Besides the force of gravity, the atmosphere, which many celestial bodies havenaturally that of the Earth, in particularalso plays an extremely important role for space travel. While the atmosphere is very valuable for the landing, it, on the other hand, forms a particularly significant obstacle for the ascent.
According to observations of falling meteors and the northern lights phenomena, the height of the entire atmosphere of the Earth is estimated at several hundred (perhaps 400) kilometers (Figure 8). Nevertheless, only in its deepest layers several kilometers above the Earth, only on the "bottom of the sea of air" so to speak, does the air density exist that is necessary for the existence of life on Earth. For the air density decreases very quickly with increasing altitude and is, by way of example, onehalf as great at an altitude of 5 km and only onesixth as great at an altitude of 15 km as it is at sea level (Figure 9).
Figure 8. Assuming that the atmosphere is approximately 400 km high, the diagram shows its correct ratio to the Earth.
This condition is of critical importance for the question of space travel and is beneficial to it because, as is well known, air resists every moving object. During an increasing velocity of motion, the resistance increases, however, very rapidly, and more specifically, in a quadratic relationship. Within the dense air layers near the Earth, it reaches such high values at the extreme velocities considered for space travel that as a result the amount of work necessary for overcoming the gravitational field during ascent, as mentioned previously, is increased considerably
and must also be taken into consideration to a substantial degree when building the vehicle. However, since the density of the air fortunately decreases rapidly with increasing altitude, its resistance also becomes smaller very quickly and can as a result be maintained within acceptable limits. Nevertheless, the atmosphere is a powerful obstacle during ascent for space travel. It virtually forms an armored shield surrounding the Earth on all sides. Later, we will get to know its importance for returning to Earth.