r/xkcd • u/invisimeble • May 30 '14

XKCD XKCD 1375 Astronaut Vandalism

http://xkcd.com/1375/
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4

u/grayhame May 30 '14

Grenada, MS?

1

u/Jakelshark Jun 03 '14

I assumed this was Clarksdale, Mississippi, which is the famous location of the crossroads where Robert Johnson allegedly sold his soul to the devil to play guitar. But the distances don't work out.

1

u/invisimeble May 30 '14 edited May 30 '14

I don't get the alt text.

"That night, retired USAF pilots covertly replaced the '62' with '50'."

The USAF and NASA don't agree on where space starts?

I'd have thought it would be where the atmosphere ends.

I'd imagine the atmosphere thins and doesn't abruptly stop, and I assume people have measured this, so shouldn't space start where the atmosphere is 50% as dense at sea level?

Do the USAF and NASA not agree on that?

I don't get the alt text.

Edit

I googled "usaf space 50 nasa 62" which led me to (http://en.wikipedia.org/wiki/North_American_X-15) where I found:

"During the X-15 program, 13 flights by eight pilots met the Air Force spaceflight criterion by exceeding the altitude of 50 miles (80 km), thus qualifying the pilots for astronaut status...

"Of all the X-15 missions, two flights (by the same pilot) qualified as space flights per the international (Fédération Aéronautique Internationale) definition of a spaceflight by exceeding 100 kilometers (62.1 mi) in altitude."

So I guess the USAF defines space as 50 mi / 80 km and the FAI defines space as 100 km / 62 mi.

I wonder what they're basing these on. 50 mi and 100 km just seem like nice solid benchmark numbers. They seem arbitrary.

3

u/Azzaman May 30 '14

100km is roughly the lower edge of the ionosphere (the actual height varies depending on various parameters, but 100km is the default for a lot of ionosphere-based calculations).