Not surprisingly, atmospheric pressure decreases
every 18,000 foot change in altitude. At 75,000 feet, our
near spacecraft measures an air pressure less than 6% of
the air pressure found at sea level, or about 95% vacuum.
At 100,000 feet, our near spacecraft measures an air
pressure only 1% of the air pressure at sea level. It is 99%
The “Stack” — Component parts of the integrated
launch vehicle and near spacecraft.
NUTS & VOLTS
vacuum at 100,000 feet! As a result of these low air pressures, the sky becomes inky black in color. The reduction
in air pressure (or more specifically, air density) has one
more effect and that deals with cosmic rays. In near
space, with fewer air molecules to create a shield, the cosmic ray flux is over 100 times greater than at sea level.
The third condition found in near space is what happens to the earth's horizon. There are three amazing
First, the distance to the horizon increases.
Remember that a six foot tall adult sees a horizon that is
typically three miles away. At an altitude of 75,000 feet,
our near spacecraft sees a horizon that is 335 miles away.
At 100,000 feet, the horizon is closer to 400 miles away.
So at 100,000 feet, our near spacecraft can see our entire
state in a single glance.
Another effect of altitude on the earth's horizon is
that it makes the earth's curvature noticeable.
Photographs taken of the earth's horizon from near space
show the edge of the earth to be curved and this should
be enough to satisfy the members of the Flat Earth
The final effect that altitude has on the earth's horizon is that the horizon gets lower. This effect is sometimes
called depression of the horizon. The effect is very noticeable to astronauts on the Space Shuttle where they orbit
earth at an altitude of 300 nautical miles. You don't have
to orbit the earth, though, to see this effect. Even our near
spacecraft can detect this effect. At an altitude of 100,000
feet, our near spacecraft sees a horizon that is more than
five degrees lower than it is at sea level. So the angular
distance from horizon, to zenith, to opposite horizon
spans more than 190°.
The final condition found in near space that I will discuss is gravity. The higher our near spacecraft climbs, the
less gravitational force earth exerts.
Now, this is not the same effect noticed by astronauts. When in orbit about the earth, an astronaut is in a
state of constant freefall. As a result, s/he feels weight-less. This weightlessness overwhelms the reduction of
gravity due to the distance that the Space Shuttle orbits
from the earth's center.
However, in near space, we can detect a change in
the earth's gravity. At an altitude of 100,000 feet, the
acceleration due to gravity is 1% less. Consequently, our
near spacecraft weighs only 99% of its weight at sea
You can see that the conditions in near space look
and feel much like space. There is no means for the hobbyist to create these conditions on a large scale. If we
want to experience space vicariously, then the amateur
near space program is the only game in town.
Getting Into Near Space
Now that you're familiar with the location and conditions found in near space, let's talk about how we get
there. There are two elements to getting into near space: