It is useful to think about all the energy in the problem. There is a case where the only energy in the problem is the kinetic energy of the spaceship and the

It is useful to think about all the energy in the problem. There is a case where the only energy in the problem is the kinetic energy of the spaceship and the potential energy from gravity. That is the case of firing the spaceship with a giant catapult so that it goes and hits the other planet. In this case, it will be going really fast when it leaves earth. It will slow down as it moves away from earth, leaving its gravity. Then, it accelerates as it falls towards the heavier planet. Then splat, even faster than it was moving when it left earth. At any point, the change in kinentic energy will be the negative of the change in potential energy. A more common scenario, at least in our imagination, is that a thruster on the spacecraft is adding and removing lots of energy from the spacecraft. The math will still work out. And this way there is hope that the spacehip ends up at rest, not destroyed, on the larger planet. But this time the thruster did lots of work and has to be included in the energy balance.