Here's my take to explain it: SPOILERS
The crew never left the ship. We hear the problem with the air intake and the issues with Oxygen and CO2. I suggest that the crew became delusional, and the two men killed at the ending where Bug and Dvork although they looked to the other two like local town psychos.
That the movie did not truly answer the question, and we are forced to speculate is cheap. If stiffs the viewer out of any satisfaction in the conclusion. A few Directors and writers do this from time to time and when they do i turns me off their work.