Time and again (1951)

It is the future and Mankind has spread to the stars like seeds before the wind. One star system, though, shrouded in mystery, has defied Man's every attempt to visit it. Every expedition to 61 Cygni has found its path inexplicably deflected and has been forced to return home in frustration. In desperation, special agent Asher Sutton was sent on a solo mission, but unlike the others he did not return and 61 Cygni was quietly forgotten. As the book begins, twenty years have passed and, against all odds, Asher Sutton has returned. The mystery only deepens when it is discovered that Asher's ship was damaged many years ago in a crash that left it completely disabled and ought to have killed its sole passenger. The conclusion becomes inescapable; Asher Sutton died but now he's back. As the story develops, we discover Asher is not alone and it's not clear that he's even entirely human. But most importantly, Asher returns bearing an idea that will shake Mankind's beliefs to their foundations. In Time and Again, Mankind is spread thin across the stars and to help hold the frontier he has created biological androids. Created in the lab by chemical means, androids are sterile and cannot reproduce but in all other respects are as human as their creators. None the less, androids are treated as property and bear a mark on their foreheads to distinguish them from "true" humans. Androids dream of one day being acknowledged and treated as the equals of the "humans" and Asher's idea is the key for which they have been searching. Asher soon becomes the center of a struggle between three groups; humans of the present who fear any new idea that might loosen Mankind's tenuous grip on the stars, humans of the future who, via time travel, are waging a quiet war to alter the past to maintain the current status quo, and the androids of the future who struggle to let Asher's idea be born.