Voyager 2 launched in 1977. It is now well beyond the edge of the solar system, and its power source – a set of radioisotope thermoelectric generators – loses output every year as the plutonium inside decays.
That decline is predictable and irreversible. The engineering question has always been what to switch off, and when.
What they did
In an operation the team nicknamed the Big Bang, engineers simultaneously shut down several power-hungry components and switched functions to lower-power alternatives, buying additional operating margin without losing the instruments that still return useful data.
Doing that on a spacecraft is not like reconfiguring a server. A command takes many hours to arrive. If something goes wrong, the response takes as long again. There is no way to check the result before committing.
Why it is remarkable
The hardware was designed with the technology of the early 1970s, to survey four planets, on a mission expected to last a handful of years. It has now operated for nearly five decades in an environment nobody had directly measured when it launched.
The people who wrote the original software have mostly retired. The documentation is partly on paper. Every change requires reconstructing the intent of engineers who solved a problem fifty years ago with constraints nobody remembers.
What it is still doing
Measuring the properties of interstellar space directly – magnetic fields, particle densities, plasma. There is no other source for that data. Every model of what lies beyond the heliosphere is calibrated against two spacecraft, and there are no replacements planned.
The end of it
Power output will eventually fall below what the instruments need, and then below what the transmitter needs. The exact year depends on decisions like this one, which is why they are made carefully.
After that the spacecraft continues, silent, on a trajectory that will take it past other stars in tens of thousands of years. It simply stops telling us anything.