How do we know when Voyager reaches interstellar space?

Whether and when NASA's Voyager 1 spacecraft, humankind's most distant object, broke through to interstellar space, the space between stars, has been a thorny issue. For the last year, claims have surfaced every few months that Voyager 1 has "left our solar system." Why has the Voyager team held off from saying the craft reached interstellar space until now?

Fuente : http://feeds.sciencedaily.com/~r/sciencedaily/~3/j...