On September 5, 1977, NASA launched Voyager 1 from Cape Canaveral on a Titan IIIE rocket, sixteen days after its twin, Voyager 2. The plan was a grand tour of Jupiter and Saturn, timed to a planetary alignment that happens once every 176 years and lets one spacecraft swing from planet to planet on gravity assists.
It launched second. On a faster path, it reached Jupiter first in March 1979, where it found active volcanoes erupting on the moon Io, the first ever seen on a body other than Earth, and showed that Europa is a shell of ice over something that moves. The volcanoes were a surprise. At Saturn in 1980 it flew close to Titan to study the dense atmosphere, and that manoeuvre pushed it out of the plane of the planets and ended its tour.
Both spacecraft carry the Golden Record, a copper disc with greetings in fifty-five languages, natural sounds, and ninety minutes of music, assembled by a committee led by Carl Sagan. It is a message for whoever finds the thing, and it is also the reason the mission is known outside astronomy.
Voyager 1 crossed the heliopause in August 2012 and is now in interstellar space, more than 160 astronomical units from the Sun, still returning data. Its plutonium power supply will fall below what the instruments need before long. Power will fail soon. Then the spacecraft keeps drifting, silent, for billions of years.