Hi Mark. It may be that the vacuum in space is not as complete as we think it is. There could be parts of space where there could be some friction that is causing these “space craft” to slow down.

I’m not sure as to how solar winds may affect them. It may also be a case of where things slow down because there isn’t a force to sustain the velocity for an indefinite period of time.

If it were gravity that is slowing them down then it would seem to me that there would be a point where gravity could also make them speed up. From what I read that hasn’t happened. At any rate it is beyond the capacity of my little brain to figure it out. In all likely hood I wouldn’t know if I had figured it out or not. Take Care…Don in creepy town.