Shepherd, Leslie - Interstellar Flight

 

Introduction 

 

SOMETIME IN THE near future, perhaps before the turn of the century, man will take his first step into space. He will do so in all probability without being unduly concerned about the chain of events which he will be setting into motion. The significance of the act may not have escaped him entirely, but he is not likely to he influenced by considerations of the ultimate importance of interplanetary travel upon human affairs. Scientific curiosity and the love of adventure for its own sake will be sufficient motives for the first exploratory voyages. 

Nevertheless there must he many apostles of space flight to whom these two motives are only a small proportion of the whole purpose. There must be many who cannot derive complete spiritual satisfaction from the picture of mankind spending its whole existence in one single infinitesimal planet with no contact with other species who may people countless other worlds of the universe. Many who hold to a more materialistic outlook may see in man's confinement to a single planet a factor reducing his probability of survival. Humanity dispersed over many worlds would appear more secure than humanity crowded on one single planet. 

We are going to examine the possibilities of interstellar flight mainly from the latter viewpoint, viz., the possibilities of the human race establishing colonies in other stellar systems, always assuming that there are worlds suitable for mankind to be found in such systems. We are concerned with the problem of getting small colonies across the almost endless interstellar gulfs, safely, but ultimately, no matter how or in what time. It is not necessarily a question of getting an individual from one stellar system to another, but rather a question of getting an adequate community to another system. It is important to stress this point because this profoundly affects our interpretation of what is possible and what is impossible. For if we interpret the problem of instellar flight as the problem of transporting a man from one system to another during his lifetime, then it is a much more difficult problem. 

The problem of interstellar flight is one of vast distances and interminable transits which may demand a completely new philosophy of exploration. If we accept the more general interpretation of interstellar travel then the explorer or colonist setting out for some distant system may do so in the knowledge, not only that he will never again see his native planet, but that he will not even see the planet of his destination—a privilege reserved for his descendants. Thus the philosophy of the explorer may he that of the soldier or airman setting out on a suicide raid, doing so in the knowledge that for him there can be no personal gain, only the dying knowledge that some will survive to benefit front his action. Indeed, interstellar colonization may call for the sacrifice of whole generations in the lonely reaches of space. Colonies once established may have to exist for generations in a state of complete isolation, and such communication as may exist between systems may be a very tenuous and precarious matter. 

Thus interstellar exploration and colonization may be vastly different front the exploration and colonization of our own world or even of our own system. It may require a revolution in our way of life not only socially but biologically if we are ever to become a galactic people. 

 

Distance, Time, and Energy 

We have already indicated that the dominating factor of interstellar flight is distance, vast unimaginable distance. When we discuss the achievement of space flight, of journeys to the Moon and our solar neighbours, we talk of velocities of the order of 6 miles per second. Such velocities are insignificant on the interstellar scale of magnitudes. A vehicle leaving our system for the binary Alpha-Centauri (at 4.3 light years distance,* our second nearest neighbor), (* A "light year" is the distance light travels at 186,000 miles per second in one year's time.) would take about 130,000 years to reach that destination if it travelled at such a low velocity as 6 miles per second. Thus the statement that vehicles capable of achieving interplanetary flight could also fly to other stars, while it may be true, is nevertheless completely academic. 

It is probably true to say that interstellar flight will he considered possible when transit times of 100 to 1,000 years can be realized, and not before then. In the case of a flight to Alpha-Centauri or other neighboring stars this involves mean velocities up to 6,000 miles per second. Let us see what such velocities imply in terms of known energy sources. 

A rocket is propelled forward by the ejection front its rear of a stream of matter moving at a high velocity. The velocity attained by the rocket is proportional to the velocity of this stream or jet (the so-called exhaust velocity) and also depends upon the amount of matter ejected. A high exhaust velocity pays off a considerable bonus in rocket performance. 

In the present-day chemical rocket, the exhaust velocity is determined by the available chemical energy of the propellants and the efficiency of the motor in converting this chemical energy into kinetic energy (the word "kinetic." means "matter in motion") of the gases ejected front the nozzle. The most favorable estimate of the exhaust velocity attainable with known chemical propellants, is about 2.5 miles per second. 

