why does all space have to be a vacuum, i mean that space not occupied by planets and stars and other things.. :) My point is, in reference to pioneer, that if gravity gets weaker and weaker the further you get from a sun, why cant groups of particles exists between solar systems in essence like a solar system atmosphere, which in turn provides the source of 'drag' witnessed from the pioneer data. Maybe vacuum is the wrong term, I know it is more in reference to lack of preasure then lack of things in space... Everything is spiraling away from the center, so obviously gravity is still effecting the area between solar systems as well as the systems themselves. That doesnt mean there is nothing between solar systems, it just means the 'stuff' between solar systems hasnt been caught in a stong enough gravity well to become dense enough for us to see. Now assuming there is 'stuff' between solar systems effected by the low gravity of the galaxy, and even lower gravity of the universe.. What happens when you throw a high denisty object like the pioneer through it at high speed? Now obviously the particles in the space between solar systems is rare, so the colisions with those particles should be miniscule depending on the velocity. What about the density of the pioneer itself in a low gravity enviorment? Wouldnt it create its' own minute gravity that would effect those particles? If it did, wouldnt that create more 'drag' then just the surface area of colisions with rare particles? I imagine the collection of particles build up faster and faster in such a low gravity enviornment, which would definately make the 'drag' noticeable. The counter argument is why cant we detect it.. My response to that is who says we cant? The density of those particles is so low it would be like trying to see the air between your eye and your computer screen. If you wave your hand in that same space fast enough you can feel the drag, so you know its there. I think the drag of the pioneer is very similar to our hane, thats how we know it is there.