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 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 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. 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.

[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.

* 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

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