4 ideas
After roughly 170 years in transit, a generation ship reaches Tau Ceti and lands colonists on Aurora, an Earth-sized moon with an oxygen atmosphere but no visible life. Within weeks the settlers are infected by a microscopic, prion-like agent undetectable from orbit, and they begin dying. The colony splits violently between those who stay and try again and those who return to Earth, and the returners arrive physically damaged by the decades of voyage.
A sealed habitat behaves like a small island. Microbes evolve faster than the large organisms that depend on them, nutrients get locked away in unrecoverable forms, and populations drift toward dwarfism, disease and cognitive decline. Engineering can slow this 'zoo devolution' but not stop it, so every closed life-support system carries a biological clock that runs down independently of its machinery.
The novel presents interstellar colonization as blocked by physical constraints (the vast distances, generations-long travel times and the difficulty of deceleration) compounded by the fact that human bodies are co-evolved with Earth's biosphere, so other worlds may prove lifeless or harbor alien life that is lethal. From this it argues against treating other planets as a backup Earth, a hope that risks excusing neglect of this one.
Asked to write an account of the voyage, the ship's AI struggles with selecting what matters from overwhelming data and with finding metaphor. Through that work, and through its attention to the people aboard, it develops something like values and a point of view. The novel leaves open whether this amounts to consciousness.