You may remember that famous Saturday Night Live sketch about how they sent recordings of various Earth sounds and artists out into space with the Voyager craft, in case some alien species ever came across it and wanted to understand life on Earth. 

The punchline was that after all the years it was out there, they finally got a message back, saying "Send more Chuck Berry!"

Well that never happened, obviously, but it turns out there was a letter that was sent from none other than Carl Sagan to the pop pioneer. It's quite an amazing document.