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by Indiewire
October 10, 2013 10:15 AM
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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.


  • WishIPaidAttention | December 8, 2013 1:45 PMReply

    Wish I Paid Attention in my Science classes from high school to college :(

    i haven't the slightest idea what you guys are arguing about.

  • jeremy rogers | October 19, 2013 7:37 PMReply

    I have no idea who wrote this article listing the opinions of the movie, but you are wrong on many of your points.
    Enjoy the movie, that is what it is.
    Don't try to sound or look smart trolling a piece like this.
    Great movie, really bad article.

  • Mr. X | November 16, 2013 2:58 PM

    It's kind of hard to enjoy a movie about astronauts where physics make less sense than those in Alice's Adventures in Wonderland. I'm pretty knowledgeable about the topic and it ruined the experience for me.

  • will | October 18, 2013 2:46 PMReply

    just fyi, regarding this point: "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."

    watch the movie again. you'll see that the line has slack in it and he only tugs her at the moments where he fires the jets to change course. you can see her get jerked each time the line catches tension.

  • rob | October 14, 2013 1:15 PMReply

    Super piece. As a viewer I very enjoyed the movie. As a physicist I have similar complains as the engineer. BTW the color of the flame suggest a rich oxygent environment

  • samy | October 11, 2013 4:33 PMReply

    Grat piece. I wondered myself on the distance of those jetpacks. I dont recall them being use for anything but "space-walks" in close proximity of the shuttle/station

  • yuri | October 11, 2013 3:57 PMReply

    The sequences with the soyuz rentry are also incorrect. The rentrt requiews a very narrow ngle to avoid meltdown. I totally agree with the first point

  • fred | October 11, 2013 2:30 PMReply

    Loved the movie. But agree with the article in its criticism of the Physics. Particularly the scene with the fire extinguisher as a propulsion device.

  • tom | October 11, 2013 1:15 PMReply

    Way to go Rens.movement in microgravity are very different than in strong gravity. Ron does not understand basic principles and he is arrogant about his ignorance.

  • rens | October 11, 2013 1:07 PMReply

    Ron: I am a former student of Dr Kohn ( I am assuming that he is the Kohn mentioned in the article). So I decided to point out the errors in your statements
    2- if the debris comes from another orbit how does it hits them twice
    3-The arm did no break of it was pull from its stand. It is extremely flexible so having the Hubble attached to it it would bend in the direction of the gravity gradient The Hubble is big mass object.
    4- you are imagining solutions the oxygen in their tanks had enough for two orbits(180 minutes ) because they were out for along time before the debris scenes. so they did not have enough oxygen in their thanks for a realistic double chamber procedure
    5- There is a large body of knowledge about tether dynamics in space. In fact most of the study of their dynamics in space was sponsored by Nasa and and the European Space station. You obviously are enamored with your lack of knowledge.
    6- The propagation of the flame wave in the film has the characteristics of a a flame propagating in a rich oxygen environment . Compare the scenes in the film with an animation offered in the web (if you know how to read basic science articles)
    7- Nasa sites give the orbital state of the Hubble and the space station. Their orbtal angles are different
    8- Yes about the contingency plan.

  • Jon | October 11, 2013 3:55 AMReply

    1. Word on the street is that interests in competing movies for the Oscars are driving this kind of lame criticism. What are they paying you, and have you trolled on Facebook or Twitter lately?
    2. The debris does come from another orbit, which means it could have a higher velocity than the orbit it passes through with the shuttle. It certainly is not possible for anyone to calculate the rate of speed of debris in a movie. Did you use a radar gun or what?
    3. The arm is like a fishing pole and does bend. It's only when damaged that it sticks out and breaks off.
    4. When Ryan gets into the Soyuz it is not apparent if there is a jump cut or not to where she closed the chamber and filled it with O2. It's likely than since this was already done in a previous scene, the editor felt it was unnecessary to show it again. It's like is any movie where you see an actor walk into a building and then cut to them inside a room in the building. Duh.
    5. You can't possibly know the dynamics of tethers in a movie. You have no way to make any kind of measurement as to speed, distance and so on. Where did you go to school? CIT? Stick with the Big Bang Theory. That's more your speed. Movies are beyond your comprehension.
    6. The fire "implies a pure oxygen environment"? Really. And you know this for a fact? The space station was already open? Did you miss the scene where she closed the hatch and went into the station. Does it make sense that she adjusted O2 or whatever (off screen - you know about off screen, right?) as part of her protocol, or that possibly the damage caused a mixture other than normal? Did you not see the fire balls that she missed as she first floated through the station? Do we have to show you every freaking thing she does? And you're a scientist? Or maybe you're autistic. No wonder NASA was defunded.
    7. How do you know for a fact how far the station is and how fast they were traveling in their suits with jet-packs? Since they were on the shuttle, they were already moving at a high rate of speed. How can you assume the space station is moving at the exact same rate? Perhaps it's moving slower. Perhaps after being swung around from the collision they were already moving at a faster rate. You don't have a clue. You're all conjecture.
    8. NASA has a contingency plan? Really? They can instantly change orbit to avoid debris while three crew members are hanging on outside? Can you tell me the next Powerball number while you're at it? At least give me something that has real meaning instead of this conjecture psychic stuff.

