This doesn't mean to say that we must remain forever restricted to the Earthly realm and to its nearest celestial bodies. Because, if we succeeded in increasing further the exhaust velocity beyond the 4,000 (perhaps 4,500) meters per second when generating the thrust, the highest attainable in practice at the present time, or in finding a possibility of storing on board very large quantities of energy in a small volume, then the situation would be completely different.
And why shouldn't the chemists of the future discover a propellant that surpasses in effectiveness the previously known propellants by a substantial degree? It might even be conceivable that in the course of time mankind will succeed in using those enormous amounts of energy bound up in matter, with whose presence we are familiar today, and in using them for the propulsion of space vehicles. Perhaps we will someday discover a method to exploit the electrical phenomenon of cathode radiation, or in some other way attain a substantial increase of the exhaust velocity through electrical influences. Even using solar radiation or the decay of radium, among others, might offer possibilities to satisfy this purpose.
In any case, natural possibilities for researchers and inventors of the future are still available in many ways in this regard. If success results from these efforts, then probably more of those alien worlds seen by us only as immensely far away in the star- studded sky could be visited by us and walked on by humans.
An ancient dream of mankind! Would its fulfillment be of any use to us? Certainly, extraordinary benefits would accrue to science. Regarding the practical value, an unambiguous judgement is not yet possible today. How little we know even about our closest neighbors in the sky! The Moon, a part of the Earthly realm, our "immediate homeland" in the universe, is the most familiar to us of all the other celestial bodies. It has grown cold, has no atmosphere, is without any higher life form: a giant rock- strewn body suspended in space, full of fissures, inhospitable, dead-a bygone world. However, we possess significantly less knowledge about that celestial body, observed the best next to the Moon, about our neighboring planet Mars, even though we know relatively much about it in comparison to the other planets.
It is also an ancient body, although considerably less so than the Moon. Its mass and, consequently, its gravitational force are both considerably smaller than that of the Earth. It has an atmosphere, but of substantially lower density than the terrestrial one (the atmospheric pressure on its surface is certainly significantly lower than even on the highest mountain top on Earth). Even water is probably found on Mars. However, a fairly large part of it is probably frozen, because the average temperature on Mars appears to be substantially below that of the Earth, even though in certain areas, such as in the Martian equatorial region, significantly warmer points were detected. The temperature differences between night and day are considerable due to the thinness of the atmosphere.
The most unique and most frequently discussed of all Martian features are the so-called "Martian canals." Even though in recent times they have been considered mostly as only optical illusions, it is still unclear just exactly what they are.
In any case, the present knowledge about Mars does not provide sufficient evidence for a final judgement as to whether this celestial body is populated by any form of life, or even by intelligent beings. For people from Earth, Mars would hardly be inhabitable, primarily because of the thinness of its atmosphere. From a scientific point of view, it would certainly offer an immensely interesting research objective for space travellers. Whether walking on Mars would have any practical value can still not be determined with certainty today; however, this does not appear to be very probable.
It is an altogether different situation with the second planet directly adjacent to us, Venus, the brightly shining, familiar "morning and evening star." Its size as well as its mass and accordingly the gravitational field existing at its surface are only slightly smaller than the Earth's. It also has an atmosphere that should be quite similar to the terrestrial atmosphere, even though it is somewhat higher and denser than the Earth's. Unfortunately, Venus can be observed only with difficulty from the Earth's surface, because it is always closer to the sun and, therefore, becomes visible only at dawn or dusk. As a result, we know very little about its rotation. If Venus rotates in approximately 24 hours roughly like the Earth, a situation assumed by some experts, then a great similarity should exist between Venus and Earth.
In the case of this planet, finding conditions of life similar to terrestrial conditions can be expected with high probability, even if the assumption should be valid that it is continually surrounded by a cloud cover. Because even on Earth, highly developed forms of plant and animal life already existed at a time when apparently a portion of the water now filling the seas and oceans was still gaseous due to the slow cooling of the globe millions of years ago and, therefore, continually surrounded our native planet with a dense cloud cover. In any case, Venus has the highest probability of all the celestial bodies closely known to us of being suitable for colonization and, therefore, of being a possible migration land of the future. Furthermore, since it is nearest to us of all planets, it could be the most likely and tempting destination for space travel.
Mercury offers even more unfavorable conditions for observation than Venus because it is still closer to the sun. Of all the planets it is the smallest, has an atmosphere that is no doubt extremely thin and surface conditions apparently similar to those of the Moon. For this reason and especially due to its short perihelion distance (solar radiation about 9 times stronger than on the Earth!), extremely unfavorable temperature conditions must exist on it. Consequently, Mercury should be considerably less inviting as a destination.
While it was still possible when evaluating the celestial bodies discussed above to arrive at a fairly probable result, our current knowledge about the more distant planets, Jupiter, Saturn, Uranus and Neptune, is hardly sufficient to achieve this. Although we have been able to determine that all of them have dense atmospheres, the question of the surface conditions of these planets is, however, still entirely open: in the cases of Jupiter and Saturn, because they are surrounded by products of condensation (clouds of some kind) so dense that we apparently cannot even see their actual surfaces; and in the cases of Uranus and Neptune, because their great distances preclude precise observation.
Therefore, anything regarding their value as a space flight destination can only be stated with difficulty. But the following condition by itself is enough to dampen considerably our expectations in this regard: a relatively very low average density has been determined for these planets (1/4 to 1/5 of that of the Earth), a condition indicating physical characteristics quite different from those on Earth. It would be perhaps more likely that several of the moons of these celestial bodies (primarily, those of Jupiter would be considered in this connection) offer relatively more favorable conditions. One thing is certain in any case: that their masses are considerably greater than the Earth's and that, therefore, the powerful gravitational fields of these planets would make a visit to them extraordinarily difficult, especially in the cases of Jupiter and Saturn.
Regarding the remaining, varying types of celestial bodies that still belong to the solar system, it can be said with a fair degree of certainty today that we would hardly be able to benefit in a practical sense from a trip to them. We see then that, generally speaking, we should not indulge in too great hopes regarding the advantages that could be derived from other celestial bodies of our solar system. In any case, we know far too little about them not to give free reign to the flight of thoughts in this regard:
Of course, it could turn out that all of these worlds are completely worthless for us! Perhaps, however, we would find on some of them a fertile soil, plant and animal life, possibly of a totally alien and unique nature for us, or perhaps of a gigantic size, as existed on Earth long ago. It would not be inconceivable that we would meet even humans or similar types of life, perhaps even with civilizations very different from or even older than those of our native planet. It is highly probable that life on other planets if it exists there at all is at another evolutionary stage than that on Earth. We would be able then to experience that wonderful feeling of beholding images from the development of our own terrestrial existence: current, actual, living and yet images from an inconceivable, million-year old past or from an equally distant future.
Perhaps we would discover especially valuable, very rare Earthly materials, radium for example, in large, easily minable deposits? And if the living conditions found there are also compatible with long-term human habitation, then perhaps even other celestial bodies will one day be possible as migration lands regardless of how unbelievable this may sound today. That such planets exist among those of our solar system is, however, only slightly probable according to what has been stated previously, with the exception of Venus, as already noted.