At first I thought that people might try getting into space much earlier, but then I remembered that many people assumed venus and mars were inhabitable anyway and the earth itself seemed much larger and unexplored 70 years ago. It was a big kick in the pants and it was discovered that mars was so airless and barren. So things might have proceeded much as now until Apollo and then the next goal of the world's space programs would have been sister earth.

It wouldn't have been too difficult to get people down to the surface of a Venus sized world after Apollo, but a mission that was capable of bringing them back again would be an enormous undertaking. So it's possible that people would be sent on a one way mission and expected to colonize the planet. This might sound extreme, but if the Soviet Union was about to do it then the U.S. might do it. It would of course be crazy dangerous.

There would be a debate about contamination. If we were going to send a mission today, the debate would be massive. If we were going there in the 50's the debate would probably not be so masssive.