I'm skeptical about Portman in such a role as this. Very skeptical.
First, her frailness is not exactly what one pictures for such a character. (an astronaut)
Second, perhaps Cuaron can motivate her better than Lucas could (one would hope so) - but my impression from the prequels was also that she was more of a "reactor" than an "actor"
. (and by that I mean someone who has a hard time getting into the role without being surrounded by proper sets and other actors in character) But I confess I have not seem all of her work. She clearly can do very well in some parts, but she is not a self-motivated actor. The director has to coax it out of her.