The salient known physical data on the principal objects of interest in the solar system are given in table 1. Many other, more detailed, characteristics of the planets such as the eccentricities and inclinations of their orbits, the inclination of their axes, their densities, albedoes, etc., are available in standard books on astronomy.

For a general appreciation of the environment of space travel the following characteristics of the solar system should be noted:



The general disposition of planetary orbits is illustrated in figure 1

TABLE 1.Physical data on principal bodies of solar system
Body Mean
from Sun
1.00 or
1 a.u.)
(st. mi)
force at
of sun-
light at
sity at
of day
of year
ber of
























88 days
24 hours
24.6 hours

10 hours
11 hours
16 hours


27 days

88 days
225 days
365 days
1.9 years

12 years
29 years
84 years
165 years

248 years






1 No solid surface
2 Location of solid surface ( below the thousands of miles depth of dense atmospheric gases covering these planets) is not known; hence, surface gravity figures are meaningless for the four giant planets.


drawing of the nine planets of our solar system in their relative orbits to our sun

Fig.1 - The Solar System



In astronomical terms, the Sun is a "main sequence" star of spectral type G-zero with a surface temperature of about 11,000° F.1 Although a medium-small star, it is over a thousand times as massive as Jupiter, and over 300,000 times as massive as the Earth. Its energy output, as light and heat, is extremely constant, probably varying no more than about 0.5 percent from the average value.2 However, it is much more variable in its Production of ultraviolet radiation, radio waves (solar static), and charged particles. At infrequent intervals, extremely intense solar outbursts of charged particles (cosmic rays) have been observed. The most recent of these outbursts, which occurred on February 23, 1956, and lasted about 18 hours, resulted in a peak intensity of ionizing radiation above the atmosphere estimated at about 1,000 times the normal value.3

All usable forms of energy on the Earth's surface, with the exception of atomic and thermonuclear energy, are directly or indirectly due to the storing or conversion of energy received from the Sun.


The planet closest to the Sun, Mercury, is difficult to observe because of its proximity to that body; hence, our knowledge of its physical characteristics is less accurate than for some of the other members of the solar system. Mercury has no moon, and its mass is not known with precision, but is of the order of one-twentieth that of the Earth. This much is known, however: It is a small rocky sphere, about half again as large as the Earth's moon, and it always keeps the same side turned toward the Sun. The sunlit half is thus extremely hot, probably having maximum surface temperatures as high as 750° F., while the side in perpetual darkness is extremely cold, cold enough to retain frozen gases, with temperatures approaching absolute zero. Mercury is not known to have any atmosphere, nor would a permanent gaseous envelope be expected to occur under the conditions existing on the planet. Its rocky surface is probably somewhat similar to that of our Moon.

Even less is known with confidence about the surface conditions on Venus. Therefore, many statements about it are necessarily more speculative than definitive. In dimensions and mass it is slightly smaller than the Earth, but no astronomer has ever seen its solid surface, since its dense and turbulent atmosphere, containing white particles in suspension, is opaque to light of all wavelengths. Neither free oxygen nor water vapor has been detected on Venus, but carbon dioxide is abundant in its atmosphere, as determined by spectrographic analysis of the light reflected from the upper reaches of its visible cloud deck. On the basis of all the available evidence, it may be presumed that the surface of Venus is probably hot, dry, dusty, windy, and dark beneath a continuous dust storm; that the atmospheric pressure is probably several times the normal barometric pressure at the

1 Hoyle, F., Frontiers of Astronomy, Harper & Bros., New York 1955

2 Roberts, W. O., The Physics of the Sun, the Second International Symposium on the Physics and Medicine of the Atmosphere and Space, San Antonio, Tex., November 12, 1958.

3 Schaefer, H. J., Appraisal of Cosmic-Ray Hazards In Extra Atmospheric Flight, Vistas in Astronautics, Pergamon Press, 1958.


surface of the Earth; and that carbon dioxide is probably the major atmospheric gas, with nitrogen and argon also present as minor constituents. 4

