Which takes more fuel, a voyage from the earth to the moon or from the moon to the earth?
The gravitational potential energy of a voyager of mass m at a distance r from the earth is 
where M is the mass of the earth. The gravitational potential energy of the voyager at a distance r from the moon is 
where Mmoon is the mass of the moon. As r and m are constants and M > Mmoon, then numerically, W > Wmoon, i.e. the work needed to make a voyage from the earth to the moon is greater than that from the moon to the earth. So it takes more fuel to make the voyage from the earth...
