Moving in zero gravity

Suppose there are no objects nearby that the astronaut could push off from. Can he turn in the direction he needs using only the movements of his own limbs? Let's say an astronaut wants to turn 180 degrees to see what’s behind him. If he stood with his feet on the ground, it wouldn’t be difficult. But in space, the movements of his upper and lower body begin to compensate each other: he turns his chest in one direction, and his legs immediately turn in the opposite direction. So as a result, he cannot turn around.

A similar situation can be reproduced under conditions of earth gravity - try to turn around on an office chair or a twist board without pushing off any other object.

Cascadeur is a physics-based animation program. Rig characters consist of many rigid bodies that have mass and tensor of inertia, used by the program to calculate the physical behavior of the characters.

To test how different movements work in zero gravity, we first created animations of these movements, then we applied our physics tools to them and set the gravity value to zero.

An important point in conditions of zero gravity is that the speed of the center of mass cannot change without an outside force. If the character’s center of mass was initially static, it will remain in the same place in space regardless of the movements performed. The character can only revolve around his center of mass!

In the original animation (first), the character is looking behind his back, turning his upper body considerably.

After using physical tools to apply zero gravity to this animation (second), the movement looks a little different. The character doesn’t have the same range of movement and his hips also begin to turn, compensating for the movement of the upper body. At the same time, character’s poses in each frame remain exactly the same as in the original animation!

So how do you turn around in zero gravity using only your own movements? We’ve checked videos of real astronauts and analyzed their movements to find out.

This is a video that we took as a reference for our animation. Our first impression was that the astronaut turns around mostly because of his arms movement. So we made a draft animation with all the poses from the video:

But once we applied our physical tools, the result was disappointing. The animation was nothing like the reference!

The conclusion was that in order for the character to move as in the reference, it is fundamentally important to correctly do the first part of his movement - a turnover to his back. So we had to investigate what and how affects this turnover and recreate these movements in the animation.

To research this, we experimented by fixing and moving various body parts, while checking the effect with our physical tools. Finally, we discovered that we can get the needed result, if the character twists his legs.

Here is an animation in which the upper part of the character remains fixed, while the legs are pulled to the stomach making a circle.

After applying physical tools, the center of mass remains in place, the upper body compensates for the movements of the legs, and the whole character begins to rotate. So we flipped our character on his back using two circular leg movements! The animation now looks like this:

This proves that circular movements of the limbs work well for turning around, but you need to use different types of movement, depending on the axis on which you need to turn.

For rotating around the transversal axis you have to rotate your limbs in a sagittal plane:

Knowing the principles of these movements, you can turn around in zero gravity in any desired direction without any external influence!

You can test it yourself by visiting the ISS or by installing the free beta version of Cascadeur :)