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# Posted 6:54 AM by Patrick Belton

BUT WHERE DOES SPACE BEGIN? As Josh and many other blogosphere commentators have noted, SpaceDev technology's SpaceShipOne vessel yesterday became the first private craft to cross into space. More interesting, though, is the question of when precisely it did so.

Common usage in the United States accords the title of 'astronaut' to any person travelling above an altitude of 50 miles, or 80 km. By comparison, a Boeing 747-400 most typically cruises at an altitude 10 kilometers, or 32,900 feet (6.23 miles). More officially, the Fédération Aéronautique Internationale, based in Lausanne, describes the boundary of space as being at 62 miles, or 100 km. SpaceShipOne's trajectory yesterday peaked at 100.12 kilometers (62.214 miles, or 328,491 feet) - meaning Michael Melvill had only 124 meters each way in which to enjoy officially stamping his passport in space.

The atmosphere thins gradually through its upper reaches, making it difficult to identify a clear delineation between it and space, but re-entering orbital craft begin to encounter noticeable atmospheric effects at 75 miles, or 120 km. (For terminology buffs, the portion of the earth's atmosphere from 50 to 85 km above the equator generally is referred to as the mesosphere, whereas the segment above 80-85 km is referred to as the thermosphere. The less exhilerating - though nonetheless stratospheric - heights lying immediately below the mesosphere are the familiar stratosphere, which ranges from 17-50 km above the equator.) Or, if you don't want to go that far, you can just go to the Stratosphere casino in Las Vegas.

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