Plenty of people have discussed your execution with you - some polite, some less so, so I'm going to avoid telling you what you already know, and instead focus on a critique of the story.
While your story drew influence from and retained much of the eerie atmosphere and tension present in movies such as Sunshine, it lacked much of the motivation. There's a line between something that's mysterious and something that is just plain unexplained, and unfortunately, this story fell on the latter side of said line. There were simply too many blanks for me to continue to take it entirely seriously. Instead of being engrossed an creeped out, I found myself continually asking 'wait, what?'
A mystery is a framework of facts creating the outline of one giant unknown. It's mysterious precisely because we know something about it, if not much. In this case, there was no such framework - merely a nebulous mass of disconnected facts and events. That sort of mass can work if, by the end, the reader (or player) has enough information to draw one of any number of conclusions, but in your story, there was simply not enough similar data. Let me show you what I mean.
At the start, John is lonely, but stable, but he suddenly begins to see this mysterious dead astronaut and a number of accompanying visions that are clearly hallucinatory. The first conclusion one might draw is that John was going mad from the isolation, but the sudden onset and severity of the hallucinations seems to disprove that. So, is it a disease? Is it some kind of outside force manipulating his mind? Is the reality around him actually breaking? You can see there are many possible conclusions here, but instead of providing facts that support one or more of them, the story simply stops providing new information and returns to what is already known: John is seeing things. And the moment that happens, the mystery is lost. The illusion of mystery is sustained by the game's masterful audioscape (bravo on that note) but the heart is gone.
This game exhibits a lot of merit and a lot of promise, hence the four out of five stars, but in future projects, I hope you heed the advice given to you by myself and others. You have the beginnings of something truly great here, and with a little work, you may be able to reach its full potential.