An electrical contactor is an electrically controlled switch which closes a power or high voltage electrical circuit. A plasma contactor changes the electrically insulating vacuum into a conductor by providing movable electrons and positive gas ions. This conductive path closes a phantom loop circuit to discharge or neutralize the static electricity that can build up on a spacecraft.
Space contains regions with varying concentrations of charged particles such as the plasma sheet, and a static charge builds up as the spacecraft moves between these regions, or as the electrical potential varies within such a region.
Static electricity may also build up on a spacecraft as a result of space radiation, including sunlight, depending on the materials used on the surfaces of the spacecraft.
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