I've tried to answer this question based on the sources listed after
and I'll piece together an explanation based on them.
Perhaps I can start by saying that 100 kms above the Earth's surface
is not really considered the edge of the Earth yet. It may be called
the start of the edge of the Earth, as Latitude below implies, for the
Earth's atmosphere with air molecules and all still present can extend
up to 1000 kms. But you could say the 100 km layer is where
significant changes take place in the conditions that affect the
flight of an object into space.
Firstly let's look at the layers. According to the "Why is the Sky
Blue?" article, 100 kms is commonly the border separating the kinds of
layers. In layers of the atmosphere according to electrical
properties, it's between the Neutral atmosphere and Ionosphere.
According to layers by temperature, it separates the Mesosphere and
Thermosphere. According to layers by composition, it separates the
Homosphere and Heterospere.
I believe that 100 km can be considered the edge of normal flight,
since at this level, the air would be so thin that it would not be
able to provide lift for aircraft, no matter how big the wings.
According to the Powerpoint presentation titled "Basic Properties of
the Atmosphere," the pressure at 100 kms up is about 0.00056 millbars
- which calculated in the Google calculator is 8.12211331 × 10 raised
to -06 pounds per square inch. Not enough to hold a wing up. So the
craft would have to be totally dependent on its propulsion system. The
propulsion system of a craft should produce power many times the
weight of the craft to be able to overcome the Earth's gravity. For
example, the Saturn V rocket produced around 7.5 million pounds of
thrust to get into space. Without that much power, it would have to
achieve Escape Velocity, the speed an object should travel so that
Earth's gravity would not pull it back down, which would be at least
11.2 km a second, or 42,000 km and hour,
Perhaps once a craft has reached 100 kms up, the gravity will also be
considerably less, so that less thrust is needed to proceed further
into space and lesser power propulsion can be used.
One other thing that might make this stage difficult to bypass is the
existence of the ionosphere, a layer composed of electrically charged
particles or ions, created by the absorption of ultraviolet light. It
is also located somewhat above that 100 km level above the earth. Any
object that passes through this layer slowly can be fried to a crisp.
Spacecraft that have passed through this layer, like the Apollo 11
mission, pass it at such speed that the ions did very little damage to
the craft passing through it. Moon landing debunkers use the
explanation that nothing could pass through the ionosphere at all, but
given enough escape speed, a craft can pass through safely. Once past
this, there can be other problems to worry about, like meteors, space
debris and space radiation.
100 kms as the border between the two layers of the atmosphere based
on composition, the Homosphere and the Heterosphere shows a
significant change. Below 100 kms, heavier molecules like oxygen and
nitrogen (which composes 70+% of the Earth's atmosphere). Above,
lighter substances like hydrogen are more abundant.
Referring to Latitude's comment, getting back down in one piece is
hard, since when you hit the atmosphere, you will dragged at high
speed through searing air friction, and retro rockets will have to
provide very high amounts of thrust to break the fall, or like the
Space shuttle, the craft will have to hit the atmosphere at a very
specific attitude and manner to avoid burnup. The tragic loss of the
Columbia demonstrated this problem.
So I believe that makes 100 kms significant is the begining of the
"edge" of space. It's not the edge per se, but forms an important part
of it. Once a craft breaks this barrier, it should start making
adjustments to adapt it to travel in different conditions in
near-space and space.
Karman line - Wikipedia, the free encyclopedia
Earth's atmosphere - Wikipedia, the free encyclopedia:
Thickness of Earth's Atmosphere:
Blue Sky - Why is the Sky Blue?
Thinkquest: The Atmosphere
Meteo 465/565 -- The Middle Atmosphere Chemical Composition
- "A transition occurs at about 100 km. Below about 100 km, the
mixing of air parcels and thus air molecules is dominated by eddy
motions. Above, 100 km, the mixing of air parcels is dominated by
molecular diffusion. With molecular diffusion in control, molecules
of different mass are no longer uniformly mixed. Thus, the region
below 100 km is called the homosphere, while the region above 100 km
is called the heterosphere." - indication of changes at 100 kms.
Basic Properties of the Atmosphere
Google search terms used:
100 km atmosphere
100 km atmosphere density
100 km atmosphere density psi
I hope this answer has been helped you a lot. Don't hesitate to ask if
you need something clarified, or have a problem with the answer. Thank