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80 MEMS and Microstructures in Aerospace Applications

sufficient electric potential, a high-energy discharge (arc) can blow away material
and deposit it on optical or other sensitive equipment. The hot, thin plasma of the
magnetosphere creates more devastating problems at the geosynchronous altitude.
In the region above 1000 km, the electromagnetic influence of plasma particles
extends over a kilometer or more. High-energy (greater than 100 keV) electron from
plasma penetrates external spacecraft surfaces, accumulating inside on well-
grounded conductors, insulators, and cables, causing strong electric fields and
ultimately breakdown. Due to their high resistivities, dielectric surfaces can be
charged to different potentials than metallic surfaces (which should be at spacecraft
ground potential). Considering the effects of internal discharges is important when a
system is expected to operate in an environment where penetrating radiation causes
charging inside the system.
Internal discharges occur when ungrounded metal or dielectric surfaces collect
enough charge from the plasma field so that the electric field generated exceeds the
breakdown strength from the point of the deposited charge to a nearby point.
Internal discharges have been suspected as the cause of a number of spacecraft
performance anomalies. The conditions for discharging are dependent on the
environment, the shielding provided by the spacecraft, the material, which is
charging, and the geometry of the charged materials.
System response to internal charging depends on the location of the discharge
and the sensitivity of the circuits. Charges that would go unnoticed on the exterior
of a space system can be significant when they occur internally. Experiments on
Long Duration Exposure Facility (LDEF) 12 have documented the phenomenon of
spacecraft charging by plasma at low altitudes. The LDEF has been a wealth of
information on the effects of the space environment. 13–17 LDEF was launched in
1984 and contained a package of 57 experiments placed in Earth orbit by the Space
Shuttle for studying the effects of exposure to the environment of space. The LDEF
was supposed to have been recovered after about 1 year. However, delays in the
shuttle program meant that the package was not brought back until January 1990,
just a few weeks before it would have reentered the atmosphere and been destroyed.
One of the experiments measured long-term current drainage of dielectric materials
under electric stress in space. Current leakage appeared to be much lower than
predicted from ground simulations. The researchers believed that instead of gradual
current drainage, instantaneous discharge to the space plasma reduced any excess
charge. Carbon residue on the samples suggested breakdown of organic materials
under the intense heat of an arcing discharge. The LDEF results suggested that
comparing results from long-term space experiments and ground simulations was
not fruitful. Simulating all space-environmental parameters during ground simula-
tions is virtually impossible. In space, other environmental variables may alter or
exacerbate plasma effects. This is an area of current research. Nonetheless, various
options are available for testing and circumventing the effects of internal charging.
For special missions, criteria can be generated that will eliminate or reduce internal
discharge concerns.
The space station, orbiting at altitudes of 400 to 500 km, could lose considerable
current to ambient plasma. Its solar arrays, 160 V cells connected end-to-end for

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