Travelling at speeds close to that of light, spacemen lived at an accelerated pace. When one of the twin boys left the starship, he grew older while his twin in space barely aged. So the starship twin left the ship to find what happened to his brother who was aging away on earth.

Why did everybody step off the ship in this strange valley and promptly drop dead? How could a well-equipped corps of tough spacemen become a field of rotting skeletons in this quiet world of peace and contentment? It was a mystery Peter and Sherri had to solve. If they could live long enough! [from the Judas Valley]Originally published in Amazing Stories, October 1956

Here are two early stories by the well known SF Author Robert Silverberg. The Happy Unfortunate was published first in Amazing Stories in 1957 and explores the angst caused when the human race reaches into space but at the cost of needing to breed a new species; specialized 'spacers' who can withstand the tremendous rigors of acceleration. The Hunted Heroes was published in Amazing stories a year earlier, in 1956. It is a futuristic story that holds great hope for the resilience of the human race after the war destroys most of the world.