Wednesday, April 2, 2014

Movie Review: Gravity

Last night I watched Gravity on Blu-ray.  I'd like to start by saying this is a visually stunning movie and gives one a feeling of being in space, in orbit about the Earth.  The performances by George Clooney and Sandra Bullock are very good, especially Bullock as you feel her fear, pathos, and final triumph.

But, while I won't recount every scientific and factual error in the movie (there's plenty of lists on the internet), I would like to say I have no idea why this movie was called Gravity.  Except for the last few scenes, the filmmakers ignored gravity completely.

People think there's no gravity in space.  If that were true then planets wouldn't orbit the sun and satellites (and space shuttles) wouldn't orbit the Earth.  Nor would the Moon.  There's lots of gravity in space and unless you're in the deep interstellar area between stars, there's enough you have to worry about it.  That means when you are in orbit about the Earth you have to deal with Earth's gravitational field.

The space shuttle and the astronauts aboard it are being affected by gravity.  To say they aren't is to say a man jumping off the Empire State Building isn't being affected by gravity.  Both that suicidal man and the astronauts are in "free fall."  That is because an orbit is sort of like Douglas Adams' description of flying: throw yourself at the ground and miss.  An object in orbit about the Earth is falling toward the surface of the Earth with whatever acceleration gravity gives it at its altitude.  But it's forward speed is such that as it falls one meter toward the surface, the surface of the Earth is one meter farther away due to the curvature of the Earth.  The object is falling toward the Earth but never hits it because of the curvature of the Earth.  (This is true for all orbits including the Earth's orbit around the Sun.)

In the movie the astronauts are working on the Hubble Space Telescope (HST) which orbits at 559 kilometers.  If you built a ladder to the altitude the the HST orbits at, you would be able to stand on it.  In fact, the gravity you would feel would be about 85% of the gravity you feel at the surface of the Earth (8.29 meters per second squared on the ladder versus 9.81 meters per second squared on the surface).  And the astronauts would fly by, floating around the cabin of the shuttle, because they are in free fall, but also in orbit.  So, yes, Virginia, there is gravity in space.  Which the filmmakers pretty much ignore.

In the movie there are scenes where astronauts move a long distance between points in orbit.  But they completely ignore orbital mechanics as if there's no gravity.  George Clooney's character uses a Manned Maneuvering Unit (MMU) to rendezvous with a distance object.  He sort of aims himself at it and accelerates (at least he didn't accelerate the whole way).  But in orbit, in order to rendezvous with something, you have to change your orbit to either speed up or slow down your velocity, then change your orbit again to match the orbit of the object you're trying to reach.  If Clooney accelerated so that he slowed down (i.e., accelerated in the direction opposite his velocity vector), this would put him in a lower orbit and his orbital velocity would increase (yes, this is counter-intuitive).  If he accelerated so that he sped up, this would put him in a higher orbit, he would slow down, and maybe the object he was trying to reach would catch up with him.  All of this was ignored in the movie as if there were no gravity.  And that was the biggest problem I had with the movie.

And then there's a scene that makes zero scientific sense when some something pulls George Clooney's character away when he had zero velocity relative to his objective and therefore had zero momentum (because momentum is mass multiplied by velocity and his velocity was zero) yet when he unhooks himself, he flies away as if some unseen force was pulling on him.  And since he moves away from the Earth, it wouldn't be gravity.

Plus I don't think an MMU has as much fuel as portrayed in the film.  Also, I really doubt NASA would let an astronaut zip around the shuttle, the Hubble Space Telescope, and other astronauts as Clooney is shown doing at the beginning of the movie.  Even if it was visually fun to watch.

But, with all that long dissertation, it was a good movie.  With better science, it could have been a great movie.


  1. Also, it was deathly boring. But yeah, science. It works for a reason. :)

    1. I didn't find it boring probably because I really really really would love to be orbiting the Earth. Either that or I was too busy correcting the science to notice it was boring.

    2. S, I will put your name in the hat for the next blast-off to the moon.
      I do want to say though, this blog is very well-written.
      Author Judith Ann McDowell

  2. Good description of orbital mechanics. The film makers did not quite grasp the concept.