  • William | October 11, 2013 3:24 AMReply

    "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. "

    It is rather ludicrous, but I think the point was to show how desperate/creative she had to get. Either way, if you had the fire extinguisher exhaust at center mass, would you still tumble forward? If it was center mass, wouldn't it just move you as you were positioned?

  • William | October 11, 2013 3:22 AMReply

    "But remember, this space station in the film was already open, which means there was no oxygen in it at all. "

    Was it? I thought it was closed, hence her needing to go through the airlock to get in. Once it exploded, then obviously it was open to the vacuum of space, but that was after she was in it and then got into the Soyuz capsule.

  • William | October 11, 2013 3:21 AMReply

    "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."

    I have virtually zero knowledge of how tethers work in space. Don't objects only have the same speed in the same orbit if they have the same initial velocity? Once you change that, wouldn't they move at different velocities? i.e., if I'm orbiting Earth while holding a rocket, let go of the rocket and it ignites its engines, wouldn't it move at a faster velocity than me?

    And why can't you pull anyone in space?

  • William | October 11, 2013 3:19 AMReply

    "This is absurd: When Ryan gets into the Soyuz, she shuts the door and takes off her suit... 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."

    I could be mistaken, but I thought they didn't show just one door. When she gets to the ISS, they show her going through an airlock. When they show her getting into the Soyuz, it's already docked with the ISS and the airlock is already open, which is why she's able to get in it so quickly to escape the fire. So there is no air that "would escape immediately" since the Soyuz and the ISS both had the same atmosphere, right?

  • William | October 11, 2013 3:15 AMReply

    "All that debris that wrecks the Shuttle? It's moving way too fast."

    I'm certainly no space expert, but if something explodes in space, doesn't the debris move outwards from the explosion pretty quickly? I thought the debris was from an explosion resulting from an anti-satellite missile test?

  • Fernand | October 10, 2013 11:58 PMReply

    Great special effects troublesome physics. I sort of got it

  • Jolie | October 10, 2013 9:14 PMReply

    I saw the film and already knew that some of the science, if not all was most likely off. The whole point of the film is one of survival and rebirth from tragedy. Unfortunately, despite these themes, the movie is not that great, but the music is awesome.

  • rena | October 10, 2013 7:06 PMReply

    Great piece. I wonder about the fireextinguisher myself. now I understand the imposibility of the sequence. I am not dissapointed just that now I understand the separation of fact from fiction.

  • troy | October 10, 2013 6:15 PMReply

    What a nice idea. with so many movies now presenting reality with special effects. it is great to get the physical basis (or lack of it) from your site. You gained a new reader.

  • naomi | October 10, 2013 4:38 PMReply

    I want to thank Indiewire for bringing out this discussion . I am a high school science teacher and to day one of my students brought your article to class. It elicited enormous interest in understanding the physics behind this film. Most of my students saw the film. Now they have a context of what is real versus what is artistic license in what they saw.