Much more complete information is available about Mars, but many questions about surface conditions still remain unanswered. With a diameter halfway between that of the Moon and the Earth, and a rate of revolution and inclination of Equator to orbital plane closely similar to those of Earth, it has an appreciable atmosphere and its surface markings exhibit seasonal changes in coloration. Its white polar caps, appearing in winter and vanishing in summer, are apparently thin layers of frozen water (frost) of the order of fractions of an inch to several inches in thickness. The atmospheric pressure at the surface has been estimated at 8 to 12 percent of Earth sea-level normal, and the atmosphere is believed to consist largely of nitrogen. No free oxygen has been detected in its atmosphere.5 Nothing definite is known about the presence or absence of marked differences in the altitude of the terrain. The "climate" would be similar to that of a high desert on Earth to an exaggerated degree (about 11 miles high, in fact) with noon summer temperatures in the Tropics reaching a maximum of perhaps 80° to 90° F., but falling rapidly during the evening to reach a minimum before dawn of the order of -100° F. The interval between 2 successive close approaches of Earth and Mars is slightly over 2 years. At opposition, that is, when the 2 planets lie in the same direction from the Sun, the approximate distance between Earth and Mars ranges from 35 million to 60 million miles.

Bleak and desert like as Mars appears to be, with no free oxygen and little, if any, water, there is rather good evidence that some indigenous life forms may exist.

The seasonal color changes, from green in spring to brown in autumn, suggest vegetation. Recently Sinton has found spectroscopic evidence that organic molecules may be responsible tor the Martian dark areas.6 The objections raised concerning differences between the color and infrared reflectivities of terrestrial organic matter and those of the dark areas on Mars have been successfully met by the excellent work of Prof. G. A. Tikhov and his colleagues of the new Soviet Institute of Astrobiology.7 Tikhov has shown that arctic plants differ in infrared reflection from temperate and tropical plants, and an extrapolation to Martian conditions leads to the conclusion that the dark areas are really Martian vegetable life.

Although human life could not survive without extensive local environmental modifications, the possibility of a self-sustaining colony is not ruled out.

The giant planets
The four members of this group of planets (Jupiter, Saturn, Uranus Neptune) have so many characteristics in common that they may

4 Dole, S. H., The Atmosphere Of Venus, The RAND Corp., paper P-978, October 12, 1956.

5 Kuiper, G. P. (Editor); The Atmospheres of the Earth and Planets, the University of Chicago Press, Chicago, Ill., 1952.

6 Sinton, W. M. Further Evidence of Vegetation on Mars, to be presented at the meeting of the American Astronomical Society, Gainesville, Fla., December 27 30. 1958.

7 Tikhov, G. A., Is Life Possible on Other Planets? Journal of the British Astronomical Association, vol. 65. No. 3, April 1955, P. 193.


well be treated together. They are all massive bodies of low density and large diameter. They all rotate rapidly. Because of their low densities (0.7 to 1.6 times the density of water) and on the basis of spectral information, they all are thought to have a "rock-in-a-snow ball" structure; that is a small dense rocky core surrounded by a thick shell of ice and covered by thousands of miles of compressed hydrogen and helium. Methane and ammonia are also known to be present as minor constituents.8 Because of the low intensities of solar radiation at the distances of the giant planets, temperatures at the visible upper atmospheric surfaces range from -200° to -300° F. A number of the satellites of Jupiter, Saturn, and Neptune are larger than the Earth's moon, and some may be as large as Mercury. Although reliable physical data on these satellites are lacking, it is possible that they might be somewhat more hospitable for space flight missions than the planets about which they orbit.

Almost nothing is known about this extreme member of the known solar system except its orbital characteristics and the fact that it is extremely cold, with a small radius and a mass about 80 percent that of the Earth.


The Moon is about 240,000 miles from the Earth, and its diameter is about 2,160 miles, a bit more than one-fourth the diameter of the Earth. The mass of the Earth is about 81.5 times that of the Moon.

The Moon has no appreciable atmosphere, and its surface is probably dry, dust-covered rock. On the basis of terrestrial experience it would be expected that this rocky surface is far from uniform in chemical composition and physical arrangement.

The face of the Moon is covered with many large craters, the origin of which is still a matter of debate. Mountains on the moon are higher than those on Earth, presumably because they are free from weathering. A Soviet astronomer recently reported observation of an erupting volcano on the moon. Whether or not the observations actually support the stated interpretation has been questioned by some authorities.


In addition to the planets and their moons there is a group of substantial bodies known as asteroids in the solar system, more or less concentrated in the region between the orbits of Mars and Jupiter. lt is possible that these chunks of material may be the shattered remains of one or more planets.

Most of the asteroids have dimensions of some miles, but quite a few are as much as 100 miles across. The largest, Ceres, is nearly 500 miles in mean diameter.

Some asteroids come within a few million miles of the earth from time to time.


