eli5: When probes are released into space they start to spin. What gives every object in space this momentum to rotate? Would it be possible to send something to space and release it very cautiously so it won’t get any energy from the releasing spacecraft and the object hence wouldn’t rotate?
In: Physics
Planets rotate because when they form the clouds of matter have a momentum of their own that is maintained when they coalesce into a single body.
As far as small bodies tending to rotate, physically speaking there is nothing preventing you to have a body moving with no spin It is however extremely *unlikely,* just like there is no physical law preventing you to balance a pen on its tip.
Just imagine having a rod perfectly still in space. It will still be subjected to gravitational forces from distant planets or winds of photons coming from distant stars, the likelihood of these forces being balanced, i.e. generating no momentum, is vanishingly small.
For the same token, an astronaut jumping of the ISS may appear to move with no spin at the beginning but if left to drift, poor man, any imbalance in the forces expressed by his/her feet will result in a rotation.
This line of reasoning does not apply only in space but also in robotics. You could ask the best engineers to build you a robot moving in a perfectly straight line but, sooner or later, it will start to drift left or right, possibly due to a tire being more inflated or any other reason.
Active guidance is required to have anything in a stable trajectory, be it on land, water, or space.
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