Rearranging the setting.

[johndallman]

Originally Posted by Prince Charon

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 . . .

Those options don't join up. If it's detected 40 years in advance, it has to be in solar orbit, and the THS of 2100 can build a terrifying amount of hardware in a few years. Any asteroid too big to divert with that much time would already be known now, in 2020.

Something from outside the solar system can be too big or too fast to be diverted, but it would hit Earth in 10 years at most after detection.