And so the rumoured casting carousel for Alfonso Cuaron’s epic sci-fi drama Gravity squeaks round to another possibility. Hot on the heels of Natalie Portman getting an offer to star in the film comes word that Sandra Bullock is being lined up as a possible choice if Portman should pass on the part.

And, according to Cinema Blend’s unnamed sources (which is why we’re treating this news strictly as a rumour for now), Bullock is actually somewhat of a controversial choice, in terms of the fact that the studio would prefer her to star instead of Portman.

Because while Portman has Cuaron’s backing, and he managed to convince Warner Bros. executives to make her the first choice after she got rave reviews for her work in Black Swan and by offering to shave a little off the budget, the studio is after someone of Bullock’s star stature, worried that an expensive effects film will need a big, recognisable name to justify its expense and carry the fact that it’s largely a one-hander.

Either star probably has the acting chops to carry off a demanding part that would see her as an astronaut stranded in space when some debris kills most of her colleagues on their orbiting station, but it remains to be seen whether Portman's current buzz or Bullock's recent Oscar will win out.

Robert Downey Jr has already filled the only other role of note, and both Cuaron and Warners are hoping that the female lead can be locked down soon, since the actor only has so much time to spare in his busy schedule.

We’ll know soon enough who has actually scored the all-important central role. Either that, or Gravity will drift off into limbo like Frank Poole in 2001…