When Earth links up with the Planescape cosmos, they're going to find one thing a bit to close to home to not defend against: spelljammers. As a result, the assorted militaries of the world have made contracts with the arcane, for spelljamming helms for military use, while the various space agencies are remolded to gain the duties of an organization not unlike the FAA; setting standards and regululations fore spelljammers and their use.
I dunno much about Spelljammer, so I can't say much about how they'll interact with the locals.
There was already a discussion about that. My opinion is to keep spelljammer away from urban planewalking and keep a modern view of the universe because... Well, it's modern