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https://en.wikipedia.org/wiki/MomentumNewton's second law of motion states that the rate of change of a body's momentum is equal to the net force acting on it. Momentum depends on the frame of reference, but in any inertial frame it is a conserved quantity, meaning that if a closed system is not affected by external forces, its total linear momentum does not change.Therefore, "in any inertial frame it is a conserved quantity, meaning that if a closed system is not affected by external forces, its total linear momentum does not change".Therefore, if the Astronaut was not effected by any external forces/energy it is expected that "its total linear momentum does not change"However, the gravity bends its linear momentum.Therefore, based on Newton's second law of motion, that astronaut must get external force/energy - even if it is called gravity, and he does't feel it.Hence, do you finally agree that in order to change the linear momentum of any object/astronaut - by gravity or by rocket - that force must come with real energy.
Based on Newton's second law of motion the orbital motion of an object must consum energy:
Therefore, based on Newton's second law of motion, that astronaut must get external force/energy