Most of the information that man is able to obtain about the universe comes to him in the form of electromagnetic radiations. Some of these radiations he can see with the naked eye. But visible light represents a very small part of the total radiation spectrum (see fig. 1); the Earth is constantly being bombarded with other forms of light," which are invisible to the human eye.
If we spread out the entire radiation spectrum as it occurs in nature, we find that a star like the Sun concentrates most of its energy in a relatively narrow band stretching from the near ultraviolet into the infrared portion of the spectrum. That is, most of the Sun's energy is visible to us by optical means. In addition, however, very high energy radiations of short wavelength in the form of X-rays, gamma rays, and cosmic rays are present, as well as radiations of low energy and long wavelength, all of which are invisible to the eye. The eye is sensitive only to those radiations that fall within a small band of wavelengths. Radiations whose wavelengths are longer or shorter than those to which the eye can respond must be "seen" with other sensors. 1
We can see into these hidden parts of the spectrum by using various kinds of detectors. Properly coated photographic plates, for example, allow us to look part of the way into the infrared region, and they also permit us to look into the ultraviolet, X-ray, and gamma-ray regions of the spectrum. Other devices, such as photoemissive cells photoconductors, and bolometers, allow us to peer further into the Infrared, while Geiger counters, scintillation counters, etc., permit us indirectly to "see" gamma radiation and some of the high-energy cosmic rays that come through the atmosphere. Radio antennas, of course, provide us with eyes to see into the longer wavelength portion of the radiation spectrum.
While we have developed sensors that will enable us to see and measure much of this very short and very long wavelength energy, we have not been able to exploit them fully because only a part of the energy in these portions of the spectrum reaches the Earth. Much of it is blocked or deflected, absorbed or screened out by the Earth's atmosphere and magnetic field.
Atmospheric turbulence, air currents and eddies make the images of the celestial bodies shimmer and dance about on the photographic plates of telescopes, so that we can only see a blur where we should see a sharp picture. The biggest telescopes on Earth normally operate with an effective resolution which is about one-twentieth of the theoretically obtainable value.
1 We can, of course, produce many of these radiations or forms of energy, on Earth. Radio and radar transmitters, light bulbs, X-ray equipment, and nuclear explosions all produce electromagnetic energy in different parts of the radiation spectrum. We cannot however, as yet produce the highest energy cosmic rays,which are really solid particles moving at tremendous velocity, here on Earth.
Low-energy radio waves are reflected and absorbed by the electrons and ions of the ionosphere, and never get through the atmosphere. In fact, most of the radio-frequency energy that envelops the Earth cannot penetrate the ionosphere.
The Earth's atmosphere is completely opaque also to wavelengths shorter than a few millimeters, and it does not "open up" until one reaches the near infrared, where ordinary light and heat rays exist. At still shorter wavelengths, because of the influence of ozone and other atmospheric gasses, the Earth's atmosphere blots out the ultra-violet radiation from the Sun and the other Stars. The so-called soft X-rays emitted by the Sun can only penetrate the outermost layers of the Earth's atmosphere. The atmosphere in general is opaque to X-rays and to gamma rays. Only the most energetic cosmic rays succeed in penetrating the Earth's atmosphere down to the ground, so that information about the lower energy primary cosmic rays is not available on the surface of the Earth. Moreover, the Earth's magnetic field repels low-energy primary cosmic rays and is another factor in keeping them from reaching the ground.
In fact, the plight of the Earth-bound observer until now may be likened to that of a man imprisoned in a stone igloo with walls and roof 10 feet thick. At the time of his imprisonment, a small hole in the roof directly overhead was his only connection with the outside world. Since then, through a great deal of labor, aided by ingenuity, he has been able to enlarge this roof opening slightly, and also to bore two small holes in the wall close to the ground, on opposite sides of his prison. Through these three "windows" come all the information that he is able to obtain about the world around him, and it is with this meager knowledge, coupled with his imagination, that he must construct a picture of the outside world.
