Paradises Lost - Ursula K. Le Guin

As the spaceship Discovery makes its slow way towards its destination - New Earth - Hsing and Luis come of age together. Born in the middle of this multigenerational journey, they have never known life beyond the ship and never intend to. But just as their lives seem to be unfolding as decreed, a revelation about Discovery's true course throws everything they know into question. 

An epic, intersteller tale from Ursula K. Le Guin, modern science fiction's most visionary storyteller, Paradises Lost is a beautifully realised novella of two young friends finding their lives a new, unexpected trajectory.