The Ghostly Tail of Comet SWAN . ... Comet SWAN , which unexpectedly flared up to naked-eye brightness last week, has been showing detail in its ion tail that might be described as ghostly . The ion tail is made of ionized gas, energized by ultraviolet light from the Sun and pushed outward by the solar wind. ... Following the wind, structure in Comet SWAN's tail can be seen to move outward from the Sun even alter its wavy appearance over time. ... NASA Official: Jay Norris. ...