There factually is no doubt that we will become two distinct species. For example, they will become much taller, and will have a different form of vision. To speed this up they need to breed. Breed young, and breed often. The sooner we get a few generations born, the sooner and more pronounced the adaptations will be.
This is important because I doubt the first gen astronauts will live very long after arriving because they are adapted to live on Earth. Their bones will degenerate, muscle will atrophy, among other things. However, the second generation onward should fare much better.