Once the Voyagers’ planetary journeys were over, it was possible to begin a new phase of the mission. After their final planetary stops, both probes reached the escape velocity of the solar system, allowing them to break free from the sun’s gravity. Since 2012 for Voyager 1 and 2018 for Voyager 2, they have become interstellar. We know this because after these dates, the probes’ sensors showed that charged particles from the sun became less numerous and energetic than those detected in the galactic environment. This was a golden opportunity to study the boundaries of the solar system and the environment outside it.
The secret of a long life
Reaching this distance is only possible with the right power source. Many probes use solar panels, but if they get too far from the sun, they become useless (the farthest probe that uses them is the Jupiter-orbiting Juno probe). The Voyagers’ secret lies in their atomic cores: both are equipped with three thermoelectric generators of radioisotopeso RTG: small power generators that can produce power directly on board. Each RTG contains 24 plutonium-238 oxide spheres with a total mass of 4.5 kilograms.
Plutonium-238 is an unstable isotope, meaning it undergoes radioactive decay. The plutonium atoms in RTGs release alpha particles, which include two protons and two neutrons, and these hit the RTG vessel, heating it up. The heat is then converted into electricity.
But as time passes, the plutonium on board depletes, so RTGs produce less and less energy. So the Voyagers are slowly dying. Nuclear batteries have a maximum useful life of 60 years.
In order to conserve the probes’ remaining power, the mission team is gradually shutting down the probes’ various instruments that are still active. For example, in October Voyager 2’s plasma science instrument—which measures electrically charged atoms passing through the probe—shut down; the Voyager 1 device itself shut down in 2007 due to a malfunction. These instruments were used to study charged particles from the solar magnetic field, and it was precisely this detector in 2018 that determined that Voyager 2 had left the heliosphere and gone interstellar.
Four active instruments remain, including a magnetometer and other instruments used to study the galactic environment, with its cosmic rays and interstellar magnetic field. But these are the last few years. In the next decade, it’s hard to say exactly when the batteries on both probes will run out forever.
This story originally appeared on Italy with cable and has been translated from the Italian.
