Gravity Sandra Bullock, George Clooney. Directed by Alfonso Cuarón. Rated PG-13. Opens Friday.

Hollywood has given us so many futuristic and action-packed space-set movies that it’s easy to forget how dangerous and unforgiving space is for the people who actually travel there. Alfonso Cuarón’s Gravity brings that danger home powerfully, with the story of two astronauts (Sandra Bullock, George Clooney) who end up stranded after their space shuttle is damaged by flying debris. There are no aliens, no gun battles, no explosions (well, a few explosions), just two people trying desperately to survive in impossible conditions.

As a story of survival, Gravity is nearly unparalleled. From the moment that Bullock’s Dr. Ryan Stone and Clooney’s Lt. Matt Kowalski receive an urgent message that debris from a destroyed satellite is headed their way at high speed, Cuarón keeps the suspense at a breathless level for nearly the entire running time. The amazing special effects, along with the ever-roving camera of cinematographer Emmanuel Lubezki, convey the immense emptiness of the characters’ environment, and every bid for safety presents new challenges and dangers.

Unlike, say, Apollo 13 (which gets a nod in the form of Ed Harris as the distant voice of mission control back on Earth), Gravity doesn’t have the comfort of a real-life happy ending to mitigate the suspense. There’s no reason to believe that Stone or Kowalski will make it through their ordeal alive, and Bullock and Clooney (whose role is actually much smaller than previews indicate) effectively express the desperation and determination that come along with such dire circumstances.

Unfortunately they’re also saddled with some incredibly sappy backstory-illuminating speeches that threaten to undermine the movie’s intensity. There’s one scene in particular toward the end of the movie that’s nearly unforgivable in its cheap sentimentality, and the score from Steven Price, so essential to the scenes of chaotic danger (Cuarón sticks to the realism of space being soundless, which means the score is all the audience hears), turns corny and overwrought when the characters reveal their emotional baggage. There’s so much visceral power in the fight for survival that it doesn’t need a typical Hollywood redemption arc. As long as Cuarón sticks to the danger and suspense, Gravity is a masterpiece.