If you plan to see Gravity, and you don’t like spoiled endings, STOP READING NOW.

Now that I’ve put in my required disclaimers, we can talk freely. I saw Gravity last night with some friends, and I loved it. What’s not to love about an epic 3D visual extravaganza set in space? Especially one with the lovely Ms Bullock battling orbital debris.

As you might expect, in the process of creating Ablative Air I’ve done quite a bit of research on orbital debris. I’d say I’m as close to an expert on the subject as one might reasonably claim as a layperson. There were a couple of the things that rang my “scientific credibility” alarm bells.

• Debris wiping out all communication with ground control.  Houston should be able to reach the astronauts for a window at least once every 90 minutes when they’re within visual range with ground antennas. Not to mention that comms sats are usually in geostationary orbit, thousands of kilometers above where the destruction belt was occurring.
• All the stations are a little too conveniently close to each other. You could stretch your imagination for the proximity of the Chinese station and ISS if they’re in nearly identical orbits since one of stations is actually hypothetical. However, the Hubble is orbiting a couple hundred km higher than the ISS in real life, and it’s pretty darn impossible for a jet pack to traverse that distance.  They certainly aren’t going to be in visual range.
• Basketball-size dents showing on the inside of the Chinese station when struck by debris? Being hit by a chunk moving at 11,000km/hour is not going to leave a dent, it’s going to vapourize a massive chunk of the station wall and turn our heroine into a tiny cloud of orbiting ashes.

I choose to excuse Alfonso Cuaró for a few of these liberties he’s taken, because it is a movie, and because there is so much that he does get right.  The zero gravity motion is impeccable. Action and reaction for everything, just like it should be. The visuals of the earth, stars, atmosphere, and aurora are beautiful, and very much in line with the research I’ve done for Ablative Air. And most impressive: the absolute silence of space. It’s something that few other directors have ever dared to try. I love when the escape ship undocks from the exploding space station and everything goes instantly silent.  Cinematically wonderful and absolutely truthful.

I can sympathize with Alfonso. There are lots of little liberties that I’ve taken with Ablative Air too. I struggled throughout development to balance scientific accuracy against the fun factor, just like Alfonso must have. Who wants to make a game/movie that’s scientifically accurate but absolutely zero fun to play/watch?

There’s one thing that I removed from the prototype of Ablative Air that Alfonso built his entire movie around. A runaway debris collision chain, otherwise known as “Kessler syndrome“. When two pieces of debris collide, they create even more debris. And when enough debris exists at a particular altitude, there will be so many collisions that more debris is created than can be cleared by a natural orbital decay. In effect, this becomes an untraversable shell of bullets zipping around the planet. This was first proposed by NASA scientist Donald Kessler in 1978, and it has been verified in simulation models.

I originally did emulate the Kessler syndrome within Ablative Air, exactly as Mr. Kessler predicted.  Not really on purpose–it spontaneously emerged out of my debris simulation. Originally when debris trajectories intersected they would collide, conserving the total mass of the two chunks but fragmenting into many smaller independent pieces. I found in playing this early prototype of the game that once you hit a certain threshold, play became impossible. Collisions were happening continually, and by the time you got rid of one chunk of debris, fifty more would be in its place. I tried putting a cap on the total debris count, or arbitrarily ending the game when the debris density got too high.

Neither of these were very good fixes. It was just too frustrating to play with collision-generated debris. Once you got anywhere near the critical debris density, any space station would be obliterated in a spray of debris within a second by a dense fog of rapidly moving projectiles. (Of course, when Gravity got to this “massive obliteration” stage near the end, it made me recall my own mini-simulations and I broke out into a big grin.)

My solution as implemented in the final game, was to test debris only for a collision with the space station, and no longer test for collisions between any two arbitrary pieces of debris. I really struggled with cutting out debris collisions since I knew it wasn’t being technically accurate, but I made the difficult choice to remove collisions because it made a much more enjoyable game. Sometimes you just have to sacrifice reality for fun.

I didn’t get Kessler syndrome right, but Gravity did. So I can forgive the other places where Gravity goes scientifically soft. I have a renewed well of sympathy for making the hard choices.