Travel within a planetary system was at first carried out by rocket-vessels propelled by normal fuels. In all the early ventures one great difficulty had been the danger of collision with meteors. Even the most efficient vessel, most skillfully navigated and traveling in regions that were relatively free from these invisible and lethal missiles, might at any moment crash and fuse. The trouble was not overcome till means had been found to unlock the treasure of sub-atomic energy. It was then possible to protect the ship by means of a far-flung envelope of power which either diverted or exploded the meteors at a distance. A rather similar method was with great difficulty devised to protect the space ships and their crews from the constant and murderous hail of cosmic radiation.
Interstellar, as opposed to interplanetary, travel was quite impossible until the advent of sub-atomic power. Fortunately this source of power was seldom gained until late in a world's development, when mentality was mature enough to wield this most dangerous of all physical instruments without inevitable disaster. Disasters, however, did occur. Several worlds were accidentally blown to pieces. In others civilization was temporarily destroyed. Sooner or later, however, most of the minded worlds tamed this formidable djin, and set it to work upon a titanic scale, not only in industry, but in such great enterprises as the alteration of planetary orbits for the improvement of climate. This dangerous and delicate process was effected by firing a gigantic sub-atomic rocket-apparatus at such times and places that the recoil would gradually accumulate to divert the planet's course in the desired direction.
Actual interstellar voyaging was first effected by detaching a planet from its natural orbit by a series of well-timed and well-placed rocket impulsions, and thus projecting it into outer space at a speed far greater than the normal planetary and stellar speeds. Something more than this was necessary, since life on a sunless planet would have been impossible. For short interstellar voyages the difficulty was sometimes overcome by the generation of sub-atomic energy from the planet's own substance; but for longer voyages, lasting for many thousands of years, the only method was to form a small artificial sun, and project it into space as a blazing satellite of the living world. For this purpose an uninhabited planet would be brought into proximity with the home planet to form a binary system. A mechanism would then be contrived for the controlled disintegration of the atoms of the lifeless planet, to provide a constant source of light and heat. The two bodies, revolving round one another, would be launched among the stars.
This delicate operation may well seem impossible. Had I space to describe the age-long experiments and world-wrecking accidents which preceded its achievement, perhaps the reader's incredulity would vanish. But I must dismiss in a few sentences whole protracted epics of scientific adventure and personal courage. Suffice it that, before the process was perfected, many a populous world was either cast adrift to freeze in space, or was roasted by its own artificial sun.
The stars are so remote from one another that we measure their distances in light years. Had the voyaging worlds traveled only at speeds comparable with those of the stars themselves, even the shortest of interstellar voyages would have lasted for many millions of years. But since interstellar space offers almost no resistance to a traveling body, and therefore momentum is not lost, it was possible for the voyaging world, by prolonging the original rocket-impulsion for many years, to increase its speed far beyond that of the fastest star. Indeed, though even the early voyages by heavy natural planets were by our standards spectacular, I shall have to tell at a later stage of voyages by small artificial planets traveling at almost half the speed of light. Owing to certain "relativity effects" it was impossible to accelerate beyond this point. But even such a rate of travel made voyages to the nearer stars well worth undertaking if any other planetary system happened to lie within this range. It must be remembered that a fully awakened world had no need to think in terms of such short periods as a human lifetime. Though its individuals might die, the minded world was in a very important sense immortal.
It was accustomed to lay its plans to cover periods of many million years.
In early epochs of the galaxy expeditions from star to star were difficult, and rarely successful. But at a later stage, when there were already many thousands of worlds inhabited by intelligent races, and hundreds that had passed the Utopian stage, a very serious situation arose. Interstellar travel was by now extremely efficient. Immense exploration vessels many miles in diameter, were constructed out in space from artificial materials of extreme rigidity and lightness. These could be projected by rocket action and with cumulative acceleration till their speed was almost half the speed of light. Even so, the journey from end to end of the galaxy could not be completed under two hundred thousand years. However, there was no reason to undertake so long a voyage. Few voyages in seach of suitable systems lasted for more than a tenth of that time. Many were much shorter. Races that had attained and secured a communal consciousness would not hesitate to send out a number of such expeditions. Ultimately they might project their planet itself across the ocean of space to settle in some remote system recommended by the pioneers.
The problem of interstellar travel was so enthralling that it sometimes became an obsession even to a fairly well-developed Utopian world. This could only occur if in the constitution of that world there was something unwholesome, some secret and unfulfilled hunger impelling the beings. The race might then become travel-mad.
Its social organization would be refashioned and directed with Spartan strictness to the new communal undertaking. All its members, hypnotized by the common obsession, would gradually forget the life of intense personal intercourse and of creative mental activity which had hitherto been their chief concern. The whole venture of the spirit, exploring the universe and its own nature with critical intelligence and delicate sensibility, would gradually come to a standstill. The deepest roots of emotion and will, which in the fully sane awakened world were securely within the range of introspection, would become increasingly obscured. Less and less, in such a world, could the unhappy communal mind understand itself. More and more it pursued its phantom goal. Any attempt to explore the galaxy telepathically was now abandoned. The passion of physical exploration assumed the guise of a religion. The communal mind persuaded itself that it must at all costs spread the gospel of its own culture throughout the galaxy. Though culture itself was vanishing, the vague idea of culture was cherished as a justification of world-policy.
Here I must check myself, lest I give a false impression. It is necessary to distinguish sharply between the mad worlds of comparatively low mental development and those of almost the highest order. The humbler kinds might become crudely obsessed by sheer mastery or sheer travel, with its scope for courage and discipline. More tragic was the case of those few very much more awakened worlds whose obsession was seemingly for community itself and mental lucidity itself, and the propagation of the kind of community and the special mode of lucidity most admired by themselves. For then travel was but the means to cultural and religious empire.
I have spoken as though I were confident that these formidable worlds were indeed mad, aberrant from the line of mental and spiritual growth. But their tragedy lay in the fact that, though to their opponents they seemed to be either mad or at heart wicked, to themselves they appeared superbly sane, practical, and virtuous. There were times when we ourselves, the bewildered explorers, were almost persuaded that this was the truth. Our intimate contact with them was such as to give us insight, so to speak, into the inner sanity of their insanity, or the core of rightness in their wickedness. This insanity or wickedness I have to describe in terms of simple human craziness and vice; but in truth it was in a sense superhuman, for it included the perversion of faculties above the range of human sanity and virtue.