Sagan quickly pitched the idea for the golden records, estimating it would cost $25,000 to make them. Casani agreed and Sagan and team member Ann Druyan set about choosing the music. They had just six weeks to assemble the album, the most symbolic music compilation project in history. It was an almost impossible task, by Sagan's own admission.

A frantic consultation with musicologists around the world ensued, as Druyan, who later became Sagan's wife, battled to track down 26 specific recordings, which reflected something of the emergence and evolution of music on Earth.

When the physician and biology writer Lewis Thomas was asked which tracks he would send he quickly replied 'the complete works of JS Bach …" before adding, after a pause " … But that would be boasting!" But The Sounds of Earth does carry more from Bach than any other single composer, with three pieces chosen to reflect the evolution of his style.

As with any mixtape project, particularly one intended to represent something of our diversity as a species and what it means to be a human, there are going to be some obvious omissions; not least the Beatles. Druyan was hoping for Here Comes the Sun but the request was turned down by the band's record company, as they presumably couldn't agree to clearance for the rights "in perpetuity, across the known Universe".

http://www.guardian.co.uk/science/2012/oct/21/voyager-mission-leave-solar-system