Spacecraft charging can detrimentally affect electrical operations on space systems. Most
communication and surveillance spacecraft are at geosynchronous altitudes, and many more such
spacecraft will be deployed in the new millennium. The plasma density in the geosynchronous
environment varies from over about 100 cm-3 to 0.1 cm-3 and the energy varies from a few eV to tens
of keV depending on local time and geomagnetic conditions. Spacecraft surface charging occurs at
high plasma energies. While surface material properties and spacecraft geometry are defined by
spacecraft design, spacecraft charging is controlled by the dynamic plasma condition which varies
hour by hour. Rapidly changing particle energy conditions play a major role in the onset of spacecraft
charging, whether it be simple surface charging or complex differential charging, The primary
factor in each case has recently been found to be the electron temperature. It had been previously
assumed that electron flux and electron density were the principal factors.
The best situational awareness regarding spacecraft operations and survivability should
include a warning when a threshold condition for spacecraft surface charging has occurred or is
imminent. No such real-time warning capability or product exists for US space environmental
operations. In fact, currently, post-anomaly assessment is done by examining the 50 to100 keV
electron fluxes for spikes believed to produce satellite charging and anomalies. This practice is
inadequate. Not only does it often give false alarms, but also it often misses real and significant
spacecraft charging events. It is often stated, without a concrete proof, that a surge of the total
electron density above an arbitrarily pre-defined level would indicate spacecraft charging. Such
statement is wrong and misleading.
A recent study [Lai and Della-Rose, 2001] using four weeks of spacecraft charging data
obtained on a Los Alamos National Laboratory (LANL) geosynchronous satellite has uncovered new
evidence for the existence of a critical temperature of the space plasma electrons for a given
spacecraft surface material. Below the critical temperature, little spacecraft charging occurs, while
above the critical temperature, the spacecraft potential increases almost linearly.
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