"The biggest news last week had nothing to do with presidential elections, overseas financial crises, or fiddly football coaches. The really big news, the only news that will be popularly remembered 500 years hence, was that a human artifact was, for the first time, on the verge of departing our solar system, en-route to who-knows-what in the interstellar infinitude. That was the Voyager 1 probe, launched from Cape Canaveral in 1977 to tour the outer planets and collect data on the “interstellar medium” — the thin soup of particles, trace gasses, and other teensy cosmic detritus in the appalling vastness between the stars. Voyager completed the former mission decades ago, and is now on the verge of assuming the latter, powered on its way by radiation emitted from our ever-more distant sun. The news of Voyager’s imminent departure from our system was carried everywhere. Unfortunately, almost none of the coverage addressed two central questions: What do we mean when we say “solar system”? And: How do we know we’re out of it?"