I'd like to say outer space begins in my head, when I walk into a room and forget why I'm there. With the success of Richard Branson's Virgin Galactic flight on July 11th, there's been chatter about exactly where outer space starts. In regards to orbital flight there are two competing definitions: the Kármán Line, an imaginary boundary located 62 miles (100 km) up, and 50 miles (80.5 km), which marks the top of the mesosphere, where most meteoroids vaporize.

If you read my earlier post about noctilucent clouds, the second definition should ring a bell; the clouds form at nearly the same height. For comparison, a commercial jet typically flies at an altitude of around 7.5 miles (12 km) near the top of the troposphere, the bottommost layer of the atmosphere.

Both definitions are arbitrary. The Kármán Line is named for Hungarian-American engineer Theodore von Kármán, who determined that above the 80-mile limit the air is too thin to provide the lift required to fly a conventional aircraft. NASA has used the 50-mile definition for decades, starting in the late 1950s, when the agency began awarding astronaut wings to pilots who broke the 50-mile limit.

The atmosphere is divided into layers. Above the troposphere, where weather happens, the stratosphere extends to 31 miles until it gives way to the mesosphere., the coldest layer. The Kármán Line is shown in red at 62 miles, about the elevation of a low, bright aurora. The space station orbits about 250 miles up in the thermosphere.. Above this layer is the exosphere which gives way to interplanetary space. (NOAA)
The atmosphere is divided into layers. Above the troposphere, where weather happens, the stratosphere extends to 31 miles until it gives way to the mesosphere., the coldest layer. The Kármán Line is shown in red at 62 miles, about the elevation of a low, bright aurora. The space station orbits about 250 miles up in the thermosphere.. Above this layer is the exosphere which gives way to interplanetary space. (NOAA)

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Coincidentally, that's also the lowest altitude at which a satellite can orbit the Earth for a modest length of time. Below 50 miles, friction with the air will quickly cause the object to lose altitude and burn up in the atmosphere.

To be precise, the satellite has to follow an elliptical (oval) orbit where it spends most of the time at the far end of its orbit and makes only quick swings through the mesosphere at the close end. This is certainly an acceptable definition because it captures the edginess of reaching space.

Branson abides by NASA's definition. Sunday's Virgin Galactic flight achieved a height of 53.5 miles (86 km), accomplishing its goal of reaching outer space. Jeff Bezos, founder of Amazon, has adopted the Kármán Line for his upcoming July 20th Blue Origin flight. It's the international standard and used by both the world governing body for aeronautic and astronautic records, the Fédération Aéronautique Internationale (FAI), and the United Nations.

Bezos will have bragging rights, assuming a successful flight, but I doubt the view out the window will look much different. Lucky passengers will thrill to the curved Earth under a black sky and maybe see a few bright stars, too.

The aurora borealis, seen here from the space station, forms in the thermosphere from about the Kármán Line to as high as 620 miles (1,000 km).  (NASA)
The aurora borealis, seen here from the space station, forms in the thermosphere from about the Kármán Line to as high as 620 miles (1,000 km). (NASA)

If you want to talk REAL outer space, the kind where Earth's atmosphere gives way to the wind of particles blowing from the sun, that's much farther out. The outermost layer of the atmosphere, the exosphere, pretty much ends at 6,200 miles (10,000 km) — 100 times beyond the Kármán Line.

The few atoms and molecules of earthly origin can travel hundreds of miles at that height without bumping into each other. That's out there! And yet it's only a fraction of the 239,000-mile-distance (384,600 km) to the moon.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.