Just exactly how the long trip through outer space can be achieved has already been indicated at the beginning of this book: in general, in free orbits around those celestial bodies in whose gravitational field the trip is proceeding. Within its realm, the sun must consequently be continually orbited in some free orbit if a space ship is to avoid falling victim to its gravitational force and crashing into its fiery sea.
However, we do not have to take any special precautions as long as we stay close to the Earth or to another heavenly body of the solar system. After all, these bodies orbit the sun in their own free orbits, as do all bodies belonging to it. At the velocity of the Earth (30,000 meters per second), the Moon, for example, also circles the sun, as will our future space station (both as satellites of the Earth). As a result, the sun's gravitational force loses its direct effect on those two satellites ("stable state of floating" compared to the sun).
Only when the space ship moves further away from the immediate gravitational region of a celestial body circling the sun would the sun have to be orbited in an independent free orbit. If, for example, the trip is to go from the Earth to another planet, then, based on previous calculations, both the course of this independent orbit and the time of departure from the Earth must be selected in such a fashion that the space ship arrives in the orbit of the destination planet approximately at the time when the planet also passes through the encounter point.
If the space vehicle is brought in this fashion into the practical effective range of gravity of the destination celestial body, then the possibility exists either to orbit the body in a free orbit as a satellite as often as desired or to land on it. Landing can, if the celestial body has an atmosphere similar to that of the Earth, occur in the same fashion as previously discussed for the Earth (Hohmann's landing manoeuver, Figures 44 and 45). If, however, a similar atmosphere is absent, then the landing is possible only by reaction braking, that is, by operating the propulsion system opposite to the direction of free fall during landing (Figure 37).
To travel to another celestial body within the solar system after escape from the original body, the orbital motion, previously shared with this body around the sun, must be altered by using the propulsion system to such an extent that the space ship enters an independent orbit around the sun, linking its previous orbit with that of the other celestial body. To implement this in accordance with the laws of celestial mechanics, the original orbital movement would have to be accelerated if the vehicle (according to the position of the target body) is to move away from the sun (Figure 98), and to be decelerated if it is to approach it. Finally, as soon as the destination is reached, the motion maintained in the "transfer orbit" must be changed into the motion that the vehicle must have as the new celestial body for effecting the orbiting or landing maneuver. The return trip would occur in the same fashion. It can be seen that repeated changes of the state of motion are necessary during a long-distance trip of this nature through planetary space. The changes would have to be produced through propulsion with an artificial force and, therefore, would require an expenditure of propellants, a point previously mentioned at the beginning of this book. As determined mathematically by Hohmann, the propellant expenditure reaches a minimum when the orbits of the original celestial body and that of the destination body are not intersected by the transfer orbit of the vehicle, but are tangential to it (touch it) (Figure 99). Nevertheless, the required amounts of propellant are not insignificant.
Besides the points discussed above, there are additional considerations if the destination heavenly body is not to be orbited, but is supposed to be landed on. These considerations are all the more important the greater the mass and consequently the gravitational force of the destination planet are, because the reascent from the destination planet when starting the return trip requires, as we already know from the discussion of the Earth, a very significant expenditure of energy. Additionally, if braking must be performed during the landing by propulsion in the absence of an appropriate atmosphere (reaction braking), then a further, significant increase of the amount of necessary propellants results.
Figure 98. If the motion of a freely orbiting body is accelerated, then it expands its original orbit and moves away from the center of gravity. If the motion is decelerated, then the body approaches the center of gravity by contracting its orbit.
Key: 1. Acceleration and, therefore, increasing distance; 2. Orbiting body (e.g., the Earth); 3. Decelerating and, therefore, approaching; 4. Center of gravity (e.g., the sun); 5. Original orbit. Figure 99. Tangential and intersecting transfer orbits in which the space vehicle must move in order to reach an alien celestial body within the solar system. The numbers in the figure indicate the following: 1. the orbit of the original body; 2. the orbit of the destination celestial body. The distance of the transfer orbit marked by heavy lines is that part of the orbit which the vehicle actually travels through.
Key: A. Sun; B. Transfer orbit.
The propellants must be carried on board from the Earth during the outward journey, at least for the initial visit to another planet, because in this case we could not expect to be able to obtain the necessary propellants from the planet for the return trip.