Gravity: I really wanted to be head over heels

By Guest Author 14/10/2013


by Paul Behrens

The new mega blockbuster is not shy of busting through a few laws of physics.

Screen Shot 2013-10-14 at 10.03.35 AMThe new, space-based action film Gravity was released in New Zealand last week to rave reviews. Starring Sandra Bullock and George Clooney the film follows the trials of two astronauts after they are cast adrift in orbit when attempting to upgrade the Hubble space satellite.

The film is visually stunning, and definitely delivers on that front. But I found the ‘Benny Hill in space’ plot draining and the shoehorned personal story clichéd (She’s a female medical doctor/astronaut, but she’s human too: so her personal trauma related to a child). The exposition devices are also pretty clumsy, and Sandra sure talks some Bullocks.

Sci-fi audiences are some of the friendliest, most forgiving audiences ever. But perhaps we’re too forgiving. Take historical films. They don’t get a free ride if the script is poor AND the events are inaccurate.

Lets face facts

I won’t quibble about some of the many smaller factual inaccuracies of the film. I won’t go into the fact that communication satellites orbit much higher than the International Space Station (330 km for the ISS rather than around 36,000 km for geostationary orbits), or that the Chinese space station is not yet in orbit, or that objects 100 miles away still look 100 miles away, even when you’re in space. By the way, as a fun example, I’ve outlined how you can calculate geostationary orbits below.

Art should be allowed in space, and so should artistic license. So I can happily gravitationally suspend my disbelief (People forgave the historical inaccuracies in Argo, except maybe in New Zealand). But for a film named Gravity, it doesn’t seem to know much about gravity, not to mention gravitas. In several scenes, the astronauts travel from object to object in orbit, once by way of jet pack and once in a capsule. In orbit, every object is travelling at a specific speed related to its distance from the Earth’s surface. Each object must be travelling fast enough to keep ‘missing’ Earth for its distance. (Remember that you are weightless in orbit not because there is no gravity, but because you are in free fall.) So to travel from one object to another (one orbit to another), you must adjust your velocity in a very specific way to travel the distance between orbits and to then match the velocity of your target. For this to happen in the film, we need to forget that none of the objects they travel to would be close to each other, that the jetpack would not have enough thrust to make those maneuvers, and more importantly, to ignore the fact that the astronauts never match the velocity of the target, they just cruise on up and grab hold.

Another moment where the physics goes awry is when Bullock and Clooney, attached by tether and holding onto a space station, experience a force akin to hanging off a cliff edge. Clooney is being pulled away, but we know from Newton’s laws that if they are holding onto the station in orbit they are all travelling at the same velocity and all forces should be balanced (i.e. the centripetal and gravitational forces).

So the physics here are more like Superman than Spaceman. Exploring some of the other issues would require a quite a bit of plot spoiling so I’ll leave the physics alone.

I dont know much about History

I think that we should be pleased that this is a film that bucks the trend of superhero films and re-releases, and in that way we should be very happy people are enjoying it so much. However, the critical response is interesting. I don’t think there are many people out there who would say the film had a fantastic script, or that it was full of engaging characters, yet this is usually a requirement for plaudits.

To me, the need for scientific accuracy in films (especially sci-fi films that are going for realism) is like the need for historical accuracy in films that seem historical. Accuracy is not always necessary, but it’s nice! Films like Argo and Lincoln get away with a bit of fact fudging because they have reasonably good scripts and engaging characters. However, in these films, the fact fudging reached national attention, even though they were generally well-received films.

It’s interesting to see Gravity given so much leeway on its plot, character development and script problems, seemingly because the visuals and the action are so engaging. But if the action is flawed and based in superhero land, and the characters are barely 2D—even while wearing 3D glasses to help them along—are the visuals alone enough?

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You can calculate the distance of orbit for geostationary satellites by using Newton’s second law in the form of this equation:

Screen Shot 2013-10-14 at 10.01.27 AM

 

 

 

Where r is the distance from the centre of the Earth, G is the gravitational constant, M is the mass of the Earth, and T is time it takes for the satellite to make one orbit.

The definition of a geostationary orbit means that a satellite has to be directly above the same point on Earth for the whole orbit. The time it takes the satellite to make one full orbit must be the same time it takes the Earth to rotate once on its axis. So T equals the number of seconds needed for one full rotation of the earth, the three values we need are:

G = 6.67 x 10-11 m3 kg-1 s-2

M = 5.98 x 1024 kg

T = 60 x 60 x 24 seconds (not totally accurate, but accurate enough).

So we find a distance of approximately 42,000 km… but hang on a second – didn’t I say it was 38,000 km before? Well, 42,000km is the distance to the centre of the Earth, we now subtract 6,000 km, the radius of the Earth to get the orbit distance above the surface.

Dr Paul Behrens is an independent research contractor, lecturer, and science communicator based in Wellington


0 Responses to “Gravity: I really wanted to be head over heels”

  • Why didn’t you post that review yesterday Paul…!
    We went to see it last night, this movie is (well quite unsurprinsingly) disappointing in so many ways. The huge physics incoherences (as you pointed) are terrible. That put me off nearly from the get go of the movie.
    Your review is accurate but you missed one thing. This movie is perfect material for a basic Physics class such as Physics 102 here at Auckland Uni. I have my next assignement sorted: “Watch Gravity and find at least 20 physics mistakes/incoherences”.

  • Sorry Claude! It is a great movie for 102 – could watch and critique. Then you could try and work out if there are any ways to make things work by waving hands.

  • Thanks Darcy. It’s very interesting to see some of his responses about the film. Not so many Physics questions there, which is also interesting.

    Also, what an awesome job – It must be so much fun! (except when people then write reviews about the physics in the films you’ve worked on!)

  • Hello Paul! I just came home from watching this movie – I didn’t read your article beforehand as I wanted to see the movie first, but typed a little list of the more glaring inaccuracies I spotted before coming here, just to compare heh.

    As you said, it was (sadly) not realistic at all…I really wanted to ignore that and enjoy the movie, but couldn’t quite manage it.

    Things that bothered me (spoiler alert, I suppose) in addition to the huge ones Paul already explained so well above were, to name a few:
    – Hubble, ISS and the Chinese station are all on exactly the same orbit and very very close to each other (essentially station keeping 100 miles from each other). That doesn’t sound like a good plan…even without talking about how expensive it would be to launch a space station from China to the same orbit as the Baikonur-Florida built ISS.
    -How does the debris make it’s comeback every few minutes?? The only scenario I can think of would be if the debris cloud was on exactly the same altitude orbit but going retrograde (180 degrees inclination difference)…hardly likely? Also space station circular orbits are very different to highly eccentric spy/comms satellite molniya orbits…but I guess that’s a minor detail.
    -Chinese space station is already entering atmosphere when Sandra gets there. Somehow we got from a nice stable 300-400km orbit to way under Karman line in an orbit or two? If they abandoned ISS today and just left it there, wouldn’t it take months, maybe years for it to drop off the sky due to orbital decay alone?