The Space site will include many topics that would also be on topic at Aerospace. Many sites have significant overlaps with each other, and that's okay. Amateur Radio has overlap with EE. Astronomy with Physics, Unix/Linux with System Fault, etc. Overlap is perfectly fine. Given that, what is important is to bring a community together.
I believe the Space Exploration community is already starting to come together. Despite starting after, it has more committers than both Astronomy and Aviation. And while there will be some overlap, there isn't as much as you would think. It also has one of the highest percentage of 200+ committers I've ever seen in a proposal, currently 122/173, or 70%.
Bottom line is, if something manmade goes in to space, or otherwise assists a person to get to space, it is on topic at Space Exploration. Where there is overlap, they can go on either site. I would imagine that questions relating to Aerodynamics and Rockets will find somewhat friendlier people at Avionics, and satellites, etc at Space Exploration. But I don't think we'll really know until both sites are launched.
The very fact that Space Exploration has been considered similar to both Astronomy and Aviation shows that while there overlap, it fits in to a realm of it's own, part science, part engineering, and part operations. I don't think anyone would consider Aviation to cover Astronomy, but Space Exploration fits somewhere in between. If any of these sites has a "failure to launch" problem, then we can discuss what to do to help them grow. And a merge might be in order. But first let's see how they can do independently.