Even present-day fuels possess more than enough energy, if only we knew how to release and use it. Just as molecular energy is so freely used to-day, so atomic energy may bring interplantary travel within easy reach to-morrow.
Of the Sun and the Moon, the Moon is plainly the more important, as it provides us with light when it is dark and most needed, whereas the Sun appears only in the daytime when it is light anyhow.