The Earth is protected from the harsh
conditions of space by a thin
layer of gases that we call our atmosphere. The
farther way from
Earth's surface you go, the less dense (thinner)
the air gets.
Eventually, the air gets so thin that scientists
consider it the
"vacuum" of space (vacuum simply refers to empty
space, containing no
matter). This transition from atmosphere to
space is found about
62-75 miles above the surface, depending on
which scientist you ask.
When spacecraft re-enter the Earth's atmosphere,
they start to "feel"
the effects of the atmosphere around 75 miles
above the surface, so
that's the figure I like to use.
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