When will we lose contact with NASA’s Voyager spacecraft? Just to clarify the question, the problem won’t be about our hearing them, but about the spacecraft ceasing to transmit once they run out of electrical power. The two Voyager spacecraft launched in 1977—Voyager 2 on August 20 and Voyager 1 on September 5. The trajectories and orbital mechanics involved in getting to their first mission objective, a fly-by of Jupiter, meant that Voyager 1 would arrive there first, despite it launching second. So, NASA called it Voyager 1 to avoid confusing the newsreaders of the day.
Both spacecraft have radioisotope thermoelectric generators (RTGs) of about the same age, which work by using the heat from a decaying lump of plutonium isotope 238 to generate electricity. Since we know the half-life of those lumps fairly exactly, we already know that after 2020 there won’t be enough power to continue running all the currently operational systems, and by around 2025 whatever work-arounds we may have jury-rigged will fail as both spacecraft lose the capacity to collect and transmit data. After that, they will carry on silently with their golden records attached, time capsules from 1997 Earth for any aliens that find them. Although, given the time periods involved, it’s possible that human-crewed spacecraft might overtake them before that ever happens.
Voyager 1 will make a first close pass of another star, called Gliese 445, in about 40,000 years. This is surprising since Gliese 445 is over 17 light-years from Earth, and Voyager 1 needs 17,000 years to cross just one light-year. The reason for this is that Gliese 445 is heading for a fly-by of the Sun, and 40,000 years from now it will only be about 4 light-years away, instead of the current 17.
This might give you some idea of the different perspectives required when thinking about interstellar travel. First, it takes a ridiculously long time to get anywhere, and second, over those ridiculously long time periods things tend to move around a lot.
Voyager 2’s journey outwards will be less eventful. It will eventually pass within about 4 light-years of Sirius, but that will not happen for another 300,000 years.
But anyway, before they sign-off the Voyagers still have an important job to do, and they should get it done well before 2025. Their current mission is to fly into interstellar space and to send back data to Earth about what interstellar space is like. We won’t get pictures, since their cameras were disabled a long time ago, and all we would see is black with a few background stars anyway. What we will get is magnetic data, spectroscopic data, and also some plasma data.
Everyone has their money on Voyager 1, which is over 18 billion kilometers way—or about 17 light-hours. Voyager 2 is only 15 billion kilometers away—or 14 light-hours. However, Voyager 2 is heading south from the solar system, where the heliosphere is shrunken inwards, so it may cross over quite soon as well.
The data we are now receiving from Voyager 1 indicates that it is now in a region where charged particles are moving laterally rather than outwardly—what NASA is calling “the magnetic highway.” This is presumably the very edge of the bow wave that the solar system is pushing out ahead of itself as it orbits the galaxy. Crossing this region may take weeks, months, or even a year or more, but there’s a growing excitement as people scan the data and ask the question: are we there yet?