Newton's Third Law in space

Question

Newton's Third Law states that forces occur in pairs of equal magnitude acting in opposite directions on opposing bodies. When we push against a wall, the wall pushes back with a force of equal magnitude and we move as a result of not being inert enough. What would the force pair be if an astronaut pushes against nothingness in space? Is the reaction pair within the body, and the fist accelerates orders of magnitude more noticeable than the body due to difference in inertia?

Answer 1

As already pointed out, pushing against "nothingness" is meaningless (no pun intended).

But the astronaut can perform various motions with his/her body like swinging or extending the arms and legs where different parts of the body exert equal and opposite forces on other parts of the body per Newton's 3rd law. In this case the equal and opposite force pairs are all internal to the body.

Since there are no net external forces on the astronaut (assuming negligible gravity forces) there can be no change in translational motion (acceleration) of the center of mass (COM) of the astronauts body. However, rotational motion can be attained due to the movement of the limbs due to conservation of angular momentum as discussed in the following post:

Is there a way for an astronaut to rotate?

It should also be noted that any rotational kinetic energy attained by the astronaut is the consequence of the utilization of chemical potential energy of the astronauts muscles.

Hope this helps.