Not OP but I'm guessing OP is referring to the Voyager Golden Record attached to the first Voyager probe, specifically the playback instructions using hydrogen

Not OP but I'm guessing OP is referring to the Voyager Golden Record attached to the first Voyager probe, specifically the playback instructions using hydrogen atom to derive time units for playing the record: https://en.m.wikipedia.org/wiki/Voyager_Golden_Record#Playback https://en.m.wikipedia.org/wiki/Voyager_Golden_Record#Playba... Many of the ideas for the record came from Carl Sagan and a committee he lead working with NASA.