What is gravity in Science?

Sure, in the four and a half years it took him to make the film, Cuarón had shifted around various pieces of celestial hardware, and endowed the Chinese with a souped-up space station. And he considered explaining why during the action. But, understandably, Cuarón decided that it was better to keep a tight narrative than stretch the screenplay by 30 pages.

I watched Gravity with Britain’s starriest scientific couple, Brian Cox, who advised Danny Boyle on his sci-fi epic Sunshine, and Gia Milinovich, a seasoned television and new media expert. It must be said that, yes, there are scientific holes in Gravity big enough to fly a Saturn V rocket through. One example: an astronaut in a Manned Maneuvering Unit (MMU, essentially a rocket pack) would not burn his fuel with the gay abandon shown by George Clooney. But from the opening scenes in the silent abyss beyond the Earth’s atmosphere, Gravity provided 90 minutes of unremitting suspense that never lost its momentum. We left the cinema enthralled by the spectacle.

An astronaut tethered to the damaged space station in Gravity (Warner Bros)

Two features made the non-stop thrills of Gravity soar in scientific terms. First, it popularised a scenario that is every space engineer’s worst nightmare, known as the Kessler syndrome after Donald Kessler, formerly at Nasa’s Johnson Space Center in Houston. Back in 1978, Kessler and a colleague proposed that as the number of satellites rose, so would the risk of accidental collisions. Such disasters would spawn clouds of junk, sparking a chain reaction that would swiftly ring the Earth with debris from the thousands of satellites that once monitored our climate, helped us communicate, navigate and more.

The precipitating event in Gravity is not an accident but a Russian missile strike on an obsolete satellite (described, but not shown in the movie) and, in the aftermath of the resulting chain reaction, the film-makers put a lot of effort into the small details of the Space Shuttle, Hubble Space Telescope, and International Space Station thanks to the vast amounts of Nasa photography and technical data that is in the public domain.

Even though it was not perfect, the way the film portrays life in orbit is impressive. Many people mistakenly think that gravity does not exist in space. In fact, the typical orbits for human space flight vary between 120 and 360 miles above Earth’s surface, where the G is anything but zero. If an astronaut on the space station drops an apple it falls just like on Earth. But it doesn’t look like it’s falling because the apple, the astronaut and the space station are falling together – not towards Earth but around it. Objects inside the station float because they’re all falling at the same rate.

Sandra Bullock and George Clooney starring in Gravity (Warner Bros)

One day, no doubt, movies will be shot in the microgravity of space but until then the closest you can get is the gravity-defying parabolas of the “vomit comet”, when a plane climbs and then plummets, causing a few moments of weightlessness. Given Cuarón’s preference for long, elastic shots, Clooney and Bullock would have been left green and reeling if he’d used this approach.