Monday, October 7, 2013

Gravity: How much of it is wrong, and why that's right

(Warning: Spoilers for "Gravity")

Alfonso Cuaron's magnificent film "Gravity" finally debuted last week.  I've been waiting for this event for years, having heard what he was attempting to do and having been totally blown away by his last film "Children of Men."  I was not disappointed.

Gravity is a masterpiece of film craft in every respect.  The effects are nothing short of breathtaking, the writing is crisp and compelling, and the performances are spot-on.  I did not realize what a brilliant actress Sandra Bullock was.  She really stands out, even though George Clooney is also performing at an extremely high level.

That said, the films physics (at least in terms of orbital mechanics) are completely wrong.  I think Cuaron knew this, chose to do it incorrectly, and that it was the right choice. 

Allow me to explain:

"Gravity" does not take place in zero-gravity, it takes place in free-fall.  Kowalsky and Stone are not in deep space, they are in orbit, where the gods of orbital mechanics make the rules.  They are extremely  odd rules.

In orbit, Earth's gravity is pulling you down constantly, but your forward momentum is pushing you away, at an angle.  The combination of this angular momentum with the gravity produces a balance that keeps you at a steady altitude above the earth and makes you feel and appear to be "weightless".  Even when the astronauts seem relatively motionless, they are hurtling around the earth at very high speed.  You cannot see this because they, and the shuttle, are moving together.

In orbit, your speed is your altitude, and your altitude is your speed.  If you increase your speed you move into a higher orbit, farther from the earth.  Decrease your speed, and the opposite happens, you drop into a lower orbit. The strange part comes from the fact that a higher orbit is a larger circle, and so you will take you longer to circle the earth.  In a lower orbit it takes you less time to complete that circle, because it is smaller.  So if you speed up, you don't increase your rate of orbit, you decrease it.

If you and I are in the same orbit, but you are 100km ahead of me and I want to catch up, I cannot do so by firing my thrusters forward.  If I do, I increase my speed and move myself into a higher orbit, farther from you.  Also, you will begin to creep away from me because you are in a lower orbit than I am.  No, to catch up with you I must fire my thrusters backwards, away from you, so I will slow down, drop into a lower, smaller obit, and then wait until I catch up with you and pass you a little.  Then, a bit ahead of you and below, I fire my thrusters again but this time to speed up, thus rising into your orbit for rendezvous.

This all assumes we are both in a standard equatorial orbit.  If one of us is orbiting at a different angle, then things get really complicated.

Rendezvous in orbit is really, really hard.  A major goal of the entire Gemini program was just to figure this out.  Buzz Aldrin (who landed on the moon with Neil Armstrong) got his PhD in just this problem, which is why many called him "Dr. Rendezvous".

I am sure Cuaron knows this, as he took years to prepare this film.  He also knew that if he filmed the real process of, say, Dr. Stone piloting her Soyuz to the Chinese space station by flying away from it, the visual result would make absolutely no sense to the audience.  Everything in orbit works backwards, and explaining this in a film would make for very dry, pointless exhibition.

This is a great example of the difference between verisimilitude on screen and "realism", which often does not work well as art.  It takes careful consideration to know when do to one versus the other.  All of the "three dimensional skating rink" maneuvers that Stone does while floating in the ISS, for example, are perfectly correct.  That works because the "floating astronaut" something we all understand now in terms of its visual language.

Again, I point this out not to criticize the film or the filmmakers, but to praise it and them.  The film gave me an absolutely real "feel" of being in space, even though I came to it knowing how the mechanics really work.  I was totally invested in the drama.  That's a master at work.


  1. Very interesting, thank you.

    1. My pleasure. I find this stuff compelling, but it's nice to know I'm not the only one. :)

  2. what if you thrust down and forward at the same time? Wouldn't that be the fastest way to catch up, without respect to fuel economy?

  3. If your downward thrust was less than your forward thrust, you'd simply rise more slowly but you'd still rise. If downward was more than forward, you'd sink to a lower orbit, but this is much less efficient that simply thrusting backwards to slow down. In other words, all you'd do in either case is waste fuel to no effect.

  4. Put another way, you cannot "speed up" or "slow down" in a given orbit, because your speed determines your orbit. Remember, also, that you are actually falling at all times, and you cannot fall more slowly or more quickly in a fixed gravity field (per Mr. Galileo).

  5. Thanks for the explanations and insights - most interesting! Also hadn't heard the Dr Rendezvous thing before.

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