Totally mind-blown by ‘Gravity’ and its real physics

The other weekend I went to see Gravity, in 3D. As we left the cinema there was only one thing I could say to my wife. ‘That was f—-ing AMAZING.’

Gemini 7 from Gemini 7, 15 December 1965. NASA, public domain, via Wikipedia.

Gemini 7 from Gemini 6, 15 December 1965. NASA, public domain, via Wikipedia.

I use that word a lot, just not usually on this blog. But the intensifier’s apt. After weeks slating dumb movie physics (and more to come) I was gob-smacked. Alfonso Cuaron, Kevin Grazier and the team made a massive effort to reproduce free fall. Free fall? Absolutely. It’s not ‘weightlessness’, and the astronauts are not ‘beyond the pull of Earth’s gravity’. Not in low orbit. It’s ‘free fall’ – as in falling and missing the ground. That’s what orbiting is. Seriously. Wanna see the math?

They should also have called the movie ‘Conservation of Angular Momentum’, because that’s what the physics were – everything spun…and kept spinning, because there was no force to stop the spin… Coool.

Agena target vehicle photographed from Gemini 11. Public domain, NASA, via Wikipedia.

Agena target vehicle from Gemini 11. Public domain, NASA, via Wikipedia.

Then there was the gorgeous imagery. I felt like I’d been transported back to the National Geographics I read as a kid – wonderful glossy Kodachromes of Gemini missions with Earth floating blue and magical behind them.

All that’s overlaid by the story –  an edge-of-the-seat tension drama.  Sandra Bullock’s character was tremendous. Ever wondered what you might do in a life-or-death situation? Go to pieces? Or decide to do whatever has to be done to stay alive – even if it means dying in the attempt? She took us on that decision and journey. Damn it was good!

I think the minor reality compromises the makers made to tell the story didn’t compromise suspension of disbelief. This was an awesome movie.

Of course, I’m going to list a few of those compromises – along with ways the movie also showed up the reality. But that includes spoilers – so if you haven’t seen the movie, go see it – trust me, you WANT to see this one! Then read the rest of this post.

I’ll separate the spoilers with this photo of the real thing – which is what the movie looked like. You can see my house, adjacent to the lower box on the aerial.

Spacewalk to assemble the ISS, 12 December 2006. New Zealand is below - North Island to the right, South to the left. My house is directly under the aerial centre-frame. Photo: NASA, public domain, via Wikipedia.

Spacewalk to assemble the ISS, 12 December 2006. New Zealand is below – North Island to the right, South to the left. Photo: NASA, public domain, via Wikipedia.

So – the compromises. [Spoiler Alert!] There was the scene where Bullock had to let Clooney drift. In fact they were already stopped; she had merely to tug on the cable with her pinkie finger, and he’d have drifted slowly towards her. Then there’s flying from one satellite to another using only an MMU (Manned Manoeuvering Unit) back-pack, which was flown for real in 1984 and then retired.  Hubble and the International Space Station (ISS) orbit in different planes - different angles relative to the equator – which takes a lot of energy to alter. Not possible even with a fully fuelled MMU, which has velocity-change capability (‘delta-V’) of 24.4 metres per second. There are also the physics of rendezvous in space, which are counter-intuitive – you don’t fly from place to place like the movie. I believe the original expert on how it’s done for real is Buzz Aldrin.

[Spoiler alert!] Spacesuits are harder to take on and off than portrayed, and the underwear isn’t lycra – it’s nappies (diapers), cotton long johns, and a liquid-cooling garment. Here’s a video. Actual donning time is about 20 minutes. There’s a pressure differential; the Extra Vehicular Mobility Unit (EMU) – NASA’s term for the spacesuit, which lacks any propulsion system – is pressurised to 4.3 PSI (222.37 torr), whereas the ISS operates at 14.7 PSI (760.2 torr). This means the astronauts have to spend four hours pre-breathing oxygen to avoid dysbarism – ‘the bends’, before a space-walk. Total time in the suit might be ten hours or more. That’s why the engineers insist on the nappy.

Gemini astronaut space-walking. Public domain, NASA, via Wikipedia.

Gemini astronaut space-walking. Public domain, NASA, via Wikipedia.

[Spoiler alert!] People don’t snap-freeze in vacuum. It is a brilliant insulator and heat is lost through radiation, not conduction. That’s why thermos flasks work. In shadow at Earth’s orbital distance it’s over 160 degrees C below, but that doesn’t alter the physics. The human body is more than half water, which has high thermal energy storage capacity (334 million joules per cubic metre). You’d freeze solid providing you stayed in that shadow, but not in minutes. Or hours…

And now the realities. Get this – a lot of the mayhem they showed has actually happened before, mostly to Soviet spacecraft. [Spoiler alerts!]

