Page 104 - Intro to Space Sciences Spacecraft Applications

Spacecraft Environment
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decreases with altitude. This is due to the reradiation of energy absorbed
from the layers below which escapes into space.

Thermosphere. After the mesopause (another isothermal layer), the ther-
mosphere represents a region in which the temperature increases rapidly
with altitude. This is due, once again, to the direct molecular absorption
of solar radiation by the atmospheric constituents at these altitudes.

Exosphere. In the exosphere, temperature remains relatively constant with
altitude due to the low densities and the large mean free path (distance) that
exists between molecules. In this region, energetic molecules may actually
escape from the earth’s gravitational pull and be lost into space.

Density Variation. Decrease in the density of the atmosphere with
increasing altitude is a result of the balance between the gravitational
force on molecules of different masses and the thermal energy of these
molecules. Though relationships have been developed that relate the
change in density with altitude based on the standard temperature profile,
they do not work well at the higher altitudes at which spacecraft operate.
A good rule of thumb is that up to 100 miles altitude, the density decreas-
es by a factor of 10 every 10 miles. Above this, the decrease becomes
exponential as the lighter elements such as hydrogen and helium become
more predominant. The atmosphere extends for thousands of miles above
the earth’s surface, but in ever-decreasing densities.
The density of the atmosphere at any particular altitude also varies with
time due to the variability of solar emissions over the solar cycle and dur-
ing solar flares. Increased emissions may heat up the atmosphere causing
an increase in density at satellite altitudes which increases drag and may
affect orbital lifetimes.

Electromagnetic Propagation Through the Atmosphere. As the previous
discussions have indicated, the atmosphere absorbs much of the electro-
magnetic energy radiated from both the sun above and the earth below. It
also reradiates much of this energy in both directions as well, but the aver-
age rates of absorption and emission are about equal, which results in a
relatively constant average temperature around the globe.
The mechanisms behind absorption and emission are rooted in chem-
istry, physics, and quantum theory which show that these properties hap-
pen at relatively distinct frequencies for different atoms and molecules.