There are differing definitions. NASA says anyone who flies above 50 miles is an astronaut. The Fèdèration Aèronautique Internationale, which keeps the records, says space begins at 100 kilometers (62 miles), the Kármán line.¹ Theodore Von Kármán identified this as the approximate altitude at which there was enough air to interfere (by drag) with sub-orbital flight, but too little air to provide enough lift for planes.
The boundary was more precisely defined in 2009 by a Canadian team using an instrumented rocket that went to 200 km.² They found the altitude at which the ionosphere begins to be 118 km (73 miles). Above this altitude, air currents are driven by forces from space rather than the Earth's winds.
1. FAI Sporting Code, Section 8 (2009 edition): “2.18.1 All flights must exceed an altitude of 100 km in order to qualify for records.” A pdf of the code. The FAI website hosts an interesting discussion by S. Sanz Fernández de Córdoba of the origins of the Kármán line, https://www.fai.org/page/icare-boundary retrieved 2011-9-14.
2. University of Calgary press release
L. Sangalli, David Knudsen, M. F. Larsen, T. Zhan, R. F. Pfaff and
Rocket-based measurements of ion velocity, neutral wind, and electric field in the collisional transition region of the auroral ionosphere.
Journal of Geophysical Research vol. 114, page A04306, (2009)
A planet can have a lasting atmosphere if the force of gravity at the top of its atmosphere is strong enough to prevent the escape of molecules of gas. Gas molecules are always moving. If one rreaches escape velocity, it will Each year Earth loses about 95,000,000 kilograms of hydrogen this way. mass increases grav But mass is not the only factor compositon. massive molecules The hotter a gas becomes, the faster its molecules move, on avergae. Radiation from the star it orbits warms a planet's atmosphere. heats by rdation. A very bright star, or nearness to that star,an heat. That is why Mercury has no atmosphere. Only four bodies have atmospheres as we think of Earths' air: Venus, Earth and MArs, and, peculiarly, Titian, a moon ofThe biggest mass loss comes from escaped hydrogen and helium, which escape with 95,000 metric tons of mass and 1,600 metric tons respectively. These elements are too light to stay permanently in the gravity well, so they tend to escape into space. really comes down to how big and how close to the sun you are—or, for Titan, how close you are to a really big pl Three factors play into a gas's ability to escape the pull of a planet's gravity: temperature, molecular mass, and escape velocity, the speed a molecule needs to achieve to escape into space. Hotter, lighter, and faster particles more easily slip out of a planet's gravitational grip into space than cooler, heavier, and slower particles.
Total mass of atmosphere: 5.1480 × 10¹⁸ kilograms.¹
Mean molecular weight at sea level: 28.9644 grams/mole.²
In per cent by volume:
1. Kevin E. Trenberth and Lesley Smith.
The Mass of the Atmosphere: A Constraint on Global Analyses.
Journal of Climate, v18, pages 864–875, March 2005.
2. National Oceanic and Atmospheric Administration,
National Aeronautics and Space Administration,
United States Air Force.
U. S. Standard Atmosphere, 1976.
Washington: U.S.G.P.O., October 1976. Downloadable pdf. Page 9,
Arthur N. Cox, editor.
Allen's Astrophysical Quantities. 4th ed.
New York: Springer Science+Business Media, LLC, 2004.
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Last revised: 15 September 2011.