A man who is dying pretends to be his brother to go to space because he is dying, he is very clean and cleans up any DNA that he leaves behind and at the end of the film he goes to space.

That sounds like the plot to Gattaca (Gattaca (1997) - IMDb (http://www.imdb.com/title/tt0119177/?ref_=fn_al_tt_5) .)