What is the definition of outer space? Or, more specifically, what is the difference between national air space and outer space? The air space over each national territory is subject to that country’s sovereign control.

In outer space, claims of national sovereignty have been prohibited. How is one to be distinguished from the other? The question has received much attention in recent years, and many proposals on how it might be resolved have been put forward. As yet, no consensus has emerged. However, the progress of technology may make some solution more urgent in coming years

In direct contrast, claims of exclusive national sovereignty in outer space are prohibited by international agreement. The Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies, was concluded in 1967 under the aegis of the United Nations. Article II provides that:

Outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation or by any other means.

The actual practice of nations also indicates a difference between national air space and outer space. Hundreds of objects have now been launched into orbit around the earth; in recent years and no nation has protested such passage over its territory as a violation of its sovereignty.

In fact, no nation has explicitly reserved its position concerning the passage over its territory of a space object of another country. On the other hand, no nation has been willing to limit its air space to a specific height; to do so would define the upward extent of its sovereignty and, implicitly or explicitly, the lower limit of what it considers to be outer space.

There are two general schools of thought regarding the need for and desirability of arriving soon at a clear line of demarcation, between air space and outer space. One approach cites the need to delimit the legally binding obligations regarding the activities and authority of nations in outer space and air space, respectively.

Without such a demarcation, it is contended, there will arise, as technology advances, disputes regarding the extent and nature of the obligations nations have assumed in the international agreements related to outer space. Similarly, without agreed definitions, a nation could assert claims of sovereignty that would interfere with space activities desired by many other countries.

The other approach argues that there is no evidence that a demarcation line is needed and that to set one now would be premature and possibly counterproductive. The proponents of this point of view call attention to the rapid pace of space technology and the practical uncertainties regarding the characteristics of feasible and desirable space activities.

Trying to set a boundary now, they feel, would risk getting it too high or defined in a way that might turn out to be detrimental to future space activities. (Implicit in this viewpoint, there seems to be the expectation that the later agreement is reached, the more likely the boundary is to be set lower than it would be at present.)

Those who endorse a cautious approach, note that the lack of specific agreement has not led to any international difficulties and does not seem likely to. They also suggest that the effort to establish a definitive boundary could, itself, lead to controversy and confusion, as has happened in regard to the demarcation between territorial waters and the high seas.

Why not simply set the dividing line between air and outer space at the upper limit of the atmosphere? That would probably be one of the first questions by a layman.

Furthermore, the international conventions that regulate aircraft seem to suggest this concept in their use of such terms as "air,” "atmosphere” and "atmospheric” space. The practical difficulty, however, is that the earth’s atmosphere does not end abruptly; it gradually transforms into outer space.

Some estimates place the altitude at which air space ceases well beyond the orbits of some existing earth satellites. In fact, there is no scientific agreement on the altitude at which air space ceases.

Suggestions have been made to establish the demarcation on the basis of differentiation between the several layers into which scientists divide the atmosphere. The troposphere, the layer nearest the surface of the earth, is said to extend up to about 9 to l0½ miles at the equator and 6 to 7 miles at the poles.

It is the layer in which weather phenomena occur, and it is the field of operation for conventional aviation. The troposphere contains three-fourths of all the air surrounding the earth.

Most of the rest of the air in the atmosphere is contained in the next layer, called the stratosphere. It is above the weather and is reached only by the most advanced aircraft and research balloons. Its upper limit is about 25 miles. The troposphere and stratosphere contain about 99.7 percent of the air.

A third layer, called the mesosphere, extends to about 50 miles, and beyond that is the ionosphere. The latter is sparsely occupied by gas particles, less dense than the most complete vacuum that can be achieved on earth. The upper limit of the ionosphere is not defined.

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