Time for the stars / Robert A. Heinlein.

Enhanced descriptions from Syndetics:

This is one of the classic titles originally know as the "Heinlein Juveniles," written in the 1950 and published for the young adult market. It has since been in print for 50 years in paperback, and now returns to hardcover for a new generation.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. The only problem is that one of them has to stay behind, and that one will grow old while the other explores the depths of space.Always a master of insight into the human consequences of future technologies, this is one of Heinlein's triumphs.