* Re-entry while tumbling. The Soyuz 1 mission tumbled into the atmosphere after failure of the control systems on the maiden flight of the Soyuz 7K-OK spacecraft, 24 April 1967. The sole cosmonaut, Vladimir Komarov, never gave up – he was a fantastic pilot, and managed to control the entry in the end. But the parachute release door was damaged. Komarov released the reserve chute, but it tangled with the drag chute and he was killed when Soyuz 1 slammed into the ground near Orenburg.

Soyuz TMA. Public domain, via Wikipedia.

Soyuz TMA. Public domain, via Wikipedia.

* Re-entry without PAO separation (Priborno-agregatniy otsek = service module). Only the SA re-entry module (‘B’ in the diagram) returns to Earth, base-end first; the other modules are jettisoned before re-entry. During the mission of Soyuz 5, 18 January 1969 – the orbital module (‘A’) jettisoned normally, but the PAO (‘C’) did not separate, causing the re-entry module to hit the atmosphere nose first with the PAO behind it. Oops. Luckily the bolts burned through and the PAO broke away, allowing the re-entry module to spin and present the heat shield to the atmosphere. It happened again with Soyuz TMA-10 on 21 October 2007, and AGAIN with Soyuz TMA-11 on 19 April 2008.

* Fire in orbit. On board Mir space station, 23 February 1997. It took 14 minutes to extinguish.

* Collision between spaceship and space station solar panel. Mir again, 25 June 1997 – the Progress M-30 freighter mowed through a solar panel on the Spektr module, colliding with the module and puncturing it.

* Spacecraft sinks on splash-down and the astronaut’s spacesuit fills with water. Happened to Gus Grissom on 21 July 1961 with Mercury-Redstone 4.

Damn, Gravity was a good movie!

Have you seen Gravity yet? What did you think of it?

Copyright © Matthew Wright 2013

10 comments on “Totally mind-blown by ‘Gravity’ and its real physics

  1. The trailers look amazing and I am a sucker for a good space movie. I’ll let you know what i think after I get to it.

  2. jjspina says:

    Out of this world fantastic! I loved the movie! Couldn’t move in my seat or take my eyes off the screen except when I felt a little dizzy. It was a dizzying movie though but one of the best of the year so far! Sandra Bullock did an incredible and believable job of her role. I don’t know how she didn’t get sick with all that tumbling they made her do. Maybe she did! Or maybe it was all done in a way that she really never moved at all just the background did. I want to see it again or buy it when it comes on DVD.

    • I think she was stationary and they rotated the lighting while filming her against a green screen. Apparently it was pretty closely choreographed. The whole thing was just SO well done – for me, up there in the league of “2001″ and “Apollo 13″ in terms of trying to emphasise the “realism”.

  3. Unfortunately my plans to see it this weekend fell through, so I haven’t been yet. The spoiler-free parts of your post make me look forward to seeing it even more, though.

  4. Siobhán says:

    Waiting to see it, but still have 2 or 3 weeks before it reaches the UK.

  5. susielindau says:

    I’ve heard so many great things about this movie. I will have to see it soon!
    Thanks for bringing it to the party and for the physics lesson. Now I can be completely sucked into the plot.
    I hope you have fun clicking on links and saying heloooooOOOoooo to the guests. It’s never too late!

  6. KokkieH says:

    I finally got to see Gravity tonight (rented the DVD) and I finally got to read this post as well. It was awesome. I can’t point to a single aspect of the movie I didn’t enjoy. That opening scene totally floored me – the sheer difference in scale…

    You know, that scene where she had to let him go bothered me. I was wondering why he’d keep wanting to move when he was already standing still. I’m glad to have my suspicion confirmed that they tweaked the physics there somewhat. But the rest. Wow!

    (P.S. No comment on flying through space with a fire extinguisher WALL-E style?)

    • That movie deserves all the awards it’s fielded and more. An awesome experience to watch it (if a bit ‘spinny’)…Yeah, the bit where Bullock had to let Clooney got was a bit hokey – he’d stopped and it would only have taken a tug with a pinky finger to pull them both back towards the space station. The fire extinguisher rocket was brilliant.

      Apparently a good part of the Gemini missions in the 1960s involved finding out how to do all this for real – Ed White made it look easy in the initial effort but after that a couiple of astronauts almost couldn’t get back into the cockpit after flailing around. They finally worked out the techniques. Michael Collins’ autobiography covered it off – they ended up training the guys via a kind of floor polisher device which they could stand on. It was raised off a smooth skating-rink style floor with air jets, and off they went, trying to figure out how to stop spinning before doing it for real in space.

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