Page 92 - Intro to Space Sciences Spacecraft Applications

Spacecraft Environment
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Photosphere. The photosphere is a relatively thin layer (only about 300
km thick) of hot gas which absorbs the energy produced beneath it and then
reradiates this energy into space. All electromagnetic radiation, including
the visible wavelengths we see from earth, emanates from this region of the
sun. Since this is what we perceive as its shape visually, this layer is
defined as the “surface” of the sun even though the solar body extends far
beyond. The temperature of the photosphere is roughly 6,000 OK which, as
we will see, determines the characteristics of part of the solar radiation.

Chromosphere. The layer of gas above the photosphere is known as the
chromosphere and corresponds roughly to the “atmosphere” of the sun. A
temperature profile exists through this layer which initially decreases to
about 4,000 OK then increases rapidly to about lo6 OK (although temper-
ature loses its familiar meaning with such large distances between parti-
cles). The density within the chromosphere is sufficiently low that the
electromagnetic radiation from the photosphere is not appreciably affect-
ed in its outward passage through this region.

Corona. Finally, the corona is a layer of gases that extends millions of
miles away from the sun. Temperatures remain high, and the density of
the gases is on the order of lo-” kg/m3. The corona can be seen as a
streaming pattern around the edges of the moon during a full solar eclipse.

Solar Radiation

The environment in the vicinity of the earth is affected by two major
products of the sun’s nuclear furnace. The most familiar is the electro-
magnetic radiation of which visible light is a part. The second is the out-
pouring of high energy solar particles.

Electromagnetic Radiation. Electromagnetic radiation is energy which
propagates at the speed of light in a wave-like manner. Figure 4-2 depicts
a typical sine-wave pattern used to describe an electromagnetic wave and
illustrates some basic characteristics with which we will be dealing.
Wavelength (A) represents the distance (meters, microns) between suc-
cessive peaks of a particular wave pattern. Since electromagnetic energy
travels at the speed of light, the number of peaks that pass a fixed point in
a second of time is known as thefrequency (f) of the radiation. The rela-