Time for the Stars — Robert A. Heinlein

Travel to other planets is a reality, and with overpopulation stretching the resources of Earth, the necessity to find habitable worlds is growing ever more urgent. With no time to wait years for communication between slower-than-light spaceships and home, the Long Range Foundation explores an unlikely solution--human telepathy. Identical twins Tom and Pat are enlisted to be the human radios that will keep the ships in contact with Earth, but one of them has to stay behind while the other explores the depths of space.This is one of Heinlein's triumphs.