All the way up to this point I have been lead to believe that Special Relativity only describes uniform motion. If a ship turns around the motion is not uniform so Special Relativity shouldn't be able to accurately describe what happens. Surely under Special Relativity, once two observers pass each other they will never meet again. (If they do, at least one of them has changed speed and/or direction) Maybe General Relativity can be used to deal with a change of direction (acceleration), but I haven't got that far yet!

I think you're right Paul, because what if the ship literally does not change it's velocity but instead simply arcs back to Earth. Surely that would be counted as constant velocity motion. The affect of Gravity and universal expansion must be the factors that vary the velocity.