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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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Comments

Gregory Dearth

The reason there was all this debris in the first place was the russians blew up a satelite with a missile. Being they have a crapload of experience in space, why wouldnt they realize the consequences and not do that? Second, the explosion of aa satelite adds that explosive effect to the existing orbital velocity the doomed sarelite already had. This would increase the velocity of the parts of the satelite blown off by that explosion well above the standard orbiting velocity. So fast moving debris would be possible as the explosion added energy increasing the speed of the parts. With nothing to slow down these expkoded parts, they would continue outward in all directions. Some would undoubtedly end up in the path of the Shuttle. Of course, my biggest issue is the existence of the Shuttle… doesnt NASA lack a Shuttle? Is this supposed to be in the past?

WishIPaidAttention

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

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.

will

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

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

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

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

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

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

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

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

"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

"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

"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

"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

"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

Great special effects troublesome physics. I sort of got it

Jolie

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

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

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

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

So Gravity wasn't a documentary? :(

goodman

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.

DukeD

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?

nancy

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

Sharon

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

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

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

Stevo

"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!

rob

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

Meto

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

NasaBoss

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

Corvo

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

Clair

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.

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