Even time-acceleration is no problem, just think about it what it actually is... time-travel.
Even if you confine the world just to the Earth-Moon system, the available space is so vast that encounters with other players would be few and far between. If you allow your players to time-travel, you've just added another dimension, and the only time players will ever see each other is when they first start the game. After that, everyone will be on their own timeline, and even if you're at the same base as someone else, you might not see them since they're a year in the past.
Time travel/time acceleration isn't a difficult technical question, it's a difficult content question. If someone time travels on a trip to the moon, can a player still in the time of that trip see that person's ship doing that travel? If so, what happens if they interfere with that travel? If not (or if they can't interfere), then see the "would never see another player" comment.
Or, what if you have, say, an asteroid with a limited number of resources? On real-world (thenceforth RW) date January 1st 2010, at in-game (thenceforth IG) date January 1st, 2100, player A discovers this asteroid, mines and exhausts its resources, and leaves. Then, on RW date January 31st 2010, at IG date December 31st, 2099, player B goes to that same asteroid, mines and exhausts its resources.
How do you resolve this? Does player A's mining of the asteroid remove it from past timelines? At that point, you might as well not have time travel, since there's a universal "current" state. Does player A get the minerals, and player B still gets to keep his? Well, at that point you've defeated the purpose of having limited resources, and you might as well have unlimited resources, but an economy based on unlimited resources wouldn't be particularly successful

. I guess you could have the "limitedness" of the economy based upon the rate at which you can return materials to a base to sell them, rather than the presence of them, but if the player ends up spending most of their time travelling, that's a good way to lose players quickly.
I think there's a reason that no successful multiplayer games have incorporated time travel as a primary mechanic outside of very limited situations (and the limits required would be such that it wouldn't be useful to Orbiter). I think you'd need to design around the need for time acceleration by having vessels that are so sci-fi in nature that you don't need time acceleration, but then the question becomes...why are you using Orbiter for that in the first place?
Personally, I think Freelancer would've made a fantastic MMO, with a little more balancing for that purpose. I think that's how space MMOs would have to be: sci-fi (so things are interesting), and with many abstractions from reality (because reality doesn't make a very good game

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