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| | ==Explanation== | | ==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 miles (100 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}}". | + | 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". |
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| − | 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. 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}} | + | We think of Space as being so very far away. This comic puts into perspective that it's really a lot closer to get 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. Distance-wise, however, it's the same. |
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| − | 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 kilometers (62 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 miles (80 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 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". | + | The title text states that the distance to "space" was changed later from 62 miles (100 km) to 50 miles (80 km) by retired Air Force pilots. This indicates that the pilots wanted to reduce the altitude considered to be "space", so that their own high altitude flights could be considered space flights. |
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| − | 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 miles (83.6 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 miles (85 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 miles (85-100 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 kilometers (62 miles).
| + | The {{W|U.S. Air Force|USAF}} and other military branches will award {{W|astronaut wings}} to rated astronauts who fly higher than 100 kilometers (62 miles). The 100-kilometer barrier known as the {{W|Kármán line}} was designated as the official boundary between the {{W|Earth's atmosphere}} and {{W|outer space}} by the {{W|Fédération Aéronautique Internationale}} (FAI) in the mid-1950s. Notwithstanding this definition, some test pilots in the 1960s received astronaut wings by the {{w|United States Air Force|USAF}} for their greater-than-50mi (80.5 kilometers) flights on the {{w|X-15}}. |
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| − | 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.
| + | 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 the two largest cities in some randomly chosen location in the US. One possible location is Grenada, MS, which is about 100 miles from Memphis, TN and about 114 miles from Jackson, MS. |
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| − | 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.
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| | ==Transcript== | | ==Transcript== |
| | :[A signpost with three arrows.] | | :[A signpost with three arrows.] |
| − | :[Arrow pointing up:] | + | :[Arrow pointing up:] |
| | :Space 62 | | :Space 62 |
| | :[Arrow pointing right:] | | :[Arrow pointing right:] |
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| | *The development codename of Windows 98 was "Memphis". | | *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.] | | * 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 km up and that you are much closer to space than to most other places on earth! | + | *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 km up and that you are much closer to space than to most other places on earth! |
| | {{comic discussion}} | | {{comic discussion}} |
| | [[Category:Space]] | | [[Category:Space]] |