Many nuclear reactions are millions of times more energetic than the most powerful chemical reactions. If nuclear fuels were employed in a manlier analogous to chemical rocket propellants, i.e. if the energy of the nuclear reaction were concentrated entirely in the reaction products, then the exhaust velocities attained would be a thousand times, or more, greater than those attainable with chemical propellants. 

It can be shown that so long as the mass of nuclear fuel carried by the rocket is less than four times the empty weight of the rocket, it would be better to dilute this fuel to such a degree that the mass ratio (the relationship of the mass of the fuel to the mass of the empty rocket) is made equal to 5. This procedure leads to the maximum rocket performance with the minimum fuel consumption. 

Suppose that it was required to make a one-way transit to Alpha-Centauri—which is the nearest fixed star at about four light years distance—taking 250 years and using a fissionable material as our nuclear fuel. To do this in the most economical manner, we would need a mass of nuclear fuel equal to 2.4 times the mass of the empty rocket and sufficient inert propellant material to raise the mass ratio to the most efficient value. Under these conditions the exhaust velocity would be 3,780 miles per second and the rocket would be capable of accelerating to 6,000 miles per second, or alternatively 1 as would be required in this case) accelerating to 3,000 miles per second and braking down again to zero velocity. If it were possible to employ the lithium-hydrogen reaction, the same ratio of masses would lead to a transit time of 140 years in the voyage to Alpha-Centauri. 

Since this necessitates a prodigious expenditure of what, in the fission case at least, would be valuable material, it is of interest to note that the exhaust velocity varies only as the square root of the proportion of nuclear fuel. In other words if a tenfold increase in transit time could be accepted, a hundredfold reduction in the proportion of nuclear fuel would be possible. 

 

BEFORE PROCEEDING FURTHER, it is worth pausing to consider the nature of the propulsion unit which would raise the interstellar vehicle to the considerable velocities which we have been contemplating. Referring again to the example of a rocket going to Alpha-Centauri in 250 years transit time, we note that at the expense of increasing this to 350 years we could spend a 100 years in accelerating and decelerating. This corresponds to an acceleration of 60 miles per second for one year or about one three-thousandth of the acceleration due to gravity at the earth's surface. The useful power output of the propulsion system is proportional to this acceleration and also to the exhaust velocity 13,780 miles per second). In this example the power output would have to be 10 megawatts I the prefix "mega" represents one million) per ton of rocket I specific exhaust power); in other words, if the rocket weighed 10,000 tons, the power output would be 100,000 megawatts. 

This represents a very severe power requirement and is certainly not one which could be met by present-day engineering technology. It is difficult to visualize engines running at such a high power level for 50 years continuously. However, it is to be expected that interplanetary flight techniques will have reached a very advanced state before man will be ready to embark upon deeper space flight, and low thrust/high exhaust velocity vehicles may have had several hundred years of development before called upon to play this exacting role. 

 

The Ion-Rocket Principle 

In all probability the vehicle which will carry our descendants on their first mission to the stars will use the ion-rocket principle, unless, of course, some entirely new method of propulsion is forthcoming by that time. In the ion rocket, the high-velocity exhaust jet is produced by accelerating electrically charged atoms (ions) in an electric field. 

High velocities are attainable with accelerating voltages that are not outside the range commonly encountered today. Thus a velocity of 3,780 miles per second could be obtained with singly charged carbon atoms by means of an accelerating potential of 2,500,000 volts. 

It is quite possible, in view of the high specific exhaust power required, that a great deal of the inert propellant carried by the interstellar vehicle would consist of power plant and propulsion unit replacements. From time to time during the acceleration program, the old worn-out (and probably burned-out) plant could be stripped out and consigned to the ion sources. As the voyage neared its termination and the all-up mass was reduced, the number of propulsion units might be reduced by a similar process. In this way nearly all of the initial dead weight of the vehicle would serve some useful purpose in addition to its use as propellant. 

The facts show that flight to the nearest stars appears to be possible in principle provided we are willing to accept transit times greater than 100 years, and possibly 1,000 years. 

