When I first heard the term "fediverse", it immediately made me think of some sort of vast interplanetary network. And let's face it: a fediverse-like model is really what you would need if you had settlements scattered throughout the solar system. A monolithic, centralized service would be awful, given the reality of communication lag and likely limited bandwidth.
So let's say lemmy (or more generally activitypub) were to go interplanetary. How would that work out? You set up your first instance on Mars. Any content that's posted there will be immediately available to your fellow Martians. Earthlings who subscribe may also be able to view it as their instances cache the content, albeit after some delay.
But the trouble starts when Earthlings want to start contributing to the discussion. If they have to wait the better part of an hour to get a single comment lodged, it's going to get old fast.
So you would need to allow the Earth side to branch off to some extent from what's happening on Mars. Then eventually, something like a git merge would try to bring it all back together? I wonder if that would work?