It is clearly not possible, in the space of one paper, even to touch on all aspects of interplanetary navigation, and some whole fields must be ruled out from the start. This paper will not be concerned with any discussion of the space-flight as such; there is no point in considering navigational problems at all unless one can assume (at least for the sake of argument) that there is to be some practicable way of lifting a ship out of the Earth's gravitational field, and then of accelerating it out of the Earth's orbit; but even granting this major premise a considerable amount of selection is still essential.