Let's Spoil Movies Using Science - Gravity (2013)

in #steemstem2 years ago (edited)

Image credit: NASA link
Public domain image.

This post is not about trashing movies. It is about enjoying movies first and then thinking about the science and technology later.

Movies are movies, not science textbooks but it is still fun to understand what they sometimes get wrong.

Gravity is a great movie. I went to see it in 3D and it was one of the few movies where the 3D worked and wasn't just a gratuitous gimmick. Rotten Tomatoes gave it a very high score and it was well deserved. I highly recommend it.


There are spoilers in this post so if you haven't seen the movie it is not safe to read on. After you have seen the movie come back to this post to read about some of the science.

Okay then, let's get started.


Image credit: NASA link
Public domain image.

The Kessler syndrome is a postulated chain reaction effect in which orbital debris can destroy a satellite. Then the debris from that satellite can go on to destroy other satellites. The debris from those other satellites then goes on to destroy even more satellites.

Eventually all satellites at that general orbital height are destroyed. Well that's the theory.

It was tested out by the Chinese in 2007 in a military anti-satellite test in which they destroyed one of their own weather satellites. This satellite was at a height of 865 kilometres, had a mass of 750 kg and was destroyed by a kinetic kill vehicle moving at 8 km/s.

It is estimated that several hundred thousand particles were generated and the debris field continues to threaten satellites near that orbital height to this day. I am not aware of any damage caused by this debris field but then again space is big so maybe the threat of the Kessler syndrome is overblown?


The re-entry scene for the Chinese Tiangong space station and the Soyuz capsule seemed to be pretty accurate. One would expect atmospheric re-entry to heat and weaken the metal and then for the components to fall apart as the metal lost strength.

The lifting body shape of the Shenzhou/Soyuz capsule re-oriented itself automatically due to aerodynamic effects. This might be accurate but I could also see that body shape getting locked into a tumbling spin that never stops. There seemed to be no attitude thrusters helping the capsule achieve its proper orientation so maybe a Shenzhou/Soyuz capsule could just get into the right direction just from aerodynamics alone?

Also, the debris field from the space station overtook and re-entered the atmosphere faster than the capsule. This feels right as the capsule is designed to shed its re-entry speed and energy efficiently and so randomly shaped debris would likely be more streamlined and thus overtake it.


The Shenzhou/Soyuz capsule opens its pilot chute and then its main chute in a very similar manner to Soyuz capsules shown in re-entry videos for ISS astronauts coming back to Earth. Also, just before hitting the Earth the capsule fires its retro thrusters a split second before touchdown as is done with a real Soyuz capsule (this helps soften the impact for the astronauts/cosmonauts).

The only thing missing in Gravity was that these capsules vent their excess fuel after the main parachute opens and well before touchdown. This presumably is to get rid of any hazardous fuel and to keep the crew and recovery ground team safe.



The Hubble Telescope is at an altitude of about 540 kilometres. Communication satellites are typically geosynchronous and at an altitude of about 42,000 kilometres. The debris field would therefore also be at an orbital height of roughly 500 kilometres and there is no way the debris could have taken out the communication with the astronauts.

Also, the astronaut played by Sandra Bullock managed to talk to an Inuit in northern Canada on a ham radio frequency. This indicates that NASA would likely also have been able to maintain communication with the astronauts via redundant communication channels.

Image credit: NASA link
Public domain image.

In the movie, the crew is repairing the Hubble Telescope which has an orbital height of about 540 kilometres. Later Sandra Bullock and George Clooney manage to make it to the International Space Station using the manned maneuvering unit (MMU) which is at an orbital height of about 400 kilometres.

This is completely impossible as the Hubble Telescope and the ISS also have very different orbital inclinations. The delta-v (velocity change) and energy change required is far beyond what any MMU is capable of. Also the precision needed to manage to sync up with another object's orbit is beyond any human's "seat-of-the-pants" flying capability.


