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Impact of Space Environmental Factors on Microtechnologies 71

For low-Earth orbit (LEO) and geosynchronous Earth orbit (GEO) satellites, the
number and the temperature of thermal cycles experienced are dependent on the orbit
altitude. For example, in a typical 550 km LEO, there would be approximately 15
eclipsecyclesovera24-hperiod.InaGEO,therewouldbeonly90 cyclesinayearwith
a maximum shadow time of 1.2 h per day. Trans-atmospheric temperature cycling
depends on the orbit altitude and can have the same frequency as LEO; however, the
amount of time in orbit is generally very short. Thermal cycling on planetary surfaces
depends on the orbit mechanics in ascent acceleration relationship to the sun. For
example, a system on the surface of Mars would endure a day or night cycle every
24.6h.AsMarsis1.5timesfartherawayfromthesunthantheEarth,thesun’sintensity
is decreased by 43%. The lower intensity and attenuation due to the atmosphere on
Mars limits the maximum temperature to 278C. Temperature electronic assembly
cycling is performed between high and low extremes ( 65 to 125 or 1508C, typically).

4.2.2 MECHANICAL EFFECTS OF SHOCK,ACCELERATION, AND VIBRATION
Mechanical factors that must be considered are acceleration, random vibration,
acoustic vibration, and shock. The effects of these factors must be considered
during the launch phase, during the time of deployment of the system, and to a
lesser degree, when in orbit or planetary trajectory. A folded or collapsed system
or assembly is particularly sensitive to the effects of acoustic excitation generated
during the launch phase. If the system contains large flat panels (e.g., solar panels),
the effects of vibration and shock must be reviewed carefully since large flat
surfaces of this type represent the worst-case condition.
Qualification at the component level includes vibration, shock, and thermal
vacuum tests. Temperature effects precipitate most mechanically related failures;
however, vibration does find some defects, which cannot be found, by temperature
and vice versa. Data show that temperature cycling and vibration are necessary
constituents of an effective screening program.
Acceleration loads experienced by the payload consists of static or steady state
and dynamic or vibration loads. The acceleration and vibration loads (usually called
load factors) are measured in ‘‘g’’ levels, ‘‘g’’ being the gravitational acceleration
2
constant at sea level equal to 9.806 m/sec . Both axial and lateral values must be
considered. For the Shuttle program, payloads are subjected to acceleration and
vibration during reentry and during emergency or nominal landings (as well as the
normal ascent acceleration and vibration-load events).
The vibration environment during launches can reach accelerations of 10 g at
frequencies up to 1000 Hz. Vibration effects must also be considered in the design
of electronic assemblies. When the natural frequency of the system and the forcing
frequency coincide, the amplitude of the vibration could become large and destruc-
tive. Electronic assemblies must be designed so that the natural frequencies are
much greater than the forcing frequencies of the system. In general, due to the low
mass of MEMS devices, the effect of vibration will be minimal but assuredly must
be considered with the packaging. For example, long wire bond leads have reached
harmonic frequencies, causing failures during qualification tests.

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