Science in movies and video-games

The physics of Gravity

01/06/2018 ⋅ 3 comments

Par Timo van Neerden

dr bullock
Gravity is a 2013 thriller film. It stars astronauts on a mission at the Hubble Space Telescope to be fighting for survival while a cloud of space-junk is coming at them.

Even if Nasa (which the movie stars) is a science institution, the movie shows quite a lot of science imprecisions.
Its producer justified itself though that the movie was a science-fiction movie and not a documentary.

But for me, science-fiction is half-science and half-fiction, so let’s have a look at the “science” part!

The Hubble, the ISS and the Explorer space shuttle

At the beginning of the movie, the astronauts are upgrading the Hubble Space Telescope. This is actually not a fiction: some space missions using the space shuttle have happened in the past. Last one took place in 2011 on board of the Atlantis shuttle, during mission HST-SM4.

In the movie, the shuttle is bombarded with debris from a satellite and gets inoperable. The two survivors of the mission, the Dr. Stone and Lt. Kowalsky decide to shelter in the ISS.
This may sound realistic in the first place… The problem is, if this had to happen in real life, that the Hubble is located at 590 km above ground whereas the ISS is only at 350~400 km altitude. The difference is as big as 190 km.

If the ISS was right beneath the Hubble, it would not take so much time to get from the Hubble to the ISS, but this is not the case: difference in altitude also means that the ISS has to go faster to remain on its orbit. Wikipedia tells us that the ISS goes at 27'600 km/h and that the Hubble goes at 25'776 km/h. The difference is not far from 2'000 km/h! That said, if the ISS was beneath the two astronauts at the moment they decided to take shelter in the ISS, the ISS would move faster than a rifle's bullet relative to the two guys… And they had have to catch up for the ISS…

I can easily admit that there is no friction 500 km above the ground, and thus huge accelerations are possible, but obtaining that only with a half-empty jet pack ? This is not realistic at all (yes, even if George Clooney is inside it!).

Not to forget that going from the Hubble to the ISS in a hurry (without worrying whether the ISS is nearby or not) is like going from any random place on earth to another random place on earth: it would be the next door, but also at the other side of the planet. The two satellites are moving relatively to one-another and the chances are that they are not nearby at all, but separated by several thousand kilometers. Here again, one jet-pack could never be enough (at least if it is not Ironmans suite, which has unlimited power…).

Oh, and I won’t even mention the Chinese space station, Shenzhou, which is — very fortunately — also in the same area…

But things get worse…

In zero-gravity: floating hair

If you’re in orbit around the Earth or in free-fall (actually, being in orbit is some kind of free-fall) you won’t be able to feel your own weight… nor will your hair.

So, can you explain to me why the hair of Dr. Stone are weighting downwards as the other stuff is floating around?

bullock hair
In reality, even if the hair is not excessively long hair, we should have something like here.
But we could say that the Dr. Stones hair is just a bit wet or something…

In zero-gravity: floating tears

A bit further in the movie, Dr. Stone sheds a tear, which we then see floating around in the Soyuz capsule:

larmes de Stone dans l’espace
It is a sad moment in the movie, but it is wrong: unless she detached the tear herself, in reality it turns out that tears stick to the skin, or the eye in this case, due to surface tension, the same effect that — on earth this time — makes a tear fall only after it flows right down to your chin, not only on your cheek.

Like these fake tears on a picture taken onboard the ISS by Nasa astronauts:

larmes en vrai dans l’espace
If for any reason, there was no surface tension in zero-g (which might have been possible, for some reason), then the shape of the tear (or any liquid) would have been inconsistent: the spherical shape of the tear, and the fact that all the water molecules stick together is caused by surface tension.

In zero-gravity: the extinguisher

The same as Kowalski used his jet-pack earlier in the film, the Dr. Stone uses a fire-extinguisher in order to propel herself:

dr stone et l’extincteur
This one is real! In space, given the fact there is no air friction (or very little), it is very easy to propel: you only have to throw an object (or even gas) and you get propelled in the opposite direction. It’s the basic action-reaction law.
In this case, knowing the fact that most CO2 extinguishers are filled with liquid CO2 at about 50 atmospheres of pressure, it would give you a huge amount of propelling force.

For this part of the movie, which is quite scientifically accurate, I will only allow myself to criticize (my reason to be here :p) how the Dr. Stone holds the extinguisher. It would, I think, be much easier if she placed the nozzle near here navel: that is where humans have their center of mass, and doing so, she would have avoided the swirling around effect and would have gained in efficiency to go to the space station.

Sound in space

A classical mistake in science-fiction movies: in space, with no-air, there is no sound! If it is a debris that hurts the ISS, gas that is expelled from a jet-pack, you wouldn’t hear a thing.

The astronaut himself might be able to sense vibrations through his jet-pack, but for an external person that would be watching the scene, a jet-pack would be totally siliceous.

