Outer space

There is no universally accepted legal definition of outer space; see space law. A working definition accepted by many legal experts assumes that outer space generally begins at the lowest altitude above sea level at which objects can orbit the Earth, (approximately 100 km or 62.1 miles). This article is directed less at law, and more at technical issues regarding outer space and getting to and from it. A space launch vehicle, for example, is a general term for a device that can move objects into space, either for suborbital scientific work, to put things in satellite orbits, or to send objects into escape velocity from earth. "Space launch vehicle" excludes weapons, such as ballistic missiles, whose trajectory goes through outer space, but does not achieve orbit.

The case is made, not infrequently, that it may not be advisable to have a rigorous legal definition, given that conceptual space law, technology, and case law relevant to space continue to emerge. Today's working definitions tend either to define space by excluding that which is not considered space. Space law treaties do not contain a formal definition, but, with reasonable good faith by nations, appear to work.

Space also includes bodies in space, such as the Moon.

Definitions based on atmospheric layers and their characteristics

One method to define space is to define the layers of the atmosphere, and assume space is above the highest level. Unfortunately, the top of the highest atmospheric layer is not rigorously defined, and the layers themselves are defined with respect to their properties, which vary in time. The thickness of the physical layers differ at the polar and equatorial regions.

• tropopause: the layer from the surface of the earth to the bottom of the troposphere. It varies in thickness, depending on latitude and specific atmospheric conditions. The lower parts have adequate oxygen pressure for human survival, but supplementary oxygen will be needed as the pressure drops with height.

• troposphere: If the discussion does not include the tropopuase, this is nearest the surface of the earth. It extends either from the surfce, or from the top of the tropopause, and is thinner at the poles than the equator. Weather and conventional aviation take place in this layer, which contains three-fourths of all the air surrounding the earth.

• stratosphere: Above weather, this layer is only reached by advanced aircraft, balloons, and sounding rockets. It ends at approximtely 25 miles.

• mesosphere: extends to about 50 miles

• ionosphere: sparsely occupied by gas molecules and generally lower pressure than the best vacuum generated on the surafce. There is, however, enough matter in it to reflect certain radio signals. Its upper limit is not defined.

Definitions based on aircraft and satellite performance

According to the Internationl Civil Aviation Organization,an aircraft is “any machine that can derive support in the atmosphere from the reactions of the air other than the reactions of the air against the earth’s surface.” The maximum altitude at which a machine can derive support from the reactions of the air is presently estimated at about twenty one miles by the ICAO Secretariat. By exclusion, that suggests that outer space begins at approximately 21 miles, but this is a moving target as new techniques evolve for reaching high altitudes.

Another approach is to define the lowest level of outer space as the lowest altitude at which a satellite can stay in orbit, even an orbit that will degrade fairly quickly due to atmospheric resistance. That altitude is approximately 95 miles, but a satellite equipped with altitude-keeping thrusters may be able to orbit at 75 miles. A variant treats the boundary as the point at which flight depends more on orbital mechanics and less on aerodynamic effects.

A 2001 U.S. government position again excluded rather than defined, stating the geosynchronous orbit (GEO) is above the jurisdiction of any nation.