I got this idea while reading a synopsis of “Future Boy Conan” Which I haven’t watched. I’m sure the science of this is utter bull but it could work for an interesting story. I’m just going to brain dump this here for later.
There is a huge asteroid heading towards Earth, humanity has had 5-10 years notice (maybe longer) of this but it’s so big that there was nothing that could be done to prevent it. The governments of the world decide to abandon the Earth and head out into space in search of a new home, so they have built huge spaceships called Arcs each with it’s own ecosystem and enough nuclear fuel to power it for a very long time, in order to do this they’ve basically strip mined the earth.
One of the ships gets clipped by the asteroid or something else goes wrong during takeoff that screws up it’s engines and makes it either crash back down to Earth or makes them unable to travel any distance so they are forced to land on Earth after the impact.
The survivors on this space craft and the animals/plants they took with them are the only living organisms that survived the asteroids impact apart from maybe some things under the sea and micro organisms that were hardy enough to survive it.