Why the plutonium powering Voyager could fit in a milk carton, and how that tiny lump has kept humanity’s farthest signal alive for nearly fifty years – Image for illustrative purposes only (Image credits: Unsplash)
Imagine a standard one-litre milk carton on a kitchen counter. That volume roughly matches the entire plutonium-238 dioxide fuel load carried by each Voyager spacecraft when they launched in 1977. Forty-eight years later, that modest ceramic payload remains the sole reason mission controllers on Earth can still receive data from a probe now more than 25 billion kilometres away.
The two Voyagers have travelled farther than any other human-made objects. Voyager 1 sits at roughly 170 astronomical units from the Sun and continues to transmit measurements from interstellar space. Its radio signals take more than 23 hours to reach ground stations, yet the spacecraft still returns usable science despite operating on power that has steadily declined since launch.
The Fuel Chosen for Its Precise Decay Rate
Engineers selected plutonium-238 in the early 1970s because its 87.7-year half-life matched the expected duration of an outer-planet mission. Isotopes that decay faster would exhaust their heat too soon, while longer-lived materials would produce too little warmth per kilogram to be practical. Plutonium-238 also emits almost pure alpha particles, which are stopped by just a few millimetres of shielding and therefore require minimal extra mass on the spacecraft.
Each gram of the material releases about 0.56 watts of thermal energy as it decays. With 4.5 kilograms allocated to each of the three generators aboard a Voyager, the total thermal output at launch reached roughly 7,200 watts per spacecraft. That steady heat source has no moving parts and needs no sunlight, making it reliable across the cold, distant regions the probes would eventually enter.
Turning Steady Heat into Usable Electricity
The radioisotope thermoelectric generators convert the plutonium’s warmth through the Seebeck effect, a principle discovered in 1821. A temperature difference across junctions of dissimilar materials generates voltage without any mechanical components. Each generator contains 312 silicon-germanium couples, with the hot side maintained near 1,000 degrees Celsius and the cold side radiating excess heat at about 300 degrees Celsius through external fins.
At the start of the mission this arrangement produced approximately 470 watts of electricity across the three generators, enough to operate the spacecraft’s instruments and transmitter. Conversion efficiency sits at only 6.5 percent, yet that figure proved sufficient because no other power technology could survive decades beyond Saturn. Solar arrays lose effectiveness far from the Sun, and chemical batteries would have failed within months.
Power Loss Over Decades and the Latest Adjustments
Two processes reduce output each year. The plutonium itself decays at a predictable rate of about 0.79 percent annually, while the thermoelectric materials gradually lose efficiency from prolonged high-temperature exposure. Together these effects remove roughly four watts of electrical power per spacecraft every year. After nearly five decades, each Voyager now generates less than half its original electrical output.
In late February a routine manoeuvre brought Voyager 1’s power margin dangerously close to its fault-protection threshold. On 17 April 2026, controllers at the Jet Propulsion Laboratory turned off the low-energy charged particles instrument to conserve energy. Only the magnetometer and plasma wave subsystem continue to operate, returning data from a region no other spacecraft has reached.
Engineers have prepared a more extensive power-saving sequence informally called the Big Bang. Tests on Voyager 2, which retains slightly more margin, are planned for May and June. If successful, the same steps could be applied to Voyager 1 as early as July, potentially restoring limited use of the recently deactivated instrument.
Supply Challenges for Missions Still on the Drawing Board
Plutonium-238 must be manufactured rather than mined. The United States halted production in 1988 and only restarted limited output at Oak Ridge National Laboratory in the 2010s. Current annual yields remain measured in hundreds of grams, while future missions will require kilograms. This constraint now influences planning for the Dragonfly rotorcraft to Titan and the proposed Interstellar Probe that could launch in the mid-2030s.
The Voyagers demonstrate what becomes possible when designers accept that certain problems have no lightweight workaround. A compact nuclear heat source, a simple thermoelectric conversion system, and a low-power radio have together sustained the most distant human signal for nearly half a century. That architecture will continue to deliver faint data until the remaining instruments fall silent sometime in the 2030s.