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Former NASA Engineer Provides 8 Deeply Geeky Reasons Why the Physics Of ‘Gravity’ Are All Wrong

Former NASA Engineer Provides 8 Deeply Geeky Reasons Why the Physics Of 'Gravity' Are All Wrong

Gravity” opened nationwide last week to a wide array of critical praise, but viewers with a knowledge of science were less than enthused. Astrophysicist Neil Degrasse Tyson took to Twitter with a series of missives labeled “Mysteries of #Gravity” assailing the Alfonso Cuaron-directed space adventure for several implausible twists. The New York Times pointed out a key problem involving the vast distance the astronauts manage to travel in a relatively short period of time after their shuttle is destroyed in a hail of debris. Audiences eager to appreciate the movie’s stunning technical accomplishments may howl back, “It’s just a movie!,” but it’s safe to say none of them have doctorates in physics.

So even though Indiewire chief film critic Eric Kohn raved about “Gravity” in his review, we decided it was only fair to represent the other side of the story — by turning to his father, former NASA engineer Wolf Kohn. Currently a private consultant and university professor based in Seattle, Kohn worked as Chief Researcher at Lockheed Corporation at NASA Johnson Space Center from 1981 – 1986. After seeing “Gravity” over the weekend, he picked through some of the movie’s scientific inaccuracies — but pointed out that he still enjoyed the performances.

Warning: This article contains some mild spoilers involving the plot of “Gravity.”

All that debris that wrecks the Shuttle? It’s moving way too fast. The relative velocity of the debris with respect to the shuttle and, later, the space station violates basic principles of orbital mechanics. Objects in the same orbit always have about the same velocity. In “Gravity,” the debris is moving much faster than the objects it hits. When you have debris hitting something, it’s because it comes from a higher orbit to a lower orbit. In “Gravity,” the characters state that every 90 minutes the debris passes by, so the relative velocity of the debris should be small.

The arm of the Shuttle is pointed outward. That’s not possible given the objects attached to it. At the beginning of “Gravity,” Ryan Stone (Sandra Bullock) is standing on the arm of the Shuttle while repairing the Hubble. The mechanical arm, with the Hubble attached to the grapple fixture and the astronaut in the stand, should bend like a fishing pole. In the movie, the arm of the shuttle looks like a vertical pole sticking out of the Shuttle. In the real system, the arm should bend, because it has much less inertia than the body — in this case, the Hubble and the woman attached to it. I helped astronauts train for using the original version of the shuttle’s arm. It’s a big deal because the arm is like a noodle — you can’t have such a big mass attached to it and have it just stick up like a flagpole.  

The open door on the space station would drain the oxygen from the room. Every time somebody enters a vehicle in the movie, the double chamber used for accessing or leaving the vehicle is absent. This is absurd: When Ryan gets into the Soyuz, she shuts the door and takes off her suit. The way that it’s actually done — as you can see in other movies — is that astronauts enter a pre-chamber so they could pressurize the room and then they open the other door. She can’t enter one place, close the door and all of a sudden find herself in an air-based atmosphere. That doesn’t make sense. You can’t open the door and close it; the air would escape immediately.

When Clooney’s character, Matt Kowalski (George Clooney), tethers himself to Ryan, he’s able to stretch out ahead of her and tug her along. That’s impossible. The dynamic behavior of the tethers is incorrect. As I said before, when objects are in the same orbit, they move at the same speed. So you can’t have one astronaut tow another one continuously. This bothered me because I worked with tethers. In zero gravity, they don’t stretch. But you see their tethers stretching all over the place. 

At one point, the space station catches fire — and Ryan escapes with her life. It’s not that easy. The way the fire is shown inside the space station implies a pure oxygen environment. But if that was a pure oxygen environment, she would have passed out right away because there would be too much combustion. The amount of energy created by the fire would have engulfed her in seconds. If you try to light a match under normal circumstances on the space station, the flame will just die out. But remember, this space station in the film was already open, which means there was no oxygen in it at all.

The traveling from one vehicle to another using the backpack jets does not make sense. If you want to get to the space station, and it’s like 100 miles away, you can do it two ways: You can go to a lower orbit, where you’d be moving faster than it (the lower the orbit, the faster the speed). The other possibility is the opposite: You go a little higher than the station so you’re going slower than it, and when the station passes, you can hop onto it. The way it’s shown in the film, you’d require a lot more energy — and the moment you put more gas on the jet pack, you’d switch to another orbit.

The sequence with the fire extinguisher as a propulsion is ridiculous. You can’t control the movement of your body in space with something like this. It doesn’t have enough propulsion power to prevent you from tumbling forward.

Setting all this aside, if something like the inciting incident of the film were to occur, NASA has a contingency plan. If indeed there were something coming from above or below to hit the shuttle, it should have enough jet power to change its inclination and avoid the debris. But that’s a minor point compared with the violations of physics in the film.

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