It would be the very same for exploding stars: the biggest explosions in the universe are totally quiet (but they would still kill you if you found yourself to close of one of them).

A body exposed to the vacuum of space

Another thing that surprised me a bit: right after the debris shower that hit the Explorer shuttle and the Hubble, one of the astronauts gets killed: a piece of debris hit him and went right through his helmet and his head:

gore : open head
(sorry, it is gore, but it’s for the sake of science ^^)

This image might also not be accurate: if we let a body in space, it won’t expand nor explode, but according to Wikipedia, your blood will begin to lose its oxygen in a matter of seconds. Also, the body fluids would start to boil after some time, making it expand slowly as liquids get turned into gases.
Since the head is open for this poor astronaut, his blood and maybe even his internal organs would be pushed though the outside, leaving us not with a clean hole in his face, but with a mix of organs and a cloud of blood floating around his head. Yummy.

On the ground, the internal pressure of your body compensates the few tons of atmospheric pressure exerting on your skin. In space, there is no atmosphere, so you skin gets only pushed from the inside. Your body is actually mainly composed of incompressible fluids, but the depressurization might be enough to get the liquids starting to boil and expand the skin. This is why the space suits look so inflated: they are inflated, so that the astronauts get some pressure and are able to breathe.

Since this is not yet enough pain, be noticed that the speed of the ISS makes you have a trip around the earth in 90 minutes, so you would get frozen solid and boiling hot every hour and a half, which would also damage the body even more. After some hours or days, the amount of cosmic-rays that would hit you might also be damaging for your body.

Back to the earth: falling into a river?

The end of the movie looks quite epic, but is again quite debatable.
First of all, the Shenzhou capsule where Dr. Stone is in enters in the atmosphere and warms up. This is actually real: even if the temperature at that altitude is only a few kelvin, the speed of the station hitting the molecules of air provoke important chocks and warm the capsule up). Then, the capsule falls into a river. Seriously: “a river” ?

I have no doubts that if the capsule falls on the ground it would be totally destroyed, so there comes the choice of the fall ending in water, but please: 70% of the earth's surface is recovered with the oceans.
If the capsule really had fallen randomly (without previous choice of a location on which to fall on), it would have had 70% to fall into the ocean with thousands of miles from the coasts. Also, half of the 70% would count only for the pacific ocean.

As you can see, the Pacific Ocean recovers about a third of the planet Earth:

google earth screenshot
Okay, it still remains 30% of Earth not being covered with water, so falling on the ground is still 30% realistic. But what about a river?

I live in France. Here, about 0.2% of all the land is covered by lakes and rivers. If I say that this proportion is roughly the same everywhere on Earth, the probability for Dr. Stone to fall in a river is 0.06%. In other words, she would have 99.94% chances not to fall in a river.
I imagine that Dr. Stone is either very lucky (falling in the water to keep the capsule from being destroyed and very close to the ground) or very accurate by choosing the moment to begin her comeback to earth.

Secondly, when astronauts come back from a long mission into space, the long absence of gravity causes their bones to atrophy, and they have to get through weeks or months of reeducation to get their bones to recover. Nasa is even willing to pay volunteers for them to staying laid down for several months, in order to understand what happens and what could be done when your muscles and bones are not solicited.

It is thus highly improbable that the Dr. Stone swims to the beach and stands up her feet like she never left the Earth…


The movies takes almost entirely place in space and in orbit, where there is no effect of the gravity. I agree with Neil deGrasse Tyson: the movie should have been called “zero-gravity”.

To end on a good point that actually triggers the whole plot of the story, the fact that a bunch of debris from one satellite causes other satellites to be destroyed, providing even more debris, is totally possible: this is known as the Kessler Syndrome, named after Nasa's engineer that thought about it thirty years ago.

Debris might be very small indeed, but their speed is enormous: up to 36'000 km/h (~20'000 mph). At these speeds, the total energy of a 20 grams screw has the same amount of kinetic energy than a 500 kilograms car at 200 km/h… Only to say that even a tiny piece of space-junk will destroy everything it gets into. Even a coat made out of Kevlar will not resist, so imagine a Space Station made of metal and electronics…



John Doe05/06/2018 à 08:36:51

It’s producer

. "Its producer".

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Doug Bowker16/06/2018 à 20:32:26

"It is thus highly improbable that the Dr. Stone swims to the beach and stands up her feet like she never left the Earth…" All good points overall, but I'm not sure she was even in space all that long. Her only reason for being on the mission was to fix "her" section of the Hubble (they indicated she had designed a particular experiment). In that light she may have been up there for a few days only.

The river was obviously meant as creative license, no question., The whole idea was her rising out of the African river like the first land dwellers millions of years ago.

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timo20/06/2018 à 16:55:06

@Doug Bowker : Good point for the “few days only”.
I hadn’t thought of her rising like the first time life got out of the waters, that’s brilliant.

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