Linked from
The 26 pages that link to Voyager 1, each with the reason it gives.
Solar windBroader topic: Its measurements traced the solar wind’s outer boundary and transition.
Deep Space NetworkBroader topic: The network receives its weak signals from beyond the heliosphere.
Voyager Golden RecordBroader topic: It carried one copy of the Golden Record beyond the planets.
HeliophysicsBroader topic: Its measurements directly sampled the heliosphere’s outer boundary.