When you want to transfer to a planet/moon that has a lower orbit than the source planet/moon you are orbiting, then you prograde burn to transfer should be in the opposite direction as the source planet/moon motion in its orbit. Example: Transfer from Kerbin to Eve: you have to make your prograde burn to transfer to the opposite way of Kerbin's orbit around the Sun... - Check the image below http://i.imgur.com/TcOImct.png These images taken from http://ksp.olex.biz/ That is the same as orbits around kerbin. higher orbit - burn prograde, lower orbit - burn retrograde in kerbin's SOI, kerbin is the reference in planet transfers, the sun is the reference. thats why it's called orbital mechanics, cause you use the same concepts as long as you are in a defined orbit.