Comets are very loose collections of orbital material that sweep into the inner regions of the solar system from space far beyond the orbit

8 Urey, 11. C. The Planets Their Origin and Development, Yale University Press, New Haven, Conn., 1952.


of Pluto. Some return periodically; some never do. Their bodies consist of rarefied gases and dust, and their heads are thought to be frozen gases or "ices."


The Earth receives a large quantity of material from surrounding space in the form of meteoritic particles. Most of these are decomposed in the upper atmosphere, but some reach the Earth's surface.

These particles enter the Earths' atmosphere with velocities of 7 to 80 miles per second, producing visible light streaks called meteors. Estimates of numbers, sizes, and speeds of incoming meteorites are based in part on optical observation of meteors, and in part on radio wave reflections from the ionization trails left by meteorites. Data about smaller particles are deduced from other effects, such as sky glow at twilight.

Estimates, based on various assumptions, of the total volume of incoming meteoritic material range from 25 to 1 million tons per day. A very recent estimate made by the Harvard Observatory favors a value of 2,000 tons per day.9 Soviet scientists announced in August 1958 that their satellite data indicated an estimate of 800,000 to 1 million tons per day.10

The vast range of variability in these estimates is due in part to uncertainties in densities of meteoritic material experimental inadequacies, and incomplete theoretical basis for interpreting observations.

The information concerning meteoritic input to the Earth's atmosphere is very uncertain. The meteoritic content of other space regions is largely an open question, pending direct experimentation with space vehicles.


The smallest dust particles called micrometeorites are concentrated for the most part in the ecliptic, the plane of the Earth's orbit, and since they originate as cometary refuse they may also be found distributed along the orbits of comets.11

Evidence that cosmic dust is concentrated in the plane of the ecliptic consists of observations of a faint tapered band of light that can be seen at twilight extending up from the horizon centered along the ecliptic. This band of light, which can be photoelectrically traced through the complete night sky, is called the zodiacal light.

The layer of small meteoritic particles must extend from the sun well beyond the orbit of the earth, being concentrated toward the ecliptic or fundamental plane of the solar system. Further, this dust cloud must be continuously being resupplied by cometary wastage and possibly by material from asteroid collisions. It is at the same tune being drained off by the action of solar radiation which causes the particles to spiral in toward the Sun. It has been estimated that in 60 million years all particles smaller than 1 millimeter in diameter starting nearer to the Sun than the orbit of Mars, would reach the Sun due to this effect.

9 Whipple, F. L., The Meteoritic Risk to Space Vehicles, Vistas In Astronautics, Pergamon Press p. 115.

10 Nazarova I. N. Rocket and Satellite Investigations of Meteors presented at the fifth meeting of the Comité Spéciale de l'Année Géophysique Internationale Moscow. August 1958. Nazarova has very recently revised the estimate of meteor influx downward by a factor of about 1000.

11 Beard, D. B , Interplanetary Dust Distribution and Erosion Effects, American Astronautical society, Preprint No. 58-23, August 18-19 1958.



At the surface of the Earth, man and his machines are sheltered by the atmosphere from many radiations that exist in space. These radiations include X-rays, steady ultraviolet radiation, and cosmic rays.

At times of solar flares these may be great quantities of radiation less energetic than cosmic rays, in addition to the particles now known to be trapped in the geomagnetic field.

Cosmic radiation is a general term for high-speed particles from space. About 80 percent are protons, which are nuclei of hydrogen atoms carrying a unit positive charge, with the remainder distributed among a number of other subatomic particles.

Corpuscular cosmic rays, although "hard" or highly energetic radiation, are not so numerous as other corpuscular radiations within the solar system. The Explorer IGY satellites observed an encircling belt of high-energy radiation extending upward from a height of a few hundred miles. This belt ranges from at least 65° north to about 65° south latitude, although the radiation is most intense in the equatorial region.

Explorer IV data indicate that radiation intensity increases by a factor of several thousand between 180 and 975 miles, with a rapid rise beginning at about 240 miles. The level of radiation may reach as much as 10 roentgens per hour -enough to deliver an average lethal dose in 2 days to an unshielded human being.

Results from Sputnik III, which did not travel quite so far out in space but went to higher northern and southern latitudes, seem roughly compatible with the Explorer results. Tentative data from Pioneer indicate a rapid decay of radiation intensity with increasing distance beyond about 17,000 miles from the Earth.

Since the Earth's magnetic field, which stores and traps the particles comprising the radiation belt, decreases in strength with increasing distance from the Earth, it ultimately becomes too weak to store a significant amount of radiation.

