I was enjoying the sunny afternoon weather in the city of swansea in south wales when an airplane caught my eye. the plane was crossing the coast and heading west. the plane was not at a great height and although distant the details of the fuselage such as windows and colour were clear. i could clearly see a bright sphere trailing the plane. it seemed to be about half the size of the tail fin and slightly to one side. i looked like a ball bearing, very shiny. i watched until it was no longer visible. it maintained it's course and position relative to the airplane.