Terraforming Earth

In the wake of an extinction-level meteor impact, a small group of human survivors manages to leave the barren Earth and establish a new home on the moon. From Tycho Base, they're able to observe the devastated planet and wait for a time when return will become possible. Finally, after millennia of waiting, the descendants of the original refugees travel back to a planet they've never known, to try to rebuild a civilisation of which they've never been a part.
But after so much time, the question is not whether they can rebuild an old destroyed home, but whether they can learn to inhabit an alien new world - Earth. Winner of the John W. Campbell Award for best novel, 2002