Norman Spinrad’s 1974 Riding the Torch is a standalone science fiction novella.
Mistakes were made. Earth is a lifeless cinder. Before the planet was seared, a small fleet of interstellar ships managed to escape. Surely somewhere in the sky, there must be a second Earth.
A thousand years later, the torchships are still looking. On and on they travel, harvesting the materials they need to survive and prosper from the interstellar void.