 

AT FIRST SIGHT the idea of advancing mankind's frontiers to points requiring hundreds or even thousands of years to reach, might seem hopeless. It cannot indeed be regarded as a particularly satisfactory picture of interstellar exploration. However, regarded in terms of geological eras, centuries or millennia are small intervals, and provided that human life can be sustained in exploring vehicles for long periods, there is no reason why interstellar expansion should not proceed on this basis. 

An important factor in determining whether this state of affairs was adequate would be the frequency of occurrence of planets suitable for human habitation. Many theories of planetary origin suggest that only a comparatively infinitesimal proportion of the stars are blessed with planets. This being the case we might have to go much further afield than Alpha-Centauri to find alternative accommodation for mankind. However, no theory of the formation of planets, so far advanced, can be regarded as satisfactory and for all we know planets may be the rule rather than the exception. 

Recent observations on the binary stars 61 Cygni (10.7 light years) and 70 Ophiuchi 112 light years) have indicated that nonluminous bodies of almost planetary dimensions are associated with both systems. If indeed two such close stars have planets it might well indicate that planetary systems are by no means rare phenomena and among the score or so stars which lie within about a dozen light years of our own Sun there may be many planets. One thing is certain, namely that an expensive expedition taking a small community on a thousand-year voyage to another star, would not start unless it was certain that the star possessed planets of more or less terrestrial characteristics. Thus the era of astral exploration must be preceded by a period of observational astronomy the like of which we could not contemplate today. 

Instruments of performance vastly superior to those known at present would be required to survey the neighborhood of the nearest stars. The problem would be to resolve planetary images by perhaps no more than 0.5 second of arc, the planetary image having perhaps a strength of less than one billionth of the primary image. No telescope in existence today could give this performance or anything remotely approaching it, for although a moderate telescope of about 12 inches aperture could resolve two stars with this separation, the resolution of two fringe systems I the Airy Diffraction patterns, which represent the two images) when the central maximum of the one has an intensity of one billion times that of the other is quite a different matter. 

However, the eventual construction of giant telescopes on the Moon or other comparatively airless bodies might make such observations possible. Even so, it might not prove possible to do any more than measure the orbits of planets detected in this manner, observe their spectra and obtain rough values of size on the basis of reflected light intensities. A more detailed survey might require an expedition to the system in question with provision for return unless the exploring party was provided with means of signalling its findings over the vast distance separating it from the home planets. This would be possible with existing radio techniques. 

 

The 1,000-Year Voyage 

The author is not competent to deal with the biological problems of life on an interstellar vehicle undertaking a voyage lasting for a millennium. Obviously they would assume a magnitude quite as great as the engineering problems involved. In the normal way, some thirty generations would be born and would die upon the ship. It would be as though the vessel had set out for its destination under the command of King Canute and arrived with President Truman in control. The original crew would be legendary figures in the minds of those who finally came to the new world. Between them would lie the drama of perhaps ten thousand souls who had been born and had lived and died in an alien world without knowing a natural home. 

Perhaps this picture might change as a result of advances in medical science which are not yet visualized. It would be idle for a physicist to speculate upon possibilities which might exist in this field and it is no purpose of mine to do so. 

It is obvious that a vehicle carrying a colony of men to a new system should be a veritable Noah's Ark. Many other creatures beside man might be needed to colonize the other world. Similarly, a wide range of flora would need to be carried. A very careful control of population would be required, particularly in view of the large number of generations involved. This would apply alike to humankind and all creatures transported. Life would go on in the vehicle in a closed cycle; it would be a completely self-contained world. For this and many other obvious reasons the vehicle would assume huge proportions; it would, in fact, be a very small planetoid, weighing perhaps a million tons excluding the dead weight of propellants and fuel. Even this would be pitifully small, but clever design might make it a sufficiently varied world to make living bearable. 

The passage of perhaps thirty generations would pose major problems of a sociological nature. The control of population would be only one of many. Children could only be horn according to some prearranged plan, since overpopulation or underpopulation would be disastrous. The community would be subjected to a degree of discipline not maintained in any existing community. This isolated group would need to preserve its civilization, and hand on precious knowledge and culture from generation to generation and even add to the store of science and art, since stagnation would probably be the first step to degradation. 

