Question

In a certain science fiction story written for youngsters, an accident causes an untethered astronaut to float away from his spaceship. Fortunately, he manages to return safely to the ship by making swimming motions with his arms. What is wrong with this? What is the difference between swimming in water and “swimming” in space?

Answer 1

The main problem here, is that swimming action in empty space or really thin air would require a lot of effort to generate hardly any velocity. Water is a viscous fluid and by swimming a person is pushing water away and hence creating a force that is propelling the person forward in the viscous fluid. Whereas in empty space or extremely thin air there is hardly any mass to push away, so an astronaut cannot generate the required force to push oneself forward.

So "swimming" outside a space ship is merely moking the action. It has none of the physical force components that would characterise such an action as swimming.