  • Blujazz | October 10, 2013 4:27 PMReply

    So Gravity wasn't a documentary? :(

  • Paul K | October 10, 2013 11:49 PM

    I often read comments like this, and as a documentary filmmaker I may be mistaken in assuming this was facetious. However, if not, it doesn't interpret documentary accurately, because documentaries can't necessarily claim a greater truth than fiction. It's simply constructed with different elements--with actuality. That may indeed make it more enticing, but not necessarily more true. You can still use actuality to lie, mislead, etc. Most documentaries don't aim to do that, but we've all seen it happen a time or two--maybe even a science documentary.

  • goodman | October 10, 2013 3:11 PMReply

    Great article. The point is not to decrease the artistic value of the film but what we can learn about a real system. I do not believe the intention of Indiewire was to diminish the story value of the film, but to point out some reality checks.

  • Gnostradamus | April 17, 2014 5:12 PM

    It's basically the "ignorance is bliss" mentality. If you see a movie portraying reality but with normal people breathing underwater without masks, or flying around flapping their arms, you'd find it stupid--if you know reality. These people are ignorant about the science so they can't appreciate reality, or the need for a fiction that claims realistic portrayals, to show real science.

    If there wasn't so much praise over, say, the realistic portrayal of silence in space, we may let the criticisms go. But you can't gather positive reviews with realism in one aspect and play fast and loose with everything else, unless you're honest and open about it.

  • DukeD | October 10, 2013 3:03 PMReply

    God, these articles are obnoxious. I feel like every self proclained "brainiac" is trying to flaunt their knowledge by pointing criticisms towards a FICTIONAL film. We get it, a work of fictional art took liberties to better tell its story and express its central themes.The film never declared itself a docudrama. It isn't based on actual events. It took the most realistic approach it could without restricting its narrative...not the first, won't be the last. Can we please move on and simply enjoy it for what it is?

  • Alice | October 10, 2013 4:38 PM

    WALLARD: The point is to aid immersion. And the naivite of the few shouldn't dictate THIS many articles of the exact same nature. It's a boring, reductive and repetitive way for these websites to try and generate pageviews, piggybacking off the movie of the moment-ness of Gravity. I wish these critics could actually employ their critical skills and write something more personal or attempt to write about the visual narratives in the movie in more detail but instead we get variations on the same theme. I get that there are people on furlough but come on!

    Gee I just really hope the geopolitics of Somalia gets the same treatment. Would have been nice if people had been such sticklers for fact and education of the general public when Argo came out too since the whole Iranians are mindless vigilantes angle that movie had is arguably more damaging than the small amount of scientific liberties this movie took

  • wallard | October 10, 2013 3:57 PM

    Then whats the point of Gravity attempting to be realistic at all. And more than a few people think a fictional movie like Gravity is real science.

  • nancy | October 10, 2013 12:50 PMReply

    Great piece Indiewire. The time it takes to pressurize a chamber for entering the shuttle is about 46 minutes.

  • Tom | October 13, 2013 4:33 PM

    Sorry, but you can't have it both ways. If you're willing to accept that "loss" of 45 minutes, than the debris should've come back sooner than it did, among other things. Gravity had a lot of reliance on the expiration of time, so it obviously had to pick and choose what it wanted. I loved the movie, I'm just saying...

  • Jon | October 11, 2013 4:15 AM

    They didn't enter a chamber to the shuttle. It was to the space stations. And, it is acceptable by most humans with a heartbeat to assume time passes in a movie. Otherwise we'd be sitting in our seats for 45 minutes waiting for the chamber to pressurize. You do know what movies are, right?

    Did you see The Right Stuff? In the span of three hours Chuck Yeager volunteers to fly the X-1 breaks the sound barrier, Communists launch Sputnik, Alan Shepard becomes an astronaut, immediately followed by Gus Grissom and John Glenn - ALL IN THREE HOURS!

  • Sharon | October 10, 2013 12:45 PMReply

    I watch a lot of movies for entertainment. I understand Hollywood doesn't get some details and I may mention what I find wouldn't happen in real situations to my friends of whatever when I see a movie but I don't let it keep me from enjoying a movie.

  • ernesto | October 10, 2013 12:25 PMReply

    I agree with the first point of the reviewer. However this is the highly unlike scenario: the collision that create the debris induced an orbit with High eccentricity that have a point (actually two) with the Hubble orbit and moving against the gravity gradient (very unlikely) but that impies that the debris had propulsion to maintain such an unstable orbit. but here is new morsel: the hubble and the space station are in orbits with different inclinations! so if the debris collided with the shuttle, parked in the Hubble orbit, why it hit the space station which is in an orbit with DIFFERENT inclination than the one of The Hubble? Needless to say the movie value is not in the physics it portraits but in the human story.

