Barrier Unknown

The logical outcome of the space race, and the preliminary step towards the Moon, was the manned satellite, the Big Wheel moving in a stable orbit about the Earth, integrating the data necessary for a landing on the Moon, and eventually acting as a fuelling station for the Lunar rockets. After several failures, the station is ready, but even there, danger exists, unseen, unheard, invisible and terrible.
Forced to exist in the belt of cosmic radiation surrounding the planet, men die within weeks from aplastic anaemia. Seeking a solution to the problem, Doctor Paul Russell is sent up to the satellite and here learns of the two men from the previous crew who vanished without a trace after spotting an unidentified spaceship in orbit further out from Earth than themselves.