"While our bodies are slowly destroyed in this manner, the process is opposed by what we may call the natural rejuvenation factor of living substance. Experiments with radioactive isotopes as tracers show that, in a cycle of a few years, all the atoms constituting our organism are replaced by new ones. That means that there's not a single atom in us today which was there a few years ago! This not only renews the building blocks of cells which have died in any fashion, but also continuously eliminates the detritus caused by the bombardment of cosmic radiation particles.
"We can envisage a relatively simple image of the dangers of cosmic radiation in open space if we set up a frequency of occurrence comparison for primary particles plus the secondary and subsequent collision products as occurring in various altitude layers of our atmosphere.
"For some time — and the figure has been confirmed by measurements in Lunetta — we have known that every square foot of the upper atmosphere receives per minute some 21,500 primary protons and 79 heavier nuclei. These figures give us a measure of the density of cosmic rays in empty space which you and your Mars vessels will traverse.
"At 60,000 feet, the primary particles have been reduced to 6,400 by collisions with the atoms of the atmosphere. But the collisions have the effect of producing at this height a supplementary stream of radiation consisting of 17,000 secondary protons, 15,000 neutrons and 12,000 mesons. Additionally, every square foot at this altitude is penetrated by 48,000 electrons and positrons, produced by the collisions, per minute.
"At about 14,000 feet, an altitude exceeded by but a few of our mountains, the number of primary particles is reduced to a very small, not accurately known figure. But there are still some 300 secondary protons, an equal number of neutrons, and about 2,500 mesons.
"At sea level, these figures are down to 20, 20 and 1,100, respectively.
"What may we deduce from these figures?
"The first thing is that the comparison shows indubitably that the total density of radiation at 60,000 feet altitude is much higher than in empty space; the number of particles at 60,000 feet is about 86 times as great as at sea level, while in empty space it is only about 19 times as great! If we choose to consider secondary particles of cosmic radiation to be as dangerous as the faster primary particles — which may not necessarily be quite correct — it is obviously far more dangerous to fly in the atmosphere at 60,000 feet than to remain in empty space for an equal period.
"In neither case, however, do we need to be much concerned about our health. The natural regenerative processes of our organism are competent to remove the detritus of the cosmic ray bombardment and to provide new atoms as they work their continuous miracle of cell rejuvenation.
"The problem of the effects of the bombardment upon our posterity is a different and perhaps a more serious matter. In the rapid succession of generations, in the continual disappearance and regeneration of our cellular structure there is a something which is as unalterable as the Rock of Ages. This something is the "genes," the bearers of hereditary qualities transmitted from parents to children. Genes supervise the continuity of the species; they explain hereditary family traits and determine the inherited dowry which is laid with each child into the cradle. The characteristics of the one and a half million genes which actively participate in one act of fertilization determine not only the character of the infant, but they also determine each and every physical detail, down to color of eyes, facial expression and skin pigmentation.
"Genes are also constructed of atoms and molecules, so that any damage to them by cosmic radiation must be far more fateful than damage to our organism itself. Changes in the structure of the genes, such as have been proved may occur under the affect of radiation, affect not only the characteristics of immediate offspring, but are fundamentally inheritable. Such permanent alterations of inheritance are known as 'mutations.'
"We may compute the probability that a gene, a unit of our inheritance, will be damaged by cosmic radiation, for it may be statistically treated, since we know the shotdensity and the size of the target. If we assume radiation density as that of ground level and apply it to the period extending from birth to the termination of reproductively of the average man or woman, the computation shows that a gene will be struck about ten times within the many generative cells generated over that period. But only a very tiny fraction of the generative cells of a person have the opportunity to participate in determining the characteristics of a child. Hence the probability of mutations affecting the individual family is minute.
"Due to the fact that there are some 2 billion inhabitants on our Earth, there is a goodly chance for a number of hereditary changes in each generation, despite the small likelihood of a mutation in any given family.
"A mutation is by no means always negative, for we have both good and bad inheritances. While some inherited diseases, such as the hemophilia of the Russian Czars, may turn up, improvements in heritage are also possible.
"It seems to me that the Darwinian doctrine of the survival of the fittest should here be introduced. A more intelligent, stronger, better species in the long run has a better chance of survival that the obtuse, weaker and poorer. Perhaps we can interpret the development of man from Homo primigenius to modern Homo sapiens as the combined effect of mutations on one side and Darwin's selective survival on the other. This may even be a tiny peep-hole into God's mysterious laboratory…
"Whatever our attitude towards such interpretation, one thing is certain, and that is that every living thing on Earth is subject to a certain, definite, but small prospect of a mutational change by cosmic radiation. The prospect is immeasurably small for any individual, as has been strongly attested to by the amazing uniformity with which human, animal and plant species have continued themselves through the ages.
"The total cosmic radiation density in empty space is some 19 times greater than on Earth, according to the figures just presented. A gene, however, cannot be exposed to particles penetrating the body until a certain layer of the body had been traversed. The thickness of this layer will, of course, be affected by the angle at which the particle impinges. In space, the particles approach from any direction, so statistically we shall do well to figure a certain mean layer thickness. This is about equivalent to 20 centimeters of water surrounding the genes. Collisions may of course occur in this layer, preventing primary particles from reaching the genes to some extent, but also increasing the danger of their being struck by secondary products from these collisions. You will see that the body provides a shield for the genes similar to that of a thin atmospheric layer and that the density of particles is somewhat higher than that produced by the primary radiation alone.
It can be estimated that in empty space the genes are not subject to 19 times the surface level cosmic bombardment, but to one 40 times as heavy.
"On the other hand, cosmic rays are by no means the only sources of mutations. It has been scientifically confirmed that certain chemicals, of which colchicines is one, can also affect the genes. Botanists have found in experiments with plants that exposing the genes to marked changes of temperature, in particular shortly prior to the fertilization process, has much the same effect. Mutations are sometimes caused by radiation emitted by lighting. There may even be arbitrary reorientations within the complicated molecular structure of the genes.
"Therefore there are quite a variety of additional causes of mutation aside from cosmic radiation. Some of these causes, such as lightning, do not occur in empty space.