Home Box

A Home Box was present on starliners of the 51st century. It was described by the Doctor as being like a Black Box on airplanes. When something goes wrong on one of these ships, the Home Box is somehow ejected and it returns to the home planet with all the flight details.

River Song graffitied High Gallifreyan onto the Home Box of the Byzantium in an effort to contact the Doctor, knowing he would one day find it in a museum. Once translated into English, it said "Hello Sweetie". (DW: The Time of Angels)