I watched this again last night, and it’s really an outstanding film. There are some places where the gravity and/or relativity choices don’t quite make sense, but mostly the science is handled very well.
The philosophical aspects are also treated well. Do you put survival of the currently-living members of humanity above the survival of the species as a whole, or not?
The least believable thing, in a film full of challenging ideas, is the fact that our hero has been a pilot for NASA, but has somehow fallen off their radar, when they actually need people like him and have a base within a day’s drive of him.