Hohmann has studied in detail the problem of travelling to other celestial bodies. According to his results, the long-distance trip would last 146 days from the Earth to Venus and 235 days to Mars, expressed in a terrestrial time scale. A round trip including a flyby of both Venus and Mars at the relatively small distance of approximately 8 million kilometers could be carried out in about 1.5 years. Almost 2.25 years would be necessary for a visit to Venus with a landing, including a stay there of 14.5 months and the two-way travel time.
Assume now the following: in the sense of our previous considerations, the trip would start from the space station, so that only a modest amount of energy would be necessary for the complete separation from the Earth's gravitational field; the return trip would take place directly to the Earth's surface, so that no propulsive energy would have to be expended, because in this case the descent could be controlled by using only air drag braking. The load to be transported would be as follows: 2 people including the supplies necessary for the entire trip, and all instruments required for observation and other purposes.
It then follows from Hohmann's calculations that the vehicle in a launch-ready condition, loaded with all propellants necessary for traveling there and back, would have to weigh approximately the following: 144 tons for the described round trip with a flyby near Venus and Mars, of which 88% would be allocated to the propellants, 12 tons for the first landing on the Moon, 1350 tons for a landing on Venus and 624 tons for a Mars landing. For the trip to the Moon, 79% of the entire weight of the vehicle would consist of the propellants carried on board, but approximately 99% for the trips to Venus and Mars. A 4,000 meter per second exhaust velocity was assumed in these cases.
It is obvious that the construction of a vehicle that has to carry amounts of propellants on board constituting 99% of its weight would present such significant engineering difficulties that its manufacture would initially be difficult to accomplish. For the present, among our larger celestial neighbors, only the Moon would, therefore, offer the possibility of a visit with a landing, while the planets could just be closely approached and orbited, without descending to them. Nevertheless one can hope that we will finally succeed in the long run probably by employing the staging principle explained in the beginning even with technologies known today in building space rockets that permit landings on our neighboring planets.
With the above, and when considering the present state of knowledge, all possibilities are probably exhausted that appear to present themselves optimistically for space ship travel. The difficulties would be much greater confronting a visit to the most distant planets of the solar system. Not only are the distances to be travelled to those destinations much longer than the ones previously considered, but since all of these celestial bodies have a far greater distance from the sun than the Earth, the sun's gravitational field also plays a significant role in their attainability. Because if, for example, we distance ourselves from the sun (i.e, "ascend" from it), then in the same fashion as would be necessary in the case of the Earth's gravitational field, the sun's gravitational field must be overcome by expending energy, expressed as the change of the orbital velocity around the sun, and the distance from its center, as previously discussed. This is required in long- distance travels throughout planetary space.
If, however, we also wanted to descend down to one of these celestial bodies, then enormously large amounts of propellants would be necessary, in particular for Jupiter and Saturn because they have very strong gravitational fields as a result of their immense masses. In accordance with the above discussion, we naturally cannot even think of reaching the fixed stars at the present time, solely because of their enormous distance.