The conditions, however, would be considerably more favorable if a depot for propellants appropriately suspended high over the Earth and continuously circling it in a free orbit was built, as Oberth suggests, and if the trip was started from this depot instead of from the Earth's surface, because in that case only a modest expenditure of energy would be necessary for a complete separation from the Earth, and the vehicle need not, therefore, be loaded with the propellants necessary for the ascent from the Earth. It would have to carry on board only slightly more than the amount necessary for the deep space trip itself.
Since the depot would be in a weightless state as a result of its free orbital motion, the propellants could simply be stored there freely suspended in any amount and at any place. Protected against the sun's rays, even oxygen and hydrogen would remain solidly frozen for an indefinite time. Their resupply would have to be accomplished by a continuous space ship shuttle service either from the Earth where the propellants (at least liquid oxygen and hydrogen) could be produced, for example, in large power plants powered by the heat of the tropical seas; or from the Moon, as Max Valier suggests. This method would be particularly advantageous, because since the mass and consequently the gravitational force of the Moon are considerably smaller than those of the Earth, the expenditure of energy necessary for the ascent and consequently for the propellant supply for that ascent would be significantly less. However, this assumes that the required raw materials would, in fact, be found on the Moon, at least water (in a icelike condition, for instance) because it can be decomposed electrolytically into oxygen and hydrogen, the energy for this process being provided by a solar power plant. Unfortunately, the probability for this is not all that high.
If, however, this should be possible, then even the Moon, according to Hohmann's recommendation, could be used as a starting point for travel into deep space; that is, the propellant depot could be built on the Moon. Despite many advantages of this idea, Oberth's recommendation of a freely suspended depot appears to be the better one, because the complete separation from the gravitational field of the Earth (including the Moon) would require considerably less expenditure of energy from a depot of this nature. More specifically, it would certainly be the most advantageous from an energy economics point of view to build the depot one or more millions of kilometers away from the Earth, especially if the propellants must be supplied from the Earth. We want, however, to build the depot at our space station, and thus make it a transportation base, because it is already equipped with all facilities necessary for this purpose.
Of this equipment, giant telescopes, among others, would be particularly valuable because thanks to their almost unlimited capabilities they would not only make it possible to study in detail the celestial destinations from a distance, a point previously described. The space station could probably keep the space ship under constant surveillance during a large part of its trip, in many cases perhaps even during the entire trip, and could remain in at least one-way communications with it through light signals to be emitted at specific times by the space ship. Thus, the space station, besides satisfying the many assignments already discussed, would be able to satisfy those that assist not only in preparing for actual travel into the universe but also serve as a basis for the entire traffic into outer space.