On the technical side one would list the conservation of habitability as one of the major undertakings. Maintaining a reasonable atmosphere over the long period of space flight would be no small matter. Loss of air from the vehicle, and of other volatile materials for that matter, could be very serious when integrated over a thousand years. A hundred milligrams of air leaking out of a million-ton vessel in one second sounds insignificant, but in 1,000 years it would amount to a loss of 3,000 tons of material. Artificial gravity would need to be provided by rotation, and one might visualize the vessel as a huge oblate spheroid. The list of problems could be increased endlessly, but they would all add up to the fact that an interstellar expedition under such circumstances would be formidable. 

 



 

Flight at Near-Optic Velocity 

Thus far we have confined our attention to a conception of interstellar flight based upon known principles, although involving considerable extrapolation from present technical capabilities. This has been characterized by a picture of voyages taking centuries or millennia to complete, entailing a strangely planetless existence for the travellers. This is not the notion of interstellar travel envisaged by many people to whom even the speed of light would be an irksome crawl. Since we are in no position to judge what sources of energy will be exploited in the remote future it is essential that we should investigate some of the features of flight at velocities approaching that of light itself. 

It is an observed fact in our physical world that the speed of light in empty space is a constant and that no change in the motion of the Earth relative to other heavenly bodies results in any measurable change in this velocity. This fact led Lorentz, Fitzgerald, and others to formulate new equations of motion, kindred to the theory of relativity by Einstein. 

A consequence of these relativistic laws of dynamics which is usually regarded as setting a limitation upon flight to the stars is the limiting nature of the velocity of light. According to these laws a material body can never attain the velocity of light. The simple rules which govern the addition of velocities in the old Newtonian dynamics do not apply to very high velocities. To illustrate this let us consider a vehicle moving at a velocity of 0.9 c. It is convenient to measure very high velocities in terms of the velocity of light using c. as the symbol for velocity of light; thus 0.9c means nine-tenths of the velocity of light). This velocity of 0.9 c. is the value measured by some observer at rest (with respect to the surrounding stars) who is watching the vehicle. Suppose that there is a super-gun on the vehicle capable of projecting a missile with a muzzle velocity of 0.1 c. If this gun is fired in the forward direction of the vehicle's motion, then according to the old Newtonian dynamics the observer should find that missile is moving with the velocity of light. But, according to the Einstein theory of relativity the observer will find that the velocity of the missile is 0.918 c. 

Two further interesting facts are apparent from the relativistic theory. The first is, that an observer watching a vehicle which is moving relative to himself will find that the length of the vehicle appears to be reduced along the direction of motion. This effect is known as the Fitzgerald-contraction. 

The second effect of importance is the time-dilatation., According to the theory of special relativity, the clocks on the moving vehicle will appear to the observer to be running slow. That is, the frequency of events which occur on the vehicle will appear to him to he reduced. In fact, the frequency of events appears to he reduced in exactly the same proportion as the length of the vehicle appears to be reduced in the direction of motion. We shall call this reduction the y-ratio. 

Thus an observer, watching a vehicle moving relative to himself at the velocity 0.990 c., will find that its length along the direction of motion appears to be only 0.142 of its length when at rest with respect to him, and the frequency of events which occur on the vehicle will appear to be only 0.142 or one-seventh of their normal rate. 

The time-dilatation effect has been checked experimentally by observations of electrically charged particles called mesons passing through the Earth's atmosphere. These particles interact with the oxygen and nitrogen nuclei after entering the Earth's atmosphere and undergo a change, which resolves them into an electron emitting gamma rays and neutrinos. The height at which this change is made is known, as is the life of the meson. According to these factors and despite the fact that they may be traveling at the speed of light, only an immeasurably few mesons should ever reach the surface of the earth. In point of fact, a large proportion do reach the surface, a fact which can be explained only on the basis of the time-dilatation effect. 

