A marker no machine has passed
Voyager 1 will be one light-day from Earth on 18 November at 5:16 a.m. Eastern time. That is about 16 billion miles, or 26 billion kilometres, from home. A light-day is simply the distance light covers in 24 hours, and until now no human-made object had travelled that far.
The practical meaning is delay. A command sent from a ground station will take a full day to reach the spacecraft, and the answer will take another day to come back. Engineers cannot steer the probe in real time. They send instructions, wait two days for confirmation and plan the next step from there.
Built for four years, running for 49
Voyager 1 launched in 1977 on a mission planned to last four years. It flew past Jupiter and found a thin ring and two small moons, then passed Saturn and catalogued five more moons and a new ring. It overtook Voyager 2 on 15 December 1977 and became the first human-made object to enter interstellar space in 2012.
Its power comes from radioisotope generators that lose a few watts each year. Most instruments are already off. The probe now sends data at 160 bits per second, slower than an early dial-up connection, and it has used only its X-band transmitter since 1981.
What comes next
Before the mission's 50th anniversary in 2027, the team expects to shut down more heaters and instruments to keep the spacecraft alive. Managers hope to keep the cosmic ray subsystem running on Voyager 2 and the magnetometers and plasma wave instruments on both probes.
Those instruments would let the twins work like weather stations in interstellar space, measuring the plasma, magnetic fields and particles they pass through. The probes will fall silent sometime in the 2030s. Until then, every year adds distance and a little more data from a place nothing else has reached.