The radiation ionization storm just was the height of deus ex machina. We knew something was going to go wrong, but as soon as the alarm went off, I was like "Here it comes" and rolled my eyes. I'm OK with someone sacrificing themselves for the team, but that just felt so contrived after they had spent the entire film building up the oyxgen issue, which I thought was a logical complication of having a fourth person aboard. If it were up to me, I would have just eliminated the CO2 scrubber problem entirely - spaceships should have multiple backup systems for *everything,* particularly something so critical. Simply having four humans on board instead of three could have been enough to throw off the balance, as an extra passenger isn't a contingency that a ground team is necessarily going to prepare for.
Also, how did Michael get stuck behind a bolted on panel? Who bolted it on behind him? The most logical way was for him to install it himself, and have him sneak aboard because he wanted to go to space, but that's clearly not what happened. But it doesn't make sense any other way. That was something I expected the film to address in the final act in some revelation that would have made for a better climax than the one we got. But it didn't.
Anyway, great premise, good first two thirds, excellent acting (particularly by Shamier Anderson, which is notable because he was the one actor I *didn't* know beforehand), but the movie fell apart in the final act with certain problems set up earlier not paying off with deus ex machina taking over as the impetus for the final decision.