The N435 Groundwire. This helps the Di-lithium crystal maintain its phase reference to Ground (you can't say Earth because that's light years away.)
Without this wire the Di-Lithium crystal would deteriorate, (a small sugar lump sized device that keeps the entire star ship going, yet mysteriously isn't backed up by a spare, for which Capt Kirk was once heard to say: "Mr Spock, please escort Scottie down the working alleyway to number 42 airlock and space the f*****.")
Such are the vagaries of space travel.
