I would suggest that there is some hindsight (http://en.wikipedia.org/wiki/Hindsight) creeping in to your statement. Using your example of the new world, explorers had to at least deal with ocean voyages, new diseases, and possibly dangerous flora and fauna. All of these dangers were unknown until they were known. In the same vein, the dangers of a "vastly different atmosphere" were unknown, but are now know via experiments in deep sea diving, not to mention going to the moon. Death via exploration in a new environment (ocean, deep earth, space, etc) is certainly not a new risk. Looking back from this point in time in history, it is easy to suggest that those risks were minimal.