For instance, here we could emphasize the actual exploration (how we measured something) -- that's our title! If a question doesn't even mention exploration, then it should move over there. That seems to be hinted at here.
No, it is fully on topic on both sites. Overlap doesn't matter on Stack Exchange. A question always stays where it is asked, if it is on topic there. It doesn't matter if it is on topic elsewhere.
As far as where users choose to ask a question, that just depends on what community they want to answer it. If they feel that the space exploration community will provide the answers they seek, then they should ask here. If they feel the astronomy community would provide answers they seek, then there.
Planetary science is on-topic here. However, some questions might benefit with a comment like
Welcome to Space Exploration! Although your question is on-topic for this site, you may get better answers for this particular question by asking it on the Astronomy site.