Justin's reaction after the outer door to the airlock is triggered is heartbreaking. He seems to suddenly snap back to his senses and frantically pleads for his crewmates to get him out of the airlock while they can only look on in helpless horror. And then the screaming starts.
The face that Smith makes when he realizes the Lewis and Clark, which he had just finished fixing is about to explode. Right as he sees the timer on the bomb, his expression is heart-wrenching.