclear Legacy
Six Apollo missions left RTGs on the Moon. These were SNAP-27 units, designed to power the scientific instrument packages that astronauts set up on the lunar surface. The Apollo 11 mission did not carry one, but every mission from Apollo 12 through 17 did.
Apollo 13, of course, never made it to the Moon. Its service module exploded partway there, and the crew barely survived the trip home. The lunar module, which carried the RTG, was jettisoned before reentry and crashed into the South Pacific Ocean near the Tonga Trench, one of the deepest places on Earth. Somewhere down there, in the crushing darkness of the abyssal plain, nearly four kilograms of plutonium-238 sits in its ceramic casing, slowly warming the water around it. It will continue doing so for centuries.
This was not an accident in the sense of being unexpected. NASA designed the fuel casing to survive reentry and ocean impact precisely because they knew the odds of a failed mission were never zero. The plutonium is in a form called plutonium dioxide, a ceramic that does not dissolve in seawater and cannot react chemically with its environment. It simply sits there, hot and inert, slowly decaying.
Lighthouses at the End of the World
While America was sending RTGs to the Moon, the Soviet Union found a very different application for them. The Soviet Arctic coast stretches for thousands of miles, much of it unreachable by road for most of the year. Ships navigating these waters needed lighthouses and navigation beacons, but maintaining them was essentially impossible. There were no power lines, no reliable fuel deliveries, no technicians who could visit regularly.
So the Soviets built RTGs into over a thousand unmanned lighthouses along this frozen coast. The devices were based on strontium-90, a different radioactive fuel that is cheaper to produce but requires more shielding. These lighthouses operated automatically for years at a time, their lights blinking out across the ice, powered by quietly decaying atoms.
Then the Soviet Union collapsed.
For years, nobody maintained the lighthouses. Nobody checked on the RTGs. Some disappeared, possibly stolen by scrap metal thieves who had no idea what they were taking. Others were damaged by storms or shifting ice. The international community eventually stepped in, and a decades-long project to locate and remove all the RTGs began. The last ones were finally decommissioned in 2021, thirty years after the Soviet flag came down.
Nobody knows exactly what happened to the missing ones.
The Pacemaker Experiment
Perhaps the strangest application of RTG technology was inside the human body. In the 1970s, cardiac pacemakers were becoming common, but they required batteries that had to be replaced surgically every few years. What if you could install a pacemaker that would never need a new battery?
Mound Laboratory, the same facility that invented the RTG, began developing plutonium-powered pacemakers in 1966. The devices were tiny, containing a minuscule amount of plutonium-238, just enough to generate the milliwatts needed to keep a heart beating in proper rhythm. About 140 people received these nuclear pacemakers, and many outlived their expected lifespans precisely because their hearts never stopped receiving that steady electrical pulse.
The program was canceled in 1972, not because the pacemakers did not work, but because of a problem nobody had fully considered: cremation. When a person with a nuclear pacemaker died, the device had to be surgically removed before the body could be cremated. If it was not removed, the plutonium would be released into the crematorium's exhaust system and dispersed into the air.
There was simply no way to guarantee that every mortician, in every funeral home, would know to check for and remove these devices. Modern lithium batteries improved enough to make the nuclear pacemaker unnecessary, and the program quietly ended. As of 2007, at least nine people were still walking around with plutonium in their chests.
The Chemistry of Power
Not every radioactive material can power an RTG. The fuel must meet several demanding criteria, and the intersection of all these requirements narrows the field to just a handful of candidates.
First, the half-life must be long enough to provide power for the duration of the mission, but short enough to produce useful amounts of heat. A material with a half-life of a billion years would barely warm up; one with a half-life of days would burn out before it could be used. Most RTG fuels have half-lives measured in decades.
Second, the decay must produce the right kind of radiation. Alpha particles are ideal because they are heavy, slow-moving, and easily stopped by a thin layer of material, converting all their energy to heat without requiring heavy shielding. Beta particles can also work but tend to produce secondary X-rays when they slow down, requiring more shielding. Gamma rays and neutrons pass right through most materials and would require massive lead casing to contain, defeating the purpose of a lightweight power source.
Third, the material must be dense enough to pack a useful amount of energy into a small volume. A space probe cannot carry tons of fuel.
Fourth, the material must be chemically stable and not react with its container over decades of operation.
Plutonium-238 meets all these criteria better than any other isotope. It has a half-life of 87.7 years, long enough for multi-decade missions but short enough to produce serious heat. It decays almost entirely by alpha emission, requiring minimal shielding. In many designs, the fuel casing itself provides all the radiation protection needed. It can be formed into a ceramic, plutonium dioxide, that is chemically inert and mechanically sturdy.
The number 238 is important here. Plutonium comes in several isotopes, and they behave very differently. Plutonium-239 is the isotope used in nuclear weapons and reactors, capable of sustaining a chain reaction. Plutonium-238 cannot sustain a chain reaction under any circumstances. It is simply hot. You cannot make a bomb out of it, no matter how much you have.