Trapping of electrons or protons in the Earth's magnetic field is not expected to be of importance in very high latitudes, and, therefore, the radiation hazard will be much less near the poles than in the lower altitude radiation belt.

The apparent dose rate at altitudes between about 300 and 400 miles lies within the accepted AEC steady-state tolerance level for human beings of 300 milliroentgens per week (1.79 milliroentgens per hour). Therefore, this radiation belt does not interdict low-altitude manned satellites. It does imply that manned satellites orbiting at altitudes greater than 300 to 400 miles would require some shielding, the weight of shielding increasing up to the greatest altitude for which we have fairly firm information (roughly 1,200 miles). Beyond this altitude, the radiation levels are uncertain, but it is expected that at some altitude a maximum must be reached after which the dosage rates should diminish.

Depending upon the extent of the equatorial radiation belt, manned space flights could use the technique of leaving or returning to the Earth via the polar regions, or could penetrate directly through the radiation belt with adequate shielding to protect human beings during the transit,


The Sun, in addition to its steady radiations, also delivers great out bursts with radiation levels as much as 1,000 times normal at the times of large solar flares, which appear roughly about once every 3 years, as well as smaller increases almost daily.

At the present time, it is not possible to predict exactly when these large solar flares will occur. A man in a vehicle above the Earth's protective atmosphere, if exposed to the radiation from such a flare during its entire period of activity, might absorb enough radiation to make him ill (not immediately but by a week or two later). Since our knowledge is incomplete as to the peak intensities of solar flares, their frequency of occurrence, and their durations, the risk to human beings from this source cannot now be assessed with any real accuracy.

The constant radio noise background emitted by the Sun is also greatly enhanced during solar flares. This solar noise is reinforced somewhat by radio noise from distant galaxies. Some localized areas of far space radiate very intensely. Such a source is in the region of the Crab nebula.


Although not a theater of operations in the first phases of the space age, it is worthwhile to mention the larger setting in which the solar system itself figures.

The nearest neighbor of our solar system is the star system Alpha Centauri, a bright object in the southern sky at a distance of about 4 light-years. (Pluto is 5½ light-hours from the Sun.) Alpha Centauri is a double star whose two main components orbit about one an other. A third star called "Proxima" is also associated with this system and is actually at the present time the star closest to our solar system. ("Proxima" itself may also be doubled.)

It is not known whether the Alpha Centauri system has any planets, but observations of some other nearby stars, e. g., 61 Cygni, indicate, from wobbles in their motion, the possible presence of orbiting dark bodies with masses comparable to Jupiter's. There is, then, what might be considered indirect evidence for the existence of other planetary systems. Within 20 light-years of the Sun there are known to be about 100 stars with possibly 2 or 3 planetary systems, if the interpretation of the "wobbles" are correct. Kuiper estimates on the basis of the ratio of the masses of components of double stars that not more than 12 percent of all stars may have planetary systems.12 When we realize that there are some 2 x 1011 stars in our galaxy, this would give 1 to 10 billion with planetary systems. It seems reasonable to speculate that out of this vast number there surely must be some systems with earth-like planets, and that on some of these planets life similar to our own may have evolved.13-16

12 Kuiper, O. P., The Formation of the Planets, Journal of the Royal Astronautical Society of Canada, vol. 50, No. 2, p. 167.

13 Jeans, Sir James, Life on Other Worlds, A Treasury of Science, Edited by H. Shapley, Harper & Bros., New York, 1954.

14 Shapley, H., Of Stars and Men: The Human Response to an Expanding Universe, Beacon Press, Boston, 1958.

15 Planets: Do Other "Humans" Live? Newsweek, vol. LII, No. 20, November 17, 1958 p. 23.

16 Haldane, J. B, S., Genesis of Life, The Earth and Its Atmosphere, edited by D. R. Bates, Basic Books, Inc., New York, 1957.


With our present state of knowledge, however, communication with such planetary systems is a matter of speculation only. When we recall that our galaxy is some 100,000 light-years in diameter, the Sun being an insignificant star some 30,000 light-years from the galactic center, circling in an orbit of its own every 200 million years as the galaxy rotates, we realize that even trying to visualize the tremendous scale of the universe beyond the solar system is difficult, let alone trying to attempt physical exploration and communication. Nor is the interstellar space of the galaxy the end, for beyond are the millions of other galaxies all apparently rushing from one another at fantastic speeds; and the limits of the telescopically observable universe extend at least 2 billion light-years from us in all directions.