Page 100 - Intro to Space Sciences Spacecraft Applications

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Spacecraft Environment
Sunspots seem to occur only around mid-solar latitudes, between
around 60” north and south of the equator, forming closer to the equator
as the cycle approaches minimum. A reversal of the magnetic polarity of
the sun, from positive to negative lines of magnetic flux emanating from
the geographical poles, occurs over each solar cycle. This variability in
intensity, location, and magnetic polarity over time manifests itself in a
changing and difficult-to-predict environment in the vicinity of the earth.

THE EARTH

The environment on and around the earth is affected in many ways by
the electromagnetic and particulate radiations from the sun. Spacecraft
must operate in and through the resulting environment, which includes the
earth itself and its atmosphere as well as the space around it. Consequent-
ly, the following sections describe some of the properties of the earth as
well as the interaction of the earth with the solar radiations.

Electromagnetic Interaction

We previously determined the amount of electromagnetic energy that
reaches the vicinity of the earth when we calculated the solar constant. This
energy retains the blackbody spectral characteristics it had when it left the
sun, the only difference being that the energy levels over the range of
wavelengths has decreased. As this energy encounters the earth, the inter-
actions affect both the earth and the energy itself. In some cases, these
interactions are useful to the space systems user. For example, remote sens-
ing techniques measure the absorption or transmission of solar radiation
through the atmosphere to infer some of the characteristics of the atmos-
phere from space, and most spacecraft use solar panels to convert some of
this radiation to electricity to power the spacecraft systems. In other cases,
solar radiations produce undesired effects. For instance, spacecraft may
absorb excess infrared frequencies, necessitating the use of thermal control
devices, and the ionosphere, described next, may introduce errors in trans-
mitted signals. Spacecraft systems, including power generation and ther-
mal control, and spacecraft applications, such as remote sensing and com-
munications, are described more thoroughly later in the text.

The Ionosphere. The short wavelength (X-ray, UV) radiations from the sun
have enough energy to “knock” electrons out of some of the constituents of