Astronaut Vandalism
Astronaut Vandalism
[A signpost with three arrows.] [Arrow pointing up:] Space 62 [Arrow pointing right:] Jackson 115 [Arrow pointing left:] Memphis 98
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 miles (100 km), due to "astronaut vandalism". The 62 mile distance is the Kármán line , one of the conventional demarcations of the beginning of " 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 miles as being an easy trip on the ground, but that same 62 miles is incredibly hard when going vertically, against the force of gravity.
The title text references the fact that while the Fédération Aéronautique Internationale (FAI) defines the Kármán line , the boundary between Earth's atmosphere and outer space (i.e., the start of space), to be 100 kilometers (62 miles) above mean sea level, the U.S. Air Force and other military branches will award astronaut wings to rated astronauts who fly higher than 50 miles (80 km).  In 2005 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 X-15 retroactively received astronaut wings for their greater-than-50 mi (80.5 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 Theodore von Kármán , who originally calculated the height at which an 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 miles (83.6 km) - closer to the USAF definition than to what is now called the Kármán line.  Additionally, the boundary between the mesosphere and the thermosphere is traditionally taken to be 53 miles (85 km), also close to the Air Force definition.  On the other hand, some newer research suggests the mesopause (the line between the mesosphere and thermosphere) may have peaks between 53 and 62 miles (85-100 km).  Also the 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 kilometers (62 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 km (80-93 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 miles from Memphis, TN and about 114 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.
[A signpost with three arrows.] [Arrow pointing up:] Space 62 [Arrow pointing right:] Jackson 115 [Arrow pointing left:] Memphis 98