In previous considerations, we did not leave the confines of the dominant force of the Earth's attraction its "territory in outer space," so to speak. What about the real goal of space flight: completely separating from the Earth and reaching more distant celestial bodies?
Before we examine this subject, a brief picture of the heavenly bodies is provided, seen as a future destination from the standpoint of space travel. In the first place, we must broaden the scope of our usual notions, because if we want to consider the entire universe as our world, then the Earth, which previously appeared to us as the world, now becomes just our "immediate homeland." Not only the Earth! But everything that it holds captive by virtue of its gravitational force, like the future space station; even the Moon must still be considered a part of our immediate homeland in the universe, a part of the "Earth's empire." How insignificant is the distance of about 380,000 km to the Moon in comparison to the other distances in outer space! It is only a thousandth of the distance to Venus and Mars, located next to us after the Moon, and even the Earth together with the Moon's entire orbit could easily fit into the sun's sphere.
The next larger entity in the universe for us is the solar system, with all its various, associated heavenly bodies. These are the 8 large planets or "moving" stars, one of which is our Earth, (Figures 96 and 97) and numerous other celestial bodies of considerably smaller masses: planetoids, periodic comets, meteor showers, etc. Of the planets, Mercury is closest to the sun, followed by Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune, the most distant. Together with the Moon, Venus and Mars are the planets nearest to the Earth.
All of these celestial bodies are continuously held captive to the sun by the effect of gravity; the bodies are continually forced to orbit the sun as the central body in elliptical orbits. The planets together with the sun form the "sun's empire of fixed stars," so to speak. They form an island in the emptiness and darkness of infinite space, illuminated and heated by the sun's brilliance and controlled at the same time by the unshakable power of the sun's gravitational force, and are thus linked in an eternal community. That island is our "extended homeland" within the universe. A realm of truly enormous size: even light needs more than 8 hours to traverse it and it is racing through space at a velocity of 300,000 km per second!
Figure 96. A schematic of the orbits of the 8 planets of our solar system in their relative sizes.
Key: 1. Sun; 2. Mercury; 3. Earth; 4. Neptune.
Figure 97. Enlarged rendition taken from Figure 96 of the orbits of Mars, the Earth, Venus and Mercury.
Key: 1. Earth; 2. Mercury.
And yet, how tiny is this world compared to the incomprehensible distances of the universe, from which those many white hot celestial bodies familiar to us as fixed stars send their shining greetings of radiation. Even the one closest to us, the fixed star Alpha Centauri, is 4.3 light years away; i.e., around 4,500 times as far as the diameter of the entire solar system! All of the others are still many more light years away from us, most of them hundreds and thousands of light years. And if there are fixed stars closer to us that are already burnt out, then we are unaware of them in the eternal darkness of empty space.
From this discussion, it can be seen that only those heavenly bodies belonging to the solar system can be considered for the trip to alien celestial bodies, at least according to the views held today.