(Spoiler)...Why did they film two ending scenes (happy and tragedic ones)?
I read an article that there were special bonus features included on the DVD and Blu-ray, among them an alternate ending scene in which Norah rewires the damaged escape pod and manages to escape before the explosion.
You can see it here:-
https://www.youtube.com/watch?v=IlWWnH7jAr8
Also, there is an allusion in this scene. Norah uses the locket with her fiancee's picture in it to fix the power failure, meaning that, their love saves her.
I do not know why did the producers choose the sad ending, did they think that breaking the viewer's heart, makes the movie more interesting.