The above illustration provides the 1995 location of spacecraft that have passed the orbits of Pluto/Neptune. It appears from the illustration that Pioneer 10 was travelling along the "tail" and may never definatively reach the heliopause. Do we know which direction the tail points, or is the illustrtation taking a guess at that direction?
We sure do. Basically, the Sun moves through the galaxy at about 220 km/sec. The bow shock is created in the direction of motion and is the place where there is a pressure balance point between the Interstellar medium and the Interplanetary medium. Magnetic fields get involved, so its a little more complex than that, but that's the general idea. The tail is formed opposite of the direction of motion. There was a really great Hubble pic of a bow shock in Orion on the Astronomy Picture of the Day on this site a few weeks back. A very striking photo. If I remember correctly from my grad school days, I don't think the tail is as big as the picture dictates, I think the scale is a bit off.
Despite the fact that it is mostly boring dust, molecules and atoms, the Local Interstellar medium is more interesting than the field in general. Did you know that the Solar System lies in a region slightly more dense than the surrounding material, which is a vacuum called the Local Bubble? The Local Interstellar Map is a good idea where everything lies within a thousand light years or so. The bow shock in your picture is too small to be on that map, but it is in the direction of the arrow, and the tail is in the opposite direction.
From here http://quest.arc.nasa.gov/sso/cool/pioneer10/general/amonetxt.html
Now if you go to the next slide, we see an Artist's rendition of the heliospheric boundary. This is the next mission, try to find out where they are. The heliopause in the center there is a meeting surface of the solar wind and that region outside the influences of the sun where interstellar space begins. The interstellar space is of unknown composition, really. But it does contain energyic flow and particles whose cosmic flow, as indicated here by the Galactic cosmic rays, have been detected as far inward as the earth and Venus. The solar wind is a flow of gases that, from the generation by the sun, it's a plasma subatomic particle, and it travels at the speed of 1,600,000 kilometers per hour or 1 million miles per hour. It undergoes a solar wind termination shock somewhere between originating and the heliopause, in which the flow goes from subsonic -- supersonic very abruptly to subsonic. And we see that the motion, in this picture right to left, there is a shock and it goes downstream.
There are a few other spacecraft doing the search, Voyager 1 and 2, but Pioneer 10 is unique as being the only spacecraft in the downstream direction. Unique also at this moment, because it's the farthest away.
Before we launched Pioneer 10, we felt that the extent of the solar wind was perhaps five times the effect -- fell off about at the distance five times the distance from the sun as the earth, or five AU, astronomical units, that is the distance between the sun and earth. 150 million kilometers or 193 million miles. The outermost planet is at about 40 AU. Pioneer 10 is now at 65 AU. And we have yet to find the heliospheric boundaries, but we feel we are getting close. Scientists now estimate that these boundaries are anywhere from 70 AU to 120 AU.
this was from a conference a few years ago.
I am most likely out of date on this subject.
It's probably affected by the Great Attractor (Things out there are rushing toward it. No-one knows what 'it' is, intriguing.)