The slowing down of the tempo of events upon the moving vehicle relative to the tempo of the same events for the observer at rest, has an important bearing upon interstellar flight. If X is a traveler on an interstellar rocket which leaves the earth at near-optic velocity and returns with the same speed, then the time which he records for the voyage does not agree with that of an observer whom we shall call Y, and who remains on the home planet. 

To take an example, suppose X goes from our system to Procyon (10.4 light years) and back with a velocity 0.990 c. The result is that, while the observer Y records X's return 21 years later, X is aware only of a passage of 3 years. Indeed if X could get close enough to the speed of light he could circumnavigate the universe in his life-time, though he would find on returning that perhaps 10 billion years had elapsed and the solar system and stars that he knew had changed beyond recognition. This would in fact be one-way time travel. 

 

CLEARLY, the attainment of velocities close to that of light would make interstellar travel a much more promising proposition than would the speeds which we have considered in the first part of this discussion. It would become possible for a man to leave his native system, journey to a star—even fairly distant ones would be within reach—and return within a few years of his time. The only shortcoming would be the fact that a long time would actually have elapsed at home during his seemingly short voyage, and friends whom he left in the bloom of their youth would be found in their dotage. It is possible that human society, with the help of medical science, could adapt itself to such a state of affairs, but it is no purpose of mine to speculate on such matters here. 

The first requirement of a vehicle designed to reach velocities close to that of light would be a source of energy far more potent than anything known today. The best that we could imagine in terms of our present-day knowledge would be the wholesale conversion of mass into energy. In modern nuclear and cosmic ray physics we know of a small number of processes whereby particles are completely converted into radiation, the oldest known process being the mutual annihilation of the electron and positron, with the complete conversion of their mass into electromagnetic energy in the form of two equally energetic photons. It is not known whether nucleons (ie, protons and neutrons) can undergo similar processes; searches for the negative proton have so far proved fruitless. However, it is possible that such a particle can exist, if only for a short time, in which case the complete conversion of nuclear matter into energy is possible at least in principle. It would be quite pointless to speculate upon the possibilities of releasing energy in useful amounts by annihilation, since there is no process known to us today which might produce such an effect. However, it is interesting to investigate the behavior of a rocket making use of such powerful energy sources. In a conventional rocket, decrease of mass occurs by virtue of the fact that matter is ejected in the exhaust jet. On the other hand, in the super-rocket utilizing the wholesale conversion of matter into energy, mass is lost both through the expulsion of material in the jet and also the disappearance of matter which is converted into energy. If the full cycle of energy conversion is 100% efficient, however, it can be shown that the rocket behaves as though all the matter is expelled in the jet. 

A case of particular interest is a rocket propelled by photons, i.e. by electromagnetic radiation. In such a rocket the available matter would be converted into radiation, which would then be directed into a beam leaving the tail of the rocket. This would be, in fact, a propulsive jet with an exhaust velocity c. The energy conversion would be 100% efficient, if all the radiation was beamed in the same direction. 

A comparison of the classical and relativistic equations of motion shows that, while the latter places restrictions upon interstellar flight from the point of view of the stationary observer, it actually favors the traveler because of the time-dilatation effect. To illustrate this, let us consider a rocket with a mass ratio of 7.4, 100% efficient energy conversion, and a photon drive with exhaust velocity c. According to classic theory, such a rocket could attain a velocity 2c., or alternatively accelerate up to a velocity c. and then retard to zero. Thus on the classical theory, neglecting periods of acceleration and retardation, a rocket would reach Alpha-Centauri in 4.3 years. 

In the relativistic theory, however, the actual velocity reached by the rocket will be 0.76 c., whence in the rest system the transit time would be 5.61 years. On the other hand the transit time in the traveler's frame of reference is only 3.67 years. Thus when we speak of the principles of relativistic mechanics being a barrier to interstellar flight, we are not being strictly correct. The most serious factor restricting journeys to the stars, indeed, is not likely to be the limitation on velocity but rather limitation on acceleration. 

