Imagine a planet in elliptical orbit around a star, without any perturbation from third bodies and moving in perfect vacuum. The only force acting in the system is gravity. As the planet gets closer to the star, gravity acts in the direction of its motion, so it gains speed and KE but loses PE. As it gets away from the star, gravity slows it down and it loses KE but gains PE. Hence mechanical energy is conserved. But if we now consider that the planet is a macroscopic object with a significant volume and a certain composition, doesn’t this have an impact on the analysis? The side closer to the star will be subject to a stronger force than the opposite side. It seems to me that the difference gets wider as the planet gets farther from the star, so the planet is stretched. And such difference gets narrower as the planet gets closer to the star, so in this leg of the trip it compresses. And if the composition of the planet does not enable it to be perfectly elastic and recover its original size, can’t there dampening arise, i.e. a supplementary loss of KE that may make it finally collapse against the star? Is it so? And if so, it is because the principle of conservation of mechanical energy only applies to ideal point masses? Or, rather, am I misunderstanding the principle and mixing things that have nothing to do with each other?