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Microsystems in Spacecraft Thermal Control 189

TABLE 9.2
Passive and Active Thermal Control Hardware

Thermal Control Hardware
Passive System Active System
Thermal surface finishes Heaters
Multilayered insulation Louvers
Radiators Heat switches
Mountings and interfaces Fluid loops
Phase change materials Thermoelectric coolers
Heat pipes or loops

solar exposure, that radiate excess heat into space. Other common passive thermal
control elements include specialized thermal surface finishes, multilayered insula-
tion blankets, conduction enhancing or retarding materials, phase change materials,
heat pipes, and bimetallic louvers, which open and close according to the radiator
temperature.
Active thermal control is required when the temperature needs to be tightly
controlled, or when the thermal environment is highly variable. A summary of
passive and active thermal control hardware is shown in Table 9.2. Active control
provides the thermal design engineer flexibility, tighter control, and faster design
turnaround. Small satellites may benefit well from this approach since they are
more likely to be mass produced and need a thermal design which can meet a range
of mission criteria.

9.3.2 HEAT TRANSFER IN SPACE
Radiation heat transfer is an important process between an orbiting spacecraft and its
surrounding environment. This heat exchange is a final energy balance between heat
absorption on spacecraft surfaces and heat rejection to space. In addition to internal
heat generation, spacecraft external surfaces receive radiation from the space envir-
onment.Thequantityoftheradiationabsorbedisrelatedtotheintensityoftheexternal
radiation, the area affected, and the solar absorptivity (a) of the surface. The quantity
of heat rejected is proportional to the radiator area, temperature differential between
the radiator, the ‘‘effective sink temperature’’ of what it is viewing, and the infrared
emissivity («) of the surface. The ratio of solar absorptivity and infrared emissivity
(a/«) is important in determining the spacecraft surface temperature.
For passive thermal control, designing and selecting surface materials with
desired a and « is an effective way to obtain an optimal heat balance. Unfortunately,
these properties are fixed once surface materials are selected. Long-term exposure
to space environments degrades thermal control surfaces by increasing solar absorp-
tivity (sometimes very significantly) with the result of increasing spacecraft
surface temperature. During normal operation, spacecraft temperatures may be

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