– The summer of 2013 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 3-D 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.

“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.”

Up until a week before shooting, she was led to believe that the production would be airborne, but Cuarón and company found other means to achieve the appearance of weightlessness.

Stars in the sky

“Gravity” begins with a spectacular uninterrupted 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 3-D space, to unproven digital technologies, to marionette-like cable systems for the stars.

The actors’ challenge was “not only knowing it was going to mean long, long, long, takes,” Cuarón said, “but it was going to be in really uncomfortable positions. Full credit goes to Sandra and George because they gave reality to the shots.”

“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.”

To match the ever-changing light sources from the Earth and sun, cinematographer Emmanuel Lubezki designed a 9-by-14-foot box whose interior was lined with LED bulbs. The box was big enough to hold one actor at a time while projecting shifting light.

“It was the most bizarre series of contraptions,” Bullock said. “Beyond the box, there were the arms that make the cars in Detroit, with the camera on it, which would rush toward you while you’re clamped from the waist down. Then there was the 12-wire system where you’re basically floating, while being manipulated by puppeteers.” She performed on a bicycle seat welded to a pole, balancing with one leg strapped down so her body could be free to simulate weightlessness, and sitting in an office chair on a hydraulic lift. “It was genius what they came up with.”

Lengthy casting process

Because the process of setting up the film was so protracted, potential stars came together and fell apart. Angelina Jolie, Natalie Portman and Robert Downey Jr. all were attached to the project before Bullock and Clooney.

Cuarón met the actress following her Oscar win for “The Blind Side” and her painful split from then-husband Jesse James. 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.’ ”

Bullock spent six months in physical training to prepare for shooting. She underwent a transformation to suggest that her character, who is plagued by memories of a private tragedy, had consciously stripped away her femininity, becoming “a shell of a woman.”

“You’re getting your story not just from what the humans are saying but visually what’s happening,” Bullock said. “What is her background physically, emotionally, how is she in that environment?” Working for much of the shoot in high-tech solitude actually helped her identify with the character, she said. “The technology was helpful because you would feel so frustrated, so lonely, so panicked about the long time you’re attached to something and you can’t use your body the way you’re used to.”

It wasn’t all hardship. Bullock’s star perk on the film set outside London was a play area for her 3-year-old son, Louis. Clooney’s was a tiki bar.

“Both my children knew exactly what they wanted,” she said with a laugh.