Interstellar is a vague term which simply means "among (or between if you must) the stars." If you mean "outside the heliopause" you need to say just that. I'm "inside" the heliopause (i.e., I'm under the influence of the sun-related things that stop influencing me when I go beyond it), but I'm not part of, or in or on, the sun, therefore I'm interstellar if I want to put it that way. It simply isn't a scientific term unless qualified. It's a nice word though, as are helicopter and mellifluous. Musk and Branson may be able to help you one day.