Edited sentence: However, if later they did find out that they were in a different universe return would probably not have been possible.In some ways, it was fortunate that when the original switch occurred in "Mirror, Mirror" that Kirk and his fellow crew members originally entered a universe so drastically different than their own.In the dealings with the Mirror Universe, there are Prime Directive considerations at work as well.It enabled Kirk and his crew to realize from the start that something was wrong.It is debatable how the Prime Directive would apply to alternate universes - whether or not future contamination of the universe should be permitted.It's questionable if they would have ever known that they were in a different universe.The crew might have been beamed into a universe where any differences between that universe and ours might have been unnoticeable at first to either Kirk and his crew, and their counterparts