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Impact of Space Environmental Factors on Microtechnologies 79

solids is enhanced by the fact that oxygen and other free ions are abundant in many
orbits. The existence of free ions and active elements in the Earth’s
upper atmosphere makes it a much harsher environment than a laboratory on the
Earth’s surface. Two actions essential for enhancing the reliability of a satellite
under such adverse conditions are: where possible use hermetically sealed parts and
avoid the use of materials which outgas excessively or react to create corrosive
material.
Outgassing of volatiles and toxic gases must be extremely low in the crew
compartment areas. The maximum allowable levels for nonmetallics are defined in
NASA specification MSFC-PA-D-67-l3. For manned space-flight (such as Apollo),
conditions of 5 psi oxygen and 72 h of exposure, the total organics evolved must be
less than 100 ppm.
To assure part performance in a zero-pressure environment, thermal vacuum
testing is usually required at the component level. Zero-pressure environments
cause more severe thermal stresses on parts. It was reported by Gibbel 11 that
thermal or vacuum testing may yield a greater than 208C temperature rise (at the
high extreme) over a regular thermal or atmosphere test. The variance between how
the piece part is tested and the environment in which the part will be used
demonstrates the importance of temperature de-rating. Many times, extreme test
temperatures are used to accelerate failure mechanisms. The near-perfect vacuum of
the space environment provides little or no convective air cooling. All heat must be
dissipated from the vehicle through radiation. Space-borne electronic equipment is
cooled by conductive heat transfer mechanisms which transport dissipated heat to
external radiating surfaces of the spacecraft. These conducting paths typically
consist of thermally conductive pads, edge-connecting mechanisms, circuit-card
fixtures, metal racks, and the system chassis.
The reduced pressure encountered in high-altitude operations can result in a
reduced dielectric strength of the air in nonhermetically sealed devices. This
permits an arc to be struck at a lower voltage and to maintain itself for longer,
and may lead to contact erosion. Use of vented or nonhermetically sealed devices in
high altitude or vacuum applications requires special precautions, such as additional
de-rating.
In a low-pressure environment the likelihood of voltage flashover between
conductors is increased. The voltage at which flashover occurs is related to gas
pressure, conductor spacing, conductor material, and conductor shape. These rela-
tionships are plotted as Paschen’s curves. Flashover resulting from corona discharge
does not occur at voltages less than 200 V. Above that level, conductor separation,
insulation, and conductor shapes must be carefully selected.
Within several hundred kilometers of the Earth, molecules in the upper atmos-
6
phere are ionized by solar ultraviolet and x-ray to form dense (up to 10 particles/
3
cm ) low-energy plasma. In this region, known as the ionosphere, plasma particles
behave collectively because of the small range of individual particle influence
(1 mm at shuttle orbit). Charged particles accumulate on spacecraft surfaces, creating
differential charging and strong local electric fields. If a surface builds up

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