Applying this idea to the radiation spectrum in figure 1, we may say that man's knowledge of the universe has come to him through three "windows" in the spectrum. Through a radio window he "sees" a wide range of radio frequencies from different types of celestial objects: the Sun, the planets, possibly other stars, clouds of gas in our own galaxy, exploding stars, and outside universes. Through an optical window, he has been able to obtain other information about the universe with the aid of telescopes, spectroscopes, cameras, etc. And finally, there is the cosmic-ray window, through which still other bits of information have come to him. Except for these apertures, however, the wall of the atmosphere shuts him in. 2
A prime question of concern to astronomers is the composition of the universe; that is, the relative abundance of different species of atoms. The most abundant elements in the universe are thought to be the lighter ones: Hydrogen, helium, carbon, nitrogen, oxygen, neon, etc. Unfortunately, their strongest spectral lines are in the far ultra
2 The areas beneath the curves in the three open regions indicate the amount of energy at various wavelengths that reaches the Earth. While the total quantity of energy that reaches us in the form of visible light is quite high, the amounts in the radio wave and cosmic-ray portions of the spectrum are relatively low. Most of these energies lie in a region of the spectrum to which we are denied access from the ground.
violet region of the radiation spectrum, which can never be observed from the surface of the earth. While indirect methods have given us some preliminary data, the fundamental questions of how much of these substances exists and whether the stars differ in composition from one another must still be considered as unsolved. The answers would be of the utmost importance to astronomy. These spectra, especially in the bright and very hot stars, could be observed with apparatus in the few-hundred-pound satellite-payload range, and the results telemetered to earth with equipment that is now available. The total amount of information that must be transmitted to obtain the first spectroreconnaissance of the stars is in fact quite small, and useful observations could be done within today's technology.
Obviously, the nearest star, the Sun, is worthy of the most detailed study. This is true not only because the Sun is the prime source of the Earth's energy, but because the ultraviolet, X-ray, and gamma-ray radiation of the Sun will be controlling factors of man's environment in space, and they may provide hazards to man's survival which are of a nature still unknown and unsuspected.
Moreover, the effect of the Sun's ultraviolet radiation dominates the ionization of the outermost atmosphere of the Earth, which has an important effect on communication, and possibly a very much more important one on the weather. The gain for pure scientific research, however, that could be obtained through detailed observation of the solar ultraviolet spectrum will come mainly in explanations of the origin of the hot outermost layers of the Sun. While the surface of the Sun is at a temperature of about 6,000°C., the temperature rises as one goes out from the Sun, reaching at least a million degrees at a distance of some 20,000 kilometers from the Sun's surface. We have no explanation of the origin of this heating; we do not know where the Sun's outer envelope stops, and there is a good chance that the Earth itself is immersed in this corona.
The Sun is subject to violent storms, manifested by so-called sun- spots, prominences, and solar flares. These disturbed outer regions of the Sun have an important effect on the Earth's ionosphere, geomagnetism, and, in the long run, ordinary weather. It is technically feasible to map the Sun in terms of its emissions in the ultraviolet or even soft X-ray portions of the spectrum. Such observations unobstructed by the atmosphere, would provide us with a detailed history of what happens during solar storms and flares. Pictures could be telemetered back on a more-or-less continuous basis, providing warning of solar disturbances which could affect the Earth.
To obtain a true picture of the total cosmic-ray energy enveloping the Earth, particularly that which reaches us from the Sun during solar storms or flares, it will be necessary to reach beyond the atmosphere and through the Earth's magnetic field to a distance of perhaps 25,000 miles. The longer wavelength primary cosmic radiation, where in fact most of the cosmic-ray energy is concentrated, must be investigated experimentally if we are to obtain information about the origin of these particles, their effect on the Earth and its inhabitants, and the hazards they may present to space travel.
The question of photographing other objects in our universe from space, and the possibility of our finding new kinds of objects, is an important one. Even low-altitude orbiting satellites, and certainly
high-altitude satellites, would permit us to make detailed observations of the structure of the surfaces of the planets and the composition of their atmosphere. At the present time there are indications from ultraviolet measurements made from Naval Research Laboratory rockets that there are in space large clouds of gas of an unknown nature which shine in the ultraviolet. A photograph of even minimum resolution taken in the far ultraviolet would decide this question. In particular, astronomers have found that the most abundant element in the universe, hydrogen, is present throughout our Milky Way. This is detectable so far by radio observations only, although it is known that perhaps 10 percent of the total mass of our universe is in the form of gaseous hydrogen in space. Direct photographs, or spectra, of this interstellar hydrogen can be obtained once a vehicle has penetrated the atmosphere of the Earth, photography being important both in the ultraviolet and in the infrared.
