Rearranging the setting.

[Prince Charon]

A couple of possible ATE variants (at least on Earth):

1. In 2050, astronomers discover a large asteroid on a course that will lead it to collide with Earth a a high enough relative velocity that there isn't time to shove it off course. When this happens depends on things about the tech of the setting that I don't know well enough, but i was thinking it hits in the 2080s or 2090s, giving a few decades to evacuate the regions likely to be hardest hit (they should be able to pretty quickly determine roughly where on earth it will hit, even if they don't know the exact location)... but there's no way to get everyone, nor can they get the majority off the planet. Building large numbers of habitats and expanding existing ones would need to be done as soon as possible, but they would need to be ready before the people arrive, and the number left on Earth when the Dinosaur-Killer hits would still be probably at least an order of magnitude more than those in space.

Then you have the issue that everyone is going to try to get on, and some of them will decide that if they can't go, no-one can, which means the need for massive security to keep people from destroying the vessels or letting off bioengineered plagues or... well, the Last Days would be a horror on top of horror, and then there's all the culture clashes out in space. Even if the game is set decades after the meteor hit, they'll still be adjusting in some areas. Meanwhile, depending on how bad things are on Earth, there may still be people living like Mad Max down there, or there may be no-one, or it may have been not so bad as expected.


2. Imagine something like the Genetics Wars of the Psi Trek setting, just (probably) without the psi, and with the outer space parts being far closer to TS. If you want, some of the bioroids and parahumans could resemble Star Trek races, but I was mostly thinking of a 'we did it to ourselves' sort of post-apocalypse. The number of people making it off earth would be far lower, but depending on how the wars went, the number on Earth might be quite small (in Psi Trek, Earth's population went from more than 9.5 billion in 2050 to around 5.2 billion in 2060, and kept falling for decades after that, hitting a low point in 2095 of 2.37 billion). Likewise, some of the stations that refugees went to might be uninhabited due to hidden nanoplagues and bioplagues or smuggled bombs (also a problem in the other idea), vessels might have been turned away and left to die, or constantly wandering, doing odd jobs for fuel and other resources.