My question is, take Mars, say, and install a huge rocket engine on the ground pointing out the planet (obviously) and in the opposite direction of its velocity, so that if it is turned on, the planet is slowed. The rocket starts to work and Mars is slowed. If the loss of velocity goes little by little, could we make Mars follow another stable orbit or would it just fall to the Sun? If you can go more in depth: if the answer to the question asked before is yes, what equation tells me exactly how little the loss of velocity must be in order to keep the planet through another stable orbit? What defines the stability of the orbit? Can you calculate every possible orbit by its stability given two bodies like the Sun and Mars? Thank you very much in advance!