For a long time to come, the major part of the human race will continue to live on the surface of the Earth. Yet, at some time in the future, new human colonies may be started on planets of other stars, and, eventually, the number of human beings living elsewhere may exceed the population of the Earth.
In his recent book, The Next Million Years, Charles G. Darwin did not even mention space flight but confined his attention to the future history of the human population of the Earth. In the long run, however, space flight may prove to be the most significant departure in the history of mankind. This may not necessarily be true for the vast majority of the Earth's population who will remain at home; but it certainly should be true for those few (and their descendants) who will journey into space to find new homes among the stars and start new colonies and new bases from which to launch still other expeditions into a gradually expanding volume of space, the next frontier. The population of the Earth can not continue to grow indefinitely at the present rate (between and 2 per cent each year). An upper limit must be reached within the next several hundred years, when the Earth's population will somehow become more or less stabilized numerically (barring a man-made catastrophe). Whether the Earth will be such a pleasant place to live at that time is problematical. It will undoubtedly be much more crowded than it is now, and there will inevitably be increasing incentives for pioneers to seek new lives among the stars for themselves and their families. However, as others have already pointed out, we can not look to space flight to solve the problem of the Earth's population explosion. This is obvious, of course, from the present rate of increase in the world's population. In mid-1962 the world's population reached the 3-billion mark, and the net annual rate of increase was estimated at 1.8 per cent, or over 50 million people per year. Just to hold the Earth's population constant at the present time would require the emigration of almost 150,000 people per day—clearly not a reasonable concept. In another century, if the present rate of natural increase continued, the emigration rate would have to be stepped up to 900,000 per day to keep the Earth's population constant at 18 billion people. Admittedly, however, experience has shown that it is dangerous to assume that present population growth trends can be used with confidence to predict future population levels.
If man learns to travel through space at, say, one-quarter or one-half the speed of light, even with long planetary stopovers on star-hopping expeditions making the net advance only one-tenth the speed of light, the entire Galaxy could be explored and all its habitable planets settled within the next million years. Unquestionably, many technological advances will occur before this much time has elapsed, and the spread of mankind throughout the Galaxy may take place more rapidly. The future history of man may well be written among the stars.
Each stage in the progress of man as he star-hops into new, unexplored regions of the Galaxy will be accompanied by an important kind of distillation process. Always, those volunteering for the next expedition into the unknown will tend to be adventurous, self-reliant, inquisitive, courageous, and hardy pioneers, while those selected to go will be chosen on the basis of good health, high professional competence, emotional stability, reliability of judgment, et cetera. This is based on the assumption that such expeditions will always be large, expensive projects— projects too important, whether privately or publicly financed, to be entrusted to any but the most competent and reliable people. And, in the main, these characteristics will be passed on to their descendants, so a kind of selection process will take place, with those at the frontier of the wave through the Galaxy always representing some of the best qualities of mankind.
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