As the maps are supposed to cover multiple planets, moons and asteroids, it might be a neat idea to allow players join and leave to allow one long massive session. I know literally none RTS that would ever let you do that and would be a super sweet feature which would make this game even more unique. From the technical aspects its probably problematic to get a snapshot of the current state of game, aka dumping the gamestate and send what is needed to the player. For a game that has THAT scale it might even cause performance issues on not very advanced machines. Of course another factor would be balancing, a new player could be bashed right in his moment of join, so some kind of "stealth" would be needed. Alternatively a new planet could bet spawned or moved into the system after the player had the chance to build up his base for a time that's based on the strength of the others players military. Obviously, this is probably rather complicated to implement, especially the snapshot part could be very problematic, maybe you don't even want it because of the game play in general, but from what it looks like now, you will receive enough money to outreach even your last stretch goal, and this feature, would definitively would make PA a even more unique and fantastic game. I mean, your motto is "shooting for awesome" and I think having a game session that features dozens of huge battles over a few hours would be absolutely *awesome*.
The joining and leaving has been confirmed multiple times. I am unsure of the specifics of starting as a new player in a running game. Currently the only thing that has been specificaly confirmed is joining a "player" as another controler.
There have been a few mentions of 24+ hour games, during which clan mates / friends would switch in/out to sleep Eg: Buddy A plays for 4 hours. Buddy B joins and shares control with Buddy A. Buddy A explains what's going on then leaves. Cycle repeats with Buddy C, and so on. This essentially allows a game to go on forever, as long as there are people to sustain it. I imagine there will be a few servers made just to do this. I quite like the idea of an endless war server. There could be dozens of planets, and each team would get their own massive, heavily fortified homeworld. The conflict would revolve around control of the minor planets, then when dominance is achieved by one team, assaults on the homeworld would begin. Each team could have multiple players in control. Some just for logistics, others for battles, others for sneak attacks and defense. This game has the potential to be absolutely amazing