It is clear, therefore, that the limiting nature of the velocity of light is not necessarily the most serious barrier in the attainment of interstellar flight. In fact in most respects it is no barrier at all. The real difficulty, always assuming that we can find suitable energy sources for the job, lies in the unfavorable ratio of power dissipation to acceleration as soon as we become involved with high relative velocities. The problem is fundamental to any form of propulsion which involves nonconservative forces e.g., the thrust of a rocket jet to produce the necessary acceleration. The only method of acceleration conceivable that would not be subject to this difficulty would he that caused by an external field of force. 

It might be argued of course that emitters at a temperature of 100,000° Kelvin are not fundamentally impossible and that such temperatures are not even outside the range of our experiences today. However, the fact is that the utilization of radiators at this temperature is quite inconceivable in terms of existing techniques and we are therefore in no position to speculate profitably. 

 

The Effect of Interstellar Matter 

As soon as we consider motion through space at velocities comparable to that of light, we can no longer regard interstellar space as a complete vacuum. Interstellar matter is known to exist with an average density equivalent to about 1 hydrogen atom per cm3, though with variations of up to 1,000 between regions of lowest and highest densities. This matter occurs in two forms, (a) the interstellar gas, and (b) interstellar dust, it being probable that the latter accounts for approximately 1% of the total interstellar matter which consists mainly of gaseous hydrogen and helium. 

M. W. Ovenden has examined the possibilities of collisions between an interstellar vehicle and the dust particles. He has found, on the basis of the available evidence, that the probability of collision is so low as to be no risk at all. We shall confine our attention to encounters between the vehicle and the interstellar gas. 

At a speed of 8,100 miles per second a proton striking the vehicle would penetrate only a few microns into the hull of a ship. A small amount of radioactivity would be produced as a result of nuclear disintegrations caused by the bombardment, but this would he of no consequence. We see that a hull 1 cm thick of some material such as aluminum would be effective protection even at velocities up to 60,000 miles per second. 

The problem becomes serious at velocities of 120,000 miles per second or more, when the oncoming particles represent a flux of "cosmic radiation" of great intensity. It would prove necessary to dispose a considerable mass of material in front of the living quarters of any vehicle traveling at near-optic velocities. This material could of course be used up in the closing stages of the retardation program when the velocity had been reduced to a safe level. 

It is evident from the above considerations that the existence of interstellar matter could not be ignored in vehicles traveling near the speed of light. Precautions would have to be taken to ensure that the interstellar particles or any of the secondary radiations (mesons. etc.) produced by them could not penetrate to the living quarters, and that the heat produced from these particles did not result in excessive temperatures within the vehicle. 

 

Conclusions 

There does not appear to be any fundamental reason why human communities should not be transported to planets around neighboring stars, always assuming that such planets can be discovered. However, it may transpire that the time of transit from one system to another is so great that many generations must live and die in space, in order that a group may eventually reach the given destination. There is no reason why interstellar exploration should not proceed along such lines, though it is quite natural that we should hope for something better. To achieve a more satisfactory performance, however, we should need sources of energy far more powerful than any utilized or known today. Nothing less than the complete conversion of matter into energy would suffice to bring about speeds where the traveler could exploit the relativistic time-dilatation effect which would reduce interstellar transit times to quite moderate proportions. Even if we assume that such energy conversion will become possible it is by no means certain that we should be able to make effective use of it. A probable stumbling block in this direction is the extremely unfavorable ratio of power to acceleration which results from the use of exhaust velocities approaching the speed of light. It is quite impossible at the present time to come to any conclusion regarding the possibility of interstellar flight along these more ambitious lines. 

The notion that the impossibility of velocities greater than c. places any serious restriction upon interstellar communication is based upon a rather narrow outlook. It is true that if we think in terms of interstellar vacations, business trips to Capella and interstellar warfare, a certain inconvenience would be encountered. But these are minor features, and taking the broad view of things, the fact that it may take many years to go from one system to another is no great restriction. Indeed, so far as the traveler is concerned the limiting nature of the speed of light is more than offset by the time-dilatation effect; from his viewpoint the speed of light is infinite. And, though man can never travel faster than light, his frontiers will be limitless if he can approach it ever closer.