The Plutonium Problem
There is, however, a significant problem with plutonium-238: there is almost none of it.
Unlike plutonium-239, which accumulates in nuclear reactors as a byproduct of power generation, plutonium-238 must be deliberately manufactured. This is done by bombarding neptunium-237 with neutrons in a special reactor, a process that is slow, expensive, and not a priority for anyone whose primary mission is generating electricity.
The United States stopped producing plutonium-238 in 1988 when the Savannah River Site ended its production. For years, NASA relied on purchases from Russia, which still had stockpiles from the Soviet era. But Russian supplies are finite, and the cost per gram has climbed steadily.
In 2012, the Department of Energy restarted domestic production at Oak Ridge National Laboratory, but output remains limited. As of recent years, production has reached only about 400 grams per year, enough for perhaps one major mission annually. The Curiosity rover alone used 4.8 kilograms.
This scarcity has driven research into alternative fuels. Americium-241 has attracted particular interest because it is a byproduct of plutonium-239 decay and thus accumulates in spent nuclear fuel. It has a longer half-life of 432 years, meaning lower power density but longer mission potential. The European Space Agency has been developing americium-based RTGs for future deep space missions.
Strontium-90, the Soviet choice for their arctic lighthouses, is another option. It is abundant in nuclear waste and cheap to produce, but it emits beta radiation that generates X-rays, requiring heavier shielding. For terrestrial applications where weight matters less, it remains attractive.
What an RTG Is Not
It is worth being clear about what an RTG is not: it is not a nuclear reactor.
A nuclear reactor, whether in a power plant or a submarine, works by splitting atoms in a controlled chain reaction. Each fission event releases neutrons that trigger more fission events, releasing more neutrons, and so on. This chain reaction can be controlled by absorbing some of the neutrons with special materials, speeding up or slowing down the reaction as needed. It can also, if something goes wrong, run out of control.
An RTG has no chain reaction. The atoms decay on their own schedule, one at a time, with no connection between one decay event and the next. There is no way to speed it up, slow it down, or turn it off. There is also no way for it to explode or melt down. The fuel simply sits there and radiates heat until it is gone, a process that takes centuries.
The safety record reflects this fundamental simplicity. Every RTG that has experienced a launch failure or reentry accident has performed exactly as designed, keeping its radioactive contents contained. The Apollo 13 fuel capsule survived not only the mission failure but the violent reentry and ocean impact, settling intact into the deep ocean. Subsequent underwater surveys have detected no radiation leakage.
This is not to say RTGs are perfectly safe. If someone were to deliberately crack one open and disperse the contents, the radioactive material could cause harm. The missing Soviet lighthouse units represent a genuine concern. But as nuclear technology goes, RTGs are remarkably benign.
Into the Darkness
The Voyager probes, after nearly fifty years of flight, are now beyond the heliopause, the boundary where the Sun's influence gives way to interstellar space. Their RTGs, originally producing about 470 watts each, now generate less than half that. By the late 2020s, there may not be enough power to run any scientific instruments at all. By the 2030s, the probes will fall silent entirely, tumbling endlessly through the galaxy, their plutonium still warm but no longer able to power their voices.
But Voyager's success has inspired thinking about even longer journeys. Various proposals for interstellar precursor missions, spacecraft that would travel far beyond our solar system, have considered RTGs as their power source. The challenge is that even plutonium-238 decays too quickly for voyages measured in centuries.
Enter americium-241. With its 432-year half-life, an americium RTG could potentially power a probe for a thousand years, long enough to reach nearby stars. The Innovative Interstellar Explorer, a NASA concept study, proposed exactly this: a small probe accelerated to tremendous speed, running on americium for a millennium, sending back data from distances no human artifact has ever reached.
The physics works. The engineering is understood. The only question is whether anyone will build it.
The Eternal Battery
There is something almost philosophical about an RTG. It is a device that converts entropy directly into usefulness. The radioactive atoms are falling apart, sliding toward lower energy states, and as they do, we capture a fraction of that descent and turn it into electricity. We are not fighting entropy; we are riding it like a wave.
No other power source works this way. Solar panels depend on the sun shining. Wind turbines need wind blowing. Fossil fuel generators require a continuous supply of fuel. Even nuclear reactors need their chain reactions carefully managed. An RTG needs nothing. It produces power simply because certain atoms cannot remain as they are.
The first RTG launched into space in 1961 is still in orbit, its plutonium long since decayed below useful levels but the basic structure intact, a tiny monument to an idea that worked exactly as its inventors hoped. The Voyagers carry that same idea toward the stars, still broadcasting, still exploring, still powered by the quiet disintegration of matter that began before they launched and will continue long after we stop listening.
It is a curious kind of immortality. Not the immortality of unchanging permanence, but the immortality of useful decline. The RTG does not fight time. It spends time, converting duration into discovery, one decaying atom at a time, until there is nothing left to give.