6.2 ELECTRICAL SPACECRAFT CHARGING AT GEOSYNCHRONOUS ORBIT Under certain conditions, when the sun is active (^3 percent of the time) hot (kev) plasma can be generated in the tail of the geomagnetosphere (local midnight region of space) and propelled toward earth. This can have the following results: • Plasma will produce a net electron current (N amps/cm^) on exposed spacecraft surfaces. • Since surface electrical capacitances are small (p farads/cm^), spacecraft will acquire surface voltages at ^100 v/sec. • If no electrical breakdowns occur, voltages of >10 kev may result. • If electrical breakdown occurs, the discharge energy may find its way to spacecraft components, affecting them. The plasma is slowed down by the magnetic field of the earth until it has reached an altitude where the energy density of the earth's magnetic field exceeds that of the plasma. At such an altitude (^3 Re) the plasma is torn apart, the negative constituent (electrons) being pushed toward the dawn gradient of space, and the positive constituent (protons) being pushed toward the dusk gradiant (Figure 6.2-1). Spacecraft is near geosynchronous orbits which become immersed in this hot plasma will acquire a negative charge (and thus a negative voltage). The effects of such charging include: • Spacecraft will acquire negative voltage (plasma probes, ion spectrometers affected). • Surface electrical fields will result (de voltages on antennas, across insulators). • Electrical discharges may result across insulators (typical spacecraft windows, surfaces can stand ^kv). • Discharge energy can affect sensitive components (a/c for thermal control surfaces may increase, coatings on optical windows may lose transparency, electrical circuits may be tripped, overloaded). If the spacecraft can withstand surface voltages (and.voltage differences of ^20 kv) there is usually no problem because at such negative voltages the electron and proton currents are approximately equal and the charging stops. However, if the spacecraft surfaces cannot withstand voltages (and many spacecraft surfaces break down at 'vlO kv) discharges will occur. If part of this discharge energy (generally <1 joule) can find its way into sensitive electronic components (via conduction or radiation) such components may be adversely affected. As shown in Table 6.2-1, this has been the case in the past. Four DSCS-II spacecraft, four DSP spacecraft, 5 INTELSAT-III spacecraft, four INTELSAT-IV spacecraft, and two TELSAT spacecraft have been affected by hot
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