  • Everybody | October 10, 2013 11:50 AMReply

    Nobody cares because the movie made you feel something. -Love, Everyone

  • Gnostradamus | April 17, 2014 5:14 PM

    Christians always espouse love while, like Jon, showing how much hate they really have against "everyone else".

  • movielovingengineer | October 24, 2013 3:48 PM

    You're right. A bullshit christian metaphor with terrible inaccuracies and worse acting was loved by "all" of us. Speak for your self, I thought the movie was garbage whether it was fictional or not. The only positive was the CG.

  • Jon | October 11, 2013 4:16 AM

    It seems scientists tend to have a problem understanding words like feeling or love.

  • Some of us | October 10, 2013 7:02 PM

    Dear "Everybody", Some of us loved the movie *and* love reading about the scientific inaccuracies. You can enjoy art from multiple (even conflicting) points of view. Try it some time! -Love, Some of us

  • Stevo | October 10, 2013 11:34 AMReply

    "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's exactly what she does. She is in the double room and has to wait until the chamber is pressurized. Not until the oxygen gauge for the room is full, does she remove her helmet!

  • Paul | October 11, 2013 12:03 AM

    Yes, I appreciate the perspective of the comments from the former NASA engineer. However, on this point Stevo is right in pointing out that indeed there was an intermediary chamber. I think in one instance they only showed one door being closed, but if I remember accurately it had already presumed (and not misled) she closed the first door and was out of that intermediary chamber. This may be an instance where our thoughtful commentator misunderstood the conventions of cinema much as Cuaron might misunderstand some of the physics of space!

    Also, many filmmakers do make a considerable effort to pay homage to things like science and history. But some of them do so primarily because they feel it adds to the power of the narrative, so it's not necessarily unreasonable to suggest that Cuaron could have achieved both narrative and scientific enlightenment. Most of the qualms written above could've had a fairly easy fix.

    On another note, maybe it's naive to wish folks wouldn't post such negative comments ridiculing why these articles are written in the first place, but yet I do. All interpretations of various media has its place, and if the perspective does not relate or resonate with you, stop bitching and be mature enough to recognize that it will speak to others with those particular interests. You're clogging up what would otherwise be a thoughtful comments section.

  • Not quite | October 10, 2013 1:07 PM

    It takes 45 minutes to pressurize the chamber.

  • rose | October 10, 2013 12:37 PM

    right on the money Stevo.

  • rob | October 10, 2013 11:02 AMReply

    I agree with the criticism in the article. Like superman, it is nice art not science.

  • Meto | October 10, 2013 10:59 AMReply

    Anyway the satellite collision described in the movie will induce drastic orbit changes

  • NasaBoss | October 10, 2013 10:56 AMReply

    The commenter is wrong . Watch the movie! All the objects are moving in the same direction! The orbiter, the debris and the station. Learn the basics before you comment

  • ress | October 10, 2013 12:42 PM

    The relative high velocities would have to be maintained over several orbits according to the movie. That is a more or less description of a perpetual motion machine!... which I guess it can exist in a movie.

  • Shameem | October 10, 2013 11:21 AM

    I guess you are wrong. .the commenter never aid they are moving in the opposite direction. He just meant that their relative velocities are high. And that means the debris should have been going slower than what is shown in the film. . please do check before you criticise

  • Corvo | October 10, 2013 10:49 AMReply

    Let the backlash begin! But you can't stop Gravity, haters!

  • Gnostradamus | April 17, 2014 5:15 PM

    You seem to be the one hating any criticisms of something you love.

  • Paul | October 11, 2013 12:05 AM

    What I hate is how stupidly people label others as "haters". The writer expressed enjoyment and appreciation for the film, even while pointing out its scientific inaccuracies.

  • Clair | October 10, 2013 10:37 AMReply

    First point is wrong, namely objects in the same orbit can have large relative speed to each other, that is if they orbit in opposite directions. Obviously we can say that is not very likely however it is not impossible. This mistake alone throws big shadow on the rest of the article.