Time for the Stars (Library Edition)

Travel to other planets is now a reality, and with overpopulation stretching the resources of Earth, the necessity of finding habitable worlds is growing ever more urgent. There's a problem though--because the spaceships are slower than light, any communication between the exploring ships and Earth would take years.

Tom and Pat are identical twin teenagers. As twins they've always been close, so close that it seemed like they could read each other's minds. When they are recruited by the Long Range Foundation, the twins find out that they can, indeed, peer into each other's thoughts. Along with other telepathic duos, they are enlisted to be the human transmitters and receivers that will keep the ships in contact with Earth. But there's a catch: one of the twins has to stay behind--and that one will grow old--while the other explores the depths of space and returns as a young man still.