As per Newton's II law every action has equal and opposite reaction
So when Astronaut is detached from the spaceship and she is at distance 50 m from the spaceship then there is no force on the Astronaut and she is in isolated state
In this way if Astronaut want to reach back on her spaceship then she required a force towards the spaceship
Here the force on her can not be applied by any other mean as she is isolated situation
So she must have to throw an object in opposite direction with some force due to which she will also get a force due to Newton's III law
This will lead her towards the spaceship
So with the help of Newton's III law we will say that she will be able to reach at her spaceship