The average spacecraft is crammed with electronic and mechanical equipment so that it can achieve its mission. Most of this equipment, especially the electronics, has been developed from similar domestic and industrial items that were designed to operate down here on the ground. In other words, they have a design heritage such that they work best when they operate at room temperature. One main reason for requiring thermal control onboard a spacecraft is to produce a room temperature environment for the various equipments so that they operate reliably for the lifetime of the spacecraft mission. It is the same with home electronics. For example, if we put our TV set in the freezer or in the oven, its useful lifetime will be considerably reduced. Similarly, reliable, long-term performance of most spacecraft components requires them to operate within thermal tolerances. Table 9.3 shows the approximate temperature range for the reliable operation of some items of spacecraft equipment, which the thermal control engineer has to consider when designing the thermal control subsystem. (The definition of room temperature is quite broad.)
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