In the mid-1950s it seemed like the road to the stars was just within reach. The rocket testing of the late 1940s and early 1950s had confirmed that rockets were a powerful new way to explore.
First off in these books beyond the background to spaceflight was a voyage to the moon. It seemed like it was simply a question of will and technology and these books introduce this goal as attainable in the next 10-20 years.
Post world war 2 it seemed like the Earth had been spoken for. If a nation was going to grow it needed to explore new frontiers and establish new colonies.
Furthermore beyond the Moon we were meant to travel into the solar system. Mars was waiting for us and we just needed to expand our horizons.