George Clooney Replaces Robert Downey, Jr. In 'Gravity' With Sandra Bullock (VIDEO)

They're already stars, and now, George Clooney and Sandra Bullock are headed into space.

The Hollywood Reporter is reporting that Clooney has joined the cast of the upcoming film 'Gravity,' concluding an epic and winding search for the movie's leads. Clooney replaces Robert Downey, Jr., who had been attached to the project for some time.

Clooney joins Sandra Bullock in a film about a space shuttle disaster and its survivors' struggles to get back to earth safely. Bullock and Clooney will both play astronauts, with Bullock billed as the number one lead.

THR chronicles the long, difficult effort in casting the movie -- initially, Angelina Jolie and Downey were supposed to take Bullock and Clooney's roles. Within the last few months, Marion Cotillard, Scarlett Johansson, Blake Lively and Natalie Portman all were considered for the part that eventually went to Bullock.

In 2002's 'Solaris,' Clooney played a psychologist that goes on a space station and receives astronaut training, and he donned an astronaut suit a deleted scene in his hit film 'Up In The Air,' which is featured below.