ASTRONOMY AND ARTIFICIAL SATELLITES—GOLDBERG 287 
No radiation of wavelengths shorter than about 2,900 A. ever reaches 
the surface of the earth, either at sea level or at the tops of high moun- 
tains. The absorption of ultraviolet radiation and X-rays is caused 
by atoms and molecules of nitrogen and oxygen and by molecules of 
ozone which are created by the action of sunlight on oxygen molecules. 
At infrared wavelengths, the atmosphere is partially transparent in 
a number of so-called wavelength “windows” from about 1 to 24 
microns. A small amount of radiation also leaks through at 1 or 2 
millimeters. At much longer wavelengths, the atmosphere again 
becomes transparent to radio waves from a few millimeters to about 
30 meters long, although observations have occasionally been made at 
wavelengths of about 100 meters through transient “holes” in the 
earth’s ionosphere. The infrared radiation and the shortest radio 
waves are blocked by molecules of water and carbon dioxide, whereas 
the longest radio waves are reflected back into space by the charged 
upper layers of the atmosphere, the ionosphere. The diagram illus- 
trates quite clearly how severe have been the handicaps under which 
astronomers have been working, especially since the radio window 
has been used effectively only during the past 15 years. 
From knowledge of the composition of the earth’s atmosphere and 
from laboratory experiments on the absorbing power of gases, it 1s 
possible to calculate with reasonable precision the altitudes at which 
the atmosphere becomes transparent to radiation of various wave- 
lengths. The results of one such calculation are shown in figure 2. 
The curve shown there gives the altitude at which the atmosphere 
becomes 50 percent transparent for each wavelength of the ultra- 
violet spectrum. For wavelengths greater than about 2,200 A., the 
radiation is absorbed by ozone concentrated mainly in a layer between 
heights of about 10 to 40 kilometers. From 2,200 A, to 900 A. the 
absorption is caused by oxygen and nitrogen molecules, primarily 
the former. The reason for the plateau in the curve between 1,400 A. 
and 1,800 A. is that, at about 100 kilometers or a little higher, the 
oxygen molecules have broken apart into their constituent atoms and 
hence no longer can absorb radiation at these wavelengths. The 
dip in the curve at 1,200 A. is a fortunate circumstance, since it coin- 
cides almost exactly with the position of the Lyman-e line of hydro- 
gen, which is strongly emitted by the sun and probably also by most 
other stars. At altitudes greater than 100 kilometers, radiation is 
predominantly absorbed by atoms of oxygen and nitrogen which pre- 
sent a solid wall for wavelengths between 200 and 900 A. At the 
very shortest wavelengths, X-rays are also screened by atomic oxy- 
gen and nitrogen, although they penetrate to relatively low altitudes. 
The calculations suggest that observations from a height of about 
50 kilometers would extend the solar spectrum to 2,200 A.; at 100 
kilometers altitude, the ultraviolet limit would be about 1,100 A.; 
