The effect of spacecraft size on electrical charging by the hot plasma is summarized as follows: • i Influence of static disturbances to plasma damped out in ^1 DeBye length. • DeBye length for spacecraft charging plasma on the order of 100 meters. • When size of spacecraft >1 DeBye length, central portions of spacecraft will not receive the charging current that extreme portions will. • As a first approximation, assume that only surfaces within ^100 meters of spacecraft edges will receive free space charging current (other surfaces will receive no charging current). A plasma has a characteristic dimension (the DeBye length) which is the approximate distance in a plasma that a static disturbance will propagate. This DeBye length for the hot plasma involved here is on the order of 100 meters. Since most spacecraft are smaller than this, all parts of their surfaces are exposed to the same plasma environment. However, for a spacecraft with one or more surface linear dimensions >100 meters, only the extremities of the spacecraft will be exposed to the free space plasma. The hot plasma is a rarified medium (particle density ^10^ cm"^) which produces its charging effects only because the thermal energies of the particles is fairly high (keV). The plasma density is sufficiently low that its conductivity is only %10~9 mh a/m. Thus it will not act to short out exposed electrical circuits by itself (but it will act to augment the effects of an electrical discharge which occurs because of electrons and ions generated elsewhere). Instead the plasma will cause a small leakage current (<1 ma/ meter) for a 1-cm radius conductor in an electric field of 1000 volts/cm). The hot plasma has been a problem for spacecraft in near synchronous earth orbit because it was not anticipated. By a few simple precautions it is possible to avoid problems due to this plasma: Wherever possible, have exposed spacecraft surfaces composed of conductive (<10$ ohm-cm) material. Otherwise (for covers of RF antennas, optical sensors, etc.) have insulating material which can withstand ^30 kv (solar panels are not often a problem because the plasma will charge both front and back surfaces equally). If these precautions cannot be taken for some reason, have sensitive electronic components well shielded so that any discharge energy cannot get in. Finally, use rugged electronics (not easily upset by any discharge energy which might leak in). A MIL spec which calls for "black boxes" to withstand the effects of 9 kv, 2 joule electrical discharges directed onto the outside of the boxes is one way of assuring the electronics inside the boxes is well shielded and electrically rugged. Since the discharge currents and energies are quite small compared with the size and/or rating of the electrical components, no problems are expected. In summary:
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