Regions of the atmosphere

Where does the atmosphere end and space begin?

It depends on what you mean by atmosphere! Since the atmosphere becomes thinner and thinner as you go to greater and greater heights above the Earth's surface, the transition is gradual. Depending on what physical aspect of the atmosphere is of interest, you can define various transition heights. To illustrate, here are different answers:

• When there is not enough air to breathe--depending on your health and acclimatization, this is 2 to 10 km high. At 3 km, the FAA and USAF require supplemental oxygen or pressurized cabins for aircraft. The peak of Mt. Everest is 10 km above sea level.
• When a pressurized environment is needed--air pressure at 16 to 20 km is so low that supplemental oxygen is insufficient for survival.
• When aircraft propulsion doesn't work--turbojets can function only to 32 km, and ramjets to 45 km altitude. Above this, rocket proplusion is needed.
• When the ionosphere begins--at about 80 km, plasma physics becomes important.
• USAF definition--The US Air Force defines the atmosphere-space boundary at 81 km.
• FAI definition--The Federation Aeronautique Internationale defines the atmosphere-space boundary at 100 km. As of the end of 2004, 437 people have been this high or higher (according to Encyclopedia Astronautica). Several governments use an altitude at or near this in regulating space activities.
• When you can orbit the Earth--where atmospheric drag is low enough to allow a satellite to complete an orbit; this depends on the characteristics of the satellite and the condition of the atmosphere. About 120 to 150 km is typical.
• When air molecules are not gravitational bound to the Earth--this is the beginning of the exosphere and is 300 to 600 km high, depending on solar activity.
• Where you say it does--in 1976 the nations of Columbia, Equador, Brazil, Congo, Zaire, Kenya, Uganda, and Indonesia claimed sovereignty to an altitude of 35,862 km. The UN rejected these claims in 1980.

© 2005 by Wm. Robert Johnston.
Last modified 25 March 2005.
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