Voyager 1 was launched in 1977 on a mission to fly by Jupiter and Saturn. Barring damaging encounters with cosmic dust or Saturn’s ring debris, Voyager was expected to last a few years, after which the slow decay of its radioisotope power supply would make its radio transmissions inaudibly faint. That was then.
This is now: radio detection technology has advanced more rapidly than Voyager’s transmissions have faded. Now, 33 years after launch, Voyager 1 is still on line. It is now operating 17,400,000,000 kilometers from the Sun, nearly three times as far away as Pluto, receding from us at a rate of 17 kilometers per second. Its signals take 16 hours at the speed of light to reach us. Its instruments have monitored the outward rush of the Solar Wind since launch, but now it is so far from the Sun that the interstellar medium is getting in the way. The Solar Wind sweeps out an immense “bubble” in the interstellar plasma, within which the flow is steadily outward from the Sun. Next comes a relatively thin region of interaction of the Solar Wind with interstellar plasma, a turbulent shock front. (This turbulent sheath streams out behind the Sun as it travels through the interstellar plasma at 20 kilometers per second like the tail of a comet.) Next comes the outer edge of the bow shock, beyond which no trace of the Sun’s influence survives.
Voyager 1 is now in the turbulent shock region. The plasma flow it is now measuring is at right angles to the flow from the Sun. It is leaving the heliosphere, the region in which the Sun is dominant. It is already in the disputed realm where the Sun is losing its struggle against the interstellar medium. A vast, sparse cloud of Kuiper Belt Objects lies around it, and the Oort cloud of frozen, inactive comets lies ahead, but they are so widely scattered and so faint that Voyager will not see them. To its sensors, it is leaving the Solar System behind. In 4 or 5 years it will be beyond the shock front, cruising the void between the stars. This is interstellar space, the stuff of science fiction.
But space is vast. If Voyager had been aimed at the nearest star (it wasn’t), it would be less than 0.1% of the way there. On its present course it will be as far away as the nearest star in about 50,000 years. Will we still be listening then?