Just a few days ago, we reported that Natalie Portman (Black Swan, THOR) had been offered the lead role, opposite Robert Downey Jr. (Due Date, Untitled Sherlock Holmes Sequel) in Alfonso Cuarón‘s (Children of Men) new sci-fi project Gravity.

Now, Deadline reports that Portman has passed on the lead role in the film written by Alfonso Cuarón and his son, Jonas Cuarón. Portman’s reps are citing scheduling conflicts as the reason for her passing on the role, and insiders tell them that “Warner Bros. is making a full court press for Sandra Bullock to play the role.”

Gravity stars Robert Downey Jr. as the leader of a team posted at a space station. While he and a female colleague (possibly Bullock) are traveling outside the space station, the other team members are decimated by debris from an exploded satellite.

Gravity is slated to hit theaters in 2011.