It's true tat it's been a long time since I played with this stuff, I work out for a circular orbit Total energy of an orbit is -G mp mS /(2 r) mp = ma

It's true tat it's been a long time since I played with this stuff, I work out for a circular orbit Total energy of an orbit is -G mp mS /(2 r) mp = mass of planet mS = mass of sun r = distance of planet from sun Even though this is a circular orbit, the basic result should be the same: the greater the distance from the sun the more energy the planet has, per unit mass. I think I understand your basic point, however, which is that whether we speed it up or slow it down we have to do something to the mass, and how much we do depends only on difference of the orbit radii. But note that adding 100 m/s or subtracting 100 m/s gives two different differences in r.