This summer was good for science fiction. Major studios backed such blockbuster entries as “Oblivion,” “Elysium” and “Pacific Rim,” and audiences flocked to see them. But the most ambitious sci-fi entry of the year is a different beast. It’s not futuristic, politically charged, nor concerned with alien invaders.

The 3D space adventure “Gravity,” opening Friday, is a story about spacewalking astronauts stranded in Earth orbit after a barrage of satellite debris destroys their shuttle. It is a technically rigorous, “hard-science” rendering of powerful emotional themes — hope, survival and transcendence.

Sandra Bullock and George Clooney play the shipwrecked astronauts, cast away with few options and dwindling air supplies. Director Alfonso Cuarón (“Harry Potter and the Prisoner of Azkaban,” “Children of Men”) filmed in gliding, fluid takes that evoke universal feelings of anxiety and isolation.

After the film’s debut at the Toronto International Film Festival, Bullock and Cuarón discussed the film’s monumental — and potentially dangerous — technical challenges, the casting process and her fear of flying.

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“I appreciate not being in my comfort zone because that means I’ve gotten as far away as possible,” Bullock said. “It unlocks things that scare you, frustrate you, but it also forces you to dig very deep. This scared me on every single level, primarily because originally it was supposed to be shot in the ‘Vomit Comet,’ ” NASA’s Reduced Gravity Aircraft, a Boeing 727 designed to make plummeting dives that send everyone aboard floating.

That was not a happy prospect for Bullock.

“I’m deathly afraid of flying. It’s one of my greatest fears. I thought it was just time to get over that fear. The whole film is an experiment in what you have that you don’t know you have,” Bullock said.

“Gravity” begins with a spectacular 17-minute shot that puts the viewer alongside the astronauts in orbit 375 miles above Earth. It was a major challenge of choreography that involved stringing up two of the world’s biggest movie stars like puppets dangling from strings.

“The biggest obstacle was the lack of gravity. When you are staging stuff, you are used to working with horizons and weight,” Cuarón said. “And here you didn’t have either. There is no up and there is no down. Everything is in constant motion.”

Designing intricate sequences of movements involving two actors and countless items of space debris was a major factor in the $100 million production’s 4½-year production.

When the film was in the planning stage, Cuarón learned that there was no technology to film the scenes he envisioned with the feel of a NASA space documentary. “We had to invent a new set of tools,” he said, from robotic rigs that would control cameras moving through 3D space, to unproven digital technologies, to marionette-like cable systems for the stars.

“It was certainly something completely new” to shoot in such unorthodox conditions, Bullock said. “More like being part of Cirque du Soleil than what we’d been used to as actors.”

Cuarón met the actress after her Oscar win for “The Blind Side.” What impressed him most, he said, was “how in tune Sandra was with the theme of the film, which is adversity and the possibility of rebirth as an outcome of adversity. I thought, ‘Wow, I have a collaborator.’ ”