Voyager 1 Reaches Interstellar Space. But Has It Left the Solar System?

Yesterday, NASA announced with some fanfare that after 36 years in space, the probe Voyager 1 has entered interstellar space. I have two things to say here, and I want to be careful. First, this is an amazing event, and well worth celebrating. Second, a lot of people are saying Voyager 1 has left the solar system, and that’s not really accurate. First things first. What scientists discovered is that in mid-to-late 2012, Voyager entered a new region of space. This all has to do with the solar wind, a stream of subatomic particles blown outward by the Sun. This wind expands outwards, and far beyond the orbit of Neptune it encounters the particles that exist between the stars. It pushes those aside, and loses momentum as it does so. At some point, the pressure from the wind is no longer strong enough to expand against the pressure of those external particles, and it slows to a stop. This region is called the heliopause. For years, we’ve known that Voyager is in the fuzzy volume of space where the heliopause lies, but it’s been maddeningly difficult to know if it had punched through. Read more here: http://www.slate.com/blogs/bad_astronomy/2013/09/13/voyager_1_space_probe_is_in_now_in_interstellar_space.html