> The question is: Where is the energy stored in the meantime? Where does the extra energy, if the ship falls into a heavier planet, come from? You can say it

> The question is: Where is the energy stored in the meantime? Where does the extra energy, if the ship falls into a heavier planet, come from? You can say it came from the fact that there existed a point in space with a lower gravitational potential than the one at the spaceship's point of departure. Each point p in space is assigned a "gravitational potential" value V(p) measured in energy / mass. The function V(p) is determined by the distribution of mass in the space. A body of mass m moving from point a to point b will gain "gravitational potential energy" equal to (V(b) - V(a)) * m. In your case, the spaceship moved from a higher to a lower gravitational potential, thus losing gravitational potential energy (and gaining kinetic energy). Note that you are only interested in the differences of values of V at different points, so you can add or subtract any constant to the function V.