The debris field destroys the Hubble Telescope and the Space Shuttle. Then it manages to orbit around and do this again and again every 90 minutes. The orbital period is sort of, maybe, correct but the debris field would also very likely have a different orbital inclination from the Hubble. The astronauts are also in orbit so the re-encounter period would likely be at a different period than in the movie.

So, it is just not likely that the debris would come around and slam them every hour and a half. The debris would destroy things, just more slowly and over a period of months not hours.

Image credit: Wikimedia user:Booyabazooka link
CC BY-SA 3.0 license

At the ISS Sandra Bullock and George Clooney manage to snag onto the ISS at the ends of the parachute suspension lines. They are both weightless and both are not moving relative to the ISS.

The situation is stable. There was no centripetal force. Then Clooney lets go to 'save' Bullock.

Huh? Why? There was no need. She could have just yanked on the line and brought him back towards the ISS and all would have been good.


The destruction of the Space Shuttle killed all of the astronauts except Bullock and Clooney. A quick camera shot of one of the dead astronauts shows them apparently frozen.

They would freeze yes, but it would take time. The only way to shed heat in space is via radiative heat transfer which is fairly slow when you are an object at 37°C (body temperature).

They would be dead yes, but still squishy.

Image credit: NASA link
Public domain image.

When Bullock strips off her astronaut suit in the space station we would have seen her in a bulky cooling garment. Astronauts on space walks also apparently wear adult diapers, you know, in case you got to go during the space walk.

Instead we see her in close fitting undergarments because, well, fit females in tight fitting clothing sell movies.


For the same reason as for the Hubble-to-ISS trip, the ISS-to-Tiangong space station trip is also unlikely. The orbital inclinations and orbital heights would be quite different and not a maneuver a re-entry capsule would be able to achieve. It would take a ground team and some computer time to work on the burn duration and exact burn timing to even have a chance of doing this.

An astronaut simply aiming straight at another space station and firing just won't do it as orbital mechanics doesn't work that way (Kerbal Space fans would know this).

If it were achievable there would have been a 'launch' window in which to fire the thrusters. That launch window, if it even existed, probably would have been days or weeks away.


The general re-entry effects seemed to be accurate. What was not accurate in the movie though was why the Chinese space station was re-entering the atmosphere in the first place?

The Tiangong space station would have had an orbital height of something like 350 kilometres and it would have had a mass of tens of thousands of kilometres.

The particular debris that hit it and caused the damage would have had a mass of a kilogram or less. Probably less.

Space is big, the debris field was very spread out and the amount of debris that would cover any particular area the size of a space station would be quite small.

The debris therefore could have damaged the space station but it would not have enough mass to alter its trajectory in any significant way. The space station would have remained in a stable orbit slowly getting eaten away by the debris over a period of weeks and months.

Plenty of time to plan a re-entry burn in the Shenzhen/Soyuz capsule.


As I said at the start of this post, the movie itself was great and I recommend that you watch it and enjoy it.

However the science is not perfect and it is just as enjoyable knowing what they got wrong and to not let that interfere with the plot and the drama of the overall story.

Thank you for reading my post.


Gravity Re-Entry Scene


I got to hear what astronaut Chris Hadfield thought of Gravity during a presentation he gave last year, it was quite funny! For a start getting in and out of spacesuit is a non-trivial exercise that is a lengthy time consuming process. It takes several days to prepare for a spacewalk for example and he described your dexterity in a suit as akin to being dressed inside a soccerball!

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Ah yes but as you said,what a good movie. The destruction of the ISS in 3d was phenomenal on a big screen.
The movie got a bit of criticism from the scientific community because it did try to get many things right.

One additional issue for example is how Bullock managed to open the airlock from space to get into the station. Apparently the hatch would normally open the other way and would be impossible to open without depressurising the interior first.

I think it's nevertheless one of the best sci-fi movies.

Its been a while since I've seen the movie, but getting between the different space stations (US to Chinese) probably would have been more delta v than she could pull off. I should go watch again, but...every 1 deg of plane change is also 100 m/s dV. An MMU can only do 20 m/s total...sooo...I need to go watch again.

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