There was an interesting tweet from the UK Space Agency yesterday…
UK Space Agency (@spacegovuk) October 06, 2012
I have to agree. While it’s difficult to say exactly where the boundary of the solar system lies, it’s certainly true that the cosmic rays which Voyager’s reading seem to have made a sudden dramatic change just before September.
To me, it shows that around the end of August, there was a sudden dramatic drop in low energy cosmic rays. The biggest source of low energy cosmic rays for several light years is the Sun, so essentially this marks the point where Voyager stopped feeling the solar wind. At the same time, there was an increase in high energy cosmic rays. That’s what I’d expect to happen as you leave the Sun’s heliosphere and enter the interstellar medium.
Admittedly, I don’t know the exact structure of the Sun’s heliosphere (I don’t think anyone decisively does, in fairness). Essentially, the Sun is constantly pushing outwards with solar wind, and the interstellar medium pushes back inwards. All stars do this, blowing little stellar wind bubbles in the galactic environment. A bit like this…
The termination shock there is where the solar wind slows from it’s normal supersonic speeds. The bow shock is formed because the Sun is moving through the galaxy. As it orbits, the Sun’s heliosphere plows through the interstellar medium just like the bow of a ship, forming a bow shock.
So, after a 35 year journey covering over 11 billion miles, has Voyager entered interstellar space? I think it’s too early to tell. The edge of the solar system is likely to be turbulent, and Voyager could still be in the outer reaches of the Sun’s influence. But in my speculation, it looks like there’s a very good chance that it is indeed now in interstellar space. The first craft humanity has sent out of the solar system itself. If true, this is a pretty monumental event!
Sister ship Voyager1 is 16 hrs 58 mins 13 secs of light-travel time from Earth (2012:281:1L)—
(@NASAVoyager2) October 07, 2012