Monolith149 Daily

Another place to see what KG is doing...


I just watched Gravity this morning. It was very entertaining and I liked the movie.

Congratulations are due for executing the physics of objects’ motions and the look of a so-called “zero gravity” (in orbit) environment so well. It was all believable. I can’t believe they didn’t shuttle the crew up to the ISS and shoot it on site, it was realistic enough.

The behaviour of tethered objects and such were pretty real.

However, there were some technical points to complain about. Here’s a quick run down. And, yes, there are lots of spoilers here.

Image: NASA Image. S116-E-06645 (16 Dec. 2006)—Astronaut Robert L. Curbeam, Jr., STS-116 mission specialist, participates in the mission’s third planned session of extravehicular activity (EVA) as construction resumes on the International Space Station.


A better title for the movie would have been “Momentum.” Once they got past the basics of orbital life and space, the physics wasn’t handled very well in some places. For example, you don’t need a certain amount of thrust to get to another object. The amount of thrust results in a speed which only determines how long it will take to get there.

Getting to the Chinese space station could be done with just thrusts from the maneuvering thrusters. Granted, it could take a while to get there and there was the race against the orbiting debris (but see below) and running out of oxygen. If something is far away in the orbital environment of space, you don’t need a bigger engine the farther away it is, unless you want to go faster and get there sooner. There is an argument to be made for having a lot of fuel for maneuvering because rendezvous is hard.

The Worst Flaw

And while talking about momentum, we may as well get to what I consider the worst flaw in the movie. It occurs when Sandra Bullock’s and George Clooney’s characters let go of each other and Clooney floats off into the distance. At that point neither of them is moving. They’ve both already stopped (at least in their orbiting frame of reference). In reality, when they let go he’d just hang there.

Also, just the slightest tug would have started him moving back toward the space station. Similarly, if she could have executed a slight tug on the parachute lines, she could have started herself or both of them headed back, even if at a very slow speed.

The real problem to be solved at that moment was to make sure she didn’t lose the parachute line by her foot slipping off of it, or whatever was her current situation, and also not losing their connection to each othere.

Astronaut Behaviour

I don’t think a real astronaut would be as panicked or apparently as untrained as Sandra Bullock’s character. In real life, an astronaut would be methodical, matter of fact and execute previously practiced plans. They would be more knowledgeable and resourceful. They would know where the buttons were though they would probably follow a manual. They would think through the problem and execute.

In spite of what we might have in the near future, we don’t take passengers into space yet. Mission specialists are trained as astronauts for many months.

Helmet and Space Suit

Why, in an emergency situation, in a compromised spacecraft, and with a threat of further breaching, would you ever take your space suit and helmet off? I can buy that maybe you would have trouble getting to some parts of a space station or craft, but a trained astronaut wouldn’t remove the suit until it was absolutely necessary and then only as long as required.

Similarly, they would keep the helmet on when in the smaller craft and they certainly wouldn’t leave it bouncing around in the cabin.

Use The Fire Extinguisher

A fire in the cabin would quickly use up oxygen. No one, astronaut or not, would allow a fire to burn without using a fire extinguisher on it. Even if you were seconds away from splashing into the ocean.


Again, an astronaut would be trained well enough to execute a landing. They wouldn’t open the hatch too quickly. They would have a flotation device ready. They’d know and follow the procedure. They would use the radio to reply to the broadcasts that were, in English!, trying to make contact.

They would probably have a survival pack including a radio, marker dye and/or strobe and other provisions.

I can’t imagine they would blow the hatch, sink the capsule and end up having to swim with nothing to a convenient shore.

Chute Ejection

One problem was the Soyuz being bound to the ISS by it’s parachute. I’m pretty sure the parachute can be ejected with explosive bolts at landing. I don’t think she’d have had to unbolt it by hand, but could arm and fire the bolts by pushing a button.

Collision with Debris

An accident and collisions are not unreal, but space is big. Really, really big. The chances of such a debris cluster hitting a relatively microscopic target, even the size of the International Space Station, are astronomically small. The debris cloud itself would be expanding and getting bigger and bigger. You might impact a single bolt or piece of junk, and it could be fatal. However, I don’t think you’d have a cloud concentrated enough to stay that close together by the time it got to the ISS even if their orbits did intersect.

Rendezvousing with another space craft on purpose is extremely hard. That much random junk doing it by accident is unrealistic.

It’s also true that a typical orbit at that altitude, I think, has a period of about 90 minutes. However, if you rendezvous with something at the same altitude, it’s going to take anything but 90 minutes. If two bodies were going in opposite directions it would take at most about 45 minutes. If they were going in the same direction but at slightly different speeds, then it would take a really long time, much longer than the 90 minutes. And, if they are going in the same direction, the speed of impact is much lower, just the difference in their speeds.