Thinking about it some.. I really don't believe you'd have much of a forking problem, at least at a solar system scale. Realistically the majority of hashing power will be located here on Earth for the forseeable future. With that caveat, it'd almost be guaranteed the longest chain would always be the Terra chain. You are looking at the same 51% attack scenario... in order for a serious fork to happen, you'd have to have close to 50% of total network hash capacity located a significant distance away from Earth. Actually, for just about any real non terra based mining, you'd have to have a huge investment to start up or you'd be guaranteed to always lose on the longest chain contest. I can't see this happening.
Sucks for those Martian miners. But their only chance to make any dough would be a mars-coin branch.
Sigg
I feel like there should be some ratio between distance and required hashing power to determine fork probability. Like if Pluto is 300 light minutes away, if you had 3.3% of the hashing power (10/300) you could have a realistic chance of getting a block beforehand and creating a Pluto fork.
But what this forgets (and you seemed to intuitively understand) is that earth is going to be continually sending those blocks about every 10 minutes, overwriting any short forks that may arise. Essentially this will make it impossible to mine the farther from earth you are, unless you are large enough to then become the primary node. It will also take 600+ minutes to confirm any transactions on Pluto.
So yes, and large fraction of distance from the primary network will make mining useless, unless the new locations approach 50%, in which case there is serious chance for the two locations to be 2-3 blocks out of sync and completely fork.
This seems to intuitively support the idea of multi-layers of bitcoin. One that has for example a 1 day average block creation to transfer between worlds.