Space is not 'space' (ie, a vaccum) in the solar system, viz http://en.wikipedia.org/wiki/Interplanetary_medium http://en.wikipedia.org/wiki/Interplanetary_med

Space is not 'space' (ie, a vaccum) in the solar system, viz http://en.wikipedia.org/wiki/Interplanetary_medium http://en.wikipedia.org/wiki/Interplanetary_medium [1] At the edge of the sun's impact, there is a transition zone. That transition zone is the subject of this discussion. Its the 'solar' that makes it a system. the solar system is sort of like the earths atmoshpere, in that regard. At a certain stage there is a transistion into the next level. This is probably oversimplified, but has the important general idea out on the table. Others can refine as appropriate. [1] http://news.ycombinator.com/item?id=4619870 http://news.ycombinator.com/item?id=4619870