It's time for us to start saying goodbye to NASA's Voyagers after almost 50 years of space flight

For those who are not familiar with them, they are the oldest and most distant probes built by humans that are still active. They are also the only probes that have left our solar system and ventured into the vast expanses of space.

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And unlike many other probes, these continue to function not thanks to solar energy, like the rest, but due to nuclear energy. But at some point, their mission will end. And right now is a good time to remember that.

The Voyagers began as planetary missions. Their objective was to carry out the so-called Grand Planetary Tour, that is, to visit the four outer planets of the solar system through a series of flybys. Jupiter, Saturn, Uranus, and Neptune were seen in a new light thanks to the robotic cameras of the probes and their numerous scientific instruments.

Everything we know about Uranus and Neptune is thanks to the Voyagers

The ice giants Uranus and Neptune, in particular, were studied for the first and only time in history by Voyager 2, while the successful observations of Jupiter and Saturn served as a basis for subsequent interplanetary missions to these worlds, such as Galileo, Juno, and Cassini-Huygens. Voyager 1, on the other hand, had as its main objective Titan, the largest moon of Saturn and one of the most intriguing satellites of the outer solar system.

Once the planetary journeys of the Voyagers were completed, it was possible to start a new phase of the mission. After their last planetary stops, both probes reached the escape velocity for the solar system, which allowed them to break free from the sun’s gravity.

Since 2012, in the case of Voyager 1, and since 2018, in the case of Voyager 2, they have become interstellar. We know this because after those dates, the probes’ sensors showed that the charged particles coming from the sun became less numerous and energetic than those detected in the galactic environment. It was a golden opportunity to study the boundaries of the solar system and the environment outside of it.

An atomic heart, its secret to longevity

Reaching such a distance is only possible with the right energy source. Many probes use solar panels, but if they get too far from the sun, they become useless (the farthest probe using them is Juno, which orbits around Jupiter).

The secret of the Voyagers lies in their atomic heart: both are equipped with three radioisotope thermoelectric generators, or RTGs, small power generators that can produce energy directly on board. Each RTG contains 24 spheres of plutonium-238 oxide with a total mass of 4.5 kilograms.

Plutonium-238 is an unstable isotope, which means it undergoes radioactive decay. The plutonium atoms in RTGs release alpha particles (composed of two protons and two neutrons) that collide with the RTG container and heat it. The heat is converted into electricity.

But over time, the plutonium on board depletes and the RTGs produce less and less energy. The Voyagers are slowly dying. The nuclear batteries have a maximum lifespan of 60 years.

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In order to conserve the remaining energy of the probes, the mission team is gradually shutting down the various instruments of the probes that are still active.

There are four active instruments remaining, including a magnetometer, as well as other instruments used to study the galactic environment, with its cosmic rays and interstellar magnetic field. But these are in their final years. In the next decade, the 2030s, the batteries of both probes will run out forever.