Jump from space? Not quite

Felix Baumgartner jumped Sunday from 24 miles in altitude, an incredible accomplishment for sure, but less than halfway to any technical definition of the "edge of space."

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Tony Rice
On Sunday, Austrian skydiver Felix Baumgartner's record-setting jump made news around the world. The point where he jumped from is being described as the "edge of space," even by NASA representatives.  Don't look for Baumgartner to be getting his astronaut wings from NASA or any other space agency however.

Baumgartner jumped from 24 miles in altitude, an incredible accomplishment for sure, but less than halfway to any technical definition of the "edge of space."

Edge is a misnomer here. There is no physical point where Earth or any other planet's atmosphere stops and space starts. The atmosphere gets progressively thinner as altitude increases. The best definition of where the atmosphere stops is where an aircraft's wings can no longer do the job of keeping it aloft.

As the air gets thinner, aircraft must go faster for wings to provide sufficient lift. At some point, the air is so thin that the aircraft must go so fast (in the neighborhood of 17,000 mph) to stay up that it's actually overcoming gravity by orbiting rather than by lift from the wings.

This point where aircraft become spacecraft and pilots become astronauts is defined at a nice round 100 km (~62 miles) above sea level and is known as the Kármán line, named for physicist Theodore von Kármán who first calculated it as an upper limit for aircraft to operate.

Despite the definitive sounding name, effects of the atmosphere are still felt above this line. The International Space Station orbits at about 400 km (248 miles) but it still must re-boost itself periodically to overcome the effects of atmospheric drag. That drag slows down the station, giving the Earth's gravity a chance to pull the station closer. That altitude was increased last year to reduce the effect the atmosphere has and improve fuel efficiency of the engines used to boost the station. There was a boost just last week to ensure the orbit was maintained and to avoid some space junk that was causing some concern.

The Swiss based Fédération Aéronautique Internationale which oversees aeronautics records worldwide, uses that 100km line to differentiate aeronautics and astronautics. Even the less stringent criteria used by the U.S. Air Force in the 1960s in determining which X-15 rocket plane pilots would receive astronaut wings used 50 miles as the deciding point. The FAA has opted for 100 km to determine who receives commercial astronaut designation. NASA generally stays out of the debate, likely to avoid discrepancy between criteria used by military and civilian pilots.

When asked what defines who is and isn't an astronaut, NASA Chief Historian Roger Launius answered simply "astronauts come from Houston." To date, the only commercial astronaut badges issued by the FAA were awarded to the crew of SpaceShipOne which topped 100 km in 2004.

Felix Baumgartner's jump was incredible and, as data is certified over the coming days, records for speed (833.9 mph / Mach 1.24), exit altitude (128,100 feet), and freefall (119,846 feet) will be recorded. However, jumping from the "edge of space" will have to remain a poetic description of the project.

Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.

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