Editing 1375: Astronaut Vandalism

{{comic
| number    = 1375
| date      = May 30, 2014
| title     = Astronaut Vandalism
| image     = astronaut_vandalism.png
| titletext = That night, retired USAF pilots covertly replaced the '62' with '50'.
}}

==Explanation==
Signs like this normally show the distance to places on earth's surface. This sign also has an arrow pointing away from earth and towards "space", with a distance of 62&nbsp;miles (100&nbsp;km), due to "astronaut vandalism". The 62 mile distance is the {{w|Kármán line}}, one of the conventional demarcations of the beginning of "{{w|outer space}}".

We think of space as being very far away. This comic puts into perspective that it's really a lot closer to space than to many destinations we're used to getting to by car or airplane. We think of 62&nbsp;miles as being an easy trip on the ground, but that same 62&nbsp;miles is incredibly hard when going vertically, against the force of gravity. And if you want to stay there for more than a moment, you need to somehow accelerate to orbital velocity—a task few vehicles available to private individuals can achieve.{{Citation needed}}

The title text references the fact that while the {{W|Fédération Aéronautique Internationale}} (FAI) defines the {{W|Kármán line}}, the boundary between {{W|Earth's atmosphere}} and {{W|outer space}} (i.e., the start of space), to be 100&nbsp;kilometers (62&nbsp;miles) above mean sea level, the {{W|U.S. Air Force}} and other military branches will award {{W|astronaut wings}} to rated astronauts who fly higher than 50&nbsp;miles (80&nbsp;km).  In 2005 {{W|NASA}} changed from using the FAI definition to using the USAF definition for consistency across organizations, and thus some NASA test pilots who had flown the {{w|X-15}} retroactively received astronaut wings for their greater-than-50&nbsp;mi (80.5&nbsp;kilometers) flights. (Air Force pilots of the X-15 in the 1960s had long since received astronaut wings for such flights.)  Thus in the title text, Air Force pilots surreptitiously change the sign to conform to their definition of "space".

Although most authorities use the FAI definition of space - the Kármán line - since the FAI is the international organization of record for aeronautics, there are good scientific reasons for the U.S. Air Force definition.  The line is named for {{W|Theodore von Kármán}}, who originally calculated the height at which a vehicle would have to travel faster than orbital velocity to generate lift from wings, therefore making the vehicle an object in orbit rather one using air to generate lift.  Von Kármán originally calculated this height as 51.9&nbsp;miles (83.6&nbsp;km) - closer to the USAF definition than to what is now called the Kármán line.  Additionally, the boundary between the {{W|mesosphere}} and the {{W|thermosphere}} is traditionally taken to be 53&nbsp;miles (85&nbsp;km), also close to the Air Force definition.  On the other hand, some newer research suggests the {{W|mesopause}} (the line between the mesosphere and thermosphere) may have peaks between 53 and 62&nbsp;miles (85-100&nbsp;km).  Also the {{W|turbopause}} - the line where gas molecules cease mixing atmospherically and begin stratifying by molecular weight as if they are in orbit - is generally taken to be about 100&nbsp;kilometers (62&nbsp;miles).

All of the atmospheric boundaries are variable, however, changing from day to day and season to season with no clear boundary.  Additionally, objects cannot reliably orbit below 130-150&nbsp;km (80-93&nbsp;miles) due to drag from even the sparse atmosphere in the lower thermosphere.  Despite this comic associating "space" with having a definite start the way you might definitely know when you cross the city limits of a town, the reality is that the transition from atmosphere to space takes place gradually over tens of kilometers.  Interestingly, since it is too high for aircraft and high altitude balloons, but too low for spacecraft in orbit, this "near space" transition region is one of the least-visited and least-used regions of the larger atmosphere. This comic thus both points out that the limit where space starts is arbitrarily chosen and also that space is often much closer than, for instance, two nearby cities in some randomly chosen location in the US. 

The two distances shown on the signpost can occur only at certain points on Earth. One possible location is Grenada, MS, which is about 100&nbsp;miles from Memphis, TN and about 114&nbsp;miles from Jackson, MS. Alternatively "Jackson" could mean Jackson, TN, in which case Tupelo, MS or Kenneth, MO are both viable options for the location of the signpost. However, in Tupelo the roads to Jackson and Memphis meet at a right angle, instead of pointing in opposite directions as in the comic.

==Transcript==
:[A signpost with three arrows.]
:[Arrow pointing up:]
:Space 62
:[Arrow pointing right:]
:Jackson 115
:[Arrow pointing left:]
:Memphis 98

==Trivia==
*See [http://www.slate.com/articles/news_and_politics/explainer/2004/09/where_does_space_begin.html Slate: Where does space begin?] for more background.
*This comic was published on the same day that Version 2 of the {{w|Dragon (spacecraft)|Dragon spaceship}} was unveiled.
*The mileage of the sign indicates the "[http://welcometheplutians.joelbonner.com/wp/?p=33 scene of the crime]" to be at Grenada, Mississippi.
*The development codename of Windows 98 was "Memphis".
* As said in related What If: [http://what-if.xkcd.com/58/ The reason it's hard to get to orbit isn't that space is high up. It's hard to get to orbit because you have to go so fast.]
*In the title text to this [http://www.qwantz.com/index.php?comic=2420 Dinosaur comic] both xkcd and Randall is mentioned in combination with the fact that space is only 100&nbsp;km up and that you are much closer to space than to most other places on earth!
{{comic discussion}}