Complex sentence: Humans born on Venus would probably have little difficulty adapting to Earth gravity should there be a reason to visit or return; contrasted to return trips from Mars where humans would likely need rehabilitation or the use of an exoskeleton.
Split into simpler sentences: Humans born on Venus would probably have little difficulty adapting to Earth gravity should there be a reason to visit or return. Contrast to return trips to from Mars where humans would likely need rehabilitation or the use of an exoskeleton.