Another very important question concerns the distribution of other systems of stars as one goes farther outward into space. Because of the apparent expansion of the universe, there is a shift of light emitted by distant objects toward the red portion of the spectrum, known as the "red shift." At the present time, external systems of stars, extragalactic nebulae, have been photographed out to distances of at least 2 million light years, and as far as we know, exist very much farther out into space. However, at great distances the red shift becomes larger and all the light of the system is shifted into the nonphotographable far infrared. There is no doubt of this effect, and what we do not know is how far out this red shift can be extrapolated. In particular, it is not necessary to photograph individual galaxies of very great faintness, but an important set of conclusions on the nature of the expansion of the universe and fundamental cosmology could be reached by merely measuring the total brightness in the far infrared of all of the galaxies together. Apparatus providing rough spectral resolution, so that quantitative measures of the brightness of the sky at many different wavelengths in the far infrared could be obtained, would perhaps settle some of the fundamental questions of the expansion of the universe, and the distance to which it stretches. This has an important bearing also on the age of our universe.
The subject of radio astronomy, which has grown rapidly in importance in the last few years, has already provided many scientific surprises. For example, the second brightest source of radio waves from outside the Earth turns out to be a pair of colliding galaxies at a distance of 300 million light years. Since most of the radio-frequency energy that occurs in nature does not penetrate the ionosphere, it can be measured only by probing above the ionosphere. Such investigations would greatly extend our knowledge of the total energy involved in radio emissions from these strange sources, perhaps giving us a clue to their origin.
One very important possibility considered by several scientists in recent years is that the existence of both extremely intense radio-frequency radiation and very high energy cosmic rays is an indication that we have still to discover some fundamental properties of the universe. One may speculate that the most fundamental processes in the universe are those involving extremely high energy particles, and that these may be produced by some as yet unknown physical mechanism. While tentative explanations of the origin of cosmic rays exist and suggest
that they are merely matter accelerated to ultrahigh velocities, differing negligibly from the velocity of light, it would seem desirable to obtain a complete survey of the total electromagnetic and particle spectra at altitudes where the Earth is no longer a disturbing factor. Theories of the origin of the Earth have suggested that all matter was produced in a primeval explosion some 10 billion years ago. Are there any relics of this explosion available still in the form of very high energy radiation ? Other theories suggest that matter is continuously produced in intergalactic space. Is this latter theory tenable? Are there any evidences in the radiations coming from space of the continuing creation of matter ?
Before mounting a large-scale attack on the space frontier, it is essential that we consider the interactions among possible experiments to be sure they are done in the proper sequence. It is conceivable, for example, that an early experiment, done merely because the means were available, could so alter the natural environment that other important experiments would no longer be possible. Experiments must be planned with due regard for leaving future parts of an overall program intact.
It must also be borne in mind that expensive scientific ventures in space will only be effective if backed up by adequate theoretical studies and laboratory research on the ground.
Experiments in space biology
Astronautics will provide new approaches to some of the fundamental problems in biological science. The study of terrestrial life forms in radically new environments (and perhaps even non terrestrial life forms) will become possible, providing opportunities for increased understanding of the nature of the life process, how it originated, how it evolves and functions, and what forms it may assume under widely different environmental conditions. An experimental program in space biology should include:
3 By astrophysical properties are meant absorption coefficients, masses, sizes, etc.-in general, those properties which would give information on how the organisms would react to radiation fields and other forces affecting their transport and physical state in space.
These laboratory tests should, of course, be supplemented by investigations under actual conditions in free space.
Experiments with sounding rockets
Vertical rockets, of the Viking and Aerobee type, will continue to be useful, as in the past, for measurements of upper atmosphere phenomena and composition. They can also be used for high-altitude observations of the Sun, Moon, planets and other celestial objects. (Balloons, of course, are also useful in this connection.)
Experiments with satellites
Uses of satellites for scientific observations have been mentioned under "Observation satellites" and "Meteorological satellites."
Satellites can remain in space permanently for long-term observation to altitudes of about 1 million miles, and would be most useful for continued mapping of the "radiation belt" disclosed by the-IGY satellites.
Some experiments appropriate to satellites of various payload classes are:
1. Less than 100 pounds:
4. Ten thousand pounds:
Some of the purposes to which lunar rocket experiments might be turned include measurements of-
The measurements that might be made on other planets are generally the same as those pertinent to the Earth itself